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94 C H A P T E R 6 Drilling and Sampling of Soil and Rock Introduction When formulating a plan for the subsurface exploration program, the geotechnical engineer or project geologist must carefully evaluate the variety of equipment, methods, and procedures available for drilling and sampling soil and rock. This careful evaluation of drilling and sampling options will assist with optimizing this phase of the subsurface exploration program to acquire the required information at the least cost. This chapter presents the information geoprofessionals need to accomplish this objective, focusing on the following topics: â¢ Field equipment â¢ Methods for advancing boreholes â¢ Soil sampling â¢ Rock coring methods â¢ Logging borings â¢ Boring closure Field Equipment A wide variety of conventional and modified drilling equipment is availableâranging from small, handheld portable drills and augers to large equipment for offshore use. The most common types of equipment for geotechnical explorations include rotary drills, percussive, hydraulic push, and sonic type systems. Selecting the most appropriate drilling equipment is an important aspect of any subsurface exploration program. The equipment must be capable of meeting the project requirements, have sufficient mobility, and be able to convert rapidly from one drilling technique to another. Consideration should be given to the nature of the formations to be penetrated (e.g., soft clay, dense sand, hard rock) and the type of sample that is required (e.g., bulk, disturbed, undisturbed). Hydraulic-feed machines are usually preferable because they can maintain a constant advance pressure through varying formations, which minimizes erosion and disturbance of the surrounding material. No single method of drilling is likely to prove satisfactory and economical for all geological formations and sampling requirements. Local practices and equipment availability will often dictate the method(s) used for many subsurface exploration programs. 6.2.1 Drilling on Land The conventional rotary drill rig can make several types of borings (including augering and rotary drilling); installing casing; obtaining drive samples and hydraulic-push samples; coring rock, and direct- push placing various in situ tests. Rotary drill rigs are quite versatile and adaptable to a variety of different geologies. The typical applications of the most common variants of rotary drill rigs are summarized in Table 6-1, and examples are shown in Figure 6-1.
95 Table 6-1. Rotary drill rigs Type of Rig Applications Truck-mounted Sites with easy access All-terrain Soft ground and difficult terrain Track-mounted Swampy soft ground and heavily wooded terrain Skid-mounted Steep terrain With conventional rotary methods, the drill rods are lowered down the borehole and removed repeatedly to change drill bits and obtain samples. Drill rod lengths of 5 ft (1.5 m) and 10 ft (3 m) are common. As the borehole advances deeper, considerable time and effort are required to repeatedly lower and remove rods in sectional lengths not exceeding 30 ft (9 m). Wireline drilling helps facilitate field operations by using a special cable tool to rapidly swap samplers, bits, and cores as the boring advances. An outer temporary casing is used to maintain borehole stability, and the cable system is located internal to this casing. The cable also transmits the rotary cutting action from the rig to the special drilling head at the bottom of the borehole. This is especially advantageous for deep boreholes to save time and effort. Percussive-type borings are made by repeated mechanical impacts that are transmitted to the bit. These impacts break up soil and rock into small particles that are removed by high air pressure. An air-track-type rig is used to quickly produce an open borehole. Percussive-type borings are often employed to define the top of bedrock. Direct-push sampling of soil is accomplished using special hydraulic rigs that use an internal plastic- lined steel mandrel to capture continuous samples. Samples are obtained in incremental strokes of 5 or 10 ft (1.5 or 3 m). Two types of direct-push hydraulic rigs are shown in Figure 6-3. These rigs often have twin anchors to provide the necessary reaction forces for thrust pushes. A single person can usually operate a direct-push rig and achieve depths of 100 ft (30 m) in approximately one hour. Depths greater than 150 ft (45 m) have also been achieved using direct-push rigs. Sonic drilling rigs can obtain continuous samples of both soil and rock rapidly and efficiently, considerably faster than rotary drilling methods. The sonic heads provide a range of vibratory motions and vertical forces down the rods to a special cutting shoe that cuts into a wide variety of geologic materials. The repeated cyclic loading prevents undisturbed samples from being obtained. But, continuous logging and recovery of soil layers, lenses, and strata are possible, and penetration through hard, cemented, or calcified seams is not hindered. With sonic drilling in soils, there is no need to introduce fluids, water, or air during soil sampling. This can be beneficial for explorations into earth dams, levees, and embankments where rotary drilling fluids can damage or erode the soils or in environmentally sensitive areas where water may aggravate contaminant transport. 6.2.2 Drilling over Water When the alignment of a highway project crosses over a body of water, special equipment may be necessary to accomplish the borings or soundings. This commonly occurs for bridge crossings over freshwater rivers, streams, and saltwater bays. The situation also occurs along the shores of lakes, seas, and oceans where the highway alignment is parallel to the coastline. For shallow-water conditions where water depths are less than approximately 6 ft (2 m), barges can be used to transport conventional drill rigs to the borehole or sounding locations. A tugboat or other seaworthy vessel is used to move the barge. For deeper water, depths of up to 40 ft (12 m), jack-up platform rigs are typically used. Jack-up platform rigs often consist of a rectangular steel frame with a working deck that have four outriggers at the corners. Like the barge setup, the frame is floated to its designated location, and a heavy-duty hydraulic system pushes long vertical poles with spud-cans to the seafloor to act as shallow foundations. The hydraulic system then raises the platform to the top of the long poles to allow drilling operations.
