PURPOSE OF SUBSURFACE EXPLORATION

CHAPTER 2

SUBSURFACE EXPLORATION

INTRODUCTION

As discussed in Chap. 1, the first step in the foundation investigation is to obtain preliminary information on the project and to plan the work. The next step is typically to perform the subsurface exploration. The goal of the subsurface investigation is to obtain a detailed understanding of the engineering and geologic properties of the soil and rock strata and groundwater conditions that could impact the foundation.

Specific items that will be discussed in the chapter are as follows:

1.Document review

2.Purpose of subsurface exploration

3.Borings, including a discussion of soil samplers, sample disturbance, field tests, boring layout, and depth of subsurface exploration

4.Test pits and trenches (Sec. 2.5)

5.Preparation of logs (Sec. 2.6)

6.Geophysical techniques (Sec. 2.7)

7.Subsurface exploration for geotechnical earthquake engineering (Sec. 2.8)

8.Subsoil profile (Sec. 2.9)

DOCUMENT REVIEW

Prior to performing the subsurface exploration, it may be necessary to perform a document review.

Examples of the types of documents that may need to be reviewed are as follows:

Prior Development. If the site had prior development, it is important to obtain information on the history of the site.

Aerial Photographs and Geologic Maps. During the course of the work, it may be necessary for the engineering geologist to check reference materials, such as aerial photographs or geologic maps. Aerial photographs are taken from an aircraft flying at prescribed altitude along pre established lines. Interpretation of aerial photographs takes considerable judgment and because they have more training and experience, it is usually the engineering geologist who interprets the aerial photographs. By viewing a pair of aerial photographs, with the aid of a stereoscope, a three-dimensional view of the land surface is provided. This view may reveal important geologic information at the site, such as the presence of landslides, fault scarps, types of landforms, erosional features, general type and approximate thickness of vegetation, and drainage patterns. By comparing older versus newer aerial photographs, the engineering geologist can also observe any man-made or natural changes that have occurred at the site.

 

 

Geologic maps can be especially useful to the geotechnical engineer and engineering geologist because they often indicate potential geologic hazards (e.g., faults landslides and the like) as well as the type of near surface soil or rock at the site.

A major source for geologic maps in the United States is the United States Geological Survey (USGS). The USGS prepares many different geologic maps, books, and charts and these documents can be purchased at the online USGS bookstore. The USGS also provides an “Index to Geologic

Mapping in the United States,” which shows a map of each state and indicates the areas where a geologic map has been published.

Topographic Maps. Both old and recent topographic maps can provide valuable site information. As shown in Fig. 2.3, the topographic map is to scale and shows the locations of buildings, roads, freeways, train tracks, and other civil engineering works as well as natural features such as canyons, rivers, lagoons, sea cliffs, and beaches. The topographic map in Fig. 2.3 even shows the locations of sewage disposal ponds, water tanks, and by using different colors and shading, it indicates older versus newer development. But the main purpose of the topographic map is to indicate ground surface elevations or elevations of the sea floor, such as shown in Fig. 2.3. This information can be used to determine the major topographic features at the site and for the planning of subsurface exploration, such as available access to the site for drilling rigs.

Building Code and Other Specifications. A copy of the most recently adopted local building code should be reviewed. Usually only a few sections of the building code will be directly applicable tofoundation engineering. For example, the main applicable geotechnical section in the International Building Code (2009) is Chap. 18, “Soils and Foundations.” Depending on the type of project, there may be other specifications that are applicable for the project and will need to be reviewed. Documents that may be needed for public works projects include the Standard Specifications for Public Works Construction (2003) or the Standard Specifications for Highway Bridges (AASHTO, 1996).

Documents at the Local Building Department. Other useful technical documents include geotechnical and foundation engineering reports for adjacent properties, which can provide an idea of possible subsurface conditions. A copy of geotechnical engineering reports on adjacent properties can often be obtained at the archives of public agencies, such as the local building department. Other valuable reference materials are standard drawings or standard specifications, which can also be obtained from the local building department.

