Unconfined Compression Test & Foundation Design

1. Theory

1.1. Background

Shear strength of the soil is best measured by the unconfined compression test. The main

advantage of the test is that it is very fast and cheap method for measurement of shear

strength. This procedure is used basically for cohesive soil. It cannot be used for the dry sand

and crumble clays.

1.2. Cohesive soils

These soils are clay type. Cohesion is basically defines as the force which stick/hold the particles

together. It is denoted by “c”.

The unconfined compressive strength is related to cohesion as given

c =qu/2

Where

c = cohesion

qu = unconfined compressive strength

The unconfined compressive strength is given as

As soil deform more as compared to concrete, the area of sample changes as the volume

remain constant. Thus the average cross sectional is calculated as

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1.3. Type of Foundation

Background knowledge of the methodology of determining the resistance of cohesive and no

cohesive soils is necessary to select the appropriate bearing capacity value for the particular

site and the particular foundation system under consideration. There are two types of

foundations.

Deep Foundations

Shallow Foundation

1.3.1. Shallow Foundations

It is the foundation type in which building loads are transferred to the earth very near to the

surface. Its depth is very less as compared to deep foundation. The different type of shallow

foundations are the isolated foundation (spread foundation), raft foundation, pad foundation,

rubble trench foundation.

1.3.2. Deep Foundations

Deep foundations are different to shallow foundation as they are deep located under the

ground. There are different technical reasons for the preferences over shallow foundation but

some common reasons are poor soil condition, large load etc. The deep foundations are piles.

They are constructed with different material such as steel, timber and concrete.

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1.4. Significance:

The unconfined shear strength Su for soils is important for the measurement of bearing capacity

of foundation design of building and dams. As we know that unconfined shear strength SU is

50% of the unconfined compressive strength qu when angle of internal friction is zero. It is very

important to know that most critical condition occur immediately after construction in soil

2. Apparatus

Unconfined Compression Test Machine

Two metal plates are used during the loading in loading frame. The lower plate is not

stationary, raised and released with the help of crank attached to loading frame. The other

plate is at top and remains stationary. The top plate is also attached to the load measuring

instrument. The loading frame is laboratory was manual/hand operated but it can be applied

with electric motor or some hydraulic system.

Dial Gauges & Load Ring

Calibrated proving ring are used to measure the loads.

Similarly, the dial gauges are used to measure the

deformations. These dial gauges are fixed to the station plate

at the top and the movement between the lower and top plate

is measured.

Auxiliary Apparatus

Other instrument used was as given below.

a) Specimen Preparation Equipment

b) Sample Extruder

c) Balance

3. Procedure

The first step in laboratory was to prepare the sample of soil for test with known value of moisture content. A soil sample was prepared with moisture content of range between 11%

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and 15%. Afterward, the sample was compacted in a mould with three layers. Each layer was compacted with 25 strokes. Then it was removed from the mould and its weight, length and diameter was measured and recorded as mention in the calculation/result section. The diameter of the sample was measure at three places (top, bottom and center) and its mean value was taken.

The soil sample was then placed into the compression test machine in between the top and bottom plate. The top plate was lowered such that it just contact with the top of soil sample. The dial gauge used for vertical deformation measurement was set to zero. The load was applied to the sample to such extent that it produced 0.5 to 2 % /mint axial strain.

For the calculation of stress strain diagram, the value of load, deformations and time was recorded at regular intervals. One thing was carefully notice that rate was defined based on that failure will not exceed 15 mints. The loading was increased until it was noticed that load value decrease with increasing strain.

References:

o ASTM Standards, Designation: D 2166 – 06, “Standard Test Method for Unconfined

Compressive Strength of Cohesive Soil”

o Matt Wade, Sarah Rohm and Seth Fink, Oct 2007, “CE 185 – Geotechnical Engineering

Laboratory”

o John T. Germaine, Amy V. Germaine ,2009, “Geotechnical Laboratory Measurements for Engineers”

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