13-09-2013, 04:32 PM
SOIL EXPLORATION AND GEOTECHINAL INVESTIGATION
EXPLORATION AND GEOTECHINAL .docx (Size: 339.02 KB / Downloads: 14)
Introduction
The knowledge about the site forms a vital role in the safe and economical development of a site. A thorough investigation of the site is an essential preliminary to the construction of any civil engineering works. Public building officials may require soil data together with the recommendations of the geotechnical consultant prior to issuance of a building permit. Elimination of the site exploration, which usually ranges from about 0.5 to1 percent of total construction costs, only to find after construction has started that the foundation must be redesigned is certainly false economy. This is generally recognized, and it is doubtful if any major structures are currently designed without exploration being undertaken.
Objectives of Site Investigation
To access the general suitability of the site. To achieve safe and economical design of foundations and temporary works. To know the nature of each stratum and engineering properties of the soil and rock, which may affect the design and mode of construction of proposed structure and foundation. To foresee and provide against difficulties that may arise during construction due to ground and other local conditions. To find out the sources of construction material and selection of sites for disposal of water or surplus material. To investigate the occurrence or causes of all natural and man made changes in conditions and the results arising from such changes. To ensure the safety of surrounding existing structures. To design for the failed structures or remedial measures for the structures deemed to be unsafe.
To locate the ground water level and possible corrosive effect of soil and water on foundation material.
Reconnaissance
This may be in the form of a field trip to the site which can reveal information on the type and behavior of adjacent sites and structures such as cracks, noticeable sags, and possibly sticking doors and windows. The type of local existing structure may influence, to a considerable extent, the exploration program and the best foundation type for the proposed adjacent structure. Since nearby existing structures must be maintained, excavations or vibrations will have to be carefully controlled. Erosion in existing cuts (or ditches) may also be observed. For highways, run off patterns , as well as soil stratification to the depth of the erosion cut , may be observed. Rock outcrops may give an indication of the presence or the depth of bedrock.
Detailed Exploration
Where the preliminary site investigation has established the feasibility of the project, a more detailed exploration program is undertaken. The preliminary borings and data are used as a basis for locating additional borings, which should be confirmatory in nature, and determining the additional samples required. If the soil is relatively uniform in stratification, a rather orderly spacing of borings at locations close to critical superstructure elements should be made. On occasion additional borings will be required to delineate zones of poor soil, rock outcrops, fills, and other areas which can influence the design and construction of the foundation. Sufficient additional samples should be recovered to redefine the design and for any construction procedure required by the contractor to install the foundation. This should avoid an excessive bid for the foundation work, cost overruns, and damage to adjacent property owners from unanticipated soil conditions discovered when the excavation is opened.
Wash boring:
It is a popular method due to the use of limited equipments. The advantage of this is the use of inexpensive and easily portable handling and drilling equipments. Here first an open hole is formed on the ground so that the soil sampling or rock drilling operation can be done below the hole. The hole is advanced by chopping and twisting action of the light bit. Cutting is done by forced water and water jet under pressure through the rods operated inside the hole.
In India the “Dheki” operation is used, i.e., a pipe of 5cm diameter is held vertically and filled with water using horizontal lever arrangement and by the process of suction and application of pressure, soil slurry comes out of the tube and pipe goes down. This can be done upto a depth of 8m –10m (excluding the depth of hole already formed beforehand)
Just by noting the change of colour of soil coming out with the change of soil character can be identified by any experienced person. It gives completely disturbed sample and is not suitable for very soft soil, fine to medium grained cohesionless soil and in cemented soil.
Limitations
Since a plate load test is of short duration, consolidation settlements cannot be predicted. The test gives the value of immediate settlements only. If the underlying soil is sandy in
nature immediate settlement can be taken as total settlement. If the soil is of clayey type, the immediate settlement is only a part of the total settlement. Load tests, therefore do not have much significance in clayey soils to determine allowable pressure on the basis of settlement criterion.
Plate load test results should be used with caution and the present practice is not to rely too much on this test. If the soil is not homogenous to a great depth, plate load tests give very misleading results.