vst and dmt

8/10/2019 VST and DMT

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Vane shear test (VST) and

dilatometer test (DMT)

Vane shear test (VST) Introduction to VST

Interpretation

Results from VST

Flat dilatometer test (DMT) Introduction to DMT

Devices and procedures

Calibration

Results from DMT

Interpretation of soil properties

ISSMGE Report: The Flat Dilatometer Test (DMT)in Soil Investigations (Appendix)


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Introduction to VST


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Introduction to VST

The vane shear test (VST), or field vane (FV), is used toevaluate the in-place undrained shear strength (suv) of soft tostiff clays & silts at regular depth intervals of 1 meter.

The test consists of inserting a four-bladed vane into the clayand rotating the device about a vertical axis, per ASTM D 2573guidelines.

Limit equilibrium analysis is used to relate the measured peaktorque to the calculated value of s

u. Both the peak and remolded

strengths can be measured; their ratio is termed the sensitivity,St.

A selection of vanes is available in terms of size, shape, andconfiguration, depending upon the consistency and strength

characteristics of the soil. The standard vane has a rectangulargeometry with a blade diameter D = 65 mm, height H = 130mm (H/D =2), and blade thickness e = 2 mm.


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Selection of vaneshear blades,pushing frames,

and torquemeterdevices


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Test procedure

ASTM D2573-01 Standard Test Method for Field Vane Shear Test inCohesive Soil

The test is best performed when the vane is pushed beneath the bottomof an pre-drilled borehole. For a borehole of diameter B, the top of thevane should pushed to a depth of insertion of at least df= 4B.

Within 5 minutes after insertion, rotation should be made at a constant

rate of 6/minute (0.1/s) with measurements of torque taken frequently. In very soft clays, a special protective housing that encases the vane isalso available where no borehole is required and the vane can be installedby pushing the encasement to the desired test depth to deploy the vane.

An alternative approach is to push two side-by-side soundings (one withthe vane, the other with rods only). Then, the latter rod friction resultsare subtracted from the former to obtain the vane readings. This alternateshould be discouraged as the rod friction readings are variable, dependupon inclination and verticality of the rods, number of rotations, and thusproduce unreliable and questionable data.


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Pros and cons

ADVANTAGES of VST

Assessment of undrained

strength, suv Simple test and equipment Measure in-situ clay sensitivity

(St)

Long history of use in practice

DISADVANTAGES of VST

Limited application to soft to

stiff clays with suv < 200 kPa Slow and time-consuming Raw suv needs (empirical)

correction

Can be affected by sand lensesand seams


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The general expression for all types of vanes

including standard rectangular (Chandler, 1988), bothends tapered (Geonor in Norway), bottom taper only(Nilcon in Sweden), as well as rhomboidal shapedvanes for any end angles is

where iT = angle of taper at top (with respect tohorizontal) and iB = angle of bottom taper.

For the commercial vanes in common use, the above

equation reduces to the following expressions forvanes with blade heights that are twice their widths(H/D = 2):

Rectangular (iT = 0 and iB = 0):

suv = 0.273 Tmax/D3

Nilcon(iT = 0 and iB = 45):

suv = 0.265 Tmax/D3

Geonor(iT = 45 and iB = 45):

suv = 0.257 Tmax/D3

( )HiDiDD

Ts

BTu 6)cos/()cos/(

12

2 ++=


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Sensitivity

After the peak suv is obtained, the vane is rotated quicklythrough 10 complete revolutions and the remolded (or

"residual") value is recorded. The in-situ sensitivity of thesoil is defined by:

Sensitivity St = su (peak)/ su (remolded)


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Vane results

A representative set of shear strength profiles in San Francisco Bay Mud derived from vane

shear tests for the MUNI Metro Station Project. Peak strengths increase from suv = 20 kPato 60 kPa with depth. The derived profile of sensitivity indicates 3 < St < 4.


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Vane correction factor

The mobilised shear strength for designuse is

mobilised = Rsuvwhere R= empirical correction factorthat has been related to plasticity indexPI and/or liquid limit (LI) based on backcalculation from failure case historyrecords of full-scale projects.

