Site Investigation - Elements of Soil Mechanics - page 158

part of the profile that is significantly affected by vegetation and the elements. The maps produced give a
good indication of the surface soil type and its drainage properties. The surface soil type can often be related
to the parent soil lying beneath, and so soil types below 1.5 m can often be interpreted from the maps.

Aerial photographs

With careful interpretation of aerial photographs it is possible to deduce information on landforms, topog-
raphy, land use, historical land use, and geotechnical behaviour. The photographs allow a visual inspection
of a site when access to the site is restricted. Freely accessible satellite imagery is now a much used source
of aerial photography.

Existing site investigation reports

These can often be the most valuable source of geotechnical information. If a site investigation has been
performed in the vicinity in the past, then information may already exist on the rock and soil types, drain-
age, access, etc. The report may also contain details of the properties of the soils and test results.

6.2.2 Site reconnaissance

A walk over the site can often help to give an idea of the work that will be required. Differences in vegeta-
tion often indicate changes in subsoil conditions, and any cutting, quarry or river on or near the site should
be examined. Site access, overhead restrictions and signs of slope instability are further examples of
aspects that can be observed during the walk over survey. The information observed during the survey is
used to complement the desk study information so that the ground investigation can be well planned.

6.2.3 Planning ield investigations and lab tests

In order to obtain quantitative data on the soil and rock types and properties, the ground investigation
is performed. This phase involves the sampling of the ground using recognised sampling procedures and
specialist equipment. The extent of the sampling, and subsequent testing, depends mainly on the size
and nature of the proposed structure, but is also influenced by the degree of variability of the soils on
the site. Investigation points are locations on the site where profiling and sampling of the ground occurs.
The ground is investigated and sampled by using various methods as described in the following section.
Guidance on the spacing and depth of the investigation points is given in EN 1997-2 and are summa-
rised in Table 6.1 and Table 6.2. As can be seen, the nature of the project influences significantly the
recommended extent and number of investigation points.
The depth of investigations can be reduced to z a = 2 m where the foundations in Table 6.2 are con-
structed on competent strata with distinct geology.

Table 6.1 Guidance values for spacing and pattern of investigation points.

Structure Spacing Layout

High-rise and industrial structures 15-40 m Grid pattern

Large area ≤ 60 m Grid pattern

Linear structures (e.g. roads, railways,
walls etc.)

20 m-200 m Linear

Special structures (e.g. bridges,
stacks, machinery, foundations)

2-6 investigation points per
foundation

Dams and weirs 25 m-75 m Along relevant sections