96 Borehole Advancement Methods A borehole (boring) is a vertical hole advanced into the ground to determine the depths, layers, zones, types, and thicknesses of the geomaterials (soil and rock). Geotechnical borings are used for the following: â¢ Define geologic stratigraphy and structure â¢ Obtain samples for index testing and information to determine engineering properties â¢ Provide information on groundwater table and conditions â¢ Conduct in situ borehole tests â¢ Install instruments â¢ Determine the characteristics of existing geotechnical structures This section will define the following commonly used borehole advancement and exploratory techniques: â¢ Manual methods â¢ Test pits â¢ Auger drilling â¢ Rotary drilling â¢ Drilling over water The quality of information obtained from the various methods varies with the character of the subsurface geologic conditions; therefore, careful consideration must be given when selecting a method. It may be necessary to use more than one method in advancing a particular borehole. 6.3.1 Manual Methods Simple manual exploratory probing and representative sampling techniques are used as preliminary or supplementary measures to determine basic ground characteristics, typically of near-surface soils. Hand probes are made to obtain reconnaissance information in wetland areas, concerning the thickness and lateral extent of soft, compressible organic soils. Small-diameter, flush-coupled, steel rods are pushed into the ground by hand to refusal in the underlying inorganic soil. There are also a variety of hand augers and digging tools available to collect representative samples of the near-surface soil conditions. Various sizes and styles of cutter heads are available, and extensions may be added for greater penetration depths. Small gasoline-engine-powered hand augers will increase the depth of penetration and decrease the difficulty of performing the work. 6.3.2 Test Pits To examine near-surface geological conditions in detail, geoprofessionals can excavate test pits and trenches by hand or with excavating equipment such as a backhoe. The exploratory test pit technique is often used to determine geologic contacts and the presence of faulting. Test pits can also be used to recover bulk samples for laboratory testing. The Occupational Safety and Health Administration (OSHA) prohibits personnel to enter into a test pit extending more than 5 ft (1.5 m) below ground surface or within any pit displaying evidence of instability, without proper sheeting and bracing. Once the test pit has served its usefulness, it should be properly backfilled according to standard practices for compacted fill. This will prevent possible issues later with subsidence or settlement problems due to improperly placed fill materials.
97 6.3.3 Auger Drilling Augering is a simple method to advance a borehole. There are two types of augers: solid-flight augers and HSAs. Augers are equipped with drill bits (e.g., finger bits or fishtail bits) at the lower end for breaking up soil. 220.127.116.11 Solid-Flight Augers Solid flight augers typically have a diameter of 3â8 in. (7.6 to 20 cm) and a length of 5 ft (1.5 m). Procedures for auger borings are given by AASHTO T 306 (ASTM D1452). Auger cuttings can indicate the soil types present at the site but cannot provide the specific depths of the soils. If sampling is required beneath the auger bottom, the solid-flight augers must be carefully removed so that the sampler can be inserted into the hole. This is only possible in soils that are stable, such as stiff clays and silts or cemented sands. When using solid-flight augers, exploration depths are usually restricted to about 10â20 ft (3â6 m) because of probable borehole instability. 18.104.22.168 Hollow-Stem Augers HSAs have continuous flights situated around a central shaft of open cylindrical steel tubing rather than a solid bar. Thus, HSAs do not need to be removed to collect drive or push samples from beneath the auger bottom. Because the augers are left in place, they essentially act as a temporary casing to prevent borehole cave-in. Two HSA systems are available: one that uses inner drill rods within the outer hollow augers and one that relies on a wireline system to transport samples and drilling tools from top to bottom (Figure 6-1). HSAs typically have outer diameters of 6â12 in. (15â30 cm) and inside diameters of 3â6 in. (7.6â15 cm). Auger sections of 5 ft (1.5 m) are common, and these are easily stacked to allow the boring to extend deeper. Boreholes made with HSA easily reach depths of up to 100 ft (30 m). Because no water is required while using an HSA, this augering method is considered a dry borehole procedure for geotechnical explorations that follow ASTM D6151.
