Lab Report Unconfined.docx

1.0 INTRODUCTION The unconfined compression test (UC test) is to determine the unconfined shear strength, qu. Undrained shear strength is interpreted as the maximum internal resistance of a soil to the applied shear force when it is sheared at constant volume. This test is done by subjecting rapid compressive loading on a cylindrical soil specimen so that no drainage takes place during the shear. There is only vertical load applied on the soil sample during this test. 2.0 OBJECTIVES 1. To determine the shear strength of the cohesive soil. 2. To perform the unconfined compression test. 3.0 THEORY The unconfined compression test is a special type of unconsolidated undrained test that is commonly used for clay specimens. In this test, the confining pressure, σ3 is 0. An axial load is rapidly applied to the specimens to cause failure. At failure, the total minor principal stress, σ3 is zero and the total major principal stress is σ1. Because the undrained shear strength is independent of the confining pressure, so

τf

=

σ1 2

=

qu 2

= cu

where, qu is the unconfined compression strength. The axial load may be applied to the specimen either by the controlled strain procedure, in which the stress is applied to produce a pre-determined rate of strain, or by the controlled stress procedure, in which the stress is applied in pre-determined increments of load. IS: 2720 (Part 10) -1973 recommend the use of controlled strain tests. The unconfined compression strength can be correlate with the consistencies of clays as shown in Table 1.

Table 1: Approximate relationship of consistency and unconfined compression strength of clays Consistency

qu (kN/m2)

Very soft

0 - 25

Soft

25 – 50

Medium

50 – 100

Stiff

100 – 200

Very stiff

200 – 400

Hard

> 400

4.0 TEST EQUIPMENTS 1. Compression device of any suitable type (loading frame of capacity 2 kN), with a constant rate movement. 2. Sample extractor 3. Split moulds 3.5 cm diameter and 7 cm long. 4. Frictionless end plates of 7.5 cm diameter (Perspex plate with silicon grease coating) 5. Oven 6. Balance Sensitive to weigh 0.1 g. 7. Containers for moisture content determination. 8. Proving ring of 0.01 kg sensitivity for soft soils and 0.05 kg for stiff soils. 9. Dial gauge (sensitivity 0.01 mm) 10. Vernier calipers 11. Soil sample = Wet clayed soil

5.0 PREPARATION OF SPECIMEN 1. The split mould (38 mm in diameter) is oiled lightly from inside. 2. Remoulded soil sample is prepared by compacting the soil at desired water content and dry density in the split mould. 3. Split mould is opened carefully and sample is taken out. 4. The specimen trimmed to the specified dimension (38 mm in diameter and 76 mm in height). 5. Procedure 1-4 repeated. Minimum three soil specimens should be prepared for test. 6.0 PROCEDURES 1. The initial 2. The

length and diameter of the soil specimen is measured.

specimen placed on the base plate of the load frame (sandwiched between the end

plates) 3.

A hardened steel ball placed on the bearing plate. The centerline of specimen adjusted such that the proving ring and the steel ball are in the same line.

4. Dial

gauge fixed to measure vertical compression of the specimen.

5. The gear

position adjusted on the load frame to give suitable vertical displacement.

6. The reading of 7. The

proving ring and dial gauge was set to zero.

load applied and the readings of the proving ring dial and strain dial for every 0.2 mm

were recorded. 8. Continue

loading till failure occurs or 20% vertical deformation is reached (15 mm

compression of a 38 mm diameter specimen). Sketch the failure pattern; measure the angle between the cracks and the horizontal if possible.