The planner has to consider the following points before making a program:
- Type, size and importance of the project.
- Whether the site investigation is preliminary or detailed.
In the case of large projects, a preliminary investigation is normally required for the purpose of
- Selecting a site and making a feasibility study of the project,
- Making tentative designs and estimates of the cost of the project.
Preliminary site investigation needs only a few bore holes distributed suitably over the area for
taking samples. The data obtained from the field and laboratory tests must be adequate to provide a fairly
good idea of the strength characteristics of the subsoil for making preliminary drawings and design. In
case a particular site is found unsuitable on the basis of the study, an alternate site may have to be chosen.
Once a site is chosen, a detailed soil investigation is undertaken. The planning of a soil
investigation includes the following steps:
- A detailed study of the geographical condition of the area which include
(a) Collection of all the available information about the site, including the collection of
existing topographical and geological maps,
(b) General topographical features of the site,
(c) Collection of the available hydraulic conditions, such as water table fluctuations,
flooding of the site etc,
(d) Access to the site.
- Preparation of a layout plan of the project.
- Preparation of a borehole layout plan which includes the depths and the number of bore
holes suitably distributed over the area.
- Marking on the layout plan any additional types of soil investigation.
- Preparation of specifications and guidelines for the field execution of the various elements
of soil investigation.
- Preparation of specifications and guide lines for laboratory testing of the samples
collected, presentation of field and laboratory test results, writing of report, etc.
The planner can make an intelligent, practical and pragmatic plan if he is conversant with the
various elements of soil investigation.
Depths and Number of Bore Holes
Depths of Bore Holes
The depth up to which bore holes should be driven is governed by the depth of soil affected by the
foundation bearing pressures. The standard practice is to take the borings to a depth (called the
significant depth) at which the excess vertical stress caused by a fully loaded foundation is of the
order of 20 per cent or less of the net imposed vertical stress at the foundation base level. The depth
the borehole as per this practice works out to about 1.5 times the least width of the foundation from
the base level of the foundation as shown in Fig. 9.24(a). Where strip or pad footings are closely
spaced which results in the overlapping of the stressed zones, the whole loaded area becomes in
effect a raft foundation with correspondingly deep borings as shown in Fig. 9.24(b) and (c). In the
case of pile or pier foundations the subsoil should be explored to the depths required to cover the
soil lying even below the tips of piles (or pile groups) and piers which are affected by the loads
transmitted to the deeper layers, Fig. 9.24(d). In case rock is encountered at shallow depths,
foundations may have to rest on rocky strata. The boring should also explore the strength
characteristics of rocky strata in such cases.
Number of Bore Holes
An adequate number of bore holes is needed to
- Provide a reasonably accurate determination of the contours of the proposed bearing
- Locate any soft pockets in the supporting soil which would adversely affect the safety and
performance of the proposed design.
The number of bore holes which need to be driven on any particular site is a difficult problem
which is closely linked with the relative cost of the investigation and the project for which it is
undertaken. When the soil is homogeneous over the whole area, the number of bore holes could be
limited, but if the soil condition is erratic, limiting the number would be counter productive.