98 Source: Jeff Farrar Figure 6-1. HSA methods: drill rod (left) and wireline (right) 6.3.4 Rotary Drilling Rotary drilling is a versatile and adaptable technique that can be used with a range of drilling equipment and sampling devices. Figure 6-2 shows the various procedural methods for creating a borehole, including rotary wash drilling and rotary auger procedures using either solid-flight augers or HSAs. Other borehole methods include hydraulic push, percussive, and sonic are also shown. Rotary drilling consists of advancing a cased or uncased borehole by applying rapid rotation and pressure on the drill bit that cuts and grinds the materials at the bottom of the borehole into cuttings. These cuttings are subsequently removed from the borehole by pumping air, water, or drilling mud from a surface reservoir through the drill rods to the bottom of the borehole (Clayton et al. 2008). When the required sampling depth is obtained, the drill string is removed from the borehole, and the desired sampling device is lowered to the bottom of the hole. Standard guidelines for producing boreholes by this method are given by ASTM D5782, ASTM D5783, and ASTM D5876 for air, fluid, and wireline variations, respectively.
99 Source: Paul Mayne Figure 6-2. Rotary drilling techniques in comparison with other borehole methods Several types of drill rods and bits are available for different types of overburden and rock materials encountered, and the driller can change drill rods and bits as the situation demands. Two-, three-, and four- wing carbide insert drag bits are usually used in relatively soft or loose soils, and heavier, tri-cone roller bits are used in denser soils and several types of bedrock. The Diamond Core Drill Manufacturers Association (DCDMA) has established standard drill rod sizes and couplings. The sizes for some of the more common rod types are summarized in Table 6-2. Table 6-2. Standard drill rod sizes Size Outside Diameter Inside Diameter (in.) (in.) EW 1 3/8 1 5/16 AW 1 3/4 1 1/4 BW 2 1/8 1 3/4 NW 2 5/8 2 1/4 HW 3 1/2 3 1/16 Note: Rod lengths are generally available in 1 ft, 2 ft, 5 ft, and 10 ft; (b) all have 3 threads per in. Source: Acker Drill Company 6.3.5 Measuring While Drilling Measuring (or monitoring) while drilling (MWD) uses continuous measurements of drilling-related parameters to evaluate subsurface stratigraphy and estimate soil and rock characteristics. Though widely used during oil and gas drilling, it is an emerging technology in geotechnical drilling. It is more widely
100 used in Europe than in the United States at present and is described in an International Organization for Standards (ISO) standard (ISO 22476-15). MWD is predominantly used with rotary or rotary-percussive drilling methods. The measurements are performed using a drill parameter recorder (DPR) installed on a drill rig. The DPR comprises sensors that measure the following: â¢ Downward axial force (or crowd), â¢ Torque, â¢ Vertical penetration rate, â¢ Rotation speed, â¢ Fluid (or mud) pressure ( ) and flow rate ( ) These quantities are typically plotted as a continuous function of penetration depth. It is also possible to calculate compound parameters that are derived from the basic measurements. For example, the specific energy (i.e., the amount of energy required to remove a unit volume of material) can be calculated (Teale 1965): = + 2 where = cross-sectional area of the borehole Karasawa et al. (2002a, 2002b) defined the drillability strength, : = 64 where = diameter of the borehole Laudanski et al. (2012) provide a summary of additional compound parameters developed for use with MWD. Because MWD provides continuous profiles of measured and compound parameters, it is useful for more accurately defining subsurface stratigraphy and evaluating variability in site conditions. In soils, Sadkowski et al. (2010) found that MWD was able to reduce the frequency and number of split spoon samples taken and ultimately increased the efficiency with which the borings were completed. MWD is also useful for identifying voids and thin, weak layers that may be missed during conventional drilling, and it can provide a continuous profile in ground conditions where CPT tests are infeasible, such as partially weathered rock. In rocks, variations in MWD drilling parameters may be interpreted to indicate the presence of fractures, changes in lithology, and competency of the bedrock. For example, Sadkowski et al. (2010) used MWD data collected during bedrock coring to select zones in fractured rock to conduct packer tests to measure the hydraulic conductivity of the fractured bedrock formation. MWD parameters can also be used to estimate soil and rock engineering properties. For example, Rodgers et al. (2018a, 2018b) used MWD to evaluate the unconfined compressive strength of limestone during drilled shaft construction for quality control purposes. They suggested that the MWD-based estimates unconfined compressive strength may lead to higher LRFD resistance factors and decreased construction costs compared to the use of conventional strength tests on cores. MWD parameters have also been empirically correlated to components of the RMR system (Lindenbach 2016).
