1. SITE INVESTIGATION

2. Introduction
Site investigation refers to the methodology of determining surface and subsurface features of the proposed area.
It consist of determining the profile of natural soil deposits at the site, taking samples and determining the properties.

3. Soil exploration
The field and laboratory studies carried out for obtaining the necessary information about the surface and subsurface features of the proposed area including the position of the ground water table.

4. Objectives
To know the geological condition of rock and soil formation.
To establish the groundwater levels and determine the properties of water.
To select the type and depth of foundation for proposed structure.
To determine the bearing capacity of the site.
To estimate the probable maximum and differential settlements.
To predict the lateral earth pressure against retaining walls and abutments.

5. To select suitable construction techniques.
To predict and to solve potential foundation problems.
To ascertain the suitability of the soil as a construction material.
To determine soil properties required for design.
Establish procedures for soil improvement to suit design purpose.
To investigate the safety of existing structures and to suggest the remedial measures.

6. Phases of a Soil Investigation
Phase I.( Reconnaissance)
Collection of available information such as a site plan, type, size, and importance of the structure, loading conditions, previous geotechnical reports, topographic maps, air photographs, geologic maps, hydrological information.

7. Phase II.
Preliminary reconnaissance or a site visit to provide a general picture of the topography and geology of the site.
visual inspection is done to gather information on topography, soil stratification, vegetation, water marks, ground water level, and type of construction nearby.

8. Phase III. Detailed soils exploration.
Detailed planning for soil exploration in the form trial pits or borings, their spacing and depth.
Type of field tests adopted and the type of sampling done, presence of water table if met with are recorded in the form of bore log.
The soil samples are properly labeled and sent to laboratory for evaluation of their physical and engineering properties.

9. Phase IV.
Write a report. The report must contain a clear description of the soils at the site, methods of exploration, soil profile, test methods and results, and the location of the groundwater.
This should include information and/or explanations of any unusual soil, water bearing stratum, and soil and groundwater condition that may be troublesome during construction.

10. Depth of Investigation
The depth of investigation depends on
The size and type of proposed structure
Sequence of proposed strata.

11. Depth of exploration
Depends on degree of variation of subsurface data in the horizontal and vertical directions.
It is governed by the depth of influence zone, which depends on type of structure, intensity of loading, shape, soil profile & physical characteristics of soil.

12. Significant depth
The investigation shall be carried out to the point at which the vertical stress due to proposed structure is equal to or less than 10% of original effective stress at the point before the structure is constructed – significant depth

13. Methods of borings Auger boring :-
preferred for shallow depths , low ground water table
Wash boring :-
high water table, deeper soil deposit
Rotary drilling:-
high quality boring, also for rock drilling
Percussion drilling:-
fast drilling, not taking samples

14. Auger Boring

15. Hand augers – 15 to 20cm depth
Mechanical augers - > 12m depth
Useful for subsurface investigation.
Main disadvantage is samples are highly disturbed.

16. Wash boring
Hole is drilled by driving a casing about 2 to 3m long and inserting drill rod with a chisel- shaped chopping bit at the lower portion.
Water is pumped down.
Hole is advanced by chopping action and jetting action.
Chopped soil and water is collected in tub

18. Rotary drilling
Bore hole is advanced by rotating a hollow drill rod which has cutting bit at lower end.
Drill head is provided at the top.
It consists of rotary mechanism and an arrangement for applying downward pressure.
Can be used in clay & sand.
The method is not suitable if the soil contains a high percentage of gravel/cobbles

20. Percussion drilling
Used for making holes in rocks, boulders & hard strata.
Heavy chisel is used which is alternatively get lifted and dropped in a vertical hole.
Point where chisel strikes is above W.T water is added.
Main disadvantage material at the bottom of the hole is disturbed by heavy blows.

21. Core drilling Used for drilling holes and obtaining rock cores.
Done using a diamond studded bit or cutting edge.
A double tube barrel is used to get good quality sample of rock.

22. Soil Sampling
Need for sampling
Sampling is carried out in order that soil and rock description, and laboratory testing can be carried out.
Laboratory tests typically consist of:
Index tests (for example, specific gravity, water content)
- Tests to determine engineering design parameters (for example strength, compressibility, and permeability).

23. Factors to be considered while sampling soil
Samples should be representative of the ground from which they are taken.
They should be taken in such a way that they have not lost fractions of the in situ soil.

24. Type of soil samples

25. Non-Representative samples
Non-Representative soil samples are those in which neither the in-situ soil structure, moisture content nor the soil particles are preserved.
They are not representative
They cannot be used for any tests as the soil particles either gets mixed up or some particles may be lost.
Samples that are obtained through wash boring or percussion drilling.

26. 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
obtained through cuttings while auguring, grab, split spoon (SPT), etc.

27. 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)
In sand is very difficult to obtain undisturbed sample

28. Design Features affecting the sample disturbance
Area ratio
Inside Clearance
Outside Clearance
Recovery Ratio
Inside wall friction
Design of non-return value
Method of applying force
Sizes of sampling tubes

29. Area ratio

31. Inside clearance

32. Recovery ratio

33. Inside wall friction
The friction on the inside wall of sampling tube causes disturbances of the sample.
Inside surface of the sampler must be smooth.
It is usually smeared with oil in order to reduce the friction.

34. Design of non- return value
The non – return value provided on the sampler should be of proper value.
It should have an orifice of large area to allow air, water or slurry to escape quickly.
It should close when the sample is withdrawn.

35. Method of applying force
The degree of disturbance depends upon the method of applying force during sampling & rate of penetration of sample.
For getting undisturbed sample, the sampler should be pushed and not driven.

36. STANDARD PENETRATION TEST

37. SUB- SOIL INVESTIGATION REPORT
It should contain the data obtained from bore holes, site observations & laboratory results.
It should give the recommendation about the suitable type of foundation, allowable soil pressure and expected settlement.

38. Report consists of
Introduction - scope of investigation
Description of proposed structure, the location and geological condition at the site
Details of field exploration programme – indicating the number of borings, their location & depth.
Details of method of exploration.

39. General description of the subsoil condition.
Details of laboratory test conducted on soil samples.
Depth of ground water table and change in water level
Discussion of the result.
Recommendation about allowable bearing pressure, the type of foundation or structure.
Conclusions.