The City College of New York

The City College of New York

Department of Civil Engineering

CE 345: Soil Mechanics

Instructor: Dr. George Mylonakis

LAB EXPERIMENT #6:

UNCONFINED COMPRESSION TEST

Introduction

The unconfined compression test is used to measure the unconfined shear strength of fine-grained soils, which is an approximate value of their undrained shear strength. This test is applicable only to cohesive soils such as saturated clays or cemented soils that retain an intrinsic strength without a confining pressure. It is not applicable to dry or cohesionless soils, such as gravels and sands. This test can be done for both undisturbed and remolded specimens. Here you will perform the test on a remolded specimen.

Equipment

1.  Loading device

2.  Loading ring

3.  Dial indicator

4.  Scale

5.  Trimmer

6.  Watch

7.  Sampling tube with extractor

8.  Harvard miniature compaction device

9.  Balance

10.  Moisture cans

11. Oven

Preparation of specimen

Weigh 115g of dry disturbed soil. Add water to reach water content of 28%. Mix thoroughly to get uniform water content. Compact the sample in Harvard miniature compaction mold. Carefully extrude the specimen.

Procedure

1.  Determine the initial length and diameter of the specimen.

2.  Immediately place the sample on the loading device. Lower the bottom platen or raise the bottom platen so that the upper platen barely touches the specimen and triggers a slight response of the load sensor. Attach the dial indicator to the loading device to measure the axial deformation of the specimen.

3.  Adjust the rate of axial displacement to obtain a strain rate of 1% per minute. The complete loading lasts about 20 min. to apply 20% of axial strain.

4.  Simultaneously record the axial displacement and load. Stop the test when load remains constant or after 20% axial strain.

5.  Record the time to peak strength and failure pattern (shear or bulge failure) and sketch the failed specimen.

6.  After the test, determine the water content, w, of the specimen.

Calculation

1. Water content w is given by

w = (Wi / Wd - 1) x 100 (%)

Wi = initial weight

Wd = dry weight

2. The axial strain e is

e = DH / Ho

DH = change of specimen height

Ho = initial specimen height

3. The stress s is given by

s = F / Ac

Ac = Ai / (1 - e)

where,

F = applied load

Ac = corrected area

Ai = initial area

General Comments

Plot a stress-strain diagram using same scale. Determine shear strength of soil. Discuss the soil tested based on results.

DATA SHEET: Unconfined Compression Test

Description of soil: ______

Group No.: ______

Date: ______

Moisture content, w: ______%

Initial length of specimen, Ho: ______in.

Initial diameter of specimen, Do: ______in.

Initial area of specimen, Ai: ______in.2

Corrected area of specimen, Ac: ______in.2

Force, F
(lb) / Displacement, DH
(in.) / Strain, e / Stress, s
(psi)

Shear Strength, t ______psi

29 L6 Unconfined Compression.doc