TITLE
Unconsolidated Untrained Triaxial Test (Quick Test). OBJECTIVES
To determine the undrained shear strength of a given soil sample.
To determine the shear strength parameters of the soil.
To determine the Mohr-Coulomb failure criteria for the given specimen.
INTRODUCTION
In an unconsolidated undrained test the sample is not allowed to drain. The sample is compressed at a constant rate (strain-controlled). The UU test is applicable to undistributed sample in which no change in moisture content from the in-situ value can be permitted. Test can be carried out over a range of moisture content to enable Mohr envelops for the required to be interpolated. The UU test procedure is use for determining the total strength parameters for soil that have suffered disturbances or moisture change during sample. Triaxial tests are conducted in order to determine the shear strength and the shear str ength parameters of soil. Shear strength is a term used to describe the magnitude of shear stress a soil can sustain. The triaxial test is considered to be one of the most popular tests as it is the most reliable and can accuratel y simulate field conditions. In the unconfined undrained test, the drainage of water is completely prohibited throughout the course of the experiment. However the test can be carried out with drainage as well depending on the criteria of the results required (which will be explained in further detail later on). The stress is applied to the sample being tested in a way in which the stresses acting along one axis is different to the stresses in perpendicular directions. This is achieved by placing the sample between two flat surfaces and applying a stress along that direction, while applying fluid pressure to apply the stress in the perpendicular directions. The application of these different compressive stresse s causes shear stress to develop in the sample, and for the stress and strains to be monitored until failure occurs.
Figure 01 - Specimen stress state during triaxial compression.
THEORY
In the UU test, drainage is not allowed during the consolidated stage, neither during the drainage stage, this is done by keeping the drainage valves closed. Therefore, this test is not very time consuming and is hence called the “Quick Test”. The pore water pressure developed in the sample can be expressed by the Skepton equation: U = Bσc + ABσd It will be seen that the deviator stress of the specimen is the same irrespective of the confining cell pressure σ 3, leading to the Mohr – Coulomb failure envelope being a horizontal, straight line, with the internal friction angle, Ф = 0, in the case of saturated clays and silts. The shear strength s, of the soil specimen can be found using the equation below: s = c + σ n tan Ф, Where c is the cohesion (in stress) units of the soil, and is the value of the intercept that the failure envelope makes with the shear stress axis. Theory behind the triaxial test is that stresses in planes for particular soils to occur failure in any plane. This could be emphasized using Mohr’s circle of stres ses. According to the Mohr – coulomb failure criteria, failure takes place when,
s =
σntan φ + c
Figure 02 - Mohr – Coulomb failure criteria in triaxial test.
APPARATUS
Triaxial cell testing system.
Soil sample.
Specimen mould (76mm diameter, 38mm length).
Rubber membrane.
O-rings.
Glass caps.
Air compressor.
Load frame with stepper motor drive.
Electronic measurement transducers.
Control panel to regulate pressure and flows.
PROCEDURE
The soil sample was compacted into the given mould, by filling the mould in layers, and compacting it with each layer (remolded specimen). Once the sample was made into the desired size, it was enclosed in a rubber membrane, making sure that no air bubbles were present between the membrane and the sample.
The sample was placed on the bottom cap and sealed with an O-ring.
The top case was then placed as well and sealed with another O-ring.
The triaxial cell was then assembled and the crews were tightened.
The loading piston was carefully placed on the top cap without applying any extra stress that could cause failure.
The cell was then filled with water, ensuring that no air bubbles were present inside.
The cell pressure was then applied.
The deviator stress was applied until failure occurred.
REFERENCES
Ernet,[Online],Available:http://www.tezu.ernet.in/dcivil/LAB/OLD%20IMAGES/0%203.%20 Triaxial%20Test%20Appar atus.jpg [01 May 2016] Vjtech, [Online], Available: http://www.vjtech.co.uk/products/ProdID/32/triaxial_cell_100mm [01 May 2016] Scribd(2015),[Online],Available:http://www.scribd.com/doc/287040247/UnconsolidatedUndra ined-Triaxial-Test#scribd [01 May 2016].