Chandler (1988) recommends:

R= 1.05 - b (PI)0.5

where b is a rate factor that dependsupon the time-to-failure (t

f

in minutes):

b = 0.015 + 0.0075 log tfFor guidance, embankments on softground are normally associated with tf

on the order of 104 minutes because ofthe time involved in construction usinglarge equipment.


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Vane shear test (VST) Introduction to VST

Interpretation

Results from VST

Flat dilatometer test (DMT) Introduction to DMT Devices and procedures

Calibration

Results from DMT Interpretation of soil properties



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Introduction to DMT


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Flat plate dilatometer test (DMT)

The flat dilatometer test (DMT) uses pressure readings from an inserted plateto obtain stratigraphy and estimates of at-rest lateral stresses, elasticmodulus, and shear strength of sands, silts, and clays.

The device consists of a tapered stainless steel blade with 18 wedge tip thatis pushed vertically into the ground at 200 mm depth intervals (or alternative300-mm intervals) at a rate of 20 mm/s. The blade (approximately 240 mmlong, 95 mm wide, and 15 mm thick) is connected to a readout pressuregauge at the ground surface via a special wire-tubing through drill rods orcone rods. A 60-mm diameter flexible steel membrane located on one side ofthe blade is inflated pneumatically to give two pressures: A-reading that is alift-off or contact pressure where the membrane becomes flush with the bladeface ( = 0); and B-reading that is an expansion pressure corresponding to = 1.1 mm outward deflection at center of membrane.

A tiny spring-loaded pin at the membrane center detects the movement andrelays to a buzzer/galvanometer at the readout gauge. Normally, nitrogen gas

is used for the test because of the low moisture content, although carbondioxide or air can also be used. Reading A is obtained about 15 secondsafter insertion and B is taken within 15 to 30 seconds later. Upon reaching

B, the membrane is quickly deflated and the blade is pushed to the nexttest depth. If the device cannot be pushed because of limited hydraulicpressure (such as dense sands), then it can be driven in-place, but this is not

normally recommended.


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Soils than can be tested by DMT

Suitable for SANDS, SILTS, CLAY (grains small vs.membrane D=60 mm). But can cross through GRAVEL

layers 0.5 m Very robust, can penetrate soft rocks (safe push on blade

25 ton)

Clays : cu = 2- 4 KPa to cu= 10 bar (marls) Moduli : 5 to 4000 bar (0.5 to 400 MPa) Penetrates fast and easily in hard soils PROVIDED

sufficient pushing capacity (e.g. 20 ton trucks).


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ADVANTAGES OF DMT

Simple and robust Repeatable & operator-

independent

Quick and economical

DISADVANTAGES OF DMT

Difficult to push in dense andhard materials Primarily relies on correlative

relationships

Need calibrations for localgeologies


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Standards and specifications

Standards ASTM Subcommittee D 18.02.10 - Schmertmann, J.H., Chairman (1986).

"Suggested Method for Performing the Flat Dilatometer Test". ASTM

Geotechnical Testing Journal, Vol. 9, No. 2, June. Eurocode 7 (1997). Geotechnical design - Part 3: Design assisted by field

testing, Section 9: Flat dilatometer test (DMT).

ASTM (2001). D6635 "Standard Test Method for Performing the Flat PlateDilatometer ". Approved Draft, 2001.

Manuals Marchetti, S. & Crapps, D.K. (1981). "Flat Dilatometer Manual". Internal

Report of G.P.E.

Schmertmann, J.H. (1988). Rept. No. FHWA-PA-87-022+84-24 to

PennDOT, Office of Research and Special Studies, Harrisburg, PA, in 4volumes.

US DOT - Briaud, J.L. & Miran, J. (1992). "The Flat Dilatometer Test".Department of Transportation - Fed. Highway Administr., Washington,

D.C., Publ. No. FHWA-SA-91-044, 102 pp.


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Devices

Flat Plate Dilatometer Equipment: (a) Modern Dual-Element Gauge System;

(b) Early Single-Gauge Readout; (c) Computerized Data Acquisition Model.


