Geological And Geophysical Investigation In Civil Engineering

GEOLOGICAL & GEOPHYSICAL INVESTIGATION IN CIVIL ENGINEERING

GEOLOGICAL INVESTIGATION

Site investigation is the process of determining the layers of natural soil deposits that will underlie a proposed structure and their physical properties.

A detailed site investigation is made to verify and supplement reconnaissance level and preliminary level investigations to provide the designer with specific and quantifiable information for use in design and construction.

Geologic structural features, such as faults, folds, and joints, must be identified and located. Sufficient information must be obtained on unconsolidated deposits to determine their location, thickness, and extent. Where drilling methods are used, the following tests can be used to characterize the in situ strength of foundation materials: standard penetration test (SPT), permeability tests, vane shear tests, and cone penetrometer tests.

A detailed geologic investigation may include any combination of the following: • conducting

subsurface investigations, including exploration holes, trenching, pitting, geophysical and seismic evaluations • obtaining soil and rock samples for laboratory analysis and performing in situ testing • evaluating the geomorphology, geologic units, and structures at and near the site • developing sediment budgets and sediment storage requirements or sediment management, including sediment production, transport, and yield

A detailed site investigation provides information on subsurface conditions that cannot be obtained by surface examination or by shallow subsurface investigation. Equipment such as backhoes, bulldozers, power augers, or drill rigs are typically used to perform detailed site investigations, sampling, and testing. The following information and assessments are required:

1. Knowledge of the foundation materials and conditions is of sufficient scope and quality to serve as a basis for geologic interpretation and structural design. 2. Fill materials are of suitable quality and are available in sufficient quantity. 3. Storage reservoir basins are free from sinks, permeable strata, and fractures or fissures that might lead to moderate or rapid water loss.

The main objective of geological investigations for most engineering projects are to determine: •identify, delineate, and correlate thegeologic materials •locate, identify, and interpret geologic features •Determine groundwater conditions • Interpret engineering properties of the materials •Determine testing and sampling locations •Perform sampling and in situ testing •determine what materials needto be sampled for design and construction

The main objective of geological investigations for most engineering projects are to determine: i. The geological structure of the area; ii. The lithology of the area; iii. The ground water conditions in the region; iv. The seismicity of the region;

The geological structure of the area, which also includes topography and geomorphology, is determined by conducting extensive and intensive geological surveys. These surveys would reveal the entire topographic details (i.e. existence of hills, slopes, valleys, plains, and plateaus), occurrence of folds, faults and major unconformities and joint systems and geomorphic details of water bodies like rivers, lakes, springs, drainage systems, and the like.

The lithological details as revealed by these investigations would include the type of rocks that make up different parts of the area, on the surface as also up to and even beyond a desired depth, their textures, major structures and physical and engineering properties like hardness, strength, modulus of elasticity, rigidity, porosity and permeability.

The groundwater conditions are of great significance in all major engineering structures. The relative position ofwater table with respect to the project must be thoroughly established and all variations in it during different periods in a year should be fully ascertained. Whether a proposed project would be much above the local and regional water table, or below it or would intercept it in some areas shall determine to a great extent, the ultimate design and stability of the structure and hence its cost.

The seismic factor has become a parameter of great concern during last sixty years or so. Many cities, towns and villages in different countries of the world have been devastated by earthquakes. In some cases, areas hitherto declared seismically safe, witnessed serious seismic activity resulting in catastrophes. For any type of civil engineering construction in a region its seismicity has also to be studied critically.

GEOPHYSICAL INVESTIGATION

Geophysical methods are generally non-invasive or non destructive methods long used in the construction industry for investigation of the subsurface. Principally, these are used for the detection of geologic anomalies such as cavities and voids, detection of buried pipes and other utilities, detection of water bearing aquifers for well development, exploitation of quarries and in determining soil stratification or layering. In addition, the methods provide a means for verifying as constructed pavement thicknesses in a continuous unbroken image of the pavement structural configuration or determining rebar embedment and layout non-destructively.

GROUND PENETRATING RADAR (GPR) is an offshoot of the military use of radar and was spurred by the need to do research in the thick ice of the Polar Ice Cap which would be difficult to investigate continuously by borings. The developed technology is now used widespread in the construction and civil engineering profession but has now also reverted to military use again in the detection of buried mines (IED’s) and arms caches particularly in Iraq and Afghanistan.

The GPR equipment being used scanning for Buried Pipes and aqueducts.

GPR signal image showing the occurrence of a waste dumpsite along the stretch of the scan.

SEISMIC REFRACTION METHODS

Seismic refraction consists of sending shock waves into the soil either by use of hammer striking a steel plate or with the use of explosives.

The vibrations induced are picked up by a Seismograph through an array of geophones which pick up the refracted and reflected signals. The velocity and travel time for these shock waves through materials with varying material stiffnesses are measured and the refracted and reflected signals are processed by an on board computer, as they travel to various media .

Comparison of advantages and disadvantages of the seismic refraction method.