ORIGINAL PAPER

Re-assessing a soil nailing design in heavily weathered graniteafter a strong earthquake

Sergio A. Villalobos • Paulo L. Orostegui •

Felipe A. Villalobos

Received: 26 August 2011 / Accepted: 18 January 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract Soil nailing is increasingly adopted in poten-

tially unstable slopes and steep excavations in Concepcion,

Chile. The paper presents a study of soil nailing design and

construction in Maicillo, a heavily weathered granite. The

stability of the soil nailed wall was analysed using a limit

equilibrium block method. The lowest factor of safety was

obtained when the nailed wall was 8 m high but without

the bottom row of nails. This was followed by the global

seismic case using an acceleration factor kh = 0.15. The

occurrence of a strong earthquake (magnitude 8.8) 2 weeks

after completion of the projected allowed a re-assessment

of the soil nailing design. Despite registering a maximum

acceleration of 0.63 g near the project site, the nailed wall

did not present any damage, probably due to the use of

undrained shear strength parameters.

Keywords Nailed wall � Heavily weathered granite

(Maicillo) � Limit equilibrium method � Stability analysis �Strong subduction earthquake

Resume Le clouage des sols est une technique de plus en

plus adoptee pour les pentes potentiellement instables et les

excavations a pentes raides a Concepcion, au Chili.

L’article presente une etude de dimensionnement et mise

en œuvre d’un clouage de sol dans une zone de granite tres

altere, nomme Maicillo. La stabilite de la paroi de sol cloue

a ete analysee en utilisant une methode d’equilibre limite.

Le plus faible coefficient de securite a ete obtenu lorsque la

paroi clouee faisait 8 m de hauteur, mais sans la rangee

inferieure de clous. Cette situation etait suivie par le cas

sous sollicitation sismique avec un facteur sismique

kh = 0,15. Un seisme de forte magnitude (M = 8,8) deux

semaines apres la realisation de l’ouvrage a permis de

reevaluer le dimensionnement de la paroi clouee. En depit

de l’enregistrement d’une acceleration maximale de 0,63 g

a proximite du site de projet, la paroi clouee ne presentait

aucun dommage, probablement en raison de l’utilisation de

parametres de resistance au cisaillement non draine.

Mots cles Paroi clouee � Granite fortement altere

(Maicillo) � Methode des equilibres limites � Analyse de

stabilite � Fort seisme de subduction

Introduction

The reduction of available urban space and the hilly

topography in the Greater Concepcion area, Chile, increase

the need for building in challenging conditions. Housing

developments and motorway projects often involve the

re-profiling of slopes, or simply that they are cut vertically

to create flat spaces. This can result in unstable slopes

which need to be retained usually with traditional gravity

or cantilever T or L-shaped reinforced concrete walls.

However, rigid wall solutions are expensive, time con-

suming and generally restricted to heights of 5 m due to the

risk of overturning.

S. A. Villalobos

Geology and Geomechanics, New Mine Level Project,

CODELCO, Coya Camp 267, Machalı, Rancagua, Chile

e-mail: svill008@codelco.cl

P. L. Orostegui

OITEC Engineering, Surveying and Geotechnics,

Lincoyan 444, Of. 309, Concepcion, Chile

e-mail: porostegui@oitec.cl

F. A. Villalobos (&)

Department of Civil Engineering, Catholic University

of Concepcion, Alonso de Ribera 2850, Concepcion, Chile

e-mail: avillalobos@ucsc.cl

123

Bull Eng Geol Environ

DOI 10.1007/s10064-013-0466-7

Alternatively, soil reinforcement construction tech-

niques may be appropriate. These have been increasingly

applied worldwide for at least 30 years and can be classi-

fied broadly into two categories:

1. Earth reinforcement structures require the use of

reinforcing elements and a fill material. The former

can be a type of geotextile, geogrid, metal rods or

meshes and the latter can be well compacted sand.

2. The second category does not incorporate a fill

material, but introduces reinforcement elements, such

as piles, micropiles, anchors and nails, within the

natural soil. For this reason, it is generally called

in situ soil reinforcement.

