Soil Exploration: Need and Execution | Soil Engineering

In this article we will discuss about:- 1. Introduction to Soil Exploration 2. Need for Soil Exploration 3. Planning and Execution.

Introduction to Soil Exploration:

Soil contrasts other civil engineering materials, such as steel, cement or concrete, brick, aggregate, or bitumen. All these materials can be manufactured with designated quality, for example, Fe500 steel, first-class bricks, 53-grade cement, 20 mm aggregate, and 90/80 bitumen. Depending upon the structural requirements, materials of suitable quality can be selected and used. The quality of these materials is well defined and unique throughout the world.

But geotechnical engineers do not have this luxury of choice. It is necessary to work with the available soil or rock at the given site. Soil and rock are natural materials and do not have any standard quality. Their properties vary from place to place, and even at the same project site, soil properties may vary within a short distance or depth. Thus, instead of specifying required properties, the task is to determine the properties of the soil existing at a site so that a suitable foundation can be designed for the proposed structure. This process is called site characterization.

Need for Soil Exploration:

The choice of a suitable type of foundation for any structure can be made only after studying the soil profile and its characteristics at the project site.

A well-planned and executed soil exploration can bring in a number of financial benefits and includes the following:

1. Selection of the most economical foundation.

2. Design of a safe and economical foundation.

3. Prediction of possible problems during construction and adoption of preventive measures so that the project delays can be avoided, thus saving costs.

4. Prediction of long-term stability problems to the foundation or structure and adoption of appropriate safety measures, etc.

The safe and economical design of foundation of any structure, supported on soil, requires the knowledge of the physical and engineering properties of the soil up to a depth of 1.5-2 times the width of the foundation or structure below the base of the foundation.

For large and important structures, such as multi-storied buildings, gas or oil storage tanks, and others, the selection of the foundation is based on the soil properties at the site up to a significant depth below ground level.

For structures which require excavation below ground level, such as sewage treatment plants, underground reservoirs, or structures constructed in coastal areas, construction is likely to be halted for several weeks or months if the foundation trenches collapse, delaying the project and soaring project costs.

For structures located in difficult soil conditions, such as expansive soils or soft marine clays, knowledge of soil properties up to a significant depth is required to design suitable ground improvement techniques to ensure a long- term stability of such structures.

The design of earth-supporting structures, such as retaining walls, sheet piles, abutments, and piers of bridges, also requires knowledge of soil properties of the underlying soil for estimation of lateral earth pressure for carrying out design and stability analysis of such structures.

The selection of suitable borrow areas for construction of earth structures, such as earth dams and embankments for roads, railways, or canals, requires determination of soil properties to choose the most suitable and economical borrow area.

The field and laboratory investigations and testing conducted to determine the physical and engineering prop­erties of the soil as well as to study the site conditions are known as soil exploration or geotechnical investigation. Although the name refers to soil exploration, its scope also includes study of rock, groundwater conditions, and any other material encountered in the course of soil exploration.

The need for a proper soil exploration is highlighted by Karl Terzaghi in one of his famous quotes:

“… Any attempt to stop the settlement without making the proposed preliminary investigation would be an irresponsible gamble. Since I have witnessed many gambles of this kind, I can state from personal experience that the savings associated with inadequate preliminary investigations are entirely out of proportion to the financial risks.”

Planning and Execution of Soil Exploration:

The data obtained from soil exploration will have a significant bearing on the stability of the structure, choice of a safe and economical foundation and its design, as well as successful completion of construction of the structure on time. The success of soil exploration depends on establishing the site and soil characteristics that are representative of the entire area of the site up to a significant depth.

The extent of soil exploration depends on the variability of soil and ground conditions at the proposed site. If the soil and ground conditions are highly variable, either in the lateral direction or across depth or both, a more detailed soil exploration program would be necessary. As the nature of variability of soil characteris­tics and ground conditions can be known only after carrying out preliminary exploration at a few important locations, the exact program of detailed soil exploration can be planned based on the results of the preliminary exploration.

The extent of soil exploration also depends on the type of structure and its cost. For small structures with limited budget, soil exploration is restricted to a limited extent. For large or important structures, detailed and thorough soil exploration would be necessary. Usually, the cost of soil exploration is slated to about 0.5%-1% of the cost of the structure.

