investigating the performance of micropiled raft in

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Investigating the performance of micropiled raft in

foundation of power transmission lines towers in cohesive soil: Experimental and numerical study

Journal: Canadian Geotechnical Journal

Manuscript ID cgj-2017-0027.R1

Manuscript Type: Article

Date Submitted by the Author: 10-May-2017

Complete List of Authors: Zekavati , Ali Asghar ; Niroo Research Institute, Structural Department

Khodaverdian, Alireza ; Niroo Research Institute, Structural Department Jafari, Mohamad ali; Niroo Research Institute, Structural Department Hosseini , Ahmad ; Iran University of Science and Technology,, School of Railway Engineering

Keyword: power transmission line, lattice tower, tensile behavior, micropiled-raft foundation, field loading test

https://mc06.manuscriptcentral.com/cgj-pubs

Canadian Geotechnical Journal

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Title:

Investigating the performance of micropiled raft in foundation of power transmission lines

towers in cohesive soil: Experimental and numerical study

Authors:

Ali-Asghar Zekavati (a)

, Alireza khodaverdian (b)

, Mohammad-Ali Jafari* (c)

, Ahmad

Hosseini (d)

(a) Research Assistant, Structural Department of Transmission and Distribution research

center, Niroo Research Institute (NRI), Tehran, I. R. of Iran, E-mail:

[email protected]

(b) Research Engineer, Structural Department of Transmission and Distribution research

center, Niroo Research Institute (NRI), Tehran, I. R. of Iran, E-mail:

[email protected]

(c) Assistant Professor, Structural Department of Transmission and Distribution research

center, Niroo Research Institute (NRI), Tehran, I. R. of Iran, E-mail: [email protected]

(d) Research Engineer, School of Railway Engineering, Iran University of Science and

Technology, Tehran, I. R. of Iran, E-mail: [email protected]

* Corresponding author: Mohammad-Ali Jafari, Niroo Research Institute (NRI), Structural

Department of Transmission and Distribution research center, Address: Dadman Blvd.,

Shahrak Ghds (Gharb), Tehran, I. R. of Iran. Tel.: 00982188079400, Fax: 00982188574786,

E-mail Address: [email protected]

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Abstract

This paper captures the behavior of micropiled raft in power transmission line tower

foundations in cohesive soil, concentrating on their uplift performance whether due to the

tower position along the line or under wind loading conditions. In this regard, first a number

of micropiles were driven into the ground of a project site at the ParehSar power plant, Gilan,

Iran. Compression and uplift loading tests were conducted according to relevant standards.

On the basis of the field data, a 3D finite-element model was developed and subsequently

calibrated and verified. The behavior of micropiled rafts subjected to uplift, which is a typical

type of loading in foundations of 230 kV four-circuit lattice towers, was then studied by

means of this model in terms of a wide-ranging parametric study. In the sensitivity analyses,

the impacts of various parameters, such as micropile spacing-to-diameter (s/d) and length-to-

diameter (l/d) ratios along with undrained shear strength of the soil have been investigated on

the uplift capacity of an individual micropile within and out of the group. Furthermore,

interaction factors were computed based on diverse values for undrained shear strength of the

soil, s/d ratio, l/d ratio, and grout-soil adhesion. From design and analysis perspectives, the

FEM outputs revealed that the efficiency coefficient of micropiled raft during uplift can be

considered equal to one. Moreover, it was found that not only does the behavior of micropiles

affect the neighboring micropiles immediately adjacent to the loaded one, it also influences

those in further rows, the result of which would be considering their significance as well.

Keywords: power transmission line; lattice tower; uplift behavior; micropiled-raft

foundation; field loading test; finite element method (FEM)

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Introduction

With the rapid development of civil infrastructures and the associated consumption of

power energy, a momentous portion of financial budget of governments is allocated to the

construction of high-voltage transmission lines. In soft soils, the majority of this

manufacturing cost is devoted to the tower foundation. This issue is owing to the fact that the

foundation dimensions should generally be enlarged in order to increase the ultimate bearing

capacity and reduce the settlement. In addition, for prevention of uplift phenomenon and

provision of higher overburden, the foundations are built into deeper levels.

In this regard, implementing a new foundation system for obviating the abovementioned

drawbacks appears to be indispensable. Among the common approaches, micropile group has

progressively been employed for execution of foundations in problematic soils. The first

presentation of micropile alludes to the retrofitting and underpinning of structures in Italy

during World War II (FHWA 2000). Micropiles have been adopted in many engineering

applications to date (Huang et al. 2007; Luna et al. 2015; Thompson et al. 2009). As a result,

considerable research and studies have been designated to investigating the performance of

these slender elements, Table 1.

Dependent on the type of soil, construction technique, grouting method, dimension of

micropile, loading type (axial compression or tension, lateral and combination of loads), the

results of the research mentioned in Table 1 are noticeably different with each other. This is

even more highlighted in the case of micropile groups in comparison with a single micropile

on account of the interactions among micropiles, soil, and raft.

In this regard, a widespread investigation on micropiles within a micropiled raft needs to

be conducted. Since the design procedure in most foundations is performed in a way so that

they would not experience tensile stresses, the research works on micropile groups have thus

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far been accomplished considering mostly compressive loadings. However, in the case of

foundations of lattice towers, one of the critical loads exerted is the uplift load due to typical

circumstances of these structures, particularly under wind loads. These loads could lead to the

entire uplift of the foundation, the result of which would be all its micropiles undergoing

uplift loading conditions. Taking such into account, in-depth analyses and studies of the

behavior of every single micropile located inside a group, as well as the raft global

performance have been carried out in this research.

In the current study, eight micropiles were driven and grouted (similar to the grouting

method type C) into the soft cohesive soil of ParehSar power plant. The site soil consists of

two clay layers overlain a sandy soil, and the area is surrounded by a significant number of

230 kV four-circuit transmission towers. After having implemented the test setup, a number

of axial compression, axial tension, and lateral load experiments were conducted on the

micropiles. On the basis of the experimental outputs, a three-dimensional finite element

model was developed, and the numerical results of an isolated micropile were calibrated.

Following its verification by field data published elsewhere, the numerical model was then

used to examine the uplift behavior of micropiled-raft foundation of tower.

According to the numerical analyses, the dimensions of tower foundation can considerably

diminish compared to the ordinary shallow foundation “pad and chimney.” Moreover, the

straightforwardness and speed of foundation execution highly improves in the case of

micropiled raft because of the reduction in earthwork operation and capability of construction

at levels above the water table.

Overall, the application of micropiles in foundation of transmission lines can be very

helpful from practical and technical points of view. For this reason, this study is devoted to

the behavior of micropiled raft foundations under uplift loadings, which also proves its

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effectiveness in enhancing the bearing capacity and in declining the foundation uplift risk. In

this respect, an experimental program including the site conditions, test setup, and field

results, as well as numerical analysis on an individual micropile and micropiled raft has been

presented in next sections.

