The information from the geotechnical survey is used to select the crossing route, drilling tools, procedures, and the drill-path design. The level of effort required for the geotechnical investigation depends on several factors, such as the pipe diameter, bore length, and the nature of the HDD crossing. For many HDD projects, especially in municipal areas, the subsurface investigation consists primarily of research of existing geotechnical data. Due to the costs and the congested nature of many urban municipal areas, test holes are not performed. This is especially true for shorter drill lengths and smaller-diameter pipe or cable. This is another area where the experience of the design and geotechnical engineers is important. In many instances, the technical feasibility of the project can be reasonably determined without the cost and disruption of test holes. However, when planning HDD crossings without testhole data of the proposed crossing, the risk element is always higher.
When test holes are required, it is important that they are conducted by qualified personnel and that the needed data is obtained from the investigation. During the survey, the geotechnical consultant will identify a number of relevant items including the following:
The length of the drill and the complexity of the strata determine the number of exploration holes required for the geotechnical survey. For drill paths shorter than 1000 feet, two soil-test borings (one on each end of the bore) may be adequate. If the data from these test borings indicate that the conditions are likely to be homogeneous on both sides of the bore, it may not be necessary to conduct any more test borings. If the test data indicate anomalies in the soil conditions or items such as large concentration of gravel, further tests should be conducted. For drill paths longer than 1000 feet, soil-test borings are typically taken at 500- to 700-foot intervals. The soil- test bores should be near the drill path to provide accurate soil data but far enough from the drill-path bore hole to avoid pressurized mud from rupturing to the ground surface through the soil-test bore hole. A general rule is to take soil-test bores at least 25 feet from either side of the drill path. Soil-test bores for geotechnical surveys are usually drilled 20 feet deeper that the pipe drill path. The following are a few suggested guidelines concerning soil-test bores to consider during the HDD planning stage:
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