101 Soil Sampling Soil sampling devices are divided into two broad categories based on the condition of the samples they recover: â¢ Disturbed samples â¢ Undisturbed samples 6.4.1 Split-Spoon (Disturbed Samples) The SPT uses an open-ended drive sampler (i.e., split-spoon or split-barrel sampler) with a shoe having an outer diameter of 2 in. (50 mm), an inner diameter of 1.38 in. (35 mm), and a length of 24 in. (610 mm). The split-spoon sampler components are shown in Figure 6-3. Source: Kulhawy and Mayne (1990) Figure 6-3. Geometry of split-spoon sampler Split-spoon (split-barrel) sampling and the SPT are conducted per AASHTO T 206 (ASTM D1586). An impact hammer system having a weight of 140 lb (623 Newtons [N]) and a drop height of 30 in. (76 cm) is used to drive the split spoon three successive increments of 6 in. (15 cm), for a total driven vertical distance of 18 in. (45 cm). Occasionally, four increments are driven for a total of 24 in. (61 cm) so that additional soil material can be recovered for laboratory testing. Split-barrel samplers can be outfitted with a plastic liner to help collect representative soil particles. In the United States, however, it has become standard practice to not use these liners. If necessary, a plastic basket or catcher sleeve can be added within the shoe of the sampler to assist with retaining the sample during recovery, which is often helpful for sampling loose sands. The principal advantages of using split-spoon samplers are their simplicity of construction and operation and their relative economy for estimating in situ soil parameters through empirical correlations with the SPT N-value. In addition, split-spoon samplers recover representative specimens suitable for classification and for certain simple laboratory testing, such as grain size and plasticity. 6.4.2 Direct-Push Sampling (Disturbed Samples) Several direct-push sampling techniques have become available in the past two decades: (i) continuous hydraulic, (ii) continuous sonic drilling, (iii) cone penetrometer sampling, and (iv) vibracore. General guidelines for direct-push sampling are given by ASTM D6282.
102 22.214.171.124 Continuous Hydraulic Push Samplers Using an anchored hydraulic pushing system, geoprobe or powerprobe rigs can direct push a long steel mandrel in strokes of 2â6 ft (0.6â1.8 m) to obtain continuous soil samples (Figure 6-4). The mandrel is lined with plastic tubing that captures the soil sample. This produces long soil samples that generally have a diameter of 1â3 in. (2.5â7.6 cm). The primary advantages of using continuous hydraulic push sampling over rotary sampling methods are that direct-push methods produce no spoil and add no water or air to the ground; they are minimally intrusive to the environment. Additionally, the process is faster than conventional augering or rotary methods, thus, higher production rates are achieved. Both single-tube and dual-tube systems are available. With the dual-tube system, the outer tube acts as a casing. Source: Paul Mayne Figure 6-4. Direct-push samples of soil 126.96.36.199 Continuous Sonic Samplers Sonic rigs obtain continuous samples of soil and rock through a combination of cyclic resonant vibrations, twisting, and downward vertical forces. No water or air or drilling muds are introduced during sonic drilling operations in soil. The primary advantages of sonic samplers are their speed (they obtain samples much faster than conventional rotary drilling or augering methods) and how little waste they produce (sonic drilling produces 70â80 percent less waste than conventional rotary methods). Sonic drilling is conducted according to ASTM D6914 and can achieve depths of up to 1,000 ft (304 m) in certain geologic settings. Sonic drilling uses hollow casings that have outside diameters ranging from 4.5 to 12.5 in. (11 to 31 cm) to obtain core sizes that range from 4 to 10 in. (10 to 30 cm). By using sonic energy transmitted through the drill rods, the core barrel can be advanced into all types of geomaterials. An outer casing is then forced over the core barrel to provide borehole stability. The core barrel is removed from the casing, and the sonic core is pulled to the surface to extract the samples of soil and rock in continuous lengths. Due to the excessive vibrations and repeated cyclic loading, soil and rock samples are likely to be disturbed and, therefore, unsuitable for many laboratory tests that require undisturbed specimens. Sonic drilling often generates considerable heat, so the measured moisture content of recovered samples may be in error. Sonic drilling may also pulverize cobbles, rock fragments, and boulders, altering the grain size distribution of the samples. An example of the continuous disturbed soil sampling by sonic methods is shown in Figure 6-5.
103 Source: Boart Longyear Figure 6-5. Example of recovered sonic soil samples 188.8.131.52 Vibracore Samplers Vibracoring is a simple means to obtain soil samples at the bottom of a body of water. An open hollow casing is dropped vertically through the water. The freefall (fall using gravity only) allows the casing to penetrate the sediment. At least 20 ft (6 m) of water is needed to use vibracore samplers. Casing lengths of 3.3â6.6 ft (1â2 m) are common, with open inside diameters of 4 in. (10 cm) or larger. The casings can be made of steel or transparent plastic. The transparent casing allows the soil layers and lithology to be seen once retrieved (Figure 6-6). Vibracoring provides a continuous core for examination and index testing. Source: Wessex Archaeology Figure 6-6. Consecutive vibracore samples showing changes in stratigraphy 6.4.3 Thin-Wall Tube Sampling (Undisturbed Samples) Thin-wall tube sampling is a method of obtaining medium-to-large undisturbed samples of soil for laboratory testing. This method consists of pressing thin seamless tubing into soft-to-firm fine-grained soils. Although loose granular soils may also be sampled with this method, sample retention may be a problem unless the sampling device is equipped with a piston that creates a vacuum to help retain the sample in the tube. Standard guidelines for thin-wall sampling have been established by AASHTO T207 (ASTM D1587). The thin-wall tubing may be used with a variety of sampling devices to obtain representative and relatively undisturbed samples. As with any sampling device or method, variations in design, operation, and ability to recover the sample depends on the character of the materials being sampled. The following are some of the more common thin-wall tube sampling devices: â¢ Shelby tubes
