Geotechnical Investigation

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:

  • Soil identification to locate rock, rock inclusions, gravelly soils, loose deposits, discontinuities, and hardpan
  • Soil strength and stability characteristics
  • Groundwater

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:

  1. Get accurate locations and elevations for each exploration bore hole.
  2. Do not locate exploration bore holes directly over the proposed HDD path—offset each 25 to 30 feet laterally.
  3. Request complete lithologic and geotechnic descriptions for all geologic strata encountered.
  4. Require descriptions of boring techniques and all equipment used.
  5. Test bores depths should go at least 20 feet below the lowest anticipated elevation of the horizontal directional bore hole.
  6. Standard penetration tests (SPT) should be taken at 5-foot intervals for all strata (cohesive and noncohesive units).
  7. Grain-size analyses are helpful—either good field estimates or laboratory sieve tests.
  8. It is important to get representative unconfined compression (UC) tests for all "harder" rock units that fail SPT tests (generally at auger refusal). Record rock-quality designations (RQD) and retain cores for visual inspection.
  9. Have the driller or geologist record free water levels in the borings and note all significant observations during the actual drilling process. These notes should become part of the final log.
  10. Have all bore holes thoroughly plugged and/or grouted upon completion of the exploration program.

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Responses

  • FILIPPO
    How are soil test borings taken?
    9 years ago
  • felicity
    How deep these geotechnical bores are?
    8 years ago
  • Flavia
    What geotech info is needed for pipelines?
    3 years ago
  • piia
    How many boreholes required for a HDD?
    2 years ago
  • aleksi laatikainen
    What geotech information is required for jack and bore?
    2 years ago
  • sabine
    How deep should a soil boring go for hdd?
    1 year ago
  • AMNA
    How deep do you go for soil borings for HDD?
    1 year ago
  • nadine
    What is the recommended number of drilling holes for geotechnical borings on 1.5 acres?
    1 year ago
  • kathrin traugott
    What kind of geotechnical recommendations are required for horizontal directional drilling?
    1 year ago
  • eyob
    What soil properties are needed for directional driling?
    1 year ago
  • steffen
    Are geotechnical surveys required for hdd in wv?
    10 months ago
  • peregrin
    What geotechnical information do you need for hdd?
    10 months ago
  • dirk
    What kind of geological survey do you need for directional drilling?
    9 months ago
  • selam gabriel
    What are horizontal boars called geotech?
    4 months ago
  • Sophie
    Is geotechnical survey necessary for hdd?
    2 months ago
  • maxine
    What parameters you need for horizontal drilling?
    20 days ago
  • thorsten
    What should be included in a geotechnical report for directional drills?
    12 days ago

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