Experimental study

In the first phase of the investigations on the behavior of micropile groups in foundations

of transmission towers, a series of field loading tests was accomplished so as to calibrate a

numerical model of the micropiles. In this section, the circumstances and properties of the

project field and its soil layers are initially described. The method utilized for installing the

micropiles along with its procedure will then be elucidated. Subsequently, the loading setup

for compression and tension experiments in conjunction with their outputs will be

represented. To end with, four distinct failure criteria have been introduced, one of which was

used to determine the micropile ultimate uplift capacity.

Site conditions and soil characteristics

The location of experiments was chosen in the vicinity of a combined-cycle power plant,

known as ParehSar. With the electricity generation capacity of 900 MW, this powerhouse is

situated in the northern part of Iran, Gilan, Rezvanshahr. In order to transfer the electrical

energy generated by this power station, 230 kV four-circuit power transmission towers have

been utilized. Due to the soft soil of this region, large shallow foundations must be

constructed in most cases for overcoming the settlements and upward movements caused by

the existing demand loads from the towers.

Figure 1 displays the specifications of soil strata and the results of standard penetration

test (SPT) that belongs to a borehole nearby the implementation site of the micropiles. As

evidenced in Figure 1, the subsurface layers are constituted by a soft cohesive soil overlain a

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dense sandy layer. The groundwater level has positioned adjacent to the ground surface on

account of the field’s close proximity to the coastline of the Caspian Sea. Tables 2-4 present

the outputs of the conducted field and laboratory experiments including grading, Atterberg

limits, consolidation, permeability and unconsolidated undrained (UU) triaxial tests on

various soil samples of the ParehSar site.

Micropile installation process

Execution of pilot micropiles in the field undoubtedly plays a constructive role in

simulating and predicting the performance of micropiles implemented beneath power

transmission towers. During the field tests, the conventional method in Iran, known as driven

micropile, was adopted, in which micropiles are driven and installed by a drop-hammer.

Using this technique, first a 230G-type perforated steel casing with the diameter of 76 mm

and thickness of 4 mm was rammed into the ground by means of a vertical sledge hammer,

Figure 2.

Depending on the borehole depth, the whole length is divided into a number of pieces for

ease of the task. It should be stated that the first piece of these two-meter casings does

possess a sharp tip so as to enable the other sections to be driven more conveniently into the

soil. After finishing each casing, the next one can be welded to the current one and the impact

process continues on the new segment. When the tip of the first casing reaches the target

depth, the grouting operation would commence by injecting grout through the micropile.

During the grouting process into the micropiles, inside of casing were located packings

with intervals of 2 m in order to have a uniform injection over the micropile. The volume of

the grout used for each micropile was 0.5 m3 on average. It should be mentioned that the

injection pressure during the process reached up to 1500 kPa. Afterwards, a 28-mm bar was

inserted into the final casing and also adjusted to be right in the middle of that. Eventually,

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the bearing plate was welded onto the head of micropile. Figure 3 depicts the implementation

steps of these types of micropiles.

Given the variety of existing loadings imposed on transmission towers, all the three kinds

of testing including compression, tension (pull-out), and lateral were conducted on the

micropiles in this research. In this respect, eight micropiles were executed in the pilot area,

according to the arrangement shown in Figure 4. In compliance with the guidelines of

standard tests on micropiles, (ASTM-D1143/D1143M-07(2013); ASTM D3689/D3689M-

07(2013)e1), performing compressive and uplift experiments each requires three micropiles

while two would suffice for carrying out a lateral test. It should be noted that considerations

will be restricted to only compression and, in particular, uplift tests as the major scope of the

current paper.

Field load test

As illustrated in Figure 5 (a), a steel frame was utilized during the compressive and uplift

tests as a reaction beam. Two micropiles, placed in the distance of 2.13 m from both sides of

the test micropile, held this frame so that the reaction force required against the anticipated

loading would be provided. In order to fulfill the experiments on these micropiles, a 600-kN

hydraulic jack for recording the load and three displacement gauges with an accuracy of 0.01

mm for reading the displacement of the tip of micropile were employed, Figure 5 (b).

The loading process was performed via the “quick test” method in compliance with the

standards mentioned in subsection 2.2, which is applied for soft soils. In this test method, the

loading continues with the successive increasing steps of 5% until the failure of micropile. It

should be emphasized that the loading on the micropile at each stage prolonged for four

minutes, and the corresponding displacement was noted at the end of each loading step.

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Upon achieving the target loading, the step-by-step unloading process could be initiated.

Unlike the loading process, each of the unloading stages continued for five minutes,

experiencing a load decrease of 20% in every phase. Considering the aforesaid conditions,

the compressive and uplift field loading and unloading tests were performed on the micropile

C1 and T1, whose outputs are respectively demonstrated in Figure 6 (a) and (b).

Failure criteria

According to (Fellenius 1996), describing the bearing capacity as a point for which rapid

displacement takes place due to a little increase in the loading is not an inclusive definition.

Alternatively, in many of research works, the bearing capacity considered is not equivalent to

the point at which the plunge failure occurs, either (Abd Elaziz and El Naggar 2014a;

Abdelrahman et al. 2003; Akgüner and Kirkit 2012; Décourt 2008; Elkasabgy and El Naggar

2014; Fellenius 1980; Gilchrist 1985).

On the basis of the graphs extracted from the loading experiments in the previous

subsection, calculating the ultimate uplift capacity of each micropile --provided the plunging

failure would not occur-- according to standard methodologies is greatly vital. Consequently,

four common and suitable approaches have been introduced to determine the uplift capacity

of micropiles from loading test results. Table 5 describes these methods along with presenting

approximate amplitudes of uplift and bearing capacities of the micropiles experimented at the

ParehSar site.

As can be seen in Table 5, the values of micropile ultimate uplift and bearing capacity

seem to be underestimated and overestimated, respectively by criteria (O'Neill et al. 1999)

and (Fuller and Hoy 1970). On the other hand, these figures are fairly close together

considering Davisson’s and Butler and Hoy’s criteria (Butler et al. 1976; Davisson 1972).

However, according to (Fellenius 1996), Davisson’s method is suitable and primarily adopted

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for analyzing the results of driven piles tested as per quick methods. For this reason,

considering the resemblance between driven piles and the micropiles utilized in the current

study, Davisson’s method has been exploited for estimating the uplift capacity. It is

noteworthy that changing the micropile diameter from 76 mm to 200 mm had a maximum

influence of 6% on its uplift capacity proposed in Davisson’s method.

