Soil investigation method statement. Drilling of boreholes and required tests.
This procedure deals with site investigation for the
proposed project for the purpose of assessing the suitability of the site for the project and
of acquiring knowledge of the characteristics of the site that affect the design and construction of such work
and security of neighboring land and property.
Primary Objects of Site Investigation.
The following are the
objectives of the site investigation:
Suitability: To
assess the general suitability of the site and environs for the proposed
works.
Design: To enable
adequate and economic designs to be prepared including the design of temporary
works
Construction: To plan the best method of construction to foresee and provide against difficulties
and delays that may arise during construction due to ground and other local conditions
in approximate cases, to explore sources
of indigenous materials for use in construction and to select sites for the
disposal of waste or surplus materials.
Effect of Changes: To determine the changes that may arise
in the ground and environmental conditions, either naturally or as a result of
the works, and the effect of such changes on the works, on adjacent works, and
on the environment in general.
Scope of works.
The scope of works comprises of the following:
Positioning of boreholes.
Prior to movement of drilling rigs, the location shall be
surveyed with GPS by soil investigation subcontractor surveyor and shall be marked with wooden peg with flag,
denoting the borehole. The site supervisor shall also ensure that safe operations are followed.
After which the drilling rigs shall be mobilized to the particular location.
Performing inspection pit.
Inspection pits shall be excavated to the specified depths
of 1.2m manually to confirm no
underground utility are encountered as per specification manually. In
case rock/ Ground water are encountered
before the specified depth, then the pit shall be terminated at that
depth.
Bulk disturbed samples shall be taken to full depth of
excavation. Visual inspection, logging,
sampling will be done parallel during excavation process.
Methods of drilling.
Drilling shall be carried out under the supervision of an
experienced Engineer by the following
methods appropriate to meet the nature of the works and existing soil
strata, and with the Engineer’s
approval.
Rotary Drilling with Coring.
The rotary drilling shall be carried out using a truck
mounted rotary rig. In rotary drilling,
the borehole is advanced by rotary
action of a drill bit attached at the end of hollow drill rods. The
drilling fluid which is pumped down to the bit through the hollow drill
rods lubricates the bit and flushes the
drill debris up the borehole. The
drilling fluid used is bentonite.
In sandy strata, standard penetration test shall be carried
out at regular intervals as detailed in Section
B.5.
Rotary coring is carried out once rock is encountered.
Rotary coring consists of cutting a cylinder of
rock using a rotating double tube core barrel type with a coring bit
attached to the bottom of outer
barrel. The drilling fluid is
used to cool the drill bit, remove the cuttings, and flush them up the borehole outside the core barrel.
The cores shall be wrapped in polythene bags to prevent loss
of moisture, placed in core boxes in the
sequence of depths and transported to the laboratory for testing.
Field tests and sampling.
Soil Samples.
Standard Penetration tests shall be carried out in the
boreholes at specified intervals in accordance
with BS 5930: 1999 or specification i.e., 0.5m interval from 1.20m depth
till 3.0m followed by 1.0m
interval. Disturbed samples
(bulk) and SPT samples shall be taken at specified intervals for necessary
laboratory testing. For gravels or
gravelly sand, the drive shoe shall be replaced by a solid 60 cone.
In such cases, disturbed samples shall be collected from the boring
tools.
The samples shall be packed in polythene bags with zipper
pack to retain in moisture condition,
labeled and transported to the laboratory for testing.
Rock Cores Samples.
Undisturbed rock core samples & cores of cemented
materials shall be obtained using rotary coring
method with double / triple tube core barrels with TC/Diamond bits. PQ
wire-line double tube core barrel shall
be used.
The cores shall be wrapped in cling film wrap to prevent
loss of moisture, placed in core boxes in the
sequence of depths and transported to the laboratory for testing.
Undisturbed Samples.
Undisturbed Sampling shall be carried out in cohesive materials
(if encountered) using thin-walled
Shelby tube samplers, which are pushed into soft/firm fine grained soil
materials.
For very weakly
cemented and mixed materials, sampling shall be carried out using push-in large
dia. Split spoon samplers with basket
catchers (Dia. 60mm x 600mm length) to obtain “undisturbed samples”.
Ground Water Samples.
Ground water samples shall be collected from the boreholes
after purging the borehole with a foot
valve / submersible pump and removing all added water / mud. Water
sampling shall be carried out using a
disposable plastic bailer once the natural ground water gets collected in the
borehole.
The samples shall be carried out in accordance with BS 5930:
1999 and the depth of ground water table
shall also be recorded. Wherever specified, standpipe Piezometers shall be
installed for ground water monitoring
and sampling.
Laboratory testing.
Laboratory Testing shall be carried out in accordance with
the provisions of relevant BS and ASTM
standards. The type & number
of tests shall be based on the ground materials encountered and the project requirements. These shall include
sieve analysis, Atterberg Limits, Unconfined compressive strength tests on rock cores, porosity,
direct shear test small box, Chemical analysis of soil & ground water samples (Sulphate, chloride & pH)
and carbonate content tests etc.,. Chemical
analysis comprising of Sulphate, chloride and pH shall be carried out on
soil/ground water samples collected from
each borehole.
Report.
A report of the findings that meet the requirements
mentioned in the project specifications shall be provided.