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Seismic dilatometer Flat dilatometer


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Pushed by truck

Pushed by a drill rig

Driven by

a drill rig

Pushed from

a fixed platformDriven bySPT Tripod

Driven or pushed by

a static/dynamic

penetrometer


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A-reading, B-reading and C-reading

p0 p1 p2

1.1 mm


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Dissipation tests

In CLAYS AND SILTS (not feasiblein sandy silt, sand and gravel)

Stop the blade at a given depth Monitor the decay of the total

contact horizontal stress hwith

time Infer the coeff. of consolidation/ permeability (ch , kh) from therate of decay of h


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Dissipation tests (cont.)

Recommended MethodTimed sequence ofA-readings (onlyAis taken, avoiding expansion to B. For

other methods see TC16 (2001)

Procedure Stop the blade at a given depth and start a stopwatch (t= 0 when

pushing is stopped). Slowly inflate the membrane to take theA-

reading. Vent the blade soon afterA. RecordA-value and stopwatchtime at the instant ofA-reading.

Continue to take additionalA-readings e.g. by a factor 2 increase intime (0.5, 1, 2, 4, 8, 15, 30 etc. minutes after stopping the blade).

Plot in the field a preliminaryAlog tdiagram (usually S-shaped).Stop the dissipation when theAlog tcurve has flattened sufficientlyto clearly identify the time at contraflexure point tflex(used for theinterpretation).


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Calibration Two calibrations are taken before the sounding to obtain corrections for

the membrane stiffness in air. These corrected A and B pressuresare respectively notated as p0 and p1 with the original calculations givenby (Marchetti 1980):

po = A + A p1 = B -Bwhere A and B are calibration factors for the membrane stiffness inair. The A calibration is obtained by applying suction to the membraneand B obtained by pressurizing the membrane in air (Note: both are

recorded as positive values). In stiff soils, the above two equations will normally suffice for calculatingthe contact pressure p0 and expansion pressure p1. However, in softclays & silts, a more accurate correction procedure is given by(Schmertmann 1986):

po = 1.05(A + A - zm) - 0.05(B -B - zm) p1 = B -B - zmwhere zm = pressure gage offset (i.e., zero reading of gage). Normallyfor a new gage, zm = 0.


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Calibration of membrane

(A & B) - Layout of connections

Positions of the

membrane(free,Aand B)

B

Afree


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Determination of A & B

To obtain A Apply vacuum by pulling back the syringe piston (vacuum

causes an inward deflection of the membrane similar to thatdue to external soil pressure at the start of the test) -buzzer becomes active.

Slowly release the piston and read Aon the low-rangegage when buzzer stops.

Note this negative pressure as a positiveAvalue, e.g. A= 15 kPa for a vacuum of 15 kPa (the correction formula forp0takes into account that a positive Ais a vacuum).

To obtain

B

Push slowly the piston into the syringe and read Bon thelow-range gage when buzzer reactivates.

Repeat several times


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Acceptance values of A & B (Eurocode 7, 1997)

Initial A, B(before inserting the blade) must be in theranges

A= 5 to 30 kPa B= 5 to 80 kPa

If not, replace the membrane before testing.

Final A, B The change of Aor Bat the end of the sounding must be 25 kPa

In not, test results must be discarded.

Typical values Of A, B A= 15 kPa

B= 40 kPa


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Vane shear test (VST)

Introduction to VST Interpretation

Results from VST

Flat dilatometer test (DMT) Introduction to DMT Devices and procedures

Calibration

Results from DMT Interpretation of soil properties



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Results from DMT

The two DMT readings (po and p1) are utilized to providethree indices that can provide information on the

stratigraphy, soil types, and the evaluation of soilparameters:

Material Index: ID

= (p1

- po

)/(po

- uo

)

Dilatometer Modulus: ED = 34.7(p1 - po)

Horizontal Stress Index: KD = (po - uo)/v0

where uo = hydrostatic pore-water pressure

v0= effective vertical overburden stress.