Forensic Engineering. Reports or other documents concerning the investigation of damaged or deteriorated structures may discuss problem conditions that could be present at the site (Day, 1999b, 2000b, 2004). Table 2.2 presents a summary of typical documents that may need to be reviewed prior to or during the construction of the project.

PURPOSE OF SUBSURFACE EXPLORATION

The general purpose of subsurface exploration is to determine the following:

1.Soil strata

a. Depth, thickness, and variability

b. Identification and classification

c. Relevant engineering properties, such as shear strength, compressibility, stiffness, permeability,expansion or collapse potential, and frost susceptibility. Investigation of the distribution, type, and physical properties of subsurface materials are, in some

form or other, required for the final design of most civil engineering structures. These investigations are performed to obtain solutions to the following groups of problems:

Foundation problems or determination of the stability and deformations of undisturbed subsurface materials under superimposed loads, in slope and cuts, or around foundation pits and tunnels; and determination of the pressure of subsurface materials against supporting structures when such are needed. Construction problems or determination of the extent and character of materials to be excavated or location and investigation of soil and rock deposits for use as construction materials in earth dams and fills, for road and airfield bases and surfacing, and for concrete aggregates.

Groundwater problems or determination of the depth, hydrostatic pressure, flow, and composition of the ground water, and thereby the danger of seepage, underground erosion, and frost action; the influence of the water on the stability and settlement of structures; its action on various construction materials; and its suitability as a water supply. There are many different types of subsurface exploration, such as borings, test pits, or trenches.

Table 2.3 presents general information on foundation investigations, samples and samplers, and subsurface exploration. Table 2.4 summarizes the boring, core drilling, sampling

and other exploratory techniques that can be used by the geotechnical engineer.

As mentioned earlier, the borings, test pits, or trenches are used to determine the thickness of soil and rock strata, estimate the depth to groundwater, obtain soil or rock specimens, and perform field tests such as Standard Penetration Tests (SPT) or Cone Penetration Tests (CPT). The Unified Soil Classification System (USCS) can be used to classify the soil exposed in the borings or test pits (Casagrande, 1948). The subsurface exploration and field sampling should be performed in accordance with standard procedures, such as those specified by the American Society for Testing and Materials

or other recognized sources.

BORINGS

A boring is defined as a cylindrical hole drilled into the ground for the purposes of investigating subsurface conditions, performing field tests, and obtaining soil, rock, or groundwater specimens for testing. Borings can be excavated by hand (e.g., hand auger), although the usual procedure is to use mechanical equipment to excavate the borings.

During the excavation and sampling of the borehole, it is important to prevent caving-in of the borehole sidewalls. In those cases where boreholes are made in soil or rock and there is no groundwater, the holes will usually remain stable. Exceptions include clean sand and gravels that may cavein even when there is no groundwater. The danger of borehole caving-in increases rapidly with depth and the presence of groundwater. For cohesive soils, such as firm to hard clay, the borehole may remain stable for a limited time even though the excavation is below the groundwater table. For other soils below the groundwater table, borehole stabilization techniques will be required, as follows:

Stabilization with Water. Boreholes can be filled with water up to or above the estimated level of the groundwater table. This will have the effect of reducing the sloughing of soil caused by water rushing into the borehole. However, water alone cannot prevent caving-in of borings in soft or cohesionlesssoils or a gradual squeezing-in of a borehole in plastic soils. Uncased boreholes filled with water up to or above the groundwater table can generally be used in rock and for stiff to hard cohesive soils.

Stabilization with Drilling Fluid. An uncased borehole can often be stabilized by filling it with a properly proportioned drilling fluid, also known as “mud,” which when circulated also removes the ground-up material located at the bottom of the borehole. The stabilization effect of the drilling fluid is due to two effects: (1) the drilling fluid has a higher specific gravity than water alone, and (2) the drilling fluid tends to form a relatively impervious sidewall borehole lining, often referred to as mudcake, which prevents sloughing of cohesionless soils and decreases the rate of swelling of cohesive soils. Drilling fluid is primarily used with rotary drilling and core boring methods.