The planning and execution of soil exploration may be considered under the following heads:

1. Fixing the objectives of soil exploration. 

2. Number and Disposition of Trial Pits and Borings.

3. Deciding the depth of exploration.

4. Site reconnaissance.

5. Preliminary exploration.

6. Detailed exploration.

1. Fixing the Objectives of Soil Exploration:

The following are the main objectives of soil exploration:

i. To determine the location and thickness of various soil and/or rock strata up to a significant depth.

ii. To collect soil or rock samples at locations that represent changes in soil characteristics across the area and depth for conducting laboratory tests to determine the basic and engineering properties.

iii. To conduct relevant in situ tests to determine the in situ properties of the soil or rock, such as standard pene­tration test (SPT), cone penetration test (CPT), plate load test (PLT), pressure-meter test (PMT), or geophysical exploration.

iv. To determine the depth of groundwater table (GWT) below ground level and also gather information about its seasonal variation.

v. To study the geological, geomorphological, and topographical information in and around the site that may have a bearing on the stability and safety of the structure.

vi To conduct the required tests in the laboratory on the samples of soil or rock collected from the field to deter­mine the physical and engineering properties of the soil or rock.

vii. To decide the depth of foundation depending on the soil profile at different depths and the type of structure from stability and economic considerations.

viii. To decide the type of foundation for the structure based on the interpretation of the results of soil exploration and the type of structure.

ix. To decide on the ground improvement techniques or measures required, if any.

x. To determine the safe bearing capacity of the soil at required depths.

xi. To identify and define special problems and concerns that may arise during or after construction, if any.

2. Number and Disposition of Trial Pits and Borings:

The number of test pits or borings and their relative location should be chosen in such a way as to reflect major changes in soil properties across the site area. It also depends on the extent of site and the type of structure.

Thus, the number and spacing of boreholes or trial pits depends upon the following:

i. The extent of the site.

ii. The nature of structures coming on it.

iii. The extent of variation of properties of soil over the site.

The following are the guidelines suggested for deciding the number and disposition of trial pits and borings as per IS – 1892-1979:

a. For small areas and less important buildings, one borehole or trial pit at the center may be sufficient.

b. For a compact building site, covering an area of about 0.4 ha (4000 m²), a total of five boreholes or test pits, that is, one borehole or trial pit in each corner and one at the center, is adequate.

c. For large and important structures, exploration is carried at important locations, as suggested by the geotechnical engineer, in addition to the corners of the site. The spacing between borings or test pits may be 10-30 m, depending on the variation in subsurface conditions and/or loading.

d. For very large areas, covering industrial and residential colonies:

i. The geological nature of the terrain may be used as a guide in deciding the number of boreholes or trial pits.

ii. Cone penetration tests (CPTs) are performed at every 50 m, by dividing the area in a grid pattern. The number of boreholes or trial pits may be decided by examining the variation in the penetration curves.

iii. In case of gravelly or boulderous strata, where CPTs may not be possible, geophysical methods are used as a guide in deciding the number and location of trial pits or borings.

e. In the case of highway or railway projects, exploration is carried along the center line of the project at a spacing of 150-300 m. When the soil properties are highly variable along the length, this spacing may be reduced up to 30 m, depending on the requirement.

Table 14.1 gives the minimum number of borings and the spacing between the borings for different types of struc­tures depending on whether the soil is uniform or erratic, as recommended by the US Bureau of Reclamation (USBR) Earth Manual.

3. Deciding the Depth of Exploration:

Soil exploration needs to be carried up to a depth where the stresses in the soil, caused by the structure, are signifi­cant. Significant depth is usually taken as the depth at which the vertical stress is 20% of the stress at the foundation level. The significant depth for various types of footings as per Boussinesq’s theory is given in Table 14.2.

Apart from the stress level, the depth of exploration required depends on the following:

i. Type of the proposed structure and its total weight.

ii. Size, shape, and disposition of the loaded areas.

iii. Soil profile.

iv. Physical properties of the soil in each stratum.

v. Type of foundation to be adopted.

The following are the guidelines used for deciding the depth of exploration:

i. For large and important structures, soil exploration should be carried up to the level of the hard rock.

ii. Normally, the depth of exploration should be 1.5 or 2 times the width of the foundation, below the foundation level. In certain cases, it may be necessary to take at least one borehole or cone test or both, to a depth of twice the width of the foundation.

iii. If the foundations or structures are in close proximity, the pressure bulbs overlap and the effect of each is addi­tive. In such cases, the whole of the area may be considered as loaded and exploration should be carried out up to 1.5 times the lower dimension.

iv. In weak soils, exploration should be continued to a depth at which the loads can be carried by the stratum in question, without undesirable settlement and shear failure.

v. Exploration should be always carried to a depth more than the depth of seasonal moisture variations.

vi. The depth of exploration, at the beginning of the work, may be decided as given in Table 14.3, as per IS – 1892-1979. After a few preliminary borings or test pits, this depth may be suitably modified, based on the soil profile encountered and based on the experience and judgment of the engineer-in-charge.

