Subsurface Geotechnical Investigations – Part 1
Location of InvestigationsAn important piece of information for all geotechnical investigations that seems obvious but commonly not given sufficient attention is the accurate determination of the location of investigation. It always is preferable to select boring and test pit locations that fully characterize geotechnical conditions. Of course, it is not always possible to locate a boring on a structure because of obstacles or right-of-entry difficulties. Heavily urbanized areas present particular difficulties in these aspects. However, it is important to keep in mind that correlations and interpretations may be subject to later scrutiny should a change of conditions claim be filed. All locations should be determined using either conventional surveying methods or by a global positioning system (GPS). A GPS has the significant advantage of having the positional information downloaded directly into a geographic information system (GIS).
Environmental ProtectionAfter the locations for field investigation work have been determined, routes of access to the area and the specific sites for borings and excavations should be selected with care to minimize damage to the environment. Operation of equipment will be controlled at all times, and the extent of damaged areas will be held to the minimum consistent with the requirements for obtaining adequate data. Local laws pertaining to permissible levels of sediment flow from the site need to be followed. After the exploratory sites have served their purpose, the disturbed areas will be restored to a natural appearance. All borings and test pits should be backfilled in accordance with state environmental regulations.
It is important to ensure that environmental consciousness is maintained during the conduct of geotechnical investigations. Unfortunately, drilling rigs are inherently dirty. Proper maintenance of drilling rigs will minimize this problem. For hazardous, toxic and radiological waste (HTRW) exploratory drilling, drilling rigs must be steam cleaned and all tools, equipment and personnel decontaminated. Fluids used in drilling operations, be they hydrocarbons that have leaked from the hydraulic system or a constituent of a drilling mud, are potentially toxic and should be controlled or eliminated wherever possible.
Purpose of BoringsBorings are required to characterize the basic geologic materials at a project. The major uses for which borings are made:
- define geologic stratigraphy and structure
- obtain samples for index testing
- obtain ground water data
- perform in situ tests
- obtain samples to determine engineering properties
- install instrumentation
- establish foundation elevations for structures
- determine the engineering characteristics of existing structures
Borings are broadly classified as disturbed, undisturbed and core. Borings frequently are used for more than one purpose, and it is not uncommon to use a boring for purposes not contemplated when it was made.
Thus, it is important to have a complete log of every boring, even if there may not be an immediate use for some of the information. If there is doubt regarding the range of borehole use or insufficient information to determine optimum borehole size, then the hole should be drilled larger than currently thought needed. A slightly larger-than-needed borehole is considerably less expensive than a second borehole.
Different MethodsMany methods are used to make borings and retrieve samples. Some factors that affect the choice of methods:
- purpose and information required
- equipment availability
- depth of hole
- experience and training of available personnel
- types of materials anticipated
- terrain and accessibility
- environmental impacts
- disruption of existing structure
Auger BoringsAuger borings provide disturbed samples that are suitable for determining soil type, Atterberg limits, Proctor testing and other index properties but generally give limited information on subsoil stratification, consistency or sensitivity. Auger borings are most useful for preliminary investigations of soil type, advancing holes for other sampling methods, determining depth to top of rock and for monitor well installation in soils.
Auger borings can be made using hand, helical, barrel, hollow stem or bucket augers. Auger samples are difficult to obtain below the ground water table, except in clays. However, hollow-stem augers with a continuous split-barrel sampler can retrieve some unconsolidated material from below the water table.
Truck-mounted auger rigs come equipped with high-yield and high tensile strength steel augers. New hydraulics technology can now apply torque pressures upward of 20,000 foot-pounds. With this amount of torque, augers are capable of boring large-size holes and of being used in soft rock foundation investigations. Because augers use no drilling fluids, they are advantageous for avoiding environmental impacts. Another advantage of using augers is the ability (using hollow stems) for soil sampling, i.e., taking undisturbed samples below the bit.
Many drilling rigs actually are a combination of auger/core/ downhole hammer units. A hollow-stem auger has the drill-through capability (i.e., the auger can drill to refusal, then a wire line core barrel and drill rods can be inserted to finish the hole). The auger acts as a temporary casing to prevent caving of the softer materials as sampling progresses. However, augers are not watertight, and water loss should be anticipated. Hollow-stem augers should not be used as temporary casing in areas where HTRW is to be anticipated. Temporary steel casing driven into the surface of competent bedrock or PVC casing permanently grouted into the competent bedrock surface is required when HTRW is anticipated.