Stabilization with Casing. The safest and most effective method of preventing caving-in of the borehole is to use a metal casing. Unfortunately, this type of stabilization is rather expensive. Many different types of standard metal or special pipe can be used as casing. The casing is usually driven in place by repeated blows of a drop hammer. It is often impossible to advance the original string of casing when difficult ground conditions or obstructions are encountered. A smaller casing is then inserted through the one in place, and the diameter of the extension of the borehole must be decreased accordingly.

Other Stabilization Methods. One possible stabilization method is to literally freeze the round and then drill the boring and cut or core the frozen soil from the ground. The reezing is accomplished by installing pipes in the ground and then circulating ethanol and crushed dry ice or liquid nitrogen through the pipes. Because water increases in volume pon freezing, it is important to establish a slow freezing front so that the freezing water can slowly expand and migrate out of the soil pores. This process can minimize the sample disturbance associated with the increase in volume of freezing water. Another method is to temporarily lower the groundwater table and allow the water to drain from the soil before the excavation of the borehole. The partially saturated soil will then be held together by capillarity, which will enable the soil strata to be bored and sampled. When brought to the ground surface, the partially saturated soil specimen is frozen. Because the soil is only partially saturated, the volume increase of water as it freezes should not significantly disturb the soil structure. The frozen soil specimen is then transported to the laboratory for testing.

From a practical standpoint, these two methods described earlier are usually uneconomical or most projects. There are many different types of equipment used to excavate borings. Typical types of borings are listed in Table 2.4 and include:

Auger boring. A mechanical auger is the simplest and fastest method of excavating a boring. Because of these advantages, augers are probably the most common type of equipment used to excavate borings. The hole is excavated through the process of rotating the auger while at the same time applying a downward pressure on the auger to help penetrate the soil or rock. There are basically two types of augers: flight augers and bucket augers (see Fig. 2.4). Common available diameters of flight augers are 2 in. to 4 ft (5 cm to 1.2 m) and of bucket augers are 1 to 8 ft (0.3 to 2.4 m). The auger is periodically removed from the hole, and the soil lodged in the blades of the flight auger or contained in the bucket of the bucket auger is removed. A casing is generally not used for auger borings and the hole may cave-in during the excavation of loose or soft soils or when the excavation is below the groundwater table.

Hollow-stem flight auger. A hollow-stem flight auger has a circular hollow core, which allows for sampling down the center of the auger. The hollow-stem auger acts like a casing and allows for sampling in loose or soft soils or when the excavation is below the groundwater table.

Wash boring. A wash boring is advanced by the chopping and twisting action of a light bit and partly by the jetting of water, which is pumped through the hollow drill rod and bit (see Fig. 2.5). The cuttings are removed from the borehole by the circulating water. Casing is typically required in soft or cohesionless soil, although it is often omitted for stiff to hard cohesive soil. Loose cuttings tend to accumulate at the bottom of the borehole and careful cleaning of the hole is required before samples are taken.

Rotary drilling. For rotary drilling, the borehole is advanced by the rapid rotation of the drilling bit that cuts, chips, and grinds the material located at the bottom of the borehole into small particles. In order to remove the small particles, water or drilling fluid is pumped through the drill rods and bit and ultimately up and out of the borehole. Instead of using water or drilling fluid, forced air from a compressor can be used to cool the bit and remove the cuttings. A drill machine and rig, such as shown in Fig. 2.6, are required to provide therotary power and downward force required to excavate the boring. Other rotary drilling details are provided in Table 2.4.