104 â¢ Laval samplers â¢ Piston samplers â¢ Rotary core samplers 184.108.40.206 Shelby Tube Thin-Wall Open Sampler The thin-walled sampling method, more commonly referred to as Shelby tube from the manufacturer's tradename, may be any thin-wall tubing or casing that is beveled to form a tapered cutting edge. The tubes are usually available in diameters ranging from 2 to 5 in. (5 to 12 cm) with lengths ranging from 24 to 36 in. (61 to 91 cm). A common size Shelby tube is made from 16-gauge tubing with a wall thickness of 0.065 in. (1.7 mm) that has an outer diameter of 3 in. (7.6 cm) and a length of 36 in. (91 cm). These tube samplers are constructed of steel that is often galvanized to mitigate rusting. Brass or stainless steel tubes are also available. 220.127.116.11 Laval Sampler The Laval sampler is a special high-end, thin-walled, open-tube sampler having an outer diameter of 8.6 in. and an inner diameter of 8.2 in. (22 cm). This sampler has a special screw-type head valve that applies a vacuum at the top of the sample to aid in sample recovery (La Rochelle et al. 1981). After the open tube is pushed into virgin soils, an outer coring tube with a diameter of 10.7 in. (27 cm) is rotated above the open tube. Laval samples are obtained within a rotary boring using drilling fluids such as bentonitic slurries. With a wall thickness of 0.2 in. (5 mm), the diameter-to-wall thickness ratio of D/t = 42 results in a very high-quality undisturbed condition (Clayton et al. 2008). 18.104.22.168 Piston Samplers A piston sampler is a modified thin-walled open-tube sampler that has a piston at the upper head for obtaining higher quality undisturbed samples. The piston keeps the front end of the sampler closed until the desired sample depth is reached and maintains a seal during sample recovery to the ground surface. Several types of piston samplers are available, including fixed piston and Osterberg (Clayton et al. 2008). Fixed (Stationary) Piston Sampler. The fixed (or stationary) piston sampler is a significant improvement over conventional Shelby tubes in that it decreases sample disturbance and improves recovery (Long 2002). Procedures are given in ASTM D6519. A fixed or stationary piston sampler is constructed similarly to the thin-wall open-tube sampler but includes a sealed piston and locking cone in the head assembly to prevent the piston from moving downward (Figure 6-7). The apparatus is more complicated and time consuming to operate than open-tube samplers. However, samples can be taken in uncased boreholes.
105 Source: AASHTO Figure 6-7. Components of a fixed-piston sampler The piston can be locked and fully sealed at the bottom of the thin-wall tube so that it can be lowered into the borehole without contamination. Once the sampler is in position, the piston, through a series of small-diameter inner actuating rods, is locked to the drill rig or the casing, and pressure is applied to the outer drill rods. The pressure forces the thin-walled tube down from the stationary piston. When the full press is completed following a stroke of 24 in. (61 cm), any pressure buildup is released through a small hole in the actuating rods. The tight seal of the piston also creates a vacuum on the sample that helps retain the sample. The sampler is rotated two full turns to shear off the soil at the bottom of the tube and then withdrawn very carefully from the borehole. A short waiting period before and after shearing allows
106 additional skin friction to develop between the sample and the tube, which minimizes potential for sample loss during recovery. Osterberg Sampler. The hydraulic Osterberg sampler is a fixed-piston sampler designed to obtain undisturbed samples of soft and potentially sensitive soils in uncased boreholes. The internal design of the sampler is considerably more complex than a standard fixed-piston sampler, in that it consists of an inner thin-wall sampler tube and outer pressure cylinder (Figure 6-8). In the sampling position, a movable piston is attached to the top of the sampling tube, and a fixed piston rests on the soil to be sampled. The sampler is activated by pumping fluids or gas through the pressure cylinder, which drives the upper piston and sampling tube down over the lower piston into the soil a fixed distance. Then the piston and the sample are withdrawn from the borehole. The Osterberg sampler is available with specially designed thin-wall sampling tubes with a diameter of either 3.2 or 5.1 in. (8 or 13 cm). The self-contained and very portable aspects of this hydraulic piston sampler make it an ideal sampling device in swamps and areas where access would be difficult for large conventional drilling equipment. Source: Chris Clayton Figure 6-8. Osterberg piston sampler 22.214.171.124 Rotary Core Sampling A variety of core barrels, which were originally developed for drilling and sampling bedrock, have been modified or adapted to obtain undisturbed overburden samples in very dense or partially cemented soils. These core barrels are used when the more conventional thin-wall samplers cannot penetrate the selected geological unit, usually because the geomaterials are too hard. There are many local variations in the types
