A SEMINAR REPORT ON METHODS OF SUBSOIL INVESTIGATION BY UDUEBOR micheal A. CVE/06/7988 SUBMITTED TO THE DEPARTMENT OF CIVIL ENGINEERING FEDERAL UNIVERSITY OF TECHNOLOGY, AKURE, ONDO STATE. IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE AWARD OF A BACHELOR OF ENGINEERING (B.ENG) DEGREE IN CIVIL ENGINEERING.
my seminar presentation for my 400 level in the college
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
A
SEMINAR REPORT
ON
METHODS OF SUBSOIL INVESTIGATION
BY
UDUEBOR micheal A.
CVE/06/7988
SUBMITTED TO
THE DEPARTMENT OF CIVIL ENGINEERING
FEDERAL UNIVERSITY OF TECHNOLOGY, AKURE,
ONDO STATE.
IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE
AWARD OF A BACHELOR OF ENGINEERING (B.ENG)
DEGREE IN CIVIL ENGINEERING.
MARCH, 2011.
ABSTRACT
In practice, natural soil deposits are not homogenous, elastic, or isotropic. In some places, the
stratification of soil deposits may even change greatly within a horizontal distance of 15 to 3 metres.
For foundation design and construction work, it is necessary to know the actual soil stratification at a
given site, the laboratory test results of soil samples obtained from various depths, and the
observations made during the construction of other structures built under similar conditions. For
most major structures, adequate subsoil exploration at the construction site must be conducted.
Several methods for carrying out investigation abound depending on the site terrain, type of soil to
be encountered, cost of operation, quality of data and quantity of samples and so many factors. A
brief overview would be given in this compendium of the various methods available for carrying out
investigation into soil and their mode of operation.
1.0 INTRODUCTION
1.1 WHAT IS SUBSOIL INVESTIGATION?
Site investigation is a process of site exploration consisting of boring, sampling and testing so as to
obtain geotechnical information for a safe, practical and economical geotechnical evaluation and
design. Generally it is an exploration or discovery of the ground conditions especially on untouched
site.
In other words the main purpose of site subsoil investigation is to determine within practical limits,
the depth, thickness, extent and compositions of each subsoil stratum, the depth and type of rock, the
depth and composition of groundwater, the strength, compressibility and hydraulic characteristics of
soil strata required by geotechnical engineers. Sometimes it is also known as geotechnical
investigation.
1.2 PURPOSES OF SUBSOIL INVESTIGATION
Subsoil investigation provides data on surface and underground conditions at the proposed site.
Samples may be obtained for visual inspection and to determine physical and index properties.
Depending on the site use, relatively undisturbed samples may be obtained to make estimates of
engineering properties for strength, stability and water flow.
According to Das (1990) the purpose of subsoil exploration includes the following;
a. determining the nature of the soil at the site and its stratification
b. obtaining disturbed and undisturbed soil samples for visual identification and appropriate
laboratory tests
c. determining the depth and nature of bedrock, if and when encountered
d. Performing some insitu field tests, such as permeability tests, vane shear tests and standard
penetration tests.
1.3 PHASES INVOLVED IN SUBSOIL INVESTIGATION
The stages involved in the subsoil investigations include
Surface Investigations:
Site inspection is carried out to assess general site condition if there are any anticipated
problems that might arise during the construction later on.
Usually the engineer is required to inspect the site to appreciate actual site and ground
problems with particular reference to terrain, vegetation, swamps, water runoff,
stratigraphical formations where it is exposed.
Sub-surface Investigations:
Ground or soil investigation by means of boring, sampling, testing, and etc. And also as to
determine the stratigraphy and pertinent properties of soil underlying the project site.
Subsurface Investigations consists of:
Boring: This refers to drilling or advancing a hole in the ground.
Sampling: This refers to removing soil from the hole. The samples can be classified as
disturbed or undisturbed samples.
Testing: This refers to determining the properties from the soil. The test can be performed
either at laboratory or at field.
2.0 METHODS OF SUBSOIL INVESTIGATION
The direct methods of testing described in this chapter are at the centre of routine ground
investigation. They provide the opportunity to obtain samples for visual description and index
testing, which are the primary ways in which the strata at a site are recognized, and for sampling and
much of the in situ testing needed for parameter determination, as well as allowing the installation of
instrumentation such as Piezometers.
`
1.4 TRIAL PITTING
Trial pits provide the best method of obtaining very detailed information on strength, stratification,
pre-existing shear surfaces, and discontinuities in soil. Trial pits, trenches and shallow excavations
are often used in site investigations, particularly during investigations for low- and medium-rise
construction, because they provide an economical means of acquiring a very detailed record of the
complex soil conditions which often exist near to the ground surface. Very high quality disturbed,
undisturbed and block samples can only be taken from trial pits and the sides can be photographed
for permanent record.
Trial pits may be excavated by either hand digging or machine excavation. In general, machine
excavation is used for shallow pits, whereas hand excavation is used for deep pits which must be
supported. In the limited space of a trial pit, which is often 1.5m x 3m in plan area and 4-5m depth at
ground level, it is usually impossible to place supports as machine excavation proceeds. Shallow trial
pits provide a cheap method of examining near-surface deposits in situ, but the cost increases
dramatically with depth, because of the need to support.
1.5 BORING
A large number of methods are available for advancing boreholes to obtain samples or details of soil
strata. The particular methods used any country will tend to be restricted, based on their suitability
for local ground conditions. The principal methods used worldwide are:
• Light percussion drilling;
• Power augering; and
• Washboring.
LIGHT PERCUSSION DRILLING
Often called ‘shell and auger’ drilling, this method is more properly termed light percussion drilling
since the barrel auger is now rarely used with this type of equipment. The drilling rig (Fig. 2.0)
consists of:
• A collapsible ‘A’ frame, with a pulley at its top;
• A diesel engine; connected via a hand-operated friction clutch (based on a brake drum
system) to
• A winch drum which provides pulling power to the rig rope and can be held still with a
friction brake which is foot-operated.
The rope from the winch drum passes over the pulley at the top of the ‘A’ frame and is used to raise
and lower a series of weighted tools on to the soil being drilled. The rig is very light, and can be
readily towed with a four-wheel drive vehicle. It is also very easy to erect, and on a level site can be
ready to drill in about 15 mm. Where access is very limited, it can be dismantled, and rebuilt on the
other side of an obstruction such as a doorway.
In clays, progress is made by dropping a steel tube known as a ‘claycutter’ into the soil (see Fig. 2.1).
This is slowly pulled out of the borehole and is then generally found to have soil wedged inside it.
The claycutter normally has a solid or slotted weight, called a sinker bar, attached to its upper end,
the top of which is connected to the winch rope. When the claycutter is withdrawn from the top of
the hole, the soil is removed with a metal bar which is driven into it through the open slot in the
claycutter side.
In granular materials, such as sands or gravels, a shell is used. At least 2 m of water is put in the
bottom of the borehole, and the shell is then surged, moving about 300mm up and down every
second or so. Surging the shell upwards causes water to be drawn into the bottom of the hole, and
this water loosens the soil at the base of the hole and forces it to go into suspension. As the shell is
dropped on the bottom of the hole the mixture of soil and water passes up the tube of the shell, past
the simple non- return valve (sometimes called a ‘clack’). As the shell is raised, the clack closes and
retains the soil, which precipitates above it. By repeatedly surging the shell up and down at the base
of the hole, soil can be collected and removed from the hole.