Percussion drilling. This type of drilling equipment is often used to penetrate hard rock, for

subsurface exploration or for the purpose of drilling wells. The drill bit works much like a jackhammer, rising and falling to break-up and crush the rock material. Percussion drilling works best for rock and will be ineffective for such materials as soft clay and loose saturated sand. It takes considerable experience to anticipate which type of drill rig and sampling equipment would be best suited to the site under investigation. For example, if downhole logging is required, then a large diameter bucket auger boring is needed (Fig. 2.4). A large diameter boring, typically 30 in. (0.76 m) in diameter, is excavated and then the geotechnical engineer or engineering geologist descends into the borehole. Figure 2.7 shows a photograph of the top of the boring with the geologist descending into the hole in a steel cage. Note in Fig. 2.7 that a collar is placed around the top of the hole to prevent loose soil or rocks from being accidentally knocked down the hole. The process of downhole logging is a valuable technique because it allows the geotechnical engineer or engineering geologist to observe the subsurface materials, as they exist inplace. Usually the process of the excavation of the boring smears the side of the hole, and the surface must be chipped away to observe intact soil or rock. Going downhole is dangerous because of the possibility of a cave-in of the hole as well as “bad air” (presence of poisonous gases or lack of oxygen) and should only be attempted by an experienced geotechnical engineer or engineering geologist. The downhole observation of soil and rock can lead to the discovery of important subsurface conditions. For example, Fig. 2.8 provides an example of the type of conditions observed downhole. Figure 2.8 shows a knife that has been placed in an open fracture in bedrock. Massive landslide movement caused the open fracture in the rock. Figure 2.9 is a side view of the same condition. In general, the most economical equipment for borings are truck mounted rigs that can quickly and economically drill through hard or dense soil. It some cases, it is a trial and error process ofusing different drill rigs to overcome access problems or difficult subsurface conditions. For example, one deposit encountered by the author consisted of hard granite boulders surrounded by soft and highly plastic clay. The initial drill rig selected for the project was an auger drill rig, but the auger could not penetrate through the granite boulders. The next drill rig selected was an air track rig, which uses a percussion drill bit that easily penetrated through the granite boulders, but the soft

clay plugged up the drill bit and it became stuck in the ground. Over 50 ft (15 m) of drill stem could not be removed from the ground and it had to be left in place, a very costly experience with difficult drilling conditions.

Some of my other memorable experiences with drilling are as follows:

1.Drilling accidents. Most experienced drillers handle their equipment safely, but accidents can happen to anyone. One day, as I observed a drill rig start to excavate the hole, the teeth of the auger bucket caught on a boulder. The torque of the auger bucket was transferred to the drill rig, and it flipped over. Fortunately, no one was injured.

2.Underground utilities. Before drilling, the local utility company, upon request, will locate their underground utilities by placing ground surface marks that delineate utility alignments. An incident involving a hidden gas line demonstrates that not even utility locators are perfect. On a particularly memorable day, I drove a Shelby tube sampler into a 4 in. (10 cm) diameter pressurized gas line. The noise of escaping gas was enough to warn of the danger. Fortunately, an experienced driller knew what to do: turn off the drill rig and call 911.

3.Downhole logging. As previously mentioned, a common form of subsurface exploration in southern California is to drill a large-diameter boring, usually 30 in. (0.76 m) in diameter. Then the geotechnical engineer or engineering geologist descends into the earth to get a close-up view of soil conditions. On this particular day, several individuals went down the hole and noticed a small trickle of water in the hole about 20 ft (6 m) down. The sudden and total collapse of the hole riveted the attention of the workers, especially the geologist who had moments before been down at the bottom of the hole. Because subsurface exploration has a potential for serious or even fatal injury, it is especially important that young ngineers and geologists be trained to evaluate the safety of engineering operations in the field. This must be done before they supervise field operations.

Rock and Soil Samplers

There are many different types of samplers used to retrieve soil and rock specimens from the boring. For example, three types of soil samplers are shown in Fig. 2.10, the California sampler, Shelby tube, and SPT sampler. One of the most important first steps in sampling is to clean-out the bottom of the borehole in order to remove the loose soil or rock debris that may have fallen to the bottom of the borehole. For hard rock, coring is used to extract specimens (see Table 2.4). The coring process consists of rotating a hollow steel tube, known as a core barrel, which is equipped with a boring bit. The drilled rock core is collected in the core barrel as the drilling progresses. Once the rock core has been cut and the core barrel is full, the drill rods are pulled from the borehole and the rock core is extracted from the core barrel. A rotary drill rig, such as shown in Fig. 2.6, is often used for the rock coring operation. For further details on rock core drill and sampling, see ASTM D 2113-99 (2004), “Standard Practice for Rock Core Drilling and Sampling of Rock for Site Investigation.” For soil, the most common method is to force a sampler into the soil by either hammering, jacking, or pushing the sampler into the soil located at the bottom of the borehole. Soil samplers are typically divided into two types.

