To determine the shear strength of clayey soil by Un-Confined Compression Test and Pocket Penetrometer Test.

OBJECT : To determine the shear strength of clayey soil by Un-Confined Compression Test and Pocket Penetrometer Test.

OBJECTIVE AND SCOPE

The purpose of this laboratory is to determine the unconfined compressive strength of a cohesive soil sample. We will measure this with the unconfined compression test, which is an unconsolidated undrained (UU or Q-type) test where the lateral confining pressure is equal to zero (atmospheric pressure).

STANDARDS

AASHTO T208-90

ASTM D2166-85

BIS1377:part 4

EQUIPMENTS

Unconfined compression apparatus
Proving ring, capacity 1 kN, accuracy 1N.
Dial gauge, accuracy 0.01 mm
Weighing balance
Oven
Stop watch
Sampling tube
Split mould, 38mm diameter, 76mm long
Sample extractor
Knife
Vernier calipers
Large mould

THEORY

The unconfined compression test is by far the most popular method of soil shear testing because it is one of the fastest and cheapest methods of measuring shear strength. The method is used primarily for saturated, cohesive soils recovered from thin-walled sampling tubes. The unconfined compression test is inappropriate for dry sands or crumbly clays because the materials would fall apart without some land of lateral confinement. To perform an unconfined compression test, the sample is extruded from the sampling tube. A cylindrical sample of soil is trimmed such that the ends are reasonably smooth and the length-to-diameter ratio is on the order of two. The soil sample is placed in a loading frame on a metal plate; by turning a crank, the operator raises the level of the bottom plate. The top of the soil sample is restrained by the top plate, which is attached to a calibrated proving ring. As the bottom plate is raised, an axial load is applied to the sample. The operator turns the crank at a specified rate so that there is constant strain rate. The load is gradually increased to shear the sample, and readings are taken periodically of the force applied to the sample and the resulting deformation. The loading is continued until the soil develops an obvious shearing plane or the deformations become excessive. The measured data are used to determine the strength of the soil specimen and the stress-strain characteristics. Finally, the sample is oven dried to determine its water content. The maximum load per unit area is defined as the unconfined compressive strength, qu. In the unconfined compression test, we assume that no pore water is lost from the sample during set-up or during the shearing process. A saturated sample will thus remain saturated during the test with no change in the sample volume, water content, or void ratio. More significantly, the sample is held together by an effective confining stress that result from negative pore water pressures (generated by menisci forming between particles on the sample surface). Pore pressures are not measured in an unconfined compression test; consequently, the effective stress is unknown. Hence, the undrained shear strength measured in an unconfined test is expressed in terms of the total stress.

PROCEDURE

Place the sampling soil specimen at the desired water content and density in the large mould.
Push the sampling tube into the large mould and remove the sampling tube filled with the soil. For undisturbed samples, push the sampling tube into the clay sample.
Saturate the soil sample in the sampling tube by a suitable method.
Coat the split mould lightly with a thin layer of grease. Weigh the mould.
Extrude the sample out of the sampling tube into the split mould, using the sample extractor and the knife.
Trim the two ends of the specimen in the split mould. Weigh the mould with the specimen.
Remove the specimen from the split mould by splitting the mould into two parts.
Measure the length and diameter of the specimen with vernier callipers.
Place the specimen on the bottom plate of the compression machine. Adjust the upper plate to make contact with the specimen.
Adjust the dial gauge and the proving ring gauge to zero.
Apply the compression load to cause an axial strain at the rate of ½ to 2% per minute.
Record the dial gauge reading, and the proving ring reading every thirty seconds up to a strain of 6%. The reading may be taken every 60 seconds for a strain between 6%, and 12% and every 2 minutes or so beyond 12%.
Continue the test until failure surfaces have clearly developed or until an axial strain of 20% is reached.
Measure the angle between the failure surface and the horizontal, if possible.
Take the sample from the failure zone of the specimen for the water content determination.

OBSERVATIONS

S.NO	Time	Dial gauge reading (Load)	Dial gauge reading (Deformation)	Def : (mm)	Load C3 x 0.125	Strain	Corrected Area (Ac)	Compressive strength (qu)