Numerical analysis

Given the goal of this paper, the numerical model is developed for two purposes. The first

is investigating the behavior and characteristics of the micropiles executed in the ParehSar

site, as well as determining the effective injection diameter by calibrating the results of the

micropile numerical model based on the field test data. Second, the micropile groups are

modeled as the foundation of power transmission lines towers, whose behavior will be

investigated as well. In order to perform modeling and achieve the aforementioned

objectives, the finite element software package, ABAQUS, has been used. In the following

subsections, different parts of numerical simulations comprising material and geometric

modeling, as well as model calibration and verification will be explained.

Material modeling

Three types of material participate in micropile modeling. These are steel, grout, and the

soil surrounding the micropile. To begin with, a linear elastic relation has been considered to

model the micropile casing. Additionally, for simulating the grout adjoining the micropile, a

tri-linear behavior has been presumed based on the results of cylinder compression tests,

Figure 7. As evidenced in this figure, the grout stress-strain diagram linearly moves until a

strain value less than 0.001, after which it nonlinearly reaches its climax at an almost strain

and stress of 0.002 and 35 MPa, and then the sample failure occurs. Finally, the soil was

modeled as an elastic-perfectly plastic continuum taking into account the failure criterion of

Mohr-Coulomb. The strata’s characteristics encompassing depth, type, density, elastic

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modulus, cohesion, and internal friction angle are presented in Table 6. These parameters

have been determined using laboratory and in-situ tests.

To simulate their interface, the interaction between micropile and grout has been supposed

as a tie constraint. Besides, the behavior of interaction between the grout and the surrounding

soil was modeled in two different ways. In the former case, a tie behavior between the grout

and cohesive soil has been chosen. In this case, the master surface with higher stiffness is

completely constrained into the slave surface with less stiffness, and their degrees of freedom

are completely fixed together (Hibbett et al. 1998). A “hard” contact behavior, which limits

the penetration of the slave surface (soil) into the master surface (grout) and precludes the

formation of tension along their interface, was used to model the cohesive soil behavior.

In the latter one, on the other hand, normal and tangential frictional behaviors were

considered between the grout and granular soil. In order to model the tangential behavior of

soil-grout, the penalty method has been used for frictional behaviors based on the Coulomb

friction model. In this model, both the surfaces are capable of bearing shear stresses up to a

certain amount prior to their sliding threshold. It is necessary in modelling the penalty

behavior that the friction angle and the maximum shear stress prior to sliding would be

determined. It should be mentioned that the friction angle between the grout and the soil for

the granular layers in tension and compression were adopted as 29° and 39°, respectively.

Accordingly, the shear stress at the interface can be modeled using the following equations

(Hibbett et al. 1998):

τ�� = μp(1) where μ is friction coefficient and p, at initial stress conditions, equals to a normal stress

acting on the contact surface, both of which, in at-rest stress conditions, are computed as

follows:

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p = k�σ�(2)

� = tan �(3)

in these equations, k� is the coefficient of horizontal soil pressure at rest, σ� is the effective

stress along the surface, and δ is the friction angle of contact surface.

Geometric modeling

Due to the fact that two types of loading (compression and tension) were conducted during

the field experiments, and because of the symmetry of the current model, a half-model was

considered for the calibration so as to reduce the computational cost. In the modeling process,

a total number of 84570 elements type C3D8R with an average size of 0.2 m have been used;

it is a hexahedral (with six sides and eight nodes) continuum stress-displacement element

with first-order interpolation. Furthermore, reduced-integration elements were used along

with hourglass control in order to decline the length of data processing.

For mesh generation and reduction of boundaries effects, the recommendation by

(Helwany 2007), which is used to model a pile, was considered. In this case, the soil element

will extend up to 30 times the pile diameter from both sides, as well as 0.75 times the pile

length from the micropile tip. The node movements normal to all boundary surfaces are

restricted except for those on the top surface. Moreover, the dimensions and the optimum

arrangement of the meshes have been computed by performing sensitivity analyses to

eliminate the influence of these parameters on the FEM results. Figure 8 indicates a

representation of the mesh configuration for the 3D model of the micropile.

Calibration for single micropile

As stated earlier, after driving a micropile into the ground by impact energy, the grouting

task has to be started. During and after this operation, the grout would exit through the holes

existing on the body of micropile, penetrate the soil surrounding the micropile, and lead to an

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increase in the micropile diameter. The grout geometry adjacent to the micropile casing was

modeled based on the suggestion of (Abd Elaziz 2012).

According to him, the numerical model can be calibrated by accurately modeling the

micropile and the surrounding soil, altering the injection diameters of the model, and finally

comparing the outputs of each scenario to the field data. To this end, the following conditions

must be met so that the calibrated model would be acceptable, and the effective injection

diameter could precisely be determined (Abd Elaziz 2012) :

• The values of injection diameters are obligated to be the same for all the three

loaded micropiles.

• If the results for two injection diameters were equivalent, the smaller diameter

must be selected.

It is noteworthy with regard to the stratification of the ParehSar zone soil that because the

bottom section of the driven micropile is situated within a saturated sandy soil, it is

anticipated that the injection diameter in this layer would be greater than those in others.

Considering the statements mentioned above, the most appropriate injection diameter based

upon the calibration results are exemplified in Figure 9. The calibration results between the

field and numerical outputs can be observed in Figure 10. As depicted in this figure, the

numerical results are in good agreement with the field ones.

Verification for micropile group

Since no field tests on a micropiled raft were performed during the current research, it was

of great significance to verify the numerical model presented in the next section by the results

of a current experiment. Thus, due to an adequate resemblance between the conditions of the

present study and those of (Abd Elaziz 2012; Abd Elaziz and El Naggar 2014b), the outputs

of their field tests and numerical model have been selected as the relevant benchmarks.

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In their field tests, two hollow-bar micropiles with the interval of 776 mm were installed

into the ground and subsequently loaded. In order to simulate the compressive loading test,

the soil was modeled as an isotropic elastic-perfectly plastic continuum with the failure load

defined by the Mohr-Coulomb yield criterion. Figures 11 shows the dimensions, boundaries

and 3D mesh generation of the model of the micropile group. The elements used in this

model were type linear hexahedral and 80873 by number. Figure 12 illustrates the results of

the field and numerical models, indicating the sufficient accuracy of the present model.

Parametric study

In this section, as the focus of the current research, the performance of micropiles under

uplift loads will be comprehensively investigated. As mentioned previously, the research and

studies carried out on micropile groups so far have mostly been limited to investigating their

behavior versus compressive loads. The reason is due to the fact that the design procedure in

many structures is performed in a way so as to virtually eliminate the risk of foundation

uplift.

In foundations of lattice towers, however, the foundation of each leg can predominantly be

subjected to the uplift or compressive loading due to various circumstances of wind blowing

or the location of tower within the transmission line. Accordingly, all the micropiles within

each foundation will also undergo pure uplift loads. In order to take this into account, in this

paper, the performance of a single micropile within groups utilized beneath 230 kV four-

circuit power transmission towers has been modeled and studied under uplift loads. In this

regard, the efforts are classified into two different parts: a) uplift capacity of individual

micropile within the group, and b) interaction of micropiles in group for calculating the

upward movement.