Drive BoringsDrive borings provide disturbed samples that contain all soil constituents, generally retain natural stratification and can supply data on penetration resistance. Drive boring is a non-rotating method for making a hole by continuous sampling using a heavy wall drive barrel. Push or drive samplers are of two types – open samplers and piston samplers. Open samplers have a vented sampler head attached to an open tube that admits soil as soon as the tube is brought in contact with the soil. Some open samplers are equipped with a cutting shoe and a sample retainer. Piston samplers have a movable piston located within the sampler tube. The piston helps to keep drilling fluid and soil cuttings out of the tube as the sampler advances. The piston also helps to retain the sample in the sampler tube. Where larger samples are required, the most suitable drill for this method is the cable tool rig. The cable tool rig has the capability to provide a downward driving force (drill stem on drive clamps) to make a hole and an upward force (drilling jars) to remove the drive barrel from the hole.
Vibratory samplers offer a means of obtaining disturbed samples of saturated, cohesionless soils rapidly and with relatively inexpensive equipment. The simplest devices consist of a small gasoline engine providing hydraulic power to a vibrating head clamped to aluminum tubing secured on a tripod. The rapid vibrations within the head drive the sampling tube into the ground and forces the soil up into the tube. A rubber packer secured into the open end of the sampling tube after driving creates a seal to retain the sample as the tube is withdrawn with a hand winch.
Another device, the Becker hammer drill, was devised specifically for use in sand, gravel and boulders by Canadian firm Becker Drilling LTD. The Becker drill uses a diesel-powered pile hammer to drive a special double-wall, toothed casing into the ground. Drilling fluid is pumped through an annulus to the bottom of the hole where it forces cuttings to the surface through the center of the casing. The cuttings are collected for examination. Becker drill casings are available in 5.5-, 6.6- and 9-inch outside diameters (OD), with sampling inside diameters (ID) of 3.3-, 4.3- and 6.0 inches, respectively.
Drive borings can be advanced quickly and economically with hollow-stem augers using a “plug” assembly that is either manually or mechanically set in the opening at the end of the auger string and then removed prior to sampling. Removal commonly is facilitated using a wire-line system of retrieval. Where overburden prohibits the use of augers to advance the boring due to boulders or resistant rock lenses or ledges, other methods can be used. Traditionally, a roller rock bit using drilling mud will advance the hole at a modest cost in time and dollars. Where extremely difficult drilling conditions exist, an ODEX (eccentric reamer) down-the-hole air hammer system or other coring advancer apparatus can be used to penetrate the toughest boulders or ledges while still permitting the use of standard penetration or even undisturbed sampling to be conducted.
Cone Penetration BoringsThe Cone Penetration Test (CPT) or Dutch cone boring is an in situ testing method for evaluating detailed soil stratigraphy as well as estimating geotechnical engineering properties. The CPT involves hydraulically pushing a special 1.4-inch diameter probe into the earth while performing two measurements – cone resistance and sleeve friction resistance. The probe is normally pushed from a special heavy-duty truck but also can be performed from a trailer or drilling rig. Because of the weight of the truck or trailer needed to conduct CPT borings, access to soft ground sites is limited. Recent developments in CPT technology make it possible to retrieve physical soil samples and ground water or soil-gas samples with the same drive string used to perform the cone penetration test.
Undisturbed BoringsTrue undisturbed samples cannot be obtained because of the adverse effects resulting from sampling, shipping or handling. However, modern samplers used with great care can obtain samples that are satisfactory for shear strength, consolidation, permeability and density tests, provided the possible effects of sample disturbance are considered. Undisturbed samples can be sliced to permit detailed study of subsoil stratification, joints, fissures, failure planes and other details. Undisturbed samples of clays and silts can be obtained as well as nearly undisturbed samples of some sands.
There are no standard or generally accepted methods for undisturbed sampling of noncohesive soils. One method that has been used is to obtain 3-inch thin-wall tube samples, drain them, and then freeze them prior to transporting them to the laboratory. Another method used consists of in situ freezing, followed by sampling with a rotary core barrel. Care is necessary in transporting any undisturbed sample, and special precautions must be taken if transporting sands and silts. For both methods, disturbance by cryogenic effects must be taken into account. Fixed-piston samplers, wherein a piston within a thin-walled tube is allowed to move up into the tube as the sampler is pushed into the soil, are adapted to sampling cohesionless and wet soils.
Undisturbed borings normally are made using one of two general methods – push samplers or rotary samplers. Push sampling types involve pushing a thin-walled tube using the hydraulic system of the drilling rig, then enlarging the diameter of the sampled interval by some cleanout method before beginning to sample again. Commonly used systems for push samples include the drill-rig drive, whereby pressure is applied to a thin-walled sampling tube through the drill rods; the fixed-piston sampler and the hydraulic piston sampler. Rotary samplers involve a double-tube arrangement similar to a rock coring operation except that the inner barrel shoe is adjustable but generally extends beyond the front of the rotating outer bit. This minimizes the disturbance to the sample from the drilling fluid and bit rotation.