Thin-Walled Soil Sampler. The most common type of soil sampler used in the United States is the Shelby tube, which is a thin-walled sampling tube consisting of stainless steel or brass tubing. In order to slice through the soil, the Shelby tube has a sharp and drawn-in cutting edge. In terms of dimensions, typical diameters are from 2 to 3 in. (5 to 7.6 cm) and lengths vary from 2 to 3 ft (0.6 to 0.9 m). The typical arrangement of drill rod, sampler head, and thin-wall tube sampler is shown in Fig. 2.11. The sampler head contains a ball check valve and vents for escape of air and water during the sampling process. The drill rig equipment can be used to either hammer, jack, or push the sampler into the soil. The preferred method is to slowly push the sampler into the soil by using hydraulicjacks or the weight of the drilling equipment. Thin-walled soil samplers are used to obtain undisturbed

soil samples, which will be discussed in the next section. For further details on thin-walled

sampling, see ASTM D 1587-00 (2004), “Standard Practice for Thin-Walled Tube ampling of Soils for Geotechnical Purposes.”

Thick-Walled Soil Sampler. Thin-walled samplers may not be strong enough to sample gravelly soils, very hard soils, or cemented soils. In such cases, a thick-walled soil sampler will be required. Such samplers are often driven into place by using a drop hammer. The typical arrangement of drill rod, sampler head, and barrel when driving a thick-walled sampler is shown in Fig. 2.11. Many localities have developed thick-walled samplers that have proven successful for local conditions. For example, in southern California, a common type of sampler is the California sampler, which is a split-spoon type sampler that contains removable internal rings, 1.0 in. (2.54 cm) in height. Figure 2.10 shows the California sampler in an open condition, with the individual rings exposed. The California sampler has a 3.0 in. (7.6 cm) outside diameter and a 2.50 in. (6.35 cm) inside diameter. This sturdy sampler, which is considered to be a thick-walled sampler, has proven successful in sampling hard and desiccated soil and soft sedimentary rock common in southern alifornia. Another type of thick-walled sampler is the SPT sampler, which will be discussed in Sec. 2.4.3. For further details on thick-walled sampling, see ASTM D 3550-01 (2004), “Standard Practice for Thick Wall, Ring-Lined, Split Barrel, Drive Sampling of Soils.”

Sample Disturbance

This section will discuss the three types of soil samples that can be obtained during the subsurface exploration. In addition, this section will also discuss sampler and sample ratios used to evaluate sample disturbance; factors that affect sample quality; x-ray radiography; and transporting, preserving, and disposal of soil samples.

Soil Samples. There are three types of soil samples that can be recovered from borings:

Altered Soil (also known as Nonrepresentative Samples). During the boring operations, soil can be altered due to mixing or contamination. Such materials do not represent the soil found at the bottom of the borehole and hence should not be used for visual classification or laboratory tests. Some examples of altered soil are as follows:

Failure to clean the bottom of the boring. If the boring is not cleaned out prior to sampling, a soil sample taken from the bottom of the borehole may actually consist of cuttings from the side of the borehole. These borehole cuttings, which have fallen to the bottom of the borehole, will not represent in situ conditions at the depth sampled.

Soil contamination. In other cases, the soil sample may become contaminated with drilling

fluid, which is used for wash-type borings. These samples are often called wash samples or wet samples because they are washed out of the borehole and allowed to settle in a sump at ground surface. These types of soil samples that have been contaminated by the drilling process should not be used for laboratory tests because they will lead to incorrect onclusions regarding subsurface conditions.

