TITLE: Determination of unconfined compressive strength
AIM: To determine the unconfined compressive strength of clayey soil using
REFERENCE: IS 2720 (Part10) 1991 (Reaffirmed 2006)
EQUIPMENT:
A. Apparatus
i.) Compression machine,
ii.) Proving ring,
iii.)Dial gauge of L.C. 0.01mm,
iv.) Split mould of internal diameter 30mm and length 76 mm.
v.) Sampling tube of internal diameter 30mm and length 200mm.
vi.) Stopwatch
vii.) Vernier calliper,
viii.) Knife,
ix.) Balance.
THEORY:
1. Unconfined compressive strength (qu): Unconfined compressive strength (qu) is the load
per unit area at which an unconfined cylindrical specimen of soil will fail in a simple
compression test.
2. Sensitivity: Sensitivity is defined as the ratio of U.C.S. of undisturbed soil sample to the
U.C.S. of remoulded sample at constant moisture content.
Sensitivity = qu(Undisturbed) / qu(Remoulded)
Notes:
i.) Unconfined compression test is the simplest and quickest for determining
cohesion and shear strength of cohesive soils.
ii.) Soil consistency can be easily known from the value of unconfined
compressive strength.
iii.) Sensitivity of a soil can be known from the undisturbed and remoulded U.C.S
respectively.
iv.) Sample should always be extracted along the same direction in which it entered the tube
in the field.
v.) Sampling tube should be properly sealed (by wax and cotton waste) at both
the ends in the field to prevent loss of field water content.
vi.) Where no maximum stress is observed, stress at 20% strain will give the unconfined
compression test
PROCEDURE:
i) Procurement of sampling tube with undisturbed sample from the site
ii) Coat the inside of the split mould with a thin layer of grease or oil to prevent adhesion
of the soil.
iii) Extrude the specimen from the sampling tube to the split mould with the help of
sample extractor and knife.
iv) Trim the two ends of the mould sample.
v) Remove the sample from the mould by splitting it in two parts and weigh it. Also
measure the length and diameter of specimen.
vi) Place the specimen on the bottom plate of the compression machine.
vii) Raise the bottom plate of the machine to make contact of the specimen with the upper
plate.
viii) Adjust the strain dial gauge and proving ring dial gauge to read zero.
ix) Apply the compression load by raising the bottom of the machine to produce axial
strain at a rate of 1.25 mm/min.
x) Record the strain and proving ring dial gauges readings every 30 sec.
Compress the specimen till it fails or 20% vertical deformation is reached which ever
is earliest.
xi) Measure the failure angle from horizontal and note the load at failure Determine the
water content of the specimen.
Note: The same procedure may be adopted for remoulded specimen.