Sequence of operation for field investigation

The sequence of operation for the field investigation was as under:
a. The location of borehole / trial pit was identified on ground with respect to existing structure, chainage or coordinates.
b. Drilling, sampling and field testing was carried out as per the guidelines given in following paras.
c. The findings of investigation, including field tests, sampling, visual description of strata encountered etc., were recorded in field borehole log.
d. The SPT samples packed in polythene bags and rock cores, duly labeled were preserved in wooden core boxes.
e. The UDS samples were sealed, labeled and preserved as per guidelines given in following paras.
f. Boreholes were terminated as per specifications.
g. Photographic evidence of machine positioned at particular location shall be collected.
h. Ground water table was recorded in each borehole after 24 hrs after completion of borehole.
i. Selected samples were carefully packed and shipped to laboratory for testing.

Boring in Soil

Boreholes of 150-mm diameter were drilled in soil. Boring was done by rotary Drilling Method and as per the procedure outlined in IS:1892-1979. Boreholes were supported using temporary casing wherever required.

Standard Penetration Test

This test is performed according to IS Codes: IS 2131- 1981

Click here for the pdf file called Undisturbed Soil Samples.

Obtaining the Samples

1. Tests shall be made at every change in stratum or at intervals of not more than l-5 m whichever is less. Tests may be made at lesser intervals if specified or considered necessary. The intervals be increased to 3 m if in between vane shear test is performed.

2. The sampler shall be lowered to the bottomof the borehole. The following information shall be noted and recorded:
a) Depth of bottom of borehole below ground level.
b) Penetration of the sampler into the soil under the combined weight of sampler and rods (to be noted from readings of the scale over the drill rod at the top.
c) Water level in the borehole or casing.
d) Depth of bottom of casing below ground level.

3. The split spoon sampler resting on the bottom of borehole should be allowed to sink under its own weight; then the split spoon sampler shall be seated 15 cm with the blows of the hammer falling through 75 cm. Thereafter, the split spoon sampler shall be further driven by 30 cm or 50 blows (except that driving shall cease before the split spoon sampler is full). The number of blows required to effect each 15 cm of penetration shall be recorded. The first 15 cm of drive may be considered to be seating drive. The total blows required for the second and third 15 cm of penetration shall be termed the penetration-resistance Jv; if the split spoon sampler is driven less than 45 cm (total), then the penetration resistance shall be for the last 30 cm of penetration (if less than 30 cm is penetrated, the logs should state the number of blows and the depth penetrated).

4. The entire sampler may sometimes sink under its own weight when very soft sub-soil stratum is encountered. Under such conditions, it may not be necessary to give any blow to the split spoon sampler and SPT value should be indicated as zero.

5. If on lowering the sampler by means of a string of rods it is found to rest at a level above the bottom of the casing, the penetration test and sampling should not be carried out at that stratum.

Removal of Sampler and Labelling

1. The sampler shall be raised to the surface and opened. A typical sample or samples of soil from the opened split spoon shall be put into jars without ramming. The jars shall have a self-sealing top, or shall be sealed with wax to prevent evaporation of the soil moisture. Jars shall be of such a size that they can be filled without deforming the sample. Typical samples shall be cut to such a size as to fill the jars and thereby reduce the water loss to the air in the jars. If packing as specified is not available, liner may be used in the sampling spoon. In such a case, the internal diameter of the sampling spoon should be SO adjusted that the total internal diameter after incorporating the liner is 35 mm. The sample in the liner shall be waxed properly at both the ends to keep up the natural moisture content during transit.

2. Labels shall be fixed to the jar or notations shall be written on the covers ( or .both ) with the following information:
a)Origin of sample.
b) Job designation.
c) Boring number.
d) Sample number.
e)Depth of sampling.
f) Penetration record.
g) Length of recovery.
c) Date of sampling.

3. The jars containing samples shall be stored in suitable containers for shipment. Samples shall not be placed in the sun.

