applications of vane shear test in geotechnical soil investigations

Strength of different materialsSteel Concrete Soil

Tensile strength Compressive strength Shear strength

Embankment

Strip footing

Shear failure of soilsSoils generally fail in shear

At failure, shear stress along the failure surface (mobilized shear resistance) reaches the shear strength.

Failure surface

Mobilized shear resistance

Retaining wall

Shear failure of soils

At failure, shear stress along the failure surface (mobilized shear resistance) reaches the shear strength.

Soils generally fail in shear

Failure surface

Mobilized shear resistance

Retaining wall

Other laboratory tests include,torsional ring shear test, plane strain triaxial test, laboratory vane shear test, laboratory fall cone test

Determination of shear strength parameters of soils (c, orc’’

Laboratory tests on specimens taken from representative undisturbed samples

Field tests

Most common laboratory tests to determine the shear strength parameters are,

1.Direct shear test2.Triaxial shear test3.Vane shear test4.Compression test

1. Vane shear test2. Torvane3. Pocket penetrometer4. Fall cone5. Pressuremeter6. Static cone penetrometer7. Standard penetration test

Vane Shear Test

What does Vane Shear Test measure?

o Shear strengthA term used to describe the maximum strength of soil at which point significant plastic deformation or yielding occurs due to an applied shear stress.

o Undrained shear strengthRefers to a shear condition where water does not enter or leave the cohesive soil during the shearing process.

o Remolded undrained shear strengthIs the peak undrained shearing resistance measured during the initial rotation of the vane.

o Peak undrained shear strengthIs the shear strength after significant failure and remolding of the initial soil structure.

o SensitivityIs the effect of remolding on the consistency of cohesive soil.

PLAN VIEW

Vane shear testThis is one of the most versatile and widely used devices used for investigating undrained shear strength (Cu) and sensitivity of soft clays

Bore hole (diameter = DB)

h > 3DB)

Vane

D

H

Applied Torque, T

Vane T

Rupture surface

Disturbed soil

Rate of rotation : 60 – 120 per minute

Test can be conducted at 0.5 m vertical intervals

Vane shear test

Since the test is very fast, Unconsolidated Undrained (UU) can be expected

T = Ms + Me + Me = Ms + 2Me Me – Assuming a uniform

distribution of shear strength

2

0

).2(

d

ue rCrdrM

2

0

32

0

2

322

d

u

d

uerCdrrCM

12832 33 dCdCM uu

e

d/2d/2

Cu

Cu

Cu

h

Surface area of the cylinder = 2rh = dh

Vane shear test


Ms – Shaft shear resistance along the circumference

22

2hdCddhCM uus

2122

32

dChdCT u

u

62

32 dhdCT u

62

32 dhdTCu

T = Ms + Me + Me = Ms + 2Me

Cu

Cu

h

Vane shear test


T = Ms + Me + Me = Ms + 2Me Me – Assuming a triangular


d/2d/2

Cu

82

32 dhdTCu

Cu

Cu

h

Vane shear test


T = Ms + Me + Me = Ms + 2Me Me – Assuming a parabolic


203

2

32 dhdTCu

d/2d/2

CuCu

Cu

h

42

32 dhdTCu

Generally, We have to use this formula for accounting

By inserting value according to the shear shape.

uC

Vane shear test


After the initial test, vane can be rapidly rotated through several revolutions until the clay become remoulded

peak ultimate

Shear displacement

StengthUltimateStengthPeakySensitivit

Cu

Cu

h

Some important facts on vane shear test

Insertion of vane into soft clays and silts disrupts the natural soil structure around the vane causing reduction of shear strength

The above reduction is partially regained after some time

Cu as determined by vane shear test may be a function of the rate of angular rotation of the vane

Correction for the strength parameters obtained from vane shear test

Bjerrum (1974) has shown that as the plasticity of soils increases, Cu obtained by vane shear tests may give unsafe results for foundation design. Therefore, he proposed the following correction.

Cu(design) = Cu(vane shear)

Where, = correction factor = 1.7 – 0.54 log (PI)

PI = Plasticity Index

Overconsolidation Ratio:

2006 –– 2007 Pre-Treatment Phase

South Fraser Perimeter Road

Geotechnical Investigation Data Report

South Delta (Volue 2)

Date: November 27, 2007

File #: 03-1411-095B/4600/4630

Version: FINAL

Geotechnical Investigation Report done by using Vane Shear Test

FIELD VANE TESTING RESULTS

Small vane – 11.0cm x 5.0cm – Constant = 0.20

Medium vane – 13.0cm x 6.5cm – Constant = 0.1

Large vane – 17.2cm x 8.0cm – Constant = 0.05

= 7.75

Small vane – 11.0cm x 5.0cm Max. Torque (T) = 22 N.m PI=60 Ɣ dry=17.30 KN/m3

Cu(design) = Cu(vane shear) = 0.74 * 24 = 17.67 kPaOCR kPa

FIELD VANE TESTING RESULTS

According to FHWA Geosynthetic Design and Construction Guidelines(Berg, et. al., 1998)

Section No. 5.6

“subgrade undrained shear strength must be not less than about 90 kPa (CBR < 3). “

From the results of vane shear test, we summarized the following notes:

-The major type of soil is Medium and soft (except in two results - Stiff) .

-All Cu values (except AH06-308B) is less than 90 Kpa, which mean that subgrade should be treated and improved.

-The project is 40 km Highway located close to the riverbank.

Summarized notes:

- Using Geotextile Placement as solution for separating the weak subgrade away from the sub-base, base course and other pavement layers and keep it away from ground water.

Recommendations:

- Refill the sub-base course by forming embankment with good compaction according to specifications with high quality materials the subgrade materials with other high quality materials or adding aggregate , additive to improve the quality of subgrade materials.

Recommendations:

- Spreading Geogrid reinforcement sheets on the top surface of the sub-grade and in the base course (if necessary).

Recommendations:

- Protect the sloped sides of embankment with replacing of Geogrid reinforcement sheets specially at the side of sea and pouring concrete slabs or placing precast concrete tiles on the outer slope.

Recommendations:

applications of vane shear test in geotechnical soil investigations

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