This paper concerns the latter, i.e. nails in residual

soils. Soil nailing is a solution to stabilise slopes or ver-

tical excavations such as those depicted in Fig. 1. The soil

nailing construction technique consists of driving or

drilling and subsequently grouting under high pressure

around inclined bars as the excavation or re-profiling

progresses downwards. Bars are normally inclined

downwards at between 10 and 30� and spaced horizon-

tally and vertically at between 1 and 2 m, depending on

the specific calculations. The exposed surface is rein-

forced with steel meshes and sprayed with shotcrete to

create a flexible structural facing with a thickness of

between 50 and 250 mm. A drain system is provided for

the evacuation of water behind the shotcreted wall. The

reinforcing bars transmit tension loads, although they can

resist limited shear and bending loads which are usually

neglected (Guilloux et al. 1983). The tensile load distri-

bution has been measured by Guilloux et al. (1983) who

found that it varies not only along the bars but also with

the depth of the bar. The maximum tensile load in the bar

is just behind the facing at the toe of the slope or exca-

vation, and gradually separates from the facing upwards.

This creates a parabolic distribution of maximum tensile

stresses, which tends to follow the potential failure

surface.

Soil nailing is increasingly being chosen around

Concepcion, in projects where residual soils are found.

However, few available publications are related to soil

nailing applications in these soils (heavily weathered

granite) and in a highly active tectonic area. This work

does not address the deformation and displacement of nails

or walls. Instead, the stability problem is solved using the

method of limit equilibrium of forces, which is normally

used in the Chilean practice.

Global stability analysis

A global stability analysis considers the development of

failure surfaces in the ground, which may or may not

intersect the nails. Ideally, a design should consider the

intersection of nails with the potential failure surface in

order to increase the soil resistance and contribute to the

ground stability. Stocker et al. (1979) carried out several

trials in nailed walls in Germany to study failure mecha-

nisms. They found that the failure surface is not actually

curved, but resembles more two linear parts, as shown in

Fig. 2. Subsequently, Gaessler and Gudehus (1981) con-

firmed the bi-linear shape of the failure mechanism in

laboratory tests of scaled soil nailing models. Figure 2

shows an application of the model proposed by Stocker

et al. (1979), where a bi-linear failure surface generates two

sliding blocks. This analysis method is also known as the

sliding block method.

The equilibrium calculations consider two rigid sliding

blocks separated by a vertical line bd; one is reinforced

with nails and the other is an active wedge. The angle h2

between bc and the horizontal is assumed to be equal to

h2 = p/4 ? //2. Then, this failure surface along bc cor-

responds to an active Rankine state. The inclination angle

h1 between ab and the horizontal is determined by iteration

until a minimum value of the safety factor is reached. The

total static active thrust Ea(2–1) due to the lateral earth

pressure distribution acting on the fictitious wall bd can be

expressed as:

Eað2�1Þ ¼1

2cL2

bd þ qLbd

� �ka � 2cLbd

ffiffiffiffiffika

pð1Þ

Fig. 1 Typical soil nailing

applications, reinforcing

a natural slopes and

b excavations

S. A. Villalobos et al.

123

where c is the unit weight of the soil, q is a uniformly

distributed overburden, c is the cohesion and ka is the

Coulomb active lateral earth pressure coefficient. To resist

Ea(2–1) the soil will mobilise its shear strength along ab and

bc. It is assumed that no shear is developed along bd, i.e.

both blocks move together. The Coulomb failure criterion

is used to evaluate the maximum soil shear strength

mobilised along the failure surface. A global factor of

safety is defined as the ratio between the sum of resisting

forces and the sum of applied forces. The resisting forces

C1 and C2 are obtained from the soil shear strength along

the bi-linear failure surface.

C ¼ C1 þ C2 ¼ cLab þ N1 tan /þ cLbc þ N2 tan / ð2Þ

where / is the angle of friction in plane conditions;

however, it is common practice to use / values determined

from drained or undrained triaxial tests, instead of, for

example, direct shear tests. The normal force Ni is given

by:

Ni ¼ ðWi þ QiÞcos hi

cos /ð3Þ

where the subindex i = 1 ? reinforced block and i = 2 ?active wedge, Wi is the weight of the block and wedge, Qi

is the overburden and hi is the inclination angle. The

tension force contribution of the n passive nails can be

expressed as:

T ¼Xn

i¼1

Tpi ð4Þ

where Tpi is the resistance force added by the nails crossing

the failure surface. The tension capacity is a structural

property of the nail material and the pull-out capacity

results from the interaction between the nail, the grouting

and the surrounding soil.

