CE 641 - SITE INVESTIGATION PRACTICE
BOREHOLE SHEAR TEST
NAME : S. M. SATHEEK
REG. NO : PG/E/GT/2020/25
Borehole Shear test (BST)
Introduction
• Shear test is an important test to determine the shear strength of soil to design
of earth slopes, calculating the earth pressure against retaining wall and
determination of foundation bearing capacity.
• The laboratory shear test is time consuming and expensive
Borehole Shear test (BST)
Introduction
• Borehole shear test is developed by Dr. R.L Handy at Iowa State University, it
provides convenient method to accurately measure the drained shear strength
of soil in-situ.
• BST test will perform inside the predrilled boreholes to determine the shear
strength parameters of soil at given depth during subsoil exploration.
• it is a simple and rapid test.
• Similar to performing direct shear test
• BST can also be used to determine the in-situ measurement of the cohesion
and friction angle of bedrock.
Borehole Shear test (BST) - component
Testing head: it contacted tightly to the wall of the borehole and apply
the vertical pressure and pullout force
• Oversize borehole can effect the accuracy of the test result, because it can
loosening/ softening the borehole sidewalls
• 76 mm (3inch) diameter Shelby tube is good to minimize the disturbance of soil
Borehole Shear test (BST)
Pulling assembly: it generate pullout force to the testing head through the extension
rod by turning the handle.
Borehole Shear test (BST)
Readout unit: it generate vertical pressure to
testing head by loading the pneumatic
pressure with hand pump or foot pump and
make measurement.
Extension rod & air hose: to generate pullout force
and vertical pressure depending on the testing
depth.
Borehole Shear test (BST)
Borehole Shear test (BST)
Arrangement of the borehole shear tester
Borehole Shear test (BST)
Method of testing
• Shear head will be lowered into the predrilled borehole, up to the depth that
required the shear strength parameters.
• The shear plate will be pressurized on the borehole wall
• Then allow some time to consolidate the soil under the vertical pressure on the
borehole wall
5 min in sand
10 – 20 min in clay
• Then, pressurized shear head will be slowly pulled upward by the connected
extension rod until failure occurs and measure the pullout force (shearing
pressure) by pressure gage
Borehole Shear test (BST)
• The test will be repeated by increasing the vertical pressure on the same place of
the borehole wall, without replacing and resetting and measure the pullout force.
Normal stress, σ = F/A
Shear stress, s = P/2A
Borehole Shear test (BST)
• Then, plot the graph and find the Cohesion (c) and friction angle (Ø )
(Mohr-Coulomb failure envelope)
𝒔 = 𝒄 + 𝝈𝒏 𝐭𝐚𝐧(∅)
Where, s – shear strength
c – cohesion
σn – normal stress on failure plan
φ – angle of internal friction
• Usually the BST will begin from the top of the borehole to downward
• Test can made with rotating the shear head by 90 degree at same depth.
Borehole Shear test (BST)
Advantage
• Test can perform in a different depth in a one borehole
• Directly evaluate the soil Cohesion (c), and friction angle (Ø) at a
particular depth
• Large no of test can perform in short time period
• Small size, light weight and single person can handle the test.
• Cheapest than laboratory shear test.
Borehole Shear test (BST)
Disadvantage
• Difficulty to fix the test rate and the drainage condition of the sample
• No collection of stress – stain data.
Borehole Shear test (BST)
Case study : Influence of Flooding on Borehole Shear Test (BST) Results in
Unsaturated soil
• This study was done to evaluating the influence of soil suction on shear stress
parameters, determined from the Borehole shear Test in unsaturated soil.
• BST was conducted before and after flooding the borehole for comparison of
results.
• The test after flooding the borehole is called as “Flooded Borehole Shear Test”
(FBST).
• The laboratory tests, - soil suction (ua – uw) was determined using the filter paper
technique on undisturbed specimens (test method ASTM D 5298)
• Multistage isotropically consolidated untrained Triaxial Test was done to compare
the friction angle result
Borehole Shear test (BST)
Case study
Methodology :-
• Conventional BST was conducted in hand augured boreholes at several depths
(depth from 1.5 ft to 7.5 ft at interval of 1 ft)
• Then, flooding the borehole with water and perform the BST again at the depth
locations used prior to flooding. The shear plate were rotated by 90 degree from
the previous orientation used before flooding and determined cohesion and
friction angle
• The Flooded Borehole shear test (FBST) were perform from the bottom of the
borehole and moving upward.
• No time lag was allowed between flooding and BST testing to minimize soil
swelling due to flooding.
Borehole Shear test (BST)
Case study
Methodology :-
Borehole Shear test (BST)
Case study
Result
Borehole Shear test (BST)
Case study - Conclusion
• Friction angle were much lower obtained from FBST than BST due to
the reduction in the suction due to flooding
• Cohesion intercepts obtained from BST & FBST were generally small,
but cohesion intercepts obtained from FBST were slightly decreased
compare to BST result.
• Effective stress friction angles determined from Multistage Triaxial Test
(MTT) test were generally in between obtained from the both BSTs and
FBSTs
• Cohesion intercept obtained from the MTTs were much higher than
those obtained from both BSTs and FBSTs
Borehole Shear test (BST)
Reference
- Manual of Borehole shear tester model BST-75, ACE Instrument co. Ltd.
- Influence of Flooding on Borehole Shear Test (BST) Results in Unsaturated soil
(Charbel N. Khoury and Gerald A. Miller)
- The American edge news letter, winter 2012
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