107 and mechanics of these core barrels, which are commercially available under a variety of trade names. Denison and Pitcher samplers are discussed in the following sections. Denison Sampler. The Denison sampler is designed to recover undisturbed thin-wall samples in dense sand and gravel soils, hard clays, partially cemented soils, or soft weathered rock. The sampler consists of a double- or triple-tube, swivel-type core barrel with a nonrotating inner steel or brass thin-wall liner designed to retain the sample during penetration and subsequent transport to the laboratory. The basic components are shown in Figure 6-9. The Denison sampler is designed for use with water, mud, or air with rotary drilling and is available in five outside diameter sizes, ranging from 2.94 to 7.75 in. (7.5 to 20 cm) The inner liner tube of the Denison has a sharp cutting edge that can be varied to extend up to about 3 in. (7.6 cm) beyond the outer rotating cutter bit. The extension variation is achieved by the options of interchangeable sawtooth cutter bits. These sawtooth cutter bits are preselected depending on the anticipated formation that will be sampled. The maximum extension is used in relatively soft or loose soils, and a cutting-edge flush with the coring bit is used in hard or cemented formations. An important feature of the Denison sampler is a system of check valves and release vents that bypass the hydrostatic pressure buildup within the inner sampling tube. These check valves and release vents improve sample recovery and minimize pressure disturbance of the sample. The Denison sampler is not practical for sampling loose sands or soft clays; the sample retention devices are usually inadequate for these materials. Cobbles and boulders will present major difficulties for penetration and recovery. Moreover, the sawtooth bit, with which the Denison is usually equipped, is not capable of coring hard boulders that may collapse the inner sampler tube if it is in an extended position. Source: Chris Clayton Figure 6-9. Denison triple-tube core barrel sampler
108 Pitcher Sampler. The Pitcher rotary core barrel sampler is a modification of the Denison sampler and consists of a single-tube, swivel-type core barrel with a self-adjusting, spring-loaded inner thin-wall sample tube that telescopes in and out of the cutter bit as the hardness of the material varies. This telescoping aspect eliminates the need to preselect a fixed inner barrel shoe length as is necessary with the Denison sampler. The inner steel or brass thin-wall liner tube has a sharp cutting edge that projects a maximum of 6 in. (15 cm) beyond the sawtooth cutter bit in its normal assembled position. As the sampler enters the borehole, a sliding valve directs the drilling fluid through the thin-wall sample tube to thoroughly flush the borehole. When the sample tube contacts the bottom of the borehole, it telescopes into the cutter barrel and closes the sliding valve. The closed sliding valve diverts the drilling fluid to an annular space between the sample tube and the cutter barrel. This sliding valve arrangement allows the drilling fluid to circulate which removes the borehole cuttings during sampling and prevents the drilling fluid from disturbing the recovered sample. The spring-loaded inner sample tube automatically adjusts to the density of the formation being penetrated. In very soft materials, the tube will extend as much as 6 in. (15 cm) beyond the cutter bit. As the formation density increases, the sample tube telescopes into the outer core barrel and compresses the control spring, which in turn, exerts a greater force on the tube to ensure adequate penetration. The Pitcher sampler is rotated into the formation in the same manner as conventional rock coring in either a cased or mudded borehole. The sampler is designed to be used with either water or mud and is available in four sizes that have outside diameters ranging from 2.5 to 5.8 in. (6 to 14.7 cm). In extremely dense formations or obstructions, the sample tube will retract completely into the outer core barrel to allow the cutter bit to penetrate the obstruction. The Pitcher sampleâs telescoping liner is a major advantage in highly variable formations, since it prevents the sample tube from collapsing. The Pitcher sampler, like the Denison, is not capable of coring hard cobbles and boulders. Rock Coring Methods The primary objective of rock coring is to obtain continuous undisturbed samples of the in situ rock mass that includes the sectional pieces of intact rock, as well as the joints, discontinuities, cracks, fissures, and any infillings or other features. These facets are important for evaluating the rock mass characteristics that will affect its performance as a foundation-bearing medium, as well as the stability for highway construction involving slopes, excavations, tunnels, and as a construction material such as rock fill or aggregate in pavements and concretes. It is essential to any site investigation program to carefully examine and log the recovered core. Logging the recovered core must be completed on-site immediately after the core has been extracted, since exposure to the air, moisture, humidity, sunlight, and ambient temperature may change the rock characteristics. In some cases (e.g., fissile clay shales), the alterations in characteristics can result in significantly different properties and features than what the actual in situ geomaterials possess. Rock coring and sampling by rotary drilling methods are detailed in ASTM D2113. Rock coring includes four components: (i) core bit, (ii) core barrel, (iii) drill rod, and (iv) drilling fluids. These components are described in detail in the following sections. 6.5.1 Core Bits Because diamond is the hardest natural material, it serves as an excellent means to cut into rock. The selection of the diamond size, bit crown contour, and number of water ports depends on the characteristics of the rock mass. Using an incorrect bit can be detrimental to the overall core recovery. Generally, fewer and larger diamonds are used to core soft formations, and more numerous, smaller diamonds mounted on the bit crown are used in hard formations. Special impregnated diamond core bits have been recently developed for use in severely weathered and fractured formations where bit abrasion can be very high.