Capacity of single micropile in group

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For investigation of the capacity of micropiled raft as the foundation of power

transmission towers, the configuration of micropiles in the raft has been assumed similar to

Figure 13. The reason hiding behind this arrangement is that the effect of micropiles in

adjacent rows on the results of analysis would be perceptible. As a consequence, to evaluate

the uplift capacity of each micropile, the micropile #1 is loaded in two separate conditions: a)

when the other micropiles are activated, and b) in the case where the other micropiles are

excluded. The aim of doing so is to compare the uplift capacity of micropiles in two cases of

within the group and out of it together. For the parametric study in this section, the space

between two micropiles (with diameter of d) is symbolized as s.

It should be remarked that on account of the symmetry of the micropile group, only a

quarter of Figure 13 has been modeled. There are two types of interactions for the raft of

micropiled foundations, raft-soil and raft-soil-micropile (Lee and Moon 2017). Due to the

fact that the raft in the current research undergoes merely uplift forces, and more importantly,

the raft cannot have any contributions in lowering the upward movement of the foundation in

terms of interaction with soil, its effect has been excluded from simulations similar to (Abd

Elaziz and El Naggar 2014b; Clancy and Randolph 1993; Mandolini and Viggiani 1997;

Poulos 1968; Poulos 1971,1979,1988; Sharnouby and Novak 1990). In addition, the

dimensions of the micropile and the grouted zone were determined in keeping with the results

of the micropile located inside the soft soil of ParehSar site. It was thus required to evaluate

the impact of major parameters on the FEM results, and hence various parametric studies

were completed based on two categories of the characteristics below.

• Properties of the surrounding soil: for evaluation of the effect of the cohesive soil

characteristics on the uplift capacity, three different types of soil with undrained shear

strength of 60, 90, and 120 kPa were adopted. In this survey, the length-to-diameter

ratio of micropile has consistently been considered as 160 for all the samples; and

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• Length-to-diameter ratio: the magnitudes of this parameter were selected as 40, 80,

and 120, which are much applicable in actual projects. Furthermore, the undrained

shear strength of the soil for all simulations has been chosen as 60 kPa.

It is significant to state that during the course of parametric studies, keeping the other

parameters constant was of great importance so that the distinct effect of each factor would

be carefully assessed. In all the analyses, the Poisson’s ratio was considered as 0.499.

Moreover, the values of elastic modulus have been figured by considering the following

common equation with regard to normally-consolidated soils (Briaud 2013).

� = 500��(4)

Where E is soil elastic modulus, and Su is the undrained shear strength of soil.

Figure 14 (a)-(c) shows the semi-bilinear load-displacement graphs in relation to the

performance of a single micropile placed in the micropile group under uplift loading

conditions, respectively for three different spacing-to-diameter ratios (s/d) of 3, 4.5, and 6, as

well as three various soil undrained shear strength of 60, 90, and 120 kPa. Obviously, in all

these graphs, the uplift capacity of each loaded micropile in the group is approximately equal

to that of an isolated micropile out of the group.

It can be concluded from these graphs that in all cases, the effect of “s/d ratio” compared

to that of soil undrained shear strength on the failure load is practically negligible. For

instance, noting to Figure 14 (a), if the distance among the micropiles is multiplied by two,

the uplift capacity will be decreased within 5%. Intensifying the soil undrained shear strength

by the same amount, on the other hand, would result in a failure load nearly two times the

original one, Figure 14 (a) and (c). This reveals the significance of the interaction between

soil and the grout around the micropile. In other words, the more the soil shear strength, the

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more the adhesion between the two contact surfaces, and as a result the more uplift capacity

will be obtained.

Moving to the second phase of parametric study of this part, the results of uplift loading on

micropile groups are illustrated in Figure 15. In this figure, substantial factors are three

different spacing-to-diameter ratios of 3, 4.5, and 6 in conjunction with three diverse length-

to-diameter ratios of 40, 80, and 120 to increase the practicality of the outputs. Similar to the

first case, the uplift capacity of an internal micropile (within the group) is equal to that of an

external one (isolated from the group).

It can thus be established that an efficiency coefficient equal to one could be designated to

the uplift capacity of a micropile within the group subjected to uplift loading conditions. It is

noteworthy that a similar conclusion has been drawn by (Abd Elaziz and El Naggar 2014b)

for the bearing capacity of micropile groups under compressive loading conditions.

According to the recommendation of existing standards such as (FHWA 2000), in the case

where the efficiency coefficient is greater than or equal to one, this coefficient must be

assumed equal to one.

In addition, the uplift capacity is not much influenced by the “s/d ratio” in various length-

to-diameter proportions. On the contrary, the latter ratio considerably affects the failure load

ranging from 8.6 to 340 kN corresponding to the length-to-diameter ratios of 40, 80, and 120,

respectively displayed in Figure 15 (a) to (c). The reason is that the effective length of

micropile would increase, leading to a better interaction of soil and grout with more plausible

failure sections.

Micropile performance in group

In order to have an inclusive understanding of the behavior of a single micropile in group

under uplift loading conditions, the interaction among them based on different values of

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spacing should be of high priority. In other words, this section aims to deal with the upward

movement of each micropile within the group in terms of soil strength, arrangement of

micropiles and interaction parameters by using a concept called interaction factor. It should

be declared that only the impacts of the micropiles #1 to #6 depicted in Figure 13 will be

investigated on the numerical results. This is due to the fact that since there exists a symmetry

in the micropiled raft, the outputs of the other micropiles will ideally be the same.

In this regard, the parametric studies have been performed in the following situations:

• Characteristics of the surrounding soil: six different types of soil with undrained shear

strength of 20, 30, 60, 90, 120, and 150 kPa were studied to gain an insight into the

effect of this parameter on the generated interaction. In this investigation, the length-

to-diameter ratio of 160 as well as the tie constraint between soil and grout has been

considered for all the numerical models in a consistent manner;

• Ratio of length to diameter: three magnitudes of 40, 80, and 120, which are relatively

prominent in actual projects, were adopted. Similar to the upper item, the

characteristics of soil (Su=60 kPa) and fully bond grout-soil interface have been kept

constant for all the samples; and

• Interface specifications: three adhesion coefficients of � = 0.8,0.9"#$1.0 were

used. The length-to-diameter ratio of 160 and soil properties (Su=60 kPa) were

identical for every simulation.

For taking the adhesive behavior between soil and grout into consideration in parametric

studies, tangential and normal behaviors existing in ABAQUS software were utilized. In

order to model the normal behavior, a hard contact was defined so that the contact elements

would not interpenetrate. Furthermore, the stress limit has been used to simulate the adhesive

behavior in tangential direction (Kim et al. 2007; Theinat 2015). In this regard, the internal

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friction angle must be considered a great number so as to completely replicate the adhesive

behavior in ABAQUS environment.

