The subsoil investigations reported herein were taken up to find out the nature of subsoil at the site of the proposed construction and to recommend the capacity and type of its foundation.
SCOPE OF WORK:
– To investigate the subsurface soil conditions at the proposed five point is carried out at the Construction area. The detailed scope of investigation for this site is as follows.
– Setting up boring rig at each bore hole location and boring 150 mm diameter bore holes through all kinds of soils.
– Conducting Standard Penetration Tests (SPT) at an interval of 1.50m and collecting soil samples from split spoon sampler.
– Arranging all soil samples in the core boxes as per the borehole logging, labeling sealing properly in sequence indicating number of SPT sample, depth, and number of blow of each SPT samples.
– Transporting the selected soil samples to the laboratory for conducting tests as per the scope of the work.
– Analysis and interpretation of field & laboratory test data for the preparation of Geotechnical investigation report
FIELD METHODOLOGY OF INVESTIGATION
Erecting and setting up of Boring rig: At each bore hole location as per the Client’s location plan, Rotary type boring rig was shifted, assembled and erected.
Boring in Overburden: Boring was done in accordance with IS: 1892 -1979. Standard rotary type drilling rig coupled with 8 H.P capacity diesel engines, which is fitted to a tripod frame and with all drilling accessories was used for boring. The rig deployed was generally suitable for all Geotechnical Investigation work and had an arrangement for driving and extraction of casing, boring and drilling by mud circulation method. Collecting D/S, UDS, conducting SPT test with this rig was possible. Rotary method of boring was used for boring in soil. Boring was commenced by driving a SX casing.
Boring was carried out in soil using 6” dia core barrel up to the top of the hard surface. Diameter of the borehole in soil was 150mm. SX casing was lowered in overburden as boring progressed. Advancement of borehole in soil was done by removing soil with help of water circulation under rotary action of the Core barrel. Boring in all types of soil was continued till the hard stratum was met with. Standard penetration tests were conducted at an interval of 1.50m.
In –Situ Tests In Overburden: During investigation Insitu tests were carried out. Standard penetration tests were conducted in overburden. Undisturbed and Disturbed soil samples were collected by split spoon sampler in SPT test for field observations and to determine the index properties from laboratory tests. The methodology of conducting SPT test was described below.
Standard Penetration Test (SPT): SPT’s were conducted as per IS 2131-1981.Disturbed Samples were collected through Split Spoon Sampler at 1.5m interval. A standard split spoon sampler was driven at the bottom of the hole. The penetration resistance in terms of blows for300mm penetration of the split spoon sampler was measured as ‘N’ Value.
The blows were imparted by a standard weight of 63.5 kg falling through a height of 750 mm. The resistance was measured for 150 mm, 300 mm and 450 mm penetrations. The resistance of first 150 mm was ignored and the resistance of next 300 mm was recorded as standard penetration value ‘N`. If the sampler was driven less than 450 mm (total) then the penetration resistance was given for the last 300 mm of penetration. If the penetration depth was less than 150 mm and the blow count was more than 50 then the ‘N’ value was considered as ‘Refusal’ or more than 100 blows for less than 30cms penetration.
INDIAN STANDARD CODES USED FOR THE FIELD INVESTIGATION
Field Geotechnical investigation was executed in accordance with the Indian standard Codes listed below.
i) IS: 1892: Code of practice for subsurface investigation of foundation
ii) IS: 1498: Classification and identification of soil for general engineering purpose.
iii) IS: 2131: Method for standard penetration test for soil
iv) IS: 6926: Code of Practice for Diamond Core Drilling for Site Investigations
v) IS: 4464: Code practice for Presentation of drilling information and core description
in foundation investigation.
vi) IS: 10208: Specification for diamond core drilling equipment.
vii) IS: 6935: Method of Determination of water level in boreholes.
Sampling
Undisturbed Soil Samples: Open drive samplers of 100-mm diameter and about 450-mm length were used for obtaining undisturbed samples of cohesive soils. The collection, sealing, labeling and transportation of the samples to the laboratory were done as per the IS guide-lines.
Disturbed Soil Samples: Disturbed soil samples were collected at suitable intervals of depth (not more than 2.5 m) and at all depths of change in the nature of the subsoil. These samples were sealed in polythene bags with proper identification labels.
Laboratory Tests: Some or all of the following laboratory tests, as necessary, were done on the collected samples. Representative soil/rock samples were selected for this from the different rock strata encountered during boring. The tests were performed as per the relevant Indian Standard Codes of Practice. Some or all of the following laboratory tests, as necessary, were done on the collected soil samples. Representative soil samples were selected for this from the different soil strata encountered during boring. The tests were performed as per the relevant Indian Standard Codes of Practice.
SOIL SAMPLE TEST (As per IS 2720)
(i) Natural moisture content Bulk density
(ii) Grain size analysis (using sieves) Specific gravity of soil solids
(iii) Laboratory CBR test
(iv) Free Swelling Index test, Consolidation Test
Shear Tests
[i] Triaxial compression test (unconsolidated – undrained), generally for fine- grained soils
[ii] Direct shear tests, generally for coarse-grained soils
Rock Sample Test
(i) Density (kg/m3) – As per IS 13030:1981
(ii) Specific gravity of Rock – IS 1124
(iii) Water Absorption & porosity (%) – As per IS 1124
(iv) Point Load Index (N/mm2) – As per ASTM D-5731
(v) Unconfined Compressive Strength (N/mm2) – As per IS 9143
Other tests as and when required.
