methods of soil strength determination

7/27/2019 Methods of Soil strength Determination

1/13

A

SEMINAR PRESENTATION

ON

METHODS OF EVALUATION OF SOIL STRENGTH

B Y

AWODELE, ADEDEJI O.(CVE/04/2864)


2/13

In every civil engineering construction project, site investigations are

usually required to be carried out to establish various engineeringproperties of the soil underlying the site on which the project is to becarried out.

Site investigation may vary from a simple inspection of the surfaceconditions of the soil with or without shallow trial pits to expensive

and elaborate in- situ tests such boring, SPT, plate bearing test andlaboratory tests.

Strength determination is very important as it is used to determinesome parameters which are used in the design or to confirmassumptions made during the design stage.

The method to be utilised in determining the strength of a soil sample depends

on the type of the soil, the use to which the soil is to be put, the extent to which

geotechnical tests are relevant in the project, availability of required

equipments and cost of the tests to be carried out.


3/13

SOIL SHEAR STRNGTH TESTS

Direct shear tests

Triaxial tests Unconfined Compression Strength tests (UCS)

SOIL BEARING CAPACITY TESTS

Standard Penetration Tests(SPT) Plate bearing tests

California Bearing Ratio(CBR)


4/13

The California Bearing Ratio was developed by the California State HighwayDepartment.

It is in essence a simple penetration test developed to evaluate the strength ofroad subgrades i.e. to determine the resistance of the subgrade todeformation under the load from vehicle wheels. Or more simply put, toanswer the question How strong is the ground upon which we are going tobuild the road?.

This is done by causing a plunger of standard area to penetrate a soil sample.The force required to cause the penetration is plotted against the recordedpenetration, the readings noted at regular time intervals ie observations aretaken between the penetration resistance (called the test load) versus thepenetration of the plunger. The penetration resistance of the plunger into a

standard sample of crushed stone for the corresponding penetration is calledthe standard load. The California Bearing Ratio is defined as the ratio ofthe test load to the standard load, expressed as percentage, for a givenpenetration of the plunger.

Mathematically, CBR = (Test load /Standard load)x 100


5/13


6/13

6 kg of the soil material is weighed andreserved for use in this test;

The measured out soil sample is then

spread out on a tray and the lumps arebroken into smaller particles using amallet;

An amount of water corresponding to

the OMC gotten from compacting thematerial is then added to the soil;


7/13

A hand trowel is first used to work the water into the soil,

thereafter, the hand is used to thoroughly mix the soil and watertogether;

The mould to be used for compaction is then weighed and thevolume is determined;


8/13

The mould is then fixed to the base

and the extending collar isassembled with it;

The mould assembly is then oiledproperly and a displacer is put onthe base of the mould, a disc of filter

paper is then placed on top of thedisplacer;

An amount of soil capable offorming a 6 cm layer is then put inthe mould and it is compacted in the

automatic compaction machine with25 blows of the 4.5 kg rammerdropping from a height of about 30cm;


9/13

The step above is then repeated for the 2nd,

3rd, 4th, and 5th layer of soil; The mould is then removed from the

compaction machine, put on a mixing trayand the extension collar removed;

The excess soil is then trimmed off using a

steel straight edge till the surface is smoothand level with the top of the mould;

A representative sample of the compactedsoil is collected from the trimmed off partand kept for moisture content

determination;


10/13

The mould is then loosened from thebase and the weight is determined and

recorded; The mould is then placed on a

perforated base plate such that thehollow part is turned up;

Two surcharge weights are then placed

on top of the soil sample in the mould;

The mould assembly is then soakedcompletely in water for 24 hours;


11/13

The mould assembly is then removed fromthe soaking tank, unloosened from its baseand allowed to drain for 20 minutes;

The mould assembly with 2 surchargeweights is then placed on the automatic CBRmachine, where a plunger of standard areais allowed to penetrate the soil at a constantrate;

The force required to give a particularpenetration is then read off the CBRmachine;

The top two preceding steps are thenrepeated for the bottom of the soil in themould;

Soil samples are then taken from the top andbottom centres and kept for moisturecontent determination;


12/13

The importance of geotechnical tests on soils for civilengineering construction cannot be overemphasized.

This tests are necessary not only for determining someparameters that are to be incorporated into the design but alsoto confirm certain assumptions made during the design stage ofthe project.

As much as possible, all projects should be accompanied byfully documented procedures of tests carried out prior to, duringand after the construction work.

Legislation should be made requiring that appropriate tests be

carried out before the construction plans are approved by therelevant authorities.


13/13

methods of soil strength determination

Documents