Field Observations

1. Information with regard to water table, elevations at which the drilling water was lost or elevations at which water under excess pressure was encountered shall be recorded.on the field logs. Water levels before and after putting the casing, where used, shall be measured. In sands, the level shall be determined as the casing is pulled and then measured at least 30 min after the casing is pulled; in silts, at least 24 h after the casing is pulled; in clays, no accurate water level determination is possible unless pervious seams are present. ‘However, the 24 h level shall be recorded for clays. When drilling x&d is used and the water level is desired, casing perforated at the lower end shall be lowered into the borehole and the borehole bailed down. Ground water levels shall be determined after bailing at time intervals of 30 min and 24 h until all traces of drilling mud are removed from inside the casing.

Corrections

1. Due to Overburden-The N value ior cohesionless soil shall be corrected for overburden as per Fig. 1 (Jv’ ).

1. Due to Dilatancy - The value obtained in 3.6.1 shall be corrected for dilatancy if the stratum consists of fine sand and silt below water table for values of N’ greater than 15,
as under ( x” ): N =15f$(JV’-l5)

REPORT

1. Data obtained in borings shall be recorded in the field and shall include the following:
a) Date of boring
a) Reference datum,
a) Job identification
a) Boring number
a) Sample number
a)Type of sampler
a) Drilling method
a) Sample elevation and recovery ratio.
a) Limits of stratuin.
a) Water table information (see 3.5).
a) Soil identification, including condition of samples.
a) Casing used.
a) Weather data.

2. The data obtairlecl shall be prepared in a final form as a soil profile to show the nature and extent of the soil strata over the area under consideration.

Undisturbed Soil Samples

Generally, unless specified undisturbed soil samples (UDS) was obtained at every 3.0 m interval starting from 1.0 m in all type of soil along the bore depth upto SPT ‘N’ Values of less than 50. Sampler was 75 mm / 100 mm internal diameter and generally 450 mm long. Thin walled open drive sampler was used in cohesive strata.

Click here for the pdf file called Undisturbed Soil Samples.

Disturbed Soil Samples

In all boreholes, if undisturbed samples could not be collected due to reasons stated above or if having non-plastic (cohesionless) soil which could not be extracted undisturbed, disturbed soil samples shall be taken at every 1.5 m interval and at significant change of stratum. Soil from split spoon sampler used for standard penetration tests were taken as disturbed samples. These samples were placed without delay in polyethylene bags adequately sealed.

Drilling in rock

Drilling in rock stratum was carried out using Nx size Bit and Double Tube core barrel. Core drilling was normally carried out with bits which were suitable for the percentage core recovery and diameter required.
Rotary core drilling produced circular cores of a minimum 54 mm diameter throughout the core length. The type and state of the drill bit, feed rates and management of the drill was such that at least 95% core recovery in any single run can be obtained. Drill runs were normally exceed 1.5m in length unless 100% recovery is being obtained and the core barrel was removed from the drill holes as often as may be required in order to get the best possible core recovery. Rock cores recovered during drilling were arranged sequentially in core boxes and duly identified. Their Core Recovery (CR) and Rock Quality Designation (RQD) were recorded in the borelogs. The rock type was identified and the description was presented in the borelogs.
The samples recovered during each run were arranged sequentially in wooden core boxes and the identifications – Borehole number and piece number was painted on them. Photographs for the rock core samples collected in wooden boxes shall be taken and submitted along with the Geotechnical Interpretative Reports.

Ground Water Level

Ground water level is recorded in each borehole after 24 hrs and depths are recorded on the Bore logs.

TRIAL PITS

The plan dimensions of the trial pit were 1 m x 1 m and excavated upto 3.00 m depth below proposed ground level (BPGL) unless the hard / competent strata is observed at shallow depth. The ‘log’ of the trial pit is recorded and presented as “Trial Pit Log”.