PRINCESS NINFA M.

MACAILING
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Geological and Geophysical Investigation in Civil Engineering

1. What is site investigation? What are the purpose of site investigation?

 The process of determining the layers of natural soil deposits that will underlie a
proposed structure and their physical properties is generally referred to as site
investigation.
1.1. Selection of the type and the depth of foundation suitable for a given
structure.
1.2. Evaluation of the load-bearing capacity of the foundation.
1.3. Estimation of the probable settlement of a structure.
1.4. Determination of potential foundation problems (for example, expansive
soil, collapsible soil, sanitary landfill, and so on).
1.5. Establishment of ground water table.
1.6. Prediction of lateral earth pressure for structures like retaining walls,
sheet pile bulkheads, and braced cuts.
1.7. Establishment of construction methods for changing subsoil conditions.

2. Give and explain the geophysical methods of investigation.

 There are many kinds of geophysical methods of investigation. These method are:
 Gravimetric method
 Magnetic method
 Electrical method
 Seismic method
 Radiometric method
 Geothermal method
 Gravity method represent a set of geophysical methods which make use of the
natural gravity field of the earth.
 Density of the material is the controlling physical property.
 In gravimetric method, the nature of distribution of gravity g on the surface is
analyzed. The gravity is influenced positively if the causative body is heavier,
larger and occurs at a shallow depth.
 The gravimeter, used in relative gravity measurement is a mass loaded spring.
If the subsurface has a relatively heavier body, the gravity pull is more there
(+g) and the spring extends becoming longer. If the subsurface has relatively a
lighter body there the gravity pull is less (-g) and the spring contracts and
become shorter.
 Magnetic Methods – like gravity methods, these investigations also take advantage
of natural magnetic field associated with the earth and its relation to subsurface
geology.
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 The main controlling physical property in magnetic method is magnetic

susceptibility.
 The magnetic methods are based on the fact that the magnetic bodies present
in the earth’s surface contribute to the magnetic field of the earth.
 In general, when the magnetic field of the earth or one of its components is
measured on the surface, bodies possessing magnetic moments different from
those of the surrounding rocks contribute to the deviations in the measured
quantities. From the magnetic anomalies, it is possible to locate anomalous
objects.
 The different parameters measured during magnetic investigations are total
magnetic field (intensity and direction) and different space components
 Magnetic surveys have a certain inherit limitations. Hence for unique and
accurate solutions, magnetic prospecting is often carried out along with the
gravity or other methods.
 Electrical Method are numerous and more versatile, they are more popular because
they are successful in dealing with a variety of problems like groundwater studies,
subsurface structure, and many others.
 In electromagnetic methods, electrical conductivity, magmatic permeability
and dielectric constant of subsurface bodies are the relevant properties.
 Seismic method of study is based on the principle that subsurface rock formations
bear different elastic properties. Because of this, the velocities of propagation of
seismic waves through the subsurface layers of earth, suffer reflection or critical
reflection arrive at the surface of the earth where they are detected by geophones.
From the time taken by the waves to travel through the subsurface formation and
from the seismic wave velocities of the media. It is possible to determine the depth of
various elastic boundaries.
 Elastic property differences in rocks is the controlling property.
 With the help of geophones fixed at suitable intervals on the ground, the
different seismic waves reaching the surface are recorded and from the times
of their arrival, time –distance curves are constructed. The direct waves are
the first to reach the geophones placed between point and the distance beyond
the point is called the critical distance.
 Depending upon whether reflected waves or refracted waves are used in the
investigation, there are two types of methods, namely, seismic reflection
method and seismic refraction method. A geophone an amplifier and a
galvanometer are the basic units required for reflected or refracted wave
registrations.
 Radiometric Methods controlling property is natural radioactivity of rocks and ores.
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
The normal radioactivity is different in different types of rocks. In igneous
rocks, it decreases with decreasing acidity. If rock contains radioactivity ore
bodies, such areas will exhibit very high radioactivity, giving rise to
anomalies during surveys. Thus based on the study of radioactivity. It is not
only possible to distinguish different rock types but also to detect radioactive
ore bodies. The profile drawn clearly brings out the subsurface lithology,
structure and ore body.
 Geothermal methods find application in deep structural studies, ore deposits,
groundwater studies, for delineation of salt-water fresh water interfaces. Etc.

