Geotechnical Drilling and Sampling
Continuous Sampling Gives Drillers the Most Accurate Information
The purpose of a drilling and sampling program is to obtain samples that reasonably represent subsurface conditions over the entire project site. Sampling type and frequency is dependent upon both the type of material encountered and the purpose of the investigation. The American Association of State Highway and Transportation Officials' (AASHTO) Manual on Subsurface Investigation provides additional detailed information. When appropriate, continuous sampling provides the most complete and accurate information.
- Auger Boring - Hand- or power-operated augering with periodic removal of material. In some cases, continuous auger may be used, requiring only one withdrawal. Changes are indicated by examination of the materials removed. Used for probe investigations to bedrock and for disturbed soil samples (less than 20 feet in depth). Typical uses include disturbed soil sampling and determining depth of overburden.
- Hollow-stem Auger Boring - Power-operated, hollow stem serves as a casing. General-purpose applications for soils and other locations requiring a cased hole. Typical uses include disturbed and undisturbed soil sampling, in situ testing and foundation and landslide investigations.
- Rotary Drilling - Power rotation of drilling bit as circulating fluid removes cuttings from hole. Changes are indicated by rate of progress, action of drilling tools and examination of cuttings in drilling fluid. Casing usually is not required except near the surface. This is a method to advance borings through a variety of materials, including large boulders and broken rock. Typical uses include obtaining rock cores, drilling probes, horizontal drains and installing instruments.
- Wire-line Drilling - Rotary-type drilling method where the coring device is an integral part of the drill rod string, which also serves as a casing. Core samples are obtained by removing inner barrel assembly from the core barrel portion of the drill rod. The inner barrel is released by a retriever that is lowered by a wire-length through drilling rod. This is an efficient method of recovering core samples of rock. Typical uses include foundations, material sources, rock cut investigation and general rock coring.
- For sand-and-gravel-type soils, split-spoon samples should be taken at 5-foot intervals and at significant changes in soil strata. Continuous split-spoon samples are recommended in the top 15 feet of borings made at locations where spread footings may be placed in natural soils.
- Representative split-spoon jar or bag samples should be laboratory-classified for verification of field visual soil identification.
- In silt-and-clay-type soils, split-spoon and undisturbed thin wall tube samples should be taken at 5-foot intervals and at significant changes in strata. Split-spoon and tube samples may be alternated in the same boring or tube samples may be taken in separate undisturbed boring. Representative split-spoon jar or bag samples should be laboratory classified for verification of field visual soil identification. Tube samples should be tested for consolidation (for settlement analysis) and strength (for slope stability and foundation bearing capacity analysis). Field vane shear testing also is recommended to obtain in-place shear strength of soft clays, silts and non-fibrous peats.
- In rock, continuous cores should be obtained in rock or shale using double- or triple-tube core barrels. For foundation investigations, core a minimum of 10 feet into the rock. Core samples should be evaluated for strength testing (unconfined compression) for foundation investigations, and valued for quality tests for quarry investigations (aggregate or riprap). Determine percent core recovery and the rock quality designation (RQD) value for each core run and record in the bore log.
- When dealing with ground water, record the water level encountered during drilling, at completion of boring and at 24 hours after completion of boring in the bore log. When water is used for the drilling fluid, adequate time should be permitted after hole completion for the water level to stabilize (more than one week may be required). In impermeable soils, a plastic pipe water observation well should be installed to allow monitoring of the water level over a period of time. Artesian pressure and seepage zones, if encountered, also should be noted in the bore log. The top 1,000 feet or so of the annular space between water observation well pipes and the borehole wall should be backfilled with grout, bentonite or a sand-cement mixture to prevent surface water inflow, which can cause erroneous ground water level readings.
Soil samples and rock cores obtained represent a considerable investment of time and money. The samples should be properly labeled, transported and stored. A detailed treatment of procedures for handling and storing samples is provided in the AASHTO Manual on Subsurface Investigations. However, any method that satisfactorily protects a sample from shock, large temperature changes and moisture loss may be used. All containers used for storage should be identified with the following markings:
- project name and number
- box number of total set
- bore hole number
- applicable depth information
The identification markings should be on the exterior as well as the interior of the storage container. Rock cores should routinely be photographed, in color, as soon as possible after being taken from the borehole and before laboratory testing. All samples not used in laboratory testing should be retained until the proposed construction is completed and/or all claims are settled.
Geotechnical Drilling Investigations
For structure foundations, there should be a minimum of one boring per structure unit. Continue borings:
1. through unsuitable foundation soils (such as peats, highly organic soils, soft fine-grained soils, loose coarse rained soils, etc.) into competent material of suitable bearing capacity and;
2. to depth where added stress due to estimated footing load is less than 10 percent of the existing effective soil overburden stress or;
3. a minimum of 10 feet into bedrock if the bedrock is encountered at shallower depth as determined by (2).
With retaining walls, there should be a minimum of one boring per wall. Additional borings should be spaced 50 feet to 200 feet. Some borings should be in front of the wall and some in back of wall. Continue borings to depth of two times wall height or a minimum of 10 feet into bedrock.
When approach embankments are placed over soft ground, at least one boring should be made at each embankment to determine the problems associated with stability and settlement of the embankment. Test borings at proposed abutment locations may serve both stability and structural investigations. The depth of the borings should be the same as described for structure foundations above. Additional shallow explorations (hand auger holes) taken at approach embankment locations are an economical way to determine depth of unsuitable surface soils or topsoil.
For cuts and embankments, space borings every 200 feet (erratic conditions) to 500 feet (uniform conditions), with at least one boring taken in each separate landform. For high cuts and fills (25 ft. or greater), it is desirable to have a minimum two borings along a straight line to establish geological cross sections for analysis. For cuts in stable materials, extend the borings a minimum 10 feet below the proposed centerline grade. In weak soils, at or near the proposed centerline grade, extend borings below grade to competent materials. For embankments, continue borings to competent material or to a depth of twice the height of the embankment.
When dealing with landslides, drill a minimum of two borings along a straight line to establish geologic cross sections for analysis. The number of sections depends on the extent of any stability problems. For an active slide, place at least one boring above and below the sliding area. Extend borings to an elevation below active or potential failure surface and into competent stratum, or to a depth for which failure is unlikely because of the geometry of the cross section.
Boreholes used to install inclinometers must be extended to competent material below the slide movement. At material sites (borrow sources and quarries), space borings or test pits on a grid pattern every 100 feet to 150 feet or change of material. Extend exploration to the base of the deposit or to depth required to provide needed quantity. For pavement rehabilitation, drill a minimum of one boring or test pit per half-mile with additional exploration as needed to define changes in subgrade material, pavement section and locally distressed areas. Extend depth to at least 2 feet below the expected subgrade.
Note: Information obtained from drilling may be supplemented by geophysical investigations.