109 6.5.2 Core Barrel Types Core barrels are manufactured in three basic types: single tube, double tube, and triple tube (Figure 6-10) with diameters that range from 1 to 10 in. (2.5 to 25 cm). These basic units all operate on the same principle of pumping drilling fluid through the drill rods and core barrel as the core bit cuts the rock. Fluids cool the diamond bit during drilling and carry the borehole cuttings to the surface. Fluids also maintain low temperatures that help protect the rock core and barrel from thermal expansion under high-speed rotation. A variety of coring bits, core retainers, and liners are used in various combinations to maximize the recovery and penetration rate of the selected core barrel. Of the three basic types of core barrels, the double-tube core barrel is most frequently used in rock core sampling for geotechnical engineering applications. The triple- tube core barrel is used in zones of highly variable hardness. (a) (b) (c) (d) Source: AASHTO Figure 6-10. Primary types of rock core barrels; (a) single tube; (b) rigid double tube; (c) swivel double tube; (d) triple tube 126.96.36.199 Single-Tube Core Barrel The simplest type of rotary core barrel is the single tube. The single-tube core barrel consists of a case- hardened, hollow steel tube with a diamond drilling bit attached at the bottom. The diamond bit cuts an annular groove or kerf in the formation to allow drilling fluid and cuttings to travel up the outside of the core barrel. However, the drilling fluid must pass over the recovered sample during drilling. The single- tube core barrel cannot be used in formations that are subject to erosion, slaking, or excessive swelling. The single-tube core barrel, because of its sample recovery and disturbance problems, is used infrequently. 188.8.131.52 Double-Tube Core Barrel The most popular and widely used rotary core barrel is the double tube. The double-tube core barrel is an outer tube barrel connected to the bit with an inner liner that holds the cut rock core. It is available with either a rigid or swivel inner liner. In the rigid types, the inner liner is fixed to the outer core barrel so that it rotates with the outer tube. In contrast, the swivel type is supported on a ball bearing carrier that allows the inner tube to remain stationary, or nearly so, during rotation of the outer barrelâa major improvement over the rigid type for sampling overburden materials. The sample or core is cut by rotation of the diamond bit. The bit is in constant contact with the drilling fluid as it flushes out the borehole cuttings. The addition of bottom discharge bits and fluid control valves
110 to the core barrel system minimizes the amount of drilling fluid and its contact with the sample which further decreases sample disturbance. Numerous sizes of cores can be obtained in rock, with the most common size for geotechnical explorations from rotary wash being NX with a diameter of 2.16 in. (5.5 cm) and wireline drilling at NQ with a diameter of 1.87 in. (4.7 cm). Figure 6-11 shows a section of rock core that is 10 ft (3 m) long obtained from rock coring operations. The recovered core is contained in a cardboard box for storage. A wide variety of core barrel designs and sizes are available. Table 6-3 provides a listing of several common sizes available for the swivel type double-tube core barrel design. Source: Paul Mayne Figure 6-11. Recovered rock core in gneissic granite bedrock Table 6-3. Core sizes from WG swivel type double-tube core barrel Size Rock Core Diameter Outer Tube Diameter (in.) (in.) EWG 7/8 1 7/16 AWG 1 1/8 1 13/16 BWG 1 5/8 2 9/32 NWG 2 1/8 2 29/32 HWG 3 3 3/4 Sources: Acker Drill Company (2015) 6.5.3 Triple-Tube Core Barrel The third and most complex type in rotary core barrel design is the triple-tube core barrel, which adds another separate, nonrotating liner to the double-tube core barrel. This liner, which retains the sample, consists of a transparent plastic solid tube or a split, thin metal liner. Each type of liner has its distinct advantages and disadvantages; however, both can increase sample recovery in inferior quality rock or partially cemented soils, with the additional advantage of minimizing sample handling and disturbance during removal from the core barrel. 6.5.4 Logging Borings Field logging soil and rock core borings is an important part of documenting the soil and rock conditions that exist at the project site. Field logging is best handled by an independent field engineer or geologist. A typical field log includes all the relevant information for the specific and unique boring that was completed. Relevant information includes the boring identification number, date of drilling, GPS coordinates, depths
111 of samples taken, types of samples (drive or tube), how the boring was advanced (auger, rotary wash, or direct push), hammer type, raw SPT N-values, and personnel on-site, as well as any other information that is deemed pertinent. In the field log, each sample is also identified by a unique designation, labeled, and classified per the visual-manual method ASTM D2488. While field logs have traditionally been recorded on standardized paper forms, commercial products are available that use an electronic tablet for digital entry of information. The digital entry of field information avoids the need to re-enter the same information later and is an important component of an effective strategy to manage geotechnical site characterization data as described in Appendix D. 6.5.5 Soil Boring Logs The field log, in situ test results, and laboratory testing results on recovered samples are used to produce the boring log, which is an engineering record. The boring log is interpretive and provides the permanent, technical documentation of the geomaterials encountered during drilling, sampling, and coring. The geotechnical engineer or geologist may group consecutive samples together based on color, soil type, corrected N-values (i.e., N60), and laboratory test results to form a layer or stratum that is consistently found in the adjacent companion borings from the site. An example of the engineering boring log is shown as Figure 6-12. In the final engineering boring logs, the soil types of the various layers are categorized according to a preselected soil classification system. The common soil classification systems in the United States are (i) Unified Soil Classification System (USCS) per ASTM D2488, (ii) AASHTO system, and (iii) U.S. Department of Agriculture (USDA). In addition to the soil types (e.g., clay, silt, sand, gravel), the color and consistency of the sample should be identified, as well as any additional facets or features that may be deemed important. These additional features might include organics, shells, calcareous nodules, peaty fibers, mottling, fabric, or natural markings that can help to identify the layer or strata during construction or excavation.