Rock Core BoringCored rock samples are retrieved by rotary drilling with hollow core barrels equipped with diamond- or carbide-embedded bits. The core commonly is retrieved in 5- to 10-foot lengths. The “N” size hole (approx. 3 in.) probably is the core size most widely used for geotechnical investigations and produces a satisfactory sample for preliminary exploration work and, in many instances, for more advanced design studies. Other hole sizes, including B (approx. 2.3 in.) and H (approx. 4 in.), also are quite satisfactory for geotechnical investigations. The decision on hole size should be based upon anticipated foundation conditions, laboratory testing requirements and the engineering information desired. A double- or triple-tube core barrel is recommended because of its ability to recover soft or broken and fractured zones. The use of wireline drilling, whereby the core barrel is retrieved through the drill rod string, eliminates the need to remove the drill rods for sampling and saves a great deal of time in deep borings.
Most rock boring is accomplished using truck-mounted rotary drilling rigs. Skid-mounted rigs sometimes are used in areas with poor access. Rotary drilling rigs are driven by the power takeoff from the truck engine or by independent engines. Boreholes are advanced by rotary action coupled with downward pressure applied to the drill bit and the cleaning action of the drilling fluid. Two types of pull-down mechanisms normally are used. Truck-mounted rotary drilling rigs equipped with a chain pull-down drive mechanism are capable of drilling to depths of 200 feet to 1,000 feet. Hydraulic feed drive rotary drilling rigs are capable of drilling to depths of 500 feet to 2,500 feet.
Core recovery in zones of weak or intensely fractured rock is particularly important because these zones typically are the critical areas from the standpoint of foundation loading and stability. The use of larger-diameter core barrels in soft, weak or fractured strata can improve core recovery and provides a statistically better size sample for laboratory testing. The advantages of larger cores must be weighed against their higher costs.
Although the majority of rock core borings are drilled vertically, inclined and horizontally oriented borings may be required to adequately define stratification, jointing and other discontinuities. A bias exists in the data favoring discontinuities lying nearly perpendicular to the boring. Discontinuities more nearly parallel to the boring are not intersected as often, and therefore, their frequency will appear to be much lower than it actually is. Inclined borings should be used to investigate steeply inclined jointing in abutments and valley sections for dams, along spillway and tunnel alignment and in foundations for other structures. In nearly vertical bedding, inclined borings can be used to reduce the total number of borings needed to obtain core samples of all strata.
If precise geological structure is to be evaluated from core samples, techniques involving oriented cores are required. In these procedures, the core is scribed or engraved with a special drilling tool so that its orientation is preserved. In this manner, both the dip and strike of any joint, bedding plane or other planar surface can be ascertained. A more common procedure for obtaining dip and strike of structural features is the use of borehole photography or television. If the orientation of bedding is consistent across the site, it can be used to orient cores from borings, which are angled to this bedding.
Large-diameter borings or calyx holes (2 ft. or more in diameter) occasionally are used in large or critical structures. Their use permits direct examination of the sidewalls of the boring or shaft and provides access for obtaining high quality, undisturbed samples. Direct inspection of the sidewalls may reveal details, such as thin, weak layers or shear planes that may not be detected by continuous undisturbed sampling. Large-diameter borings are produced with augers in soil and soft rock and with large-diameter core barrels in hard rock.
Drilling in EmbankmentsThe U.S. Army Corps of Engineers has developed a special regulation concerning drilling operations in dam and levee embankments and their soil foundations. In the past, compressed air and other drilling fluids have been used as circulating media to remove drill cuttings, stabilize bore holes and cool and lubricate drilling bits. There have been several incidents of damage to embankments and foundations when drilling with air, foam or water as the circulating medium. Damage has included pneumatic fracturing of the embankment while using air or air with foam, and erosion of embankment or foundation materials and hydraulic fracturing while using water. A summary of the guidance provided in the Corp’s new document:
Personnel involved in drilling in dam and levee embankments shall be senior and well qualified. Designs shall be prepared and approved by geotechnical engineers or engineering geologists. Drillers and mud specialists shall be experts in their fields.
Drilling in embankments or their foundations using compressed air or other gas or water as the circulating medium is prohibited.
Cable tool, auger and rotary tool methods are recommended for drilling in embankments. If the cable tool method is used, drilling tools must be restricted to hollow sampling (drive) barrels in earth embankment and overburden materials. If rotary drilling is used, an engineered drilling fluid designed to prevent caving and minimize intrusion of the drilling fluid into the embankment shall be used.