The purpose of the field exploration is to obtain the following (M. J. Tomlinson, ‘‘Foundation Design and Construction,’’ 5th ed., John Wiley & Sons, Inc., New York):
1. Knowledge of the general topography of the site as it affects foundation design and construction, e.g., surface configuration, adjacent property, the presence of watercourses, ponds, hedges, trees, rock outcrops, etc., and the available access for construction vehicles and materials.
2. The location of buried utilities such as electric power and telephone cables, water mains, and sewers.
3. The general geology of the area, with particular reference to the main geologic formations underlying the site and the possibility of subsidence from mineral extraction or other causes.
4. The previous history and use of the site, including information on any defects or failures of existing or former buildings attributable to foundation conditions.
5. Any special features such as the possibility of earthquakes or climate factors such as flooding, seasonal swelling and shrinkage, permafrost, and soil erosion.
6. The availability and quality of local construction materials such as concrete aggregates, building and road stone, and water for construction purposes.
7. For maritime or river structures, information on tidal ranges and river levels, velocity of tidal and river currents, and other hydrographic and meteorological data.
8. A detailed record of the soil and rock strata and groundwater conditions within the zones affected by foundation bearing pressures and construction operations,
or of any deeper strata affecting the site conditions in any way.
9. Results of laboratory tests on soil and rock samples appropriate to the particular foundation design or construction problems.
10. Results of chemical analyses on soil or groundwater to determine possible deleterious effects of foundation structures.
Some of the required information, such as the previous history and use of the site, can be obtained from a document review. For example, there may be old engineering reports indicating that the site contains deposits of fill, abandoned septic systems and leach fields, buried storage tanks, seepage pits, cisterns, mining shafts, tunnels, or other man-made surface and subsurface works that could impact the new proposed development. There may also be information concerning on-site utilities and underground pipelines, which may need to be capped or rerouted around the project.
During the course of the work, it may be necessary to check reference materials, such as geologic and topographic maps. Geologic maps can be especially useful because they often indicate potential geologic hazards (e.g., faults, landslides) as well as the type of near-surface soil or rock at the site. Both old and recent topographic maps can also provide valuable site information. Topographic maps are usually to scale and show 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 maps can even show the locations of sewage disposal ponds and water tanks, and by using different colors and shading, they indicate older versus newer development. But the main purpose of the topographic map is to indicate ground surface elevations. This information can be used to determine the major topographic features at the site and for the planning of subsurface exploration, such as available site access for drilling rigs.
Another important source of information is aerial photographs, which are taken from an aircraft flying at a prescribed altitude along preestablished lines. Viewing a pair of aerial photographs, with the aid of a stereoscope, provides a threedimensional view of the land surface. This view may reveal important geologic information at the site, such as the presence of landslides, fault scarps, types of landforms (e.g., dunes, alluvial fans, glacial deposits such as moraines and eskers), 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.
In order for a detailed record of the soil and rock strata and groundwater conditions at the site to be determined, subsurface exploration is usually required. There are different types of subsurface exploration, such as borings, test pits, and trenches.
Table 6.3 summarizes the boring, core drilling, sampling, and other exploratory techniques that can be used by the geotechnical engineer.
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., with a hand auger), although the usual procedure is to use mechanical equipment to excavate the borings.
Many different types of equipment are used to excavate borings. Typical types of borings are listed in Table 6.3 and include:
Auger Boring. A mechanical auger is a very fast method of excavating a boring.
The hole is excavated by rotating the auger while at the same time applying a downward pressure on the auger to help obtain penetration of the soil or rock.
There are basically two types of augers: flight augers and bucket augers. Common available diameters of flight augers are 5 cm to 1.2 m (2 in to 4 ft) and of bucket augers are 0.3 m to 2.4 m (1 ft to 8 ft). The auger is periodically removed
from the hole, and the soil lodged in the groves 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. Augers are probably the most common type of equipment used to excavate borings.
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-Type Borings. Wash-type borings use circulating drilling fluid, which removes cuttings from the borehole. The cuttings are created by the chopping, twisting, and jetting action of the drill bit, which breaks the soil or rock into small fragments. Casings are often used to prevent cave-in of the hole. Because drilling fluid is used during the excavation, it can be difficult to classify the soil and obtain uncontaminated soil samples.
Rotary Coring. This type of boring equipment uses power rotation of the drilling bit as circulating fluid removes cuttings from the hole. Table 6.3 lists various types of rotary coring for soil and rock.
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.
In addition to borings, other methods for performing subsurface exploration include test pits and trenches. Test pits are often square in plan view, with a typical dimension of 1.2 m by 1.2 m (4 ft by 4 ft). Trenches are long and narrow excavations usually made by a backhoe or bulldozer. Table 6.4 presents the uses, capabilities, and limitations of test pits and trenches.
Test pits and trenches provide for a visual observation of subsurface conditions.
They can also be used to obtain undisturbed block samples of soil. The process consists of carving a block of soil from the side or bottom of the test pit or trench.
Soil samples can also be obtained from the test pits or trenches by manually driving Shelby tubes, drive cylinders, or other types of sampling tubes into the ground.
(See Art. 6.2.3.)
Backhoe trenches are an economical means of performing subsurface exploration.
The backhoe can quickly excavate the trench, which can then be used to observe and test the in-situ soil. In many subsurface explorations, backhoe trenches are used to evaluate near-surface and geologic conditions (i.e., up to 15 ft deep), with borings being used to investigate deeper subsurface conditions.
Many different types of samplers are used to retrieve soil and rock specimens from the borings. Common examples are indicated in Table 6.3. Figure 6.1 shows three types of samplers, the ‘‘California Sampler,’’ Shelby tube sampler, and Standard Penetration Test (SPT) sampler.