4. Site Reconnaissance:

It is the first stage of soil exploration.

It is conducted to decide or collect the information about following:

i. The ground conditions and its surroundings by visiting, observing, and walking across the site.

ii. About the site and its surrounding sites or structures by collecting the available data.

iii. A program of soil exploration, that is, to assess the need and scope of preliminary or detailed investigation.

iv. Methods of exploration to be adopted.

v. Field tests to be conducted.

vi. Administrative arrangements required for soil exploration, including the human, financial, and physical resources needed.

Methods:

The following are the methods used in site reconnaissance:

A. Enquiries:

i. Available records of previous trial pits, boreholes, etc., in the vicinity of the site should be taken for study.

ii. Inspection of existing structures in the vicinity, particularly, those similar to the proposed structure.

iii. Information about any structure that was or is existing on the site.

iv. Information about the earlier use of the site, for example, old mines, waste disposal site, or cultivated land.

v. Information about the existing underground water, sewer, or power lines in the site.

B. Study of maps and other published literature, including the following:

i. Topographical maps.

ii. Geological maps.

iii. Soil survey maps.

iv. Pedological maps.

v. Aerial photographs.

C. Walking the site.

D. An inspection of the site and a study of existing topographical features, including the following:

i. Water levels in the streams, water courses, and wells.

ii. Local topography and geology.

iii. Nature of vegetation, drainage pattern, and flood marks.

iv. Evidence of erosion or landslides.

E. Study of previous records of trial pits or borings for the existing structure in the vicinity or for the old structure existing or demolished at the present site.

F. Study of plan of the proposed structure in relation to its location on the site is also studied and heavy loaded areas are identified on the site map.

G. Geophysical investigations – CPTs or geophysical investigations of the site by the electrical resistivity or seis­mic-refraction method may be conducted at the reconnaissance stage. Depending on the information obtained from site reconnaissance, detailed subsoil exploration is planned.

5. Preliminary Exploration:

Preliminary exploration consists of conducting soil exploration at a few important locations of the site. The meth­ods used for preliminary exploration include test pits, soil borings, CPTs, and geophysical exploration.

The scope of preliminary exploration is as follows:

i. To determine the depths, thickness, extent, and composition of each soil stratum.

ii. To determine the location of rock and GWT.

iii. To obtain approximate information regarding the strength and compressibility of various strata.

For small structures and in sites where soil characteristics are found to be uniform across the site, no further detailed exploration is required. The collected soil samples are tested in the laboratory and the physical and engineering properties of the soil at different depths are determined. Information about the depth of GWT and its seasonal variation is also obtained.

Based on this data, the type of foundation to be adopted for the proposed structure is selected and the allowa­ble bearing capacity based on shear failure and settlement criteria is determined. The design of foundation is then carried out based on the allowable bearing capacity.

For large or heavy structures and in sites where soil characteristics vary across the site, preliminary exploration is generally followed by detailed exploration. The methods of detailed exploration to be adopted as well as the number and spacing of test pits or borings are decided based on the data obtained during preliminary exploration.

6. Detailed Exploration:

If the results of preliminary exploration indicate a large variation in soil characteristics across the site, detailed explo­ration is necessary especially for large and important structures. Detailed exploration follows preliminary exploration and should be planned on the basis of data obtained during site reconnaissance and preliminary investigations to fulfill the scope and objectives of soil exploration. This plan may be modified suitably, as the investigations progress.

The scope of detailed investigations is to determine engineering properties of the strata, which are identified to be critical by preliminary exploration.

The objective of detailed exploration is as follows:

i. To determine the shear strength and compressibility of all types of soils, density, density index, natural moisture content, and permeability.

ii. To determine the pre-consolidation pressure of the strata from odometer tests.

iii. To determine the consolidation characteristics beyond the pre-consolidation pressure.

iv.. Appropriate shear tests are be conducted on samples subjected to ambient pressures beyond the pre-consolida­tion range.

The detailed investigation includes test pits or a boring program and detailed sampling to determine these proper­ties. Field tests, which may be performed during detailed exploration, are standard penetration test (SPT), CPT, in situ vane shear test, plate load test (PLT), pressure-meter (PMT), etc. The field permeability test and test for deter­mination of dynamic properties of soils may also be conducted, wherever necessary. More advanced methods of logging of boreholes by radioactive methods fall under the category of detailed investigations. All in situ tests are to be supplemented by laboratory testing.