The global factor of safety FSG is given by the ratio

between the resistant forces Ci, Ni and T and the applied

forces Wi, Qi, Ea(2–1) and Hi.

FSG ¼Ci þ Ni þ T

Wi þ Qi þ Eað2�1Þ þ Hið5Þ

The horizontal force Hi applied in the gravity centre of

both blocks represents a dynamic force in a pseudo-static

form. In earthquake geotechnical engineering practice, H is

represented as product between the weight (in this case the

weight of the rigid block) and an acceleration factor (kh)

representing the maximum horizontal acceleration

component of the earthquake.

H ¼ khW ¼ amaxh

gW ð6Þ

where amaxh is the maximum horizontal acceleration, usually

determined as a reasonable value for the geographical zone

under study, and g is the acceleration of gravity. As in the

analysis of other retaining structures, the resistance against

sliding along Lab and the bearing capacity of the foundation

soil under the soil nailing are also evaluated.

Case study

Geological and geotechnical background

The geological unit present in the project site of Quinta

Junge corresponds to a heavily weathered igneous intrusive

rock which forms part of the coastal batholith mountain

(see Fig. 3). This Palaeozoic granitic rock is between 250

and 570 million years old and is the result of the tectonic

activity caused by the subduction of the Nazca plate under

the South American plate. Weathering has destroyed the

bonding between the mineral grains and has transformed a

strong rock into crumbly lumps—the residual soil being

known locally as Maicillo. This type of material is very

complex to analyse as it is difficult to determine whether it

will behave as a rock or soil, or a combination of both.

When dry, the material has a high resistance attributable to

the remaining bonding of the granite rock. When it is wet/

Fig. 2 Global stability analysis

using sliding block method

showing, a forces acting on the

reinforced soil block and

b polygons of forces

Soil nailing in granite

123

saturated, the silt particles dominate and an undrained

behaviour prevails, which can lead to the sliding of blocks

along fracture planes in the original rock.

Figure 4a shows the blocks of the weathered granite

obtained for sampling and testing. It can be seen that the

material is composed of coarse grains, which may disin-

tegrate when handled (Fig. 4b).

Table 1 summarises the values of the main geotechnical

parameters of the Maicillo which consists of 30 % fine and

medium gravel, 55 % sand and 15 % silt of low plasticity.

The yellowish brown material classifies as a silty sand SM,

according to the USCS. It is worth mentioning that this

classification is obtained from testing a granular material

(Fig. 4b). However, intact samples were tested in a direct

shearbox apparatus. Only slight remoulding around the

corners was required, as can be seen in Fig. 4c.

The construction problem

The original seven storey block of flats in Quinta Junge

were constructed in an excavation supported by stone

masonry walls and shotcrete. The enabling works for a

second block of flats adjacent to the first required an 8 m

deep excavation with a back face at 70� cut into a 35� hill

slope, again supported by stone masonry walls and shot-

crete. However, during the excavation of the first 4 m

below a masonry wall, a triangular sliding failure occurred

(Fig. 5). The reduction of resistance as a consequence of

the excavation induced sliding along pre-existing planes of

failure in the weathered Maicillo. A common solution in

the zone, mainly for sandy soils, is anchored soldier pile

walls. However, this solution was not possible as the steel

soldier piles cannot be driven into Maicillo. Finally, it was

decided to adopt a temporary soil nailing system until the

building was completed and the intervening space

backfilled.

Soil nailing design

The soil nailing design was undertaken using the computer

program GGU-Stability (2008) which calculated the con-

ditions of limit equilibrium for the failure mechanism

presented in Fig. 2. The inputs used in the analysis con-

sidered a 0.15 m thick shotcreted wall with four rows of

nails. Table 2 summarises the parameter values used in the

analysis and design of the soil nailing. The 2 m high stone

masonry wall was treated as a material with a unit weight

of 25 kN/m3 as shown in Fig. 6.

