Different types of drilling heads/ tools are used in the pilot-hole process. Selection of drilling head is dependent on the type of soil strata. Soils with cobble stones or rock having voids or incomplete layers of rock
are not considered suitable for drilling. The purpose of the geo-tech investigation is not only to determine the feasibility of HDD crossing, but also to establish the most efficient way to accomplish it. On the geo-tech information governs the determination of best crossing route along with selection of drilling tools and execution methodology.
Following shall be investigated during the geo-tech survey:
(Supplementary geo-technical data may be obtained from existing records, e.g. recent nearby bridge constructions, other pipeline/ cable crossings in the area.)
- Soil identification to locate rock, rock inclusions, gravelly soils, loose deposits, discontinuities and hardpan
- Soil strength and stability characteristics
- Groundwater level
Geo-tech investigation is performed by analyzing the soil sample extracted from bore-holes drilled along the pipeline route called as bore log data. For long crossings, bore logs are typically taken at 200 m intervals. For short crossings i.e. crossings which are less than 300 m length, as few as three bore log may be sufficient. The borings should be near the drill-path to give accurate soil data, but sufficiently far from the borehole to avoid pressurized mud from following natural ground fissures and rupturing to the ground surface through the soil-test bore hole. A thumb rule is to take borings at least 10m to either side of bore path. Although these are good general rules, the number, depth and location of boreholes is best determined by the geo-tech engineer.
Geo-technical data for River Crossings:
River crossings require additional information such as a study to identify river bed, river bed depth, stability (lateral as well as scour), and river width. Typically, pipes are installed to a depth of at least 6m below the expected future river bottom, considering scour. Soil borings for geotechnical investigation are generally conducted to 12m below river bottom.