Equation (2), which is related to the displacement of a micropile group, has been used to

approximate the intensity of interactions in this research. The loading is exerted on the

micropile #1 (Figure 13), and the displacement of neighboring micropiles, #2 through #6, is

recorded at the ultimate load on the micropile #1. The ratio between the displacement of each

micropile due to the loaded micropile (#1 in this case) and that of itself alone under the same

load is considered as the interaction factor, αv. The simplified form of this parameter is (Abd

Elaziz and El Naggar 2014b):

�% = �&' ( �)*+

*,-(5)

Where �% and �&' are the upward movements of group and single micropile at the average

load applied to an individual micropile of group, respectively. The usage of the methodology

for calculating the settlement of pile groups has comprehensively been indicated by (Poulos

2006; Poulos and Davis 1980)

Figure 16 (a) to (e) exhibits the interaction factor in terms of the ratio s/d based on various

soil undrained shear strength for the micropiles #2 through #6. As shown in these graphs,

parallel to the improvement of soil shear strength, the interaction factor significantly

diminishes. Additionally, the variations of interaction factor in soft soils are steeper with

respect to that in soils with suitable strength characteristics. In fact, changing the spacing of

micropiles in stiff clay soils is not expected to cause so much discrepancy in the upward

movement of adjoining micropiles. Contrarily, in soft clay soils, the upward movement of

these micropiles is highly dependent on the spacing.

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It can be observed in the graphs of Figure 16 that overall in the ratio s/d domain of 3-15,

the interaction factor alters in the range 0.025-0.375. Interestingly, this range has also nearly

been obtained by (Abd Elaziz and El Naggar 2014b) in the case of compression loading

imposing on a micropiled raft, while the s/d ratio varies between 2.5 and 15 for stiff to soft

soils. It is also understandable that the relationship between interaction factors and s/d ratio

seems to be linear for soils with undrained shear strength more than 60 kPa the less of which

devotes to the soils that this relationship for them is nonlinear.

The graphs represented in Figure 16 have been reproduced once more in Figure 17, but

this time based upon diverse length-to-diameter ratios. As can be seen in Figure 17 (a) to (e),

for example at the point of s=3d on the horizontal axis, the less the ratio of length to diameter

is, the more the interaction factor and the effective contact have been obtained. The reason

behind this behavior is due to the fact that the micropiles with less length reach their bearing

capacity at strains much less than larger ones do. Consequently, the soil surrounding shorter

micropiles still behaves elastically in many locations due to the induced smaller strains and,

as a result, will not enter into the plastic phase. Thus, since the soil in this case acts elastically

in many points, it is expected that the micropiles would have a more pronounced impact on

each other, and the values of interaction factor would be higher. However, in some cases such

as Figure 17 (e), this trend is absolutely changed by increasing the ratio s/d, and in some

others, the results converge, as shown in Figure 15 (b) to (d). It can be found that by choosing

an optimum spacing, the length of micropiles can be substantially reduced. While the

interaction factors, regardless of the soil strength, are different from 0.025 to 0.36 in Fig. 17,

it is noteworthy that (Abd Elaziz and El Naggar 2014b) have calculated this parameter

between 0.025 and almost 0.3 for stiff soil as well as 0.0375 and 0.375 for soft soil. Although

in their study, the trend of diagrams becomes completely reversed as the ratio s/d increases.

Moreover, the ratio l/d in their study varied from 30 to 75, whereas it has been considered a

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variable between 40 and 160 in the current research. This difference is reasonable because of

the fact that the micropiles utilized in the present study were practically twice those utilized

by (Abd Elaziz and El Naggar 2014b) in terms of length.

Figure 18 illustrates the interaction factor versus the ratio s/d considering three different

values for grout-soil adhesion. It should be mentioned that in order to avoid generating

repetitive outputs, the numerical results of micropiles #5 and #6 were omitted. Moreover, a

proper linear equation can be fitted to the scattered data of these graphs, specifically up until

the point s=12d. The reason of interaction factors varying linearly with adhesion coefficients

is that the displacement generated at the element interface is negligible; because the grout and

soil have been clamped together virtually as a fixed constraint before the sliding threshold

occurs at the micropiles interaction. As the adhesion coefficient in the system modeled has

always been less than or equal to one, sliding along the micropile skin will take place prior to

when the soil reaches its plastic bearing capacity. Therefore, the critical shear stress will be

the determinant of bearing capacity and interaction factor, and it is expectable that it would

vary relatively linearly in terms of the adhesion coefficient. As shown in Figure 18 (a) to (c),

given the position of data points for each grout-soil adhesion, it is expected that the relation

between the interaction factor and this parameter would also be linear for different

amplitudes of spacing.

Generally, noting to Figures 15-17, the uplift capacity and upward movement of a

micropiled-raft foundation cannot be accurately determined by merely considering

neighboring micropiles of the first rows surrounding the loaded one, and the micropiles

located in next rows must also be taken into account in calculations. This is even more

significant especially in the case where the intervals among micropiles are relatively low. For

instance, according to Fig 16, in the micropile group with ./$ = 3 and undraind shear

strength of 20 kPa, the interaction factor for the micropiles #2 and #4 is 0.37 and 0.34,

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respectively. The closeness of these values indicates that the next rows of micropiles have

considerable effect on the loaded micropile in the group. The interaction factors presented in

this section will obviate the need for performing time-consuming simulations and, as a result,

are beneficial for civil engineers to have an assessment of the upward movement difference

as well as uplift loading distribution among micropiles, respectively for foundations with

flexible and rigid caps. The whole settlement of groups with rigid caps can also be computed

using this approach.

Conclusions

As explained earlier, uplift loads prevail over the other loads in foundations of lattice

towers in power transmission lines, particularly under extreme wind loads conditions.

Nevertheless, the performance of micropiles under such loading circumstances has not

sufficiently been appreciated, especially in terms of a comprehensive parametric study.

In this research, due to the economical and practical benefits of micropiled raft respect to

pad and chimney foundations, field and numerical assessments were carried out on an

isolated micropile and a micropiled-raft foundation, respectively. First, a number of

micropiles were implemented into the project site in the vicinity of ParehSar power plant for

loading experimentation. In the next phases, a numerical model was developed, calibrated

and verified based on the outputs of the field tests of the current study and elsewhere along

with a detailed parametric study on important factors influencing the micropiled raft

behavior. The key outcomes of this study can be mentioned as follows:

1. Considering the cohesive soils with various strength conditions, the ultimate uplift

capacity of a single micropile inside the raft is greater or equal to that of an isolated

one. In other words, the value of efficiency coefficient of micropiles subjected to

uplift loads would be more than one in foundations of power transmission towers.