Soil mixing. Soil or rock layers can become mixed during the drilling operation, such as by the action of a flight auger. For example, suppose varved clay, which consists of thin alternating layers of sand and clay, becomes mixed during the drilling and sampling process. Obviously laboratory tests would produce different results when performed on the mixed soil as compared to laboratory tests performed on the individual sand and clay layers.

Change in moisture content. Soil that has a change in moisture content due to the drilling fluid or from heat generated during the drilling operations should also be classified as altered soil.

Densified soil. Soil that has been densified by over-pushing or over-driving the soil sampler

should also be considered as altered because the process of over-pushing or over-driving could squeeze water from the soil. Figure 2.12 shows a photograph of the rear end of a Shelby tube sampler. The soil in the sampler has been densified by being over-pushed as indicated by the smooth surface of the soil and the mark in the center of the soil (due to the sampler head). In summary, any soil or rock where the mineral constituents have been removed, exchanged, or mixed should be considered as altered soil.

Disturbed Samples (also known as Representative Samples). It takes considerable experience and judgment to distinguish between altered soil and disturbed soil. In general, disturbed soil is defined as soil that has not been contaminated by material from other strata or by chemical changes, but the soil structure is disturbed and the void ratio may be altered. In essence, the soil has only been remolded during the sampling process. For example, soil obtained from driven thick-walled samplers, such as the SPT spilt spoon sampler, or chunks of intact soil brought to the surface in an auger bucket (i.e., bulk samples) are considered disturbed soil. Disturbed soil can be used for visual classification as well as numerous types of laboratory tests. Example of laboratory tests that can be performed on disturbed soil include water content, specific gravity, Atterberg limits, sieve and hydrometer tests, expansion index test, chemical composition (such as soluble sulfate), and laboratory compaction tests such as the Modified Proctor.

Undisturbed Samples. Undisturbed samples may be broadly defined as soil that has been subjected to no disturbance or distortion and the soil is suitable for laboratory tests that measure the shear strength, consolidation, permeability, and other physical properties of the in situ material. As a practical matter, it should be recognized that no soil sample can be taken from the ground and be in a perfectly undisturbed state. But this terminology has been applied to those soil samples taken by certain sampling methods. Undisturbed samples are often defined as those samples obtained by slowly pushing thin-walled tubes, having sharp cutting ends and tip relief, into the soil. Undisturbed soil samples are essential in many types of foundation engineering analyses, such as the determination of allowable bearing pressure and settlement. Many soil samples may appear to be undisturbed but they have actually been subjected to considerable disturbance of the soil structure. It takes considerable experience and judgment to evaluate laboratory test results on undisturbed soil

samples as compared to test results that may be inaccurate due to sample disturbance.

Factors that Affect Sample Quality. It is important to understand that using a thin wall tube, such as a Shelby tube, or obtaining a gross recovery ratio of 100 percent would not guarantee an undisturbed soil specimen. Many other factors can cause soil disturbance,

such as:

• Pieces of hard gravel or shell fragments in the soil, which

can cause voids to develop along the sides of the sampling

tube during the sampling process

• Soil adjustment caused by stress relief when making a

borehole

• Disruption of the soil structure due to hammering or pushing

the sampling tube into the soil stratum

• Tensile and torsional stresses which are produced in separating

the sample from the subsoil

• Creation of a partial or full vacuum below the sample as

it is extracted from the subsoil

• Expansion of gas during retrieval of the sampling tube as

the confining pressure is reduced to zero

• Jarring or banging the sampling tube during transportation

to the laboratory

• Roughly removing the soil from the sampling tube

• Crudely cutting the soil specimen to a specific size for a

laboratory test The actions listed earlier cause a decrease in effective stress, a reduction in the interparticle bonds, and a rearrangement of the soil particles. An “undisturbed” soil specimen will have little rearrangement of the soil particles and perhaps no disturbance except that caused by stress relief where there is a change from the in situ ko (at-rest) condition to an isotropic perfect sample stress condition (Ladd and Lambe, 1963). A disturbed soil specimen will have a disrupted soil structure with perhaps a total rearrangement of soil particles. When measuring the shear strength or deformation characteristics of the soil, the results of laboratory tests run on undisturbed specimens obviously better represent in situ properties than laboratory tests run on disturbed specimens. Some examples of disturbed soil are shown in Figs. 2.14 to 2.16 and described as follows:

Turning of edges. Turning or bending of edges of various thin layers show as curved down edges on the sides of the specimen. This effect is due to the friction between the soil and sampler. Turning of edges could also occur when the soil specimen is pushed out of the

back of the sampler in the laboratory. The turning ofedges can also be created when the sampler is hammered into the soil. Examples of turning of edges are shown in Figs 2.14 and 2.15.

Shear failures. Figure 2.16 shows four examples of shear failure of the soil within the sampler. This sample disturbance occurred during the pushing of Shelby tubes into medium soft silty clay.

X-ray Radiography of Soil Samples. Although rarely used in practice, one method of assessing the quality of soil samples is to obtain an x-ray radiograph of the soil contained in the sampling tube. A radiograph is a photographic record produced by the passage of x-rays through an object and onto photographic film. Denser objects absorb the x-rays and can appear as dark areas on the radiograph. Worm holes, coral fragments, cracks, gravel inclusions, and sand or silt seams can easily be identified by using radiography (Allen et al., 1978). Figures 2.17 and 2.18 present two radiographs taken of Orinoco Clay contained within Shelby tubes. These two radiographs illustrate additional types of soil disturbance:

Voids. The top of Fig. 2.17 shows large white areas, which are the locations of soil voids. The causes of such voids are often due to sampling and transporting process. The open voids can be caused by many different factors, such as gravel or shells which impact with the cutting end of the sampling tube and/or scrape along the inside of the sampling tube and create voids. The voids and highly disturbed clay shown at the top of Fig. 2.17 are possibly due to cuttings inadvertently left at the bottom of the borehole. Some of the disturbance could also be caused by tube friction during sampling as the clay near the tube wall becomes remolded as it travels up the tube.

Soil cracks. Figures 2.17 and 2.18 show numerous cracks in the clay. For example, the arrows labeled 1 point to some of the soil cracks in Figs. 2.17 and 2.18. Some of the cracks appear to be continuous across the entire sampling tube (e.g., arrow labeled 2, Fig. 2.17). The soil cracks probably developed during the sampling process. A contributing factor in the development of the soil cracks may have been gas coming out of solution, which fractured the clay.

Gas related voids. The circular voids (labeled 3) shown in Fig. 2.17 were caused by gas coming out of solution during the sampling process when the confining pressures were essentially reduced to zero. In contrast to soil disturbance, the arrow labeled 4 in Fig. 2.18 indicates an undisturbed section of the soil sample. Note in Fig. 2.18 that the individual fine layering of the soil sample can even be observed. For further details on x-ray radiography, see ASTM D 4452-02, 2004, “Standard Test Methods for X-Ray Radiography of Soil Samples.”

Transporting Soil Samples. During transport to the laboratory, soil samples recovered from the borehole should be kept within the sampling tube or sampling rings. In order to preserve soil samples during transportation, the soil sampling tubes can be tightly sealed with end caps and duct tape. For sampling rings, they can be placed in cylindrical packing cases that are then thoroughly sealed. Bulk samples can be placed in plastic bags, pails, or other types of waterproof containers. The goal of the transportation of soil samples to the laboratory is to prevent a loss of moisture. In addition, for undisturbed soil specimens, they must be cushioned against the adverse effects of transportation induced vibration and shock. Protection may also need to be provided against adverse temperature changes, such as overheating or freezing of the soil. The soil samples should be marked with the file or project number, date of sampling, name of engineer or geologist who performed the sampling, and boring number and. Other items that may need to be identified are as follows:

1.Sample orientation (if necessary)

2.Special shipping and laboratory handling instructions

3.Penetration test data (if applicable)

4.Subdivided samples must be identified while maintaining association to the original sample