112 Source: Geosyntec Consultants, Inc. Figure 6-12. Example engineering boring log
113 6.5.6 Logging Rock Core The recovered rock core is logged in the field using at least three measures: (i) rock type, (ii) core recovery, and (iii) rock quality designation (RQD). The rock type should be identified by a qualified engineering geologist or geotechnical engineer familiar with the local geologic setting. A selection of common rock types is given in Table 6-4 according to grain size and origin: sedimentary, metamorphic, and igneous. Special attention should be given to rocks that are soluble (e.g., limestone, some dolomites, and other carbonate geomaterials), since the possible dissolution and formation of sinkholes, caves, voids, and channels can cause movements, shifting, collapse, and scour (Sowers 1996). In addition to the basic rock types, special attention should be given to additional minerals or aspects found in the rock core and along and between the joints and discontinuities. These additional minerals include mica, fluorite, galena, asbestos, hematite, halite, and others. Table 6-4. Basic rock types Sedimentary Metamorphic Igneous Grain Aspects Clastic Carbonate Foliated Massive Intrusive Extrusive Coarse Conglomerate Breccia Limestone Conglomerate Gneiss Marble Pegmatite Granite Volcanic Breccia Medium Sandstone Siltstone Limestone Chalk Schist Phyllite Quartzite Diorite Diabase Tuff Fine Shale Mudstone Calcareous Mudstone Slate Amphibolite Rhyolite Basalt Obsidian Source: Paul Mayne When coring rock, an attempt is usually made to collect a certain length of rock in the core barrel. This is called the core run and is often done in 5-ft or 10-ft lengths. The total length of the rock recovered divided by the core run is the core recovery, typically reported as a percentage. The RQD is a modified core recovery and is the sum of all pieces of rock that are at least 4 in. long, divided by the total core recovered, also reported as a percent. Thus, core recovery â¥ RQD. The RQD is normally obtained on core having a nominal diameter of about 2 in. and thus includes NX, NQ, and NW core bits. The RQD serves as a simple measure of the quality of the rock mass comprising the intact pieces of (often) hard rock material and the network of joints, fissures, and discontinuities in the native formation. Table 6-5 provides a rating system for the rock mass based on the measured RQD. Figure 6-13 presents a rendering of the determination on RQD from a hypothetical section of recovered rock core. Table 6-5. Rated quality of rock mass based on RQD Measured RQD Quality 0 to 25% Very Poor 25% to 50% Poor 50% to 75% Fair 75% to 90% Good
114 Measured RQD Quality 90% to 100% Excellent Source: Goodman (1989) Source: Paul Mayne Figure 6-13. Evaluation of RQD from recovered core In addition to its use in field logging of rock cores, RQD is also used as an input parameter for rock mass classification schemes as described in Section 10.4. 6.5.7 Sample Protection The obtained soil and rock samples should be properly sealed and protected from the environment (e.g., exposure to hot sun, freezing temperatures, high winds). Samples should be placed in moisture-controlled containers and temperature-controlled environments to minimize alterations. During transport to the laboratory, the samples should not be subjected to excessive shaking, vibration, or rolling. Thin-walled tubes should be sealed using paraffin wax or plastic O-ring inserts or both. The samples should be kept vertical throughout transport from the field to the laboratory until specimens are extracted for laboratory testing. Split-spoon samples should be retained in plastic or glass jars with proper caps to prevent moisture loss. Recovered rock cores should be placed in secured wood boxes or cardboard core containers. Any missing rock core should have a placer or plastic insert to prevent the core sections from shifting during transport. Boring Closure Upon completion, the drilled borehole should be sealed in accordance with local, state, or federal regulations. In some instances, specific borehole closure laws mandate the borehole be sealed to protect against groundwater contamination and to provide protection for aquifer resources. The closure methods generally involve placing or injecting sealants (e.g., grouts, bentonites, concrete, other additives) into the open hole by means of freefall drop, tremie pipe placement, pressurized pumping, or use of special probes
115 with expendable tips and other technologies. A review of the various techniques is given by Lutenegger et al. (1995).
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