The nails used were high resistance thread steel bars of

25 mm nominal diameter with a minimum failure resis-

tance of 630 MPa and a yield stress fy between 420 and

580 MPa. This results in an allowable tension capacity of

180 kN, defined as the 90 % of the yield stress for a bar

section Ab, Ta = 0.9fyAb. The soil–nail interface shear

strength rs was estimated from a chart for sands proposed

by Bustamante and Doix (1985), where rs is correlated with

an SPT value of 30 blows/foot. The analysis assumed an

effective angle of friction /0 = 30� and a cohesion

c = 10 kPa based on the work of Ruız (2002) who carried

out undrained triaxial tests on Maicillo samples. These

values were mobilised under an axial deformation of 20 %,

representing a sliding condition. Yin et al. (2009) presented

cohesion and friction angle values from consolidated

drained triaxial tests carried out in a loose and completely

decomposed granite from Hong Kong. For Sr = 50 %,

c = 44 kPa and / = 31.9� and for Sr = 98 %, c = 5 kPa

and /0 = 34.6� for an axial deformation of 20 %.

Undrained shear strength parameter values were used to

allow for saturated and rapid loading conditions as a worse

case scenario.

Design of the construction sequence

The soil nailing construction began with the re-profiling of

the slope face to 85� for the installation of an electro

welded mesh ACMA C257 with a grid of 150 9 150 mm

of 7 mm nominal diameter steel wires with a yieldingFig. 3 General geological map of Concepcion (Poblete and Dobry

1968)

S. A. Villalobos et al.

123

stress of 500 MPa. Subsequently, nails were installed fol-

lowed by the addition of four 1.2 m long and 12 mm

diameter steel bars centred in a cross form around the nail

head. The purpose of the steel bars was to transmit the

loads taken by the nails to the shotcrete wall and reduce the

risk of flexural/shear failure. The steel bars were sand-

wiched by a second electro welded mesh ACMA C257,

resulting in a total of 514 mm2/m2 of steel mesh. Shotcrete

was injected to form a 30 mm thick layer between the soil

and the inner mesh and between the outer mesh and the

exterior wall face, resulting in a total wall thickness of

0.15 m. Finally, a 0.2 m square bearing plate and a nut

were installed.

The construction sequence is shown in Fig. 6 for the

static cases. Although the groundwater table may reach the

inner end of the nails, it is significantly further back than

the potential sliding blocks. The anticipated sliding blocks

would not extend back as far as the swimming pool, some

Fig. 4 a Heavily weathered

granite rock blocks in Quinta

Junge, b Maicillo after sieving

showing coarse and fine

components and c sample

preparation in the shear box

Soil nailing in granite

123

7.5 m from the nailed wall, except when seismic forces are

involved, as described below.

The installation of the nails was carried out using a

Comacchio MC 600 drill rig with a tricone bit. Air injected

with eight bars (800 kPa) pressure was used as a flush.

During the drilling operations, an electro-welded mesh

protected the slope against possible local failures related to

drilling vibration. The bars were emplaced using central-

isers and grouted using a water cement ratio of 0.5 and a

grout pressure of 10 bars (1 MPa).

Studying the soil nailing pull out resistance, Yin et al.

(2009) found that for a grouting pressure of 130 kPa, the

maximum average soil–nail interface pressure was around

170 kPa for 5 mm nail head displacement, for a confining

stress of 200 kPa and for a soil saturation of 50 %. The

laboratory tests were carried in an instrumented 0.6

wide 9 0.8 high 9 1.0 m long chamber able to test only

one horizontal nail (1.2 m long and 40 mm diameter)

introduced in a 100 mm diameter hole. The soil was a

compacted completely decomposed granite fill. Figure 7

shows Yin et al.’s (2009) results of average soil–nail

interface shear strength rs as a function of grouting pressure

gp, confining stress and nail head displacement where an

interpolation line is included, which can be expressed as:

rs ¼ 50 þ 0:5gp in kPa ð7ÞAccording to (Eq. 7), the value adopted in the analysis

of gp should be 300 kPa to result in rs = 200 kPa, which is

actually beyond the studied limits of Yin et al. (2009).