The most common type of soil sampler used in the United States is the Shelby tube, which is a thin-walled sampling tube. It can be manufactured to different diameters and lengths, with a typical diameter varying from 5 to 7.6 cm (2 to 3 in) and a length of 0.6 to 0.9 m (2 to 3 ft). The Shelby tube should be manufactured to meet exact specifications, such as those stated by ASTM D 1587-94 (1998). The Shelby tube shown in Fig. 6.1 has an inside diameter of 6.35 cm (2.5 in).
Many localities have developed samplers that have proven successful with local soil 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, 2.54 cm (1 in) in height. Figure 6.1 shows the California Sampler in an open condition, with the individual rings exposed. The California Sampler has a 7.6-cm (3.0 in) outside diameter and a 6.35-cm (2.50-in) 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 California.
Three types of soil samples can be recovered from borings:
1. Altered Soil. During the boring operations, soil can be altered due to mixing or contamination. For example, 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.
In other cases, the soil sample may become contaminated with drilling fluid, which is used for wash-type borings. These types of soil samples that have been mixed or contaminated by the drilling process should not be used for laboratory tests because they will lead to incorrect conclusions regarding subsurface conditions.
Soil that has a change in moisture content due to the drilling fluid or heat generated during the drilling operations should also be classified as altered 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.
2. Disturbed Samples. Disturbed soil is defined as soil that has been remolded during the sampling process. For example, soil obtained from driven samplers, such as the Standard Penetration Test 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 numerous types of laboratory tests.
3. Undisturbed Sample. It should be recognized that no soil sample can be taken from the ground in a perfectly undisturbed state. However, 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 thinwalled tubes, having sharp cutting ends and tip relief, into the soil. Two parameters, the inside clearance ratio and the area ratio, are often used to evaluate the disturbance potential of different samplers, and they are defined as follows:
In general, a sampling tube for undisturbed soil specimens should have an inside clearance ratio of about 1% and an area ratio of about 10% or less. Having an inside clearance ratio of about 1% provides for tip relief of the soil and reduces the friction between the soil and inside of the sampling tube during the sampling process. A thin film of oil can be applied at the cutting edge to also reduce the friction between the soil and metal tube during sampling operations. The purpose of having a low area ratio and a sharp cutting end is to slice into the soil with as little disruption and displacement of the soil as possible. Shelby tubes are manufactured to meet these specifications and are considered to be undisturbed soil samplers. As a comparison, the California Sampler has an area ratio of 44% and is considered to be a thick-walled sampler.
It should be mentioned that using a thin-walled tube, such as a Shelby tube, will 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
• Expansion of gas during retrieval of the sampling tube
• 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 above 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 stress condition to an isotropic ‘‘perfect sample’’ stress condition. 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.
Soil samples recovered from the borehole should be kept within the sampling tube or sampling rings. The soil sampling tube should be tightly sealed with end caps or the sampling rings thoroughly sealed in containers to prevent a loss of moisture during transportation to the laboratory. 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 depth.
There are many different types of tests that can be performed at the time of drilling.
The three most common types of field tests are discussed in this section:
Standard Penetration Test (SPT). The Standard Penetration Test (SPT) consists of driving a thick-walled sampler into a sand deposit. The SPT sampler must have an inside barrel diameter (Di) 3.81 cm (1.5 in) and an outside diameter (Do) 5.08 cm (2 in). The SPT sampler is shown in Fig. 6.1. The SPT sampler is driven into the sand by using a 63.5-kg (140-lb.) hammer falling a distance of 0.76 m (30 in). The SPT sampler is driven a total of 45 cm (18 in), with the number of blows recorded for each 15 cm (6 in) interval. The ‘‘measured SPT N value’’ (blows per ft) is defined as the penetration resistance of the sand, which equals the sum of the number of blows required to drive the SPT sampler over the depth interval of 15 to 45 cm (6 to 18 in). The reason the number of blows required to drive the SPT sampler for the first 15 cm (6 in) is not included in the N value is that the drilling process often disturbs the soil at the bottom of the borehole and the readings at 15 to 45 cm (6 to 18 in) are believed to be more representative of the in-situ penetration resistance of the sand. The data below present a correlation between the measured SPT N value (blows per ft) and the density condition of a clean sand deposit.
Relative density is defined in Art. 6.3.4. Note that the above correlation is very approximate and the boundaries between different density conditions are not as distinct as implied by the table.
The measured SPT N value can be influenced by many testing factors and soil conditions. For example, gravel-size particles increase the driving resistance (hence increased N value) by becoming stuck in the SPT sampler tip or barrel. Another factor that could influence the measured SPT N value is groundwater. It is important to maintain a level of water in the borehole at or above the in-situ groundwater level. This is to prevent groundwater from rushing into the bottom of the borehole, which could loosen the sand and result in low measured N values.
Besides gravel and groundwater conditions described above, there are many different testing factors that can influence the accuracy of the SPT readings. For example, the measured SPT N value could be influenced by the hammer efficiency, rate at which the blows are applied, borehole diameter, and rod lengths. The following equation is used to compensate for these testing factors (A. W. Skempton, ‘‘Standard Penetration Test Procedures,’’ Geotechnique 36):
Even with the limitations and all of the corrections that must be applied to the measured SPT N value, the Standard Penetration Test is probably the most widely used field test in the United States. This is because it is relatively easy to use, the test is economical as compared to other types of field testing, and the SPT equipment can be quickly adapted and included as part of almost any type of drilling rig.
Cone Penetration Test (CPT). The idea for the Cone Penetration Test (CPT) is similar to that for the Standard Penetration Test, except that instead of a thickwalled sampler being driven into the soil, a steel cone is pushed into the