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Example results from a DMT conducted in Piedmont residual soils, including the measured lift-off(p0) and expansion (p1) pressures, material index (ID), dilatometer modulus (ED), and horizontalstress index (KD) versus depth. The soils are fine sandy clays and sandy silts derived from the in-

place weathering of schistose and gneissic bedrock.

sg


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SHEAR WAVEVELOCITY

Vs (m/s)

SHEAR WAVEVELOCITY

Vs (m/s)


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I t t ti f il ti


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-soil classificationFor soil behavioral classification, layers are interpreted as

clay when ID < 0.6,

silts within the range of 0.6 < ID < 1.8, and

sands when ID >1.8.


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Pre-consolidation stress in clays


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K0 in soils

K0in clay The original correlation for K0, relative to uncemented clays

(Marchetti 1980), is:

K0= (KD/1.5)0.47 - 0.6

K0in sand Baldi et al. (1986) updated such K0-qc-KDchart (qc= CPT cone

resistance) by incorporating all subsequent calibration chamber

work. Moreover the chart was converted into simple algebraicequations:

K0= 0.376 + 0.095 KD- 0.0017 qc/'v0 (1)K0= 0.376 + 0.095 KD- 0.0046 qc/'v0 (2)

Eq. 1 was determined as the best fit of calibration chamber data,obtained on artificial sand, while Eq. 2 was obtained bymodifying the last coefficient to predict "correctly" K0 for thenatural river sand.



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-Peak friction angleWedge plasticity solutions have

been developed for determining

of clean sands using DMT assummarized by Marchetti

(1997), and these have been

calibrated with data from

different sand types as shown in

the rhs figure. Theoreticalcurves are presented for the

active (KA case), at-rest (K0),

and passive earth pressure

conditions (KP case), with thelatter giving reasonable values

of compared with theexperimental data (Mayne

2001).


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Stiffness and deformation parameters


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S


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Summary

SYMBOL DESCRIPTION BASIC DMT REDUCTION FORMULAE

p0 Corrected First Reading p0= 1.05 (A - ZM + A) - 0.05 (B - ZM- B)

p1 Corrected Second Reading p1= B - ZM- BZM= Gage reading when vented to atm.

If A & B are measured with the samegage used for current readings A & B,set ZM= 0 (ZMis compensated)

ID Material Index ID= (p1- p0) / (p0- u0) u0= pre-insertion pore pressure

KDHorizontal Stress Index KD= (p0- u0) / 'v0

'v0= pre-insertion overburden stressED Dilatometer Modulus ED= 34.7 (p1- p0) EDis NOT a Young's modulus E. ED

should be used only AFTER combining itwith KD(Stress History). First obtain

MDMT= RMED, then e.g. E 0.8 MDMT

K0 Coeff. Earth Pressure in Situ K0,DMT= (KD/ 1.5)0.47

- 0.6 for ID< 1.2

OCR Overconsolidation Ratio OCRDMT= (0.5 KD)1.56

for ID< 1.2

cu Undrained Shear Strength cu,DMT= 0.22 'v0(0.5 KD)1.25 for ID< 1.2

Friction Angle safe,DMT= 28 + 14.6 log KD- 2.1 log2KD for ID> 1.8

ch Coefficient of Consolidation ch,DMTA7 cm2/ tflex tflexfrom A-log t DMT-A decay curve

kh Coefficient of Permeability kh= chw/ Mh (MhK0MDMT)

Unit Weight and Description (see chart in Fig. 16)

MDMT= RMEDif ID0.6 RM= 0.14 + 2.36 log KD

if ID3 RM= 0.5 + 2 log KD

if 0.6 < ID< 3 RM= RM,0+ (2.5 - RM,0) log KDwith RM,0= 0.14 + 0.15 (ID- 0.6)

if KD> 10 RM= 0.32 + 2.18 log KD

M Vertical Drained ConstrainedModulus

if RM< 0.85 set RM= 0.85

u0 Equilibrium Pore Pressure u0= p2= C - ZM+ A In free-draining soils

vst and dmt

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