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2. In different length-to-diameter ratios, the ultimate uplift capacity of an individual

micropile within the group is expected to be more than that of the same one outside

the raft. In the same way, it can be concluded that by changing the length-to-diameter

proportion in cohesive soil, the efficiency coefficient does not vary and can be

assumed equal to one.

3. The interaction factor of micropiles declines with the increase of spacing. This

reduction with regard to soft soils is remarkably larger than that in the case of stiff

cohesive soils. The reason for this refers to the ratio between the displacement

difference and spacing of two adjacent micropiles (deformed shape of soil media). As

the displacement difference in soft soils is more than that in stiff soils, the less

reduction rate of interaction factors in terms of spacing in stiff soils is justifiable.

4. With the improvement of strength characteristics of cohesive soil, the interaction

factor will decrease. This means that micropiles in stiffer soils have less influence on

their adjacent ones. This is because the bearing capacity of each micropile in terms of

soil interaction increases, making them less dependent on each other. In other words,

when soil becomes stiff, its impact on the additional settlement of a micropile due to

its neighbors will reduce significantly more than the case where that micropile is

loaded alone.

5. By growing the length-to-diameter ratio in low magnitudes of spacing, the interaction

factor is increased. However, in greater intervals, this proportion and the trend of

results would not remain constant.

6. The grout-soil adhesion has virtually a linear relationship with the interaction factor;

this fact has been demonstrated in all the adjacent micropiles.

7. According to the results of interaction factor in micropiles, it seems that in addition to

the micropiles adjoining the loaded one, those in the next rows must also be

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considered when calculating the micropile ultimate uplift capacity and the raft uplift

movement.

Acknowledgment

The authors are grateful to the Thermal Power Plants Holding Company for their financial

support with regard to this research project. Thanks also go towards Dr. Ahmed Yehia Abd

Elaziz for his helpful comments regarding verifying the numerical model of the present study.

References

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cohesive soils. The University of Western Ontario.

Abd Elaziz, A.Y., and El Naggar, M.H. 2014a. Geotechnical capacity of hollow-bar

micropiles in cohesive soils. Canadian Geotechnical Journal 51(10): 1123-1138.

Abd Elaziz, A.Y., and El Naggar, M.H. 2014b. Group behaviour of hollow-bar micropiles in

cohesive soils. Canadian Geotechnical Journal 51(10): 1139-1150.

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Akgüner, C., and Kirkit, M. 2012. Axial bearing capacity of socketed single cast-in-place

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Alnuaim, A., El Naggar, H., and El Naggar, M. 2014. Performance of micropiled raft in sand

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Alnuaim, A.M., El Naggar, M.H., and El Naggar, H. 2016. Numerical investigation of the

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Benslimane, A., Juran, I., Hanna, S., Drabkin, S., Perlo, S., and Frank, R. 1998. Seismic

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Conference on Case Histories in Geotechnical Engineering. Missouri University of

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Briaud, J.L. 2013. Geotechnical Engineering: Unsaturated and Saturated Soils. Wiley.

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Choi, C., and Cho, S.-D. 2010. Field verification study for micropile load capacity. In Proc.

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Clancy, P., and Randolph, M. 1993. An approximate analysis procedure for piled raft

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Davisson, M. 1972. High capacity piles. Proceedings, Soil Mechanics Lecture Series on

Innovations in Foundation Construction: 81-112.

Décourt, L. 2008. Loading tests: interpretation and prediction of their results. In From

Research to Practice in Geotechnical Engineering. pp. 452-470.

Elkasabgy, M., and El Naggar, M.H. 2014. Axial compressive response of large-capacity

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Fellenius, B.H. 1980. The analysis of results from routine pile load tests. Ground Engineering

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Fuller, F.M., and Hoy, H.E. 1970. Pile load tests including quick-load test method,

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Huang, Y., Hajduk, E.L., Lipka, D.S., and Adams, J.C. 2007. Micropile load testing and

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Kershaw, K.A., and Luna, R. 2014. Full-Scale Field Testing of Micropiles in Stiff Clay

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5606.0000968.

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on behavior of piled raft foundation. KSCE Journal of Civil Engineering 21(1): 243-

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Foundations of Bridge during Construction. In IFCEE 2015.

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791-816.

O'Neill, M.W., Reese, L.C., Infrastructure, U.S.F.H.A.O.o., and Soils, A.A. 1999. Drilled

Shafts: Construction Procedures and Design Methods. ADSC, the International

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Poulos, H. 1968. Analysis of the settlement of pile groups. Geotechnique 18(4): 449-471.

Poulos, H. 2006. Pile group settlement estimation—Research to practice. In Foundation

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Poulos, H.G. 1971. Behavior of laterally loaded piles II. Pile groups. Journal of Soil

Mechanics & Foundations Div.

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Geoenvironmental Engineering 105(ASCE 15032).

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geotechnical engineering 114(6): 697-706.

Poulos, H.G., and Davis, E.H. 1980. Pile Foundation Analysis and Design. Wiley.

Sadek, M., Shahrour, I., and Mroueh, H. 2006. Influence of micropile inclination on the

performance of a micropile network. Proceedings of the Institution of Civil Engineers

- Ground Improvement 10(4): 165-172.

Shahrour, I., and Juran, I. 2004. Seismic behaviour of micropile systems. Proceedings of the

Institution of Civil Engineers Ground Improvement 7(1): 1-12.

Sharnouby, B.E., and Novak, M. 1990. Stiffness constants and interaction factors for vertical

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Theinat, A.K. 2015. 3D numerical modelling of micropiles interaction with soil & rock.

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List of symbols

l = micropile length

d = micropile diameter

E = soil elastic modulus

s = space between two micropiles

�% = upward movement of group

�&' = upward movement of single micropile

Su = undrained shear strength of soil

α = grout-soil adhesion coefficient

αv = interaction factor among micropiles

µ = friction coefficient of the contact surface

δ = friction angle of the contact surface

τin = shear stress at soil-grout interface

p = normal stress acting on contact surface

k0 = coefficient of horizontal soil pressure at rest

σv = effective stress along the contact surface

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List of Figure captions

Figure 1 Subsurface conditions of ParehSar power plant

Figure 2 Equipment required for micropile execution process

Figure 3 Phases of Installation of micropiles used in Iran

Figure 4 a) Schematic and b) photo indicating the arrangement of micropiles within their

implementation site

Figure 5 a) Typical test setup of a micropile experiment and b) hydraulic jack and

displacement gauges

Figure 6 The outputs of a) compressive and b) uplift loading experiments on the micropiles

C1 and T1, respectively

Figure 7 The compressive stress-strain relationship of the injected grout according to the

results cylinder compression tests along with its simplified form

Figure 8 A 3D mesh for the half model of the micropile in the ParehSar site

Figure 9 Dimensional characteristics of the micropile in the ParehSar site

Figure 10 Calibration results of the micropile numerical model and the field loading tests in

the ParehSar site: a) compression and b) uplift

Figure 11. a) 3D mesh generation along with distances towards the model adjacent

boundaries; b) general dimensions of micropile in the model

Figure 12. Verification of the present model by the field data of (Abd Elaziz and El Naggar

2014b) within the elastic phase under compression.