3. What are the geological conditions necessary for construction of dams?

 There are 4 types of dams: gravity dams, buttress dams, arch dams and earth dams.
 Gravity dam is a solid concrete or masonry structures that withstands the
water pressure, by virtue of its weight. All forces acting on the dam are
assumed to be directly transmitted to the foundation rocks. They are generally
of triangular profile and are among the safest.
 Buttress dam have a continuous upstream face, supported at regular intervals,
by buttress walls on the downstream side. They are lighter than solid dams
and likely to induce greater stresses at the foundation, since most of the load
passes through the buttress walls and is not spread uniformly over the
foundation.
 Arch dam is an arch-shaped, convex at the upstream side. Major portion of
the thrust forces acting on the dam are transmitted by arch action, onto the
abutment rocks. Structural efficiency is higher than that of gravity dams, the
presence of sound abutments is a prime necessity and uses less amount of
concrete.
 Earth dam is a non-rigid structures, built with naturally available materials
such as earth and rock. Ideal, where the dam site is weak to support concrete
dams, or where competent rocks are found at great depths.
 The geological conditions of a dam are narrow river valley, occurrence of bedrock at
shallow depth, competent rocks to offer stable foundation and proper geologic
structures.
 Narrow valley means smaller dam is required, and hence, lower costs.
Defective valleys include: deceptive narrowing due to thick superficial
deposits, narrowing due to rock outcrops and presence of soluble material like
gypsum, renders the rocks unsuitable.
 The presence of strong bedrock near the surface, reduces the cost of the
foundation. The site should be explore using electrical resistivity or seismic
refraction methods, to assess the nature of the bedrock. The presence of buried
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river valleys, huge boulders gives rise to problems, as they are composed of
lose material.
 Igneous rocks are safer than sedimentary ones Suitability of site depends on:
the existing rock type, the extent of weathering undergone, the extent of
fracturing, the occurrence of geological structures and the mode and number
of rock types.
 Proper geologic structures are undisturbed strata, disturbed strata, tilted beds,
folded strata, faulted strata and jointed strata.

4. What are the geological conditions necessary for construction of tunnels?

 The geological conditions necessary for construction of tunnels are types of rocks,
geological structures, ground water conditions and overbreak.
 Igneous rocks are competent, but difficult to work with and do not require
lining. Sedimentary rocks are less competent, compared to igneous.
Sandstones, shale etc. are soft, easy to work and requires lining. Metamorphic
rocks like gneisses are similar to granites.
 Geological structures are effect of joints, effect of faults, effect of folds and
effect of undisturbed or tilted strata.
 Tunnel axis passing entirely through impervious formations, tunnel axis
mostly above the water table and tunnel axis below the water table.
 Tunneling through hard rocks requires the removal of some rocks outside the
proposed perimeter. This excess quantity of rock removed, is called the
‘overbreak’. Geological factors governing the amount of overbreak are: the
nature of the rocks, orientation of the joints and orientation of bedding planes.

5. What are the geological conditions necessary for construction of roads?

 The geological conditions necessary for construction of roads are topography
(lithological characters, consolidated hard rock, unconsolidated material), geological
structures, weathering, and groundwater conditions.
 Topography or the landform of a region is single most important factor that
controls the selection of alignment of a road project. Topographic maps would
reveal the existence of various land features like valleys and the inflowing
streams, the hills and their undulations, the plateaus and the plains with all
their varying configuration from place to place. Obviously, knowledge of all
such features is not only important but very essential for a right alignment.
 Geological surveys should invariably provide all possible details regarding the
composition, texture, structure and origin of rocks and sediments making the
ground through which the proposed alignment of the highways has to pass.
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 The structural features of rocks, especially in those of sedimentary and

metamorphic origin, have very important bearing upon the design of cuts as
well as on the stability of the road as a whole. A given rock might be quite
hard and otherwise sound for a cut as road foundation.
 In some cases, when the strata along or under a cut is composed of layers of
rocks of different hardness, the softer layers get weathered at a faster rate than
the overlying or underlying harder rocks. This generally results in
undermining which might cause slips or falls of the whole face. Sometimes,
when the top layers are weathered too heavily, the slope might experience a
persistent rock fall or debris-fall type of situation from above.
 It is always necessary to investigate thoroughly the position of water table of
the area. Not only that, water bearing qualities should also be known along the
proposed route. It is quite likely that a water bearing zone (aquifer) might be
intersecting the base or slopes of an alignment. Specific care and design
would be required for these natural water conduits. These are always to be
taken as weak and hazardous zones in the road.