Transcript Site Investigation Prof. S. Subramanian Geotechnical Engineering What is SITE INVESTIGATION • Site Investigation is the gathering of the information about the proposed location.

Site Investigation
Prof. S. Subramanian
Geotechnical Engineering
What is SITE INVESTIGATION
• Site Investigation is the gathering of the information
about the proposed location of a project.
• Determine the profile of the nature soil deposit, take
soil samples and determine the soil properties.
Purpose ???
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Select the type and depth of foundation
Estimate bearing capacity
Establish ground water table
To find out the maximum and differential settlement
To predict and solve the potential foundation problems
Subsurface Exploration Programme
• Depends on type of structure
• Extent of exploration depends on the importance of the project
and location of the project.
• Get as much information as possible.
• Cost for site investigation varies between 0.05% to 0.2% of the
total cost of the entire structure.
Stages of Sub-surface exploration
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Desk study and Site Reconnaissance,
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Preliminary ground investigation,
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Detailed ground investigation
Desk study
• The desk study is the first stage of the site investigation process which
involves researching the site to gain as much information as possible,
both geological and historical.
• Get information about the type of structure and its purpose.
• Access the geologic map of the area
• Geotechnical report of near by projects
Site Reconnaissance
• Make an visit to the site to make a visual inspection to get the
following details.
• Topography of the site
• Stratification of soils in deep cuts
• Evidence of landslides
• Location of high flood marks
• Type of vegetation
• GWL in the nearby wells
Preliminary Ground Investigation
• Depth and thickness of each soil stratum.
• If possible, depth of bed rock.
• Geophysical methods or boreholes are used to for locating the soil
boundaries.
• So, Preliminary investigation gives information about
• The extent of disturbed strata
• River crossings or alluvial areas that may have buried soft material or
peat
• Information on suitability of soil for fills work, ground water condition
Detailed Investigation
• At this stage, the extent of the test , number of boreholes , depth of
boreholes and type of machinery to be used are selected.
• This stage consist of three steps
• Boring or in-situ testing
• Sampling
• Laboratory Investigation
Depth of Exploration
• Degree of variability of the subsurface in horizontal and vertical
direction controls the depth of exploration.
• The vertical depth of exploration is generally governed by the size of the
influence zone.
• Significant Depth – Depth upto which the stress increment due to
superimposed loads can produce significant settlement and shear stress.
• Significant depth is generally taken as the depth at which the vertical
stress is 20% of the load intensity.
• Large boulders should not be mistaken for rocks.
• For a multi storied building, the depth of exploration (D) is governed by
• D = C x S0.7 , C = 3 for light steel and concrete buildings, C = 6 for heavy steel
buildings.
• S = Number of stories
• For deep excavations, the depth of exploration below the proposed
excavation level should be 1.5 times the depth of excavation.
• In case of road fills, the minimum depth of boring should be 2m
below the ground surface or equal to the height of the fill,
whichever is greater.
• In case of gravity dams, the minimum depth of exploration should
be twice the height of the dam.
Lateral extent of Exploration
• Depends on change in strata in horizontal direction.
• For small and less important buildings, one borehole or trial pit at the
center of the building is sufficient.
• For a large, multi storied buildings, the bore holes should be drilled in
each corners and important locations. The spacing is kept between 10 to
30 meters.
• For highways, spacing of 150 to 300m is adopted along the center line
Test Pits and Trenches
• Test pit are suitable for smaller depth upto 3m.
• Useful for conducting plate load test.
• Soil samples can be obtained through proper sampling technique.
Auger Boring
Wash Boring
Percussion Drilling
• Percussion Drilling is the process of
making boreholes by striking the soil
then removing it.
• The tools are repeatedly dropped
down the borehole while suspended
by wire from the power winch.
Rotary Drilling
• This method is the most rapid
method of advancing a hole in soil
and rock.
• Rotary Drilling uses rotation of the
drill bit with the simultaneous
application of pressure to advance
the hole
Stabilizing the Borehole
• Self Supportive
• Maintaining water at higher level in borehole than the GWT.
• Filling with drilling mud to a level higher than the GWT.
• Using Casing pipes.
Split Spoon Sampler
Shelby Tubes
• Refer ASTM D 1587 for further details
Piston Sampler
Sample Disturbance
Effect of Sample Disturbance
Quiz ? ? ?
Which Design allows minimum Disturbance