Figure 13 Schematic of the micropile group considered for parametric study

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Figure 14 uplift load-displacement graphs for the micropiles located inside the soil with Su=

a) 60, b) 90, and c) 120 kPa

Figure 15 uplift load-displacement relationships for the micropiles with length-to-diameter

ratios of a) 40, b) 80, and c) 120 (Su = 60 kPa = constant)

Figure 16 Interaction factors at the uplift ultimate load of micropile #1 based upon different

soil cohesion coefficients for the micropiles a) #2, b) #3, c) #4, d) #5, and e) #6

Figure 17 Interaction factors at the uplift ultimate load of micropile #1 based on various

length-to-diameter ratios for the micropiles a) #2, b) #3, c) #4, d) #5, and e) #6

Figure 18 Interaction factors (�)) at the uplift ultimate load of micropile #1 based on various

magnitudes of adhesion (�) for the micropiles a) #2, b) #3, and c) #4

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Table 1. Recent investigations regarding the performance of micropiles.

Year Researchers Details

1998 Benslimane

et al.

Summary: investigation of the earthquake loading response and soil-

micropile interaction behavior; centrifugal model tests on group and network

systems embedded in loose to medium dry sand; group effects were

investigated for different configurations and at different levels of loading.

Key consequences: the results indicate a positive group effect, which leads to

smaller bending moments and displacements in pile group compared to a

single micropile; fundamental frequency of micropile system is strongly

affected by the micropile system configuration.; the p-y curves for micropile

group obtained under strong shaking were found to be non-linear and with a

low damping.

2004 Shahrour

and Juran

Summary: numerical and experimental study of micropile seismic

performance; investigating the effect of the main controlling parameters of the

micropile–soil–structure system, such as the kinematic interaction, group

effect, influence of micropile inclination, and performance assessment of

micropile systems in liquefiable soils.

Key consequences: reduction in micropile bending moment in group

compared to a single micropile under seismic loading; improvement of group

performance including an increase in micropile systems stiffness and

reduction in shear and bending moment in raft using an inclined micropile

system.

2006 Sadek et al.

Summary: numerical modeling of micropile group under lateral and vertical

loads; the groups were constituted by two rows of micropile with opposite

inclination; finite-element analyses were carried out using PECPLAS code.

Key consequences: inclined micropiles lead to improvement of group

performance under both lateral and vertical loads; under lateral loading

conditions, the inclination of micropiles leads to sufficient mobilization of the

axial stiffness of micropiles, and subsequently results in an increase in the

lateral stiffness of the group as well as the reduction of both the shearing

forces and bending moments in micropiles.

2010 Choi and

Cho

Summary: Field verification study for micropile load capacity; two sets

(installed to the depth of soil and rock layers) of field compressive loading

test (loose silty sand for set 1 and loose silty sand and gravel to rock layer for

set 2); utilizing two micropiles with different types A and similar to C

(geotextile-pack-micropile).

Key consequences: More uniform distribution of unit frictional resistance at

soil layer in micropile type C than A; more bearing capacity (at least 20%) of

micropile type C than A. (A, B, C, D grouting types are determined according

to FHWA (2002) criteria. A: grout is placed under gravity head only, B:

pressure through casing, C: single global postgrout, D: multiple repeatable

postgrout).

2014 Kershaw

and Luna

Summary: Full-scale field testing (two axial-compression, two lateral-load,

and four combined-load tests) of micropiles type A in stiff-to-hard clay

underlain by weathered silty/sandy shale; applying a static axial load (half of

the ultimate failure load) and incrementally increasing the lateral load until

the maximum lateral deflection of 63 mm corresponding to the ratio of 0.6

between the lateral and axial load.

Key consequences: The ultimate axial and lateral capacity of micropiles;

applying a static axial load has a minimum impact on the micropile lateral

behavior in stiff clay soil.

2014a

Abd Elaziz

and El

Naggar

Summary: Full-scale field (stiff to very stiff silty clay to clayey silt underlain

by a sandy layer with traces of silt) testing (three compression and two

tension monotonic axial tests on four micropiles type B) and numerical

analysis (A two-dimensional axisymmetric finite element (FE) model);

calibrating the model by the experimental results leading to an appropriate

failure criterion for micropiles; performing a parametric study to investigate

the effect of some important parameters on the micropile capacity.

Key consequences:

�� = �0.91� ��1 + 0.35�� + 9 �� for compressive loading;

�� = �0.91� ��1 + 0.0.275�� + 9 �� for uplift loading.

PG: geotechnical capacity of micropile, cu: undrained shear strength of the

soil, d: micropile diameter, L: micropile length, Vinc: percentage increase in

micropile volume, Ainc: percentage increase in micropile cross section.

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2014b

Abd Elaziz

and El

Naggar

Summary: Full-scale field study (four axial monotonic tests) on single

micropile and micropile group in stiff silty clay deposit as well as numerical

investigation using 3D FE analysis and calibrate-verify the model;

performing a parametric study to characterize the capacity and performance

of micropile groups.

Key consequences: Establishing the group efficiency of one and interaction

factor curves for evaluation of micropiles group capacity and response

(settlement) under vertical loading as well as the interaction between two

micropiles considering the spacing between them, the soil and pile properties,

and the slenderness ratio of micropile.

2014 Alnuaim et

al.

Summary: Five centrifuge tests at 50g acceleration on micropiled raft

located inside a sandy soil under a vertical point load; investigating the

impact of raft flexibility on its behavior by changing its thickness.

Key consequences: raft global and local settlements, raft contact pressure

and bending moment, raft-micropiles load shearing; 37% to 48% load bearing

of micropiles (reduction of raft contact pressure); applicability of Poulos-

Davis-Randolph (PDR) method for relatively stiff rafts, but not for flexible

ones (need correction factor).

2015 Alnuaim et

al.

Summary: Three centrifuge tests at 50g acceleration to assess the

performance of micropiled raft subjected to a concentrated vertical load in a

clayey soil (single micropile and single raft).

Key consequences: initial outputs similar to (Abd Elaziz and El Naggar

2014); considerable influence of micropiled raft on system axial stiffness

with respect to an isolated raft; reasonable accuracy of PDR method

prediction of axial stiffness of the relatively rigid raft (flexible rafts need

separate evaluation); converging the micropiled raft behavior to an isolated

raft as the ultimate load approaches its maximum value.