Further research is needed to study the effect of higher

grouting pressures on the soil–nail interface shear strength.

Seismic analysis

It has been found that during seismic loading, a failure

mechanism develops in the same form as in the static case,

but the sliding soil blocks would be approximately double

the size (Tufenkjian and Vucetic 2000; Hanna and Juran

2000; Gaessler 2007). Comparing Figs. 6d and 8, it is clear

that these much larger sliding blocks would extend the

position of the failure surface, reducing the length of nail

able to develop shear resistance and hence the global factor

of safety (see Table 3).

Stability results

The factors of safety considered in the stability analysis for

the temporary soil nailing project are presented in Table 4

and the results of the stability analyses in Table 3. It is

clear that while the sliding and bearing capacity factors of

safety are higher than the required limits, the global factors

of safety are closer to the limits imposed by the project.

The minimum factor of safety does not occur in the seismic

case, but for the construction stage with an 8 m high wall

and three rows of nails. This highlights the importance of

the bottom row of nails which can mobilise shear resistance

for a longer length than the other nails.

Fig. 5 Unstable 4 m excavation showing material sliding in a

triangular shape

Table 2 Design parameters for the four rows of nails

Parameter Values

Length of anchors Ls, m 8, 8, 6, 6

Inclination of anchors, degrees 25, 20, 20, 15

Perforation diameter D, mm 110

Spacing between anchors SH, Sv, m 1.5, 1.8

Allowable tension capacity of bars Ta, kN 180

Soil-nail interface shear strength rs, kPa 200

Coefficient of horizontal acceleration kh, g 0.15

Overburden q, kPa 10

Nailed wall inclination b, degrees 85

Table 1 Geotechnical parameters of Maicillo

Parameter Values

Specific gravity Gs 2.708

Particle size d10, d30, d50, d60, mm 0.04, 0.25, 0.7, 1.1

Uniformity and curvature coefficients Cu, Cc 27.5, 1.42

Permeability coefficient k, m/s 6.4 9 10-4

Dry unit weight cd, kN/m3 16.4

Humidity W, % 8.2

Void ratio e 0.62

Saturation Sr, % 36

Cohesion c, kPa 13a

Peak angle of friction /’max, degrees 41.3a

Dilation angle W, degrees 14.8, 5.4, 4.8a

a Obtained for a constant normal stress of 25, 50 and 100 kPa

S. A. Villalobos et al.

123

The calculated tension loads on the steel nails are

summarised in Table 5. The first two rows correspond to

32.8 m long nails and rows 3 and 4 to 32.6 m long nails.

No yielding of the steel nails is expected as the tension

values are lower than the allowable value of 180 kN

(Table 2).

The ultimate nail pull-out capacity Tu can be obtained

assuming a uniformly mobilised soil–nail interface shear

strength rs, and a mean perforation diameter Ds = aD,

where a is a parameter related to the type of injection (1.2

is for an Injection Global and Unique IGU).

Tu ¼ pDsLrs ¼ 1:2pDLrs ð8Þ

Using a factor of safety of 1.5, the estimated allowable

pull-out capacity for the 6 and 8 m long nails is 330 and

440 KN, respectively.

Observations after a seismic event

The 27 February 2010 Chile earthquake provided a unique

opportunity to rethink the soil nailing design. The soil

nailing project had finished just 25 days before the 8.8

magnitude earthquake occurred at 3.34 a.m. local time. The

30 km deep hypocentre was located at Lat 36.2908S, Long

73.2398W, some 100 km north of the project site (Barri-

entos 2010). Unfortunately, no instruments were installed

4 m

2 m 2 m

2 m2 m

6 m

8 m 8 m

q = 10 kPa q = 10 kPa

q = 10 kPa q = 10 kPa

groundwatertable

groundwatertable

groundwatertable

groundwatertable

(a) (b)

(c) (d)

swimming pool

swimming poolswimming pool

swimming pool

Fig. 6 Construction sequence design

Soil nailing in granite

123

on site. However, accelerations were recorded in the centre

of Concepcion and in San Pedro de la Paz (see Fig. 3).

