It depends upon type and importance of the structure and nature of soil strata. The cost involved should be in mind while planning the program. The following information is to get by conducting the exploration, the depth, thickness, extent and composition of each strata, depth of rock, and depth of ground water table. We must sought strength and compressibility of the strata for to make an estimate of safety and expected settlement of the structure. A site plan should be prepared. A layout plan of proposed building with column locations an its expected loads. Location of bore holes is decided along with other field tests. A careful planning is needed for the boring program. Trained persons in geology and geotechnical engineering is necessary.

Two important aspects of a boring program are spacing and depth of boring.

Spacing of Borings

The spacing and number of borings are related to the type, size, and loads of proposed structure. It is also depends on condition of soil horizontally and vertically. The local building code will be useful.

S. No

Nature of the Project

Spacing of Borings in meters

1

Highway (subgrade survey)

400

2

Earth dam

40

3

Borrow Pits

70

4

Buildings

20

5

Single Storey Buildings

50

The conditions of soil that is its uniformity is a factor for the spacing. IS1892, 1979 will be recommended for reference.

Depth of Borings

In order to get adequate information to predict the settlement of the structure, the boring should be deep enough to loading influence zone of the soil strata. The boring shall be gone up to the rock for huge buildings and bridges. For small buildings 15 or 20 meters is sufficient.

Previous loading history is known for settlement study. According to IS 1892, 1979 the depth of exploration required depends up on the type of the structure, its total weight, the size, shape and disposition of the loaded area, soil profile and the physical properties of the soil that constitutes each stratum.

Boring Log

The record of the information got from borings is called boring log.

Depth of borings IS 1892 1979 

S. No

Type of Foundation

Depth of Boring

1

Isolated spread footings or Raft                          

1.5 times the width

2

Adjacent footings spacing less than twice width  

1.5 times the width

3

Pile and well foundations

1.5 times the width of the structure from bearing level

4

Road cuts                                               

equal to bottom width of the cut

5

Fill

2 meters below the ground level or equal to the height of fill which is great

 

Soil Sampling

It is the process of obtaining samples of soil from the desired depth at the desired location from a natural soil deposit, for laboratory examination to assess its properties. From different stratas samples are taken. Determination of GW level is also a sampling.

There are disturbed and undisturbed samples, which is to be tested in the laboratory. The samples taken may loss its physical properties for example compaction. A fully undisturbed sample can not be taken.