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TRIAXIAL SHEAR TEST ON COHESIVE SOILS

A number of methods are used for conducting the triaxial test on cohesive soils. The commonly used methods are described as below:

  • Consolidated-undrained test

A deaired, coarse porous disc or stone is placed on the top of the pedestal in the triaxial test apparatus. Then a filter paper disc is placed over the porous stone. The prepared cohesive soil specimen is then placed over the filter paper disc of the stone. The usual size of the specimen is about 37.5 mm diameter and 75.o mm height. A porous stone is also placed on the top of the specimen. After that a deaired vertical filter strip papers are placed at regular spacing around the entire periphery of the porous stone such that these assembly touch both the stones. The cohesive soil sample in then covered in a rubber membrane, in which it can slid over the specimen with the help of the attached membrane stretcher. The membrane stretcher is completely covered with the specimen using O-rings.

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Vane Shear Test

The undrained shear strength of soft clays can be determined in a laboratory by a vane shear test. The test can also be conducted in the field on the soil at the bottom of a bore hole. The field test can be performed even without drilling a bore hole by direct penetration of the vane from the ground surface if it is provided with a strong shoe to protect it.

The vane shear test instrument consist of a vertical steel rod with four thin stainless steel blades (or vanes) fixed at its bottom end. The IS: 2720-1980 code recommends that the height, H of the vane should be equal to twice the overall diameter, D. The diameter and the length of the rod are recommended as 2.6 mm and 60 mm respectively.
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Shrinkage Limit Method To Determine Specific Gravity

Shrinkage limit is defined as the smallest water content at which the soil is saturated. It is also defined as the maximum water content at which a reduction of water content will not cause a decrease in the volume of the soil mass. In other words, at this water content, the shrinkage ceases.

In the below block diagram, stage 1 represents a soil sample which is fully saturated with a water content greater than shrinkage limit. Stage 2 represents soil sample which has water content equal to shrinkage limit. Stage 3 represents the soil sample when it is oven dried, i.e., the water content is removed completely. The total volume V3 in stage 3 is same as the total volume V2in stage 2. Let Msbe the mass of solids.
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Laboratory tests for Specific Gravity Determination

Keywords: Pycnometer, specific gravity, density, density bottle, glass jar, dry soil.

The specific gravity of solid particles can be determined in the laboratory using the following methods:
1. Density bottle method
2. Pycnometer method
3. Measuring flask method
4. Gas jar method
5. Shrinkage limit method

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ULTRASONIC TESTING OF CONCRETE

By
KAUSHAL KISHORE
Materials Engineer, Roorkee

Fundamental principle
A pulse of longitudinal vibrations is produced by an electro-acoustical transducer, which is held in contact with one surface of the concrete under test. When the pulse generated is transmitted into the concrete from the transducer using a liquid coupling material such as grease or cellulose paste, it undergoes multiple reflections at the boundaries of the different material phases within the concrete. A complex system of stress waves develops, which include both longitudinal and shear waves, and propagates through the concrete. The first waves to reach the receiving transducer are the longitudinal waves, which are converted into an electrical signal by a second transducer. Electronic timing circuits enable the transit time T of the pulse to be measured.

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