Tuesday, 31 May 2016

Standard Penetration Test (SPT)


One of the most common in-situ tests is the standard penetration test or SPT. This test which was originally developed in the late 1920s.

SPT is most commonly used in situ test, especially for cohesionless soils which cannot be easily samples. the test is extremely useful for determining the relative density and the angle of shearing resistance of  for cohesionless soils.

it can also used to determine the unconfined compressive strength of cohesive soils.

The standard penetration test is conducted in a borehole using a standard split-spoon sampler. 

When the borehole (55 to 150 mm in dia) has been drilled to the desired depth, the drilling tools are removed and the split-spoon sampler, attached to standard drill rods of required length is lowered to the bottom of the borehole and rested at the bottom.

The split-spoon sampler is then driven into the soil for a distance of 450 mm in three stages of 150 mm each by blows of a drop hammer of 63.5 kg mass falling vertically and freely through a height of 750 mm at the rate of 30 blows per minute (IS 2131 – 1981). The number of blows required to penetrate every 150-mm is recorded while driving the sampler. If full penetration is obtained, the blows for the first 150 mm is retained for reference purposes, but not used to compute the SPT value because the bottom of the boring is likely to be disturbed by the drilling process and may be covered with loose soil that may fall from the sides of the boring. The number of blows required for the next 300 mm of penetration is recorded as the SPT value. The number of blows is designated as the “Standard Penetration Value” or “Number” N.

The slit-spoon sampler is then withdrawn and is detached from the drill rods. The split barrel is disconnected from the cutting shoe and the coupling. The soil sample collected inside the split barrel is carefully collected so as to preserve the natural moisture content and transported to the laboratory for tests. Sometimes, a thin liner is inserted within the split-barrel so that at the end of the SPT, the liner containing the soil sample is sealed with molten wax at both its ends before it is taken away to the laboratory. 

Precautions

The drill rods should be of standard specification and should not be in bent condition

The split spoon sampler must be in good condition and the cutting shoe must be free from wear and tear

 The drop hammer must be of right weight and the fall should be free, frictionless and vertical.

 The height of fall must be exactly 750 mm. Any change in this will seriously affect the N value.

 

The standard penetration number is corrected for dilatancy correction and overburdon correction.

Undisturbed soil samples


Undisturbed soil samples are those in which the in-situ soil structure and moisture content are preserved.

• They are representative and also intact

• These are used for consolidation, permeability or shear strengths test (Engineering properties)

• More complex jobs or where clay exist

• In sand is very difficult to obtain undisturbed sample

• Obtained by using Shelby tube (thin wall), piston sampler, etc., 

Disturbed soil samples


Disturbed soil samples are those in which the in-situ soil structure and moisture content are lost, but the soil particles are intact.

• They are representative 

• They can be used for grain size analysis, liquid and plastic limit, specific gravity, compaction tests, moisture content, organic content determination and soil classification test performed in the lab

 • e.g., obtained through cuttings while auguring, etc.

Depth of exploration


The depth of the exploration required at a particular site depend on the degree of variation of the sub-surface data in the horizontal and vertical directions. It is not possible to fix the number, disposition and depth of borings without making a few preliminary borings or sounding at the site.

Generally exploration should be carried out to a depth upto which the increase in pressure due to structural loading is likely to cause perceptible settlement or shear failure. such a depth known as the significant depth, depent upon the type of the structure, its weight.  It is generally safe to assume the significant depth upto a level at which the net increase in vertical pressure become less than 10% of the initial overburden pressure.

Stages of the sub-surface exploration


1. Reconnaissance

site Reconnaissance is the 1st step in the exploration. it include a visit to the site and to study the maps and other relevant records. it helps in deciding the future programme of the site investigation. types of sample adopted to be taken and the laboratory testing and in-situ testing.

2. Preliminary or general exploration

the aim of the general exploration is to get an approximate picture of the sub-soil condition at the relatively low cost. the information so obtained should suffice for the design and execution of minor and routing engineering works.

the preliminary exploration are generally in the form of few borings or test pits. test are conducted inthe form of the borings or test pits. test are conducted with cone penetrometers and sounding rods to obtain information about strength and compressibility of the soil

3. Detailed exploration

it is a supplement to general exploration when large engineering works, heavy loads and complex costly foundation are involved, such as bridge, dam, and multistory building. however for small projects especially at sites where the strata are uniform, detailed exploration may not be required.

the purpose of detailed exploration is to determine the engineering properties of the soils in different strata. it include the  extensive boring programme, sampling and testing of the samples in the a laboratory.

field test, vane shear test, plate load test and permeability test are conducted to determine the properties of the soil in natural state.

the test for the determination of the dynamic properties are also carried out.

 

what is a Scope of soil investigation ?


An investigation of the site is essential for judging its suitability for the purposed engineering works and for preparing adequate and economic design. it consists of determining the profile of natural soil deposits at the site taking the soil samples and determining the engineering properties of the soils. It also include the in-situ testing of the soil.

1. to select the type and depth of the foundation for a given structure.

2. to determine the bearing capacity of the soil.

3. to estimate the probable maximum and differential settlement.

4. to predict lateral earth pressure against retaining walls and abutments.

5. to select the construction techniques.

6. to select the safety of the existing structure and to suggest the remedial measures.

Objectives of soil exploration


soil exploration is very useful for civil engineer to execute the design of construction, planning of construction, and for following purposes.

1. To know the geological condition of rock and soil formation.

2. To establish the groundwater levels and determine the properties of water.

3. To select the type and depth of foundation for proposed structure

4. To determine the bearing capacity of the site.

5. To estimate the probable maximum and differential settlements.

6. To predict the lateral earth pressure against retaining walls and abutments.

7. To select suitable construction techniques

8. To predict and to solve potential foundation problems

9. To ascertain the suitability of the soil as a construction material.

10. To determine soil properties required for design

11. Establish procedures for soil improvement to suit design purpose

12. To investigate the safety of existing structures and to suggest the remedial measures.

13. To observe the soil the soil performance after construction.

14. To locate suitable transportation routes.