Figure 9a shows that at Concepcion, 2.5 km from the pro-

ject site, the maximum horizontal acceleration was 0.39 g

above sand and silt deposits 120 m deep. In San Pedro de la

Paz, 5.5 km from the project site, the maximum horizontal

acceleration was 0.63 g above residual soils from weath-

ered metamorphic rocks (see Fig. 3). It is also worth noting

the long duration of the seismic event and the lower fre-

quency content in Fig. 9a compared with Fig. 9b.

Attention is also drawn to the fact that around Concepcion

the earthquake induced a number of landslides in hill slopes

composed of the same heavily weathered granite (Verdugo

et al. 2010). However, inspections indicated although neither

cracks nor any sign of failure were present in the soil nailed

wall or in the swimming pool, a 5 mm wide, 100 mm deep

crack was found next to the swimming pool and running

23 m parallel to the masonry wall at a distance of 5.6 m. It is

believed that at least the upper block of the failure mecha-

nism was activated, but the soil nailing did not show evi-

dence of damage let alone indication of failure.

Given that the geology and type of soil in Quinta Junge

is closer to that found in San Pedro de la Paz than that in

the Concepcion centre, it can be assumed that the maxi-

mum horizontal acceleration was closer to 0.6 g than 0.4 g.

For that reason, the same pseudo static stability analysis

was undertaken with this acceleration effect. Figure 10

shows that the global factors of safety FSG = 1.61 and 1.32

for kh = 0 and 0.15, as previously presented in Table 3,

correspond to cohesion and angle of friction of 10 kPa and

30�, respectively. However, from this analysis, a soil

nailing collapse should have occurred for kh C0.3, resulting

in FSG = 0.75 for kh = 0.6. In view of the fact that failure

did not occur, it seems that the use of an undrained angle of

friction value of 30� resulted in a conservative design. The

other two curves in Fig. 10 correspond to drained values

c = 13 kPa and / = 37.5 and 41.3�, which are more likely

to have been mobilised during the earthquake since the soil

had a moisture content of\8 % (February being the driest

month of the year). These latter values were measured in

direct shear tests (see Fig. 4c; Table 1). The use of these

drained values led to global factors of safety above 1 for

accelerations near to 0.6 g.

The lower curve in Fig. 10 represents the worst scenario

when the soil is saturated and large accelerations occur

Fig. 7 Average interface shear strength versus grouting pressure

(data from Yin et al. 2009)

q = 10 kPa

groundwatertable

8 m

2 m swimming pool

Fig. 8 Seismic design using Kh = 0.15 for temporary soil nailing

Table 3 Tempory factors of safety calculated

Case Constr.

1

Constr.

2

Constr.

3

Static Seismic

Sliding 33.1 16.8 10.5 10.5 7.8

Bearing

capacity

9.6 4.6 4.2 4.2 4.0

Global 1.44 1.33 1.28 1.61 1.32

Table 4 Factors of safety limits for the tempory soil nailing project

Case Construction Static Seismic

Sliding 1.3 1.3 1.1

Bearing capacity 2.5 2.5 2.3

Global 1.2 1.35 1.1

Table 5 Calculated tension loads in steel nails in kN

Nails row Constr. 1 Constr. 2 Constr. 3 Static Seismic

1 125 135 140 135 175

2 – 135 140 115 135

3 – – 140 115 135

4 – – – 115 140

S. A. Villalobos et al.

123

during 1 or 2 min in a strong earthquake. This situation

might occur if an earthquake takes place during the rainy

season.

Conclusions

This paper presents the design and construction of a soil

nailing project in a heavily weathered granite. The stability

analysis was based on a limit equilibrium method and the

results interpreted in terms of tension loads in the nails and

factors of safety. The failure mechanism consists of sliding

blocks which develop shear strength on the contact areas

with a geometry based on experimental findings. Nails

assist in increasing the shear resistance and hence the

stability of the excavated slope. It was found that at the final

stage of construction, when the bottom row of nails was not

yet installed, the situation is even less favourable than the

seismic case with kh = 0.15 for the finished soil nailing.

The analysis, based on calculations of resistant and

applied loads, proved adequate for the design of nailed

walls under drained conditions, even under a high seismic

loading. However, it is important to consider deformations

and displacements experienced by the soil, nails and wall,

during construction, service and under seismic loading.

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