2016 Alnuaim et

al.

Summary: Conducting a numerical study including 78 FE analyses in order

to evaluate the performance of micropiled rafts located inside sand;

calibration and verification of the model by employing the results of

centrifuge tests; studying the effects of micropile spacing, raft thickness, sand

density, and loading type on the bearing pressure tolerable for the raft, its

axial stiffness, differential settlement, load shearing, bending moment, as

well as micropile skin friction.

Key consequences: Suitable capability of PDR method to predict the

stiffness of a rigid raft with up to 3% error of the calibrated FEA results

(adjustment factor has been introduced for flexible rafts); differential

settlement reduction of 20-40% for micropile spacing-to-diameter ratios less

than 10; higher impacts of micropiles on a flexible raft in the case of loadings

with uniform distribution.

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Table 2. PSD properties and Atterberg limits of the soil at the ParehSar site.

Depth (m) Sample type

Classification

Particle size distribution (% passing) Atterberg limits (%)

76

mm

4.8

mm

0.075

mm

0.002

mm LL PL PI

2.5 Core sample 100 100 89 11

4 Undisturbed sample 48.6 27.5 21.1

7 Core sample 100 100 92 19

10 Core sample 100 100 86 15

13 Core sample 100 100 7 0

16 Core sample 100 100 25 4 36.5 25.3 11.2

Table 3. The results of consolidation and permeability tests on the soil at the ParehSar area.

Consolidation

Permeability (cm/s) Pressure range

(kg/cm2)

Coefficient of volume

compressibility

(cm2/kg×0.01)

Coefficient of consolidation

(cm2/s×0.001)

Pˊ mv cv k

0.25-0.5 5.0×10-2 1.2×10-3 6.3×10-6

0.5-1 4.5×10-2 1.3×10-3 6.0×10-6

1-2 3.1×10-2 1.2×10-3 4.0×10-6

2-4 1.7×10-2 1.5×10-3 2.6×10-6

4-8 9.0×10-2 2.0×10-3 1.8×10-6

8-16 4.7×10-2 2.4×10-3 1.1×10-6

Table 4. The results of UU triaxial tests on the soil at the ParehSar site.

Depth (m) Undrained shear

strength, su (kPa)

Undrained tangent

modulus, Eui (MPa)

Undrained secant

modulus, Eus (MPa)

2.5 19 16 7

7 18 32 20

10 20 26 11

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Table 5. Failure criteria for determining the uplift and bearing capacity of micropiles.

Failure criterion Failure load definition Remark Uplift (kN) Compression (kN)

Davisson’s offset

limit

1972

Load corresponding to total settlement

at the pile head =!"

#$+

%

&'(+ 4�**�

P: applied load, L: micropile length,

A: section area of micropile, E: elastic

modulus, d: micropile diameter.

Davisson

(1972) 166 295

Reese and O’Neil

1988

Load corresponding to total

displacement = 5% d. FHWA (1988) 134 225

Butler and Hoy

1977

Load at the intersection of tangent

sloping at 0.14mm/kN and tangent to

initial straight portion of total

settlement curve.

NYSDOT

(2008) 148 284

Fuller and Hoy

1970

Minimum load for a rate of total

settlement of 0.14 mm/kN. FHWA (2005) 172 391

Table 6. Geotechnical properties of soil strata at the ParehSar area.

Depth (m) Soil type* Density

(kN/m3)

Elastic

modulus

(MPa)

Poisson’s

ratio

Cohesion

(kPa)

Internal

friction

angle (°)

Dilation

angle (°)

0-6.5 CL 19 20 0.45 17.5 2 0

6.5-11 CL 19 30 0.45 18 5 0

11-12 SM 20 50 0.35 10 35 5

12> SP-SM 20 60 0.3 1 40 10

*soil types have been categorized according to Unified Soil Classification System (USCS), CL: Lean clay, SM:

silty sand, SP -SM: poorly graded sand with silt.

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Figure 1. Subsurface conditions of ParehSar power plant.

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Figure 2. Equipment required for micropile execution process.

Driving equipment

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Figure 3. Phases of Installation of micropiles used in Iran.

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Figure 4. a) Schematic and b) photo indicating the arrangement of micropiles within their implementation site.

(a)

(b)

T1

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Figure 5. a) Typical test setup of a micropile experiment and b) hydraulic jack and displacement gauges.

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Figure 6. The outputs of a) compressive and b) uplift loading experiments on the micropiles C1 and T1, respectively.

(a) (b)

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Figure 7. The compressive stress-strain relationship of the injected grout according to the results of cylinder compression tests along with its simplified form.

0

5

10

15

20

25

30

35

0 0.0005 0.001 0.0015 0.002 0.0025 0.003

Axia

l str

ess (

MPa

)

StrainTri-linear relationship Laboratory data

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Draft

Figure 8. A 3D mesh for the half model of the micropile in the ParehSar site.

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Figure 9. Dimensional characteristics of the micropile in the ParehSar site.

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Figure 10. Calibration results of the micropile numerical model and the field loading tests in the ParehSar site: a) compression and b) uplift.

(a) (b)

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Figure 11. a) 3D mesh generation along with distances towards the model adjacent boundaries; b) general dimensions of micropile in the model

(a) (b)

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Figure 12. Verification of the present model by the field data of (Abd Elaziz and El Naggar 2014b) within the elastic phase under compression.

0

200

400

600

800

1000

1200

1400

1600

1800

0 5 10 15 20 25 30

Load

(kN

)

Group displacement (mm)

FE model (Abd Elaziz and El Naggar (2014b)) FE model (current research) Field data

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Figure 13. Schematic of the micropile group considered for parametric study.

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Figure 1

14. Uplift loa

ad-displacem

ment graphs 60, b) 90

for the micr0, and c) 120

ropiles locate0 kPa.

ed inside thee soil with Suu= a)

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Draft

Figure

e 15. Uplift lraload-displacatios of a) 40

cement relati0, b) 80, and

onships for td c) 120 (Su =

the micropil= 60 kPa = c

les with lengconstant).

gth-to-diame

eter

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Draft

Figure 1

16. Interactiocohesion c

on factors at coefficients

the uplift ulfor the micr

timate load opiles a) #2,

of micropile, b) #3, c) #4

e #1 based up4, d) #5, and

pon differend e) #6.

nt soil

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Draft

Figure 1

17. Interactioto-diame

on factors at eter ratios fo

the uplift ulor the microp

ltimate load piles a) #2, b

of micropileb) #3, c) #4,

e #1 based ond) #5, and e

n various len) #6.

ngth-

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DraftFigure 18. Interactmagnit

tion factors (tudes of adhe

( ) at the upesion ( ) for

plift ultimater the microp

e load of micpiles a) #2, b)

cropile #1 b) #3, and c)

ased on vari#4.

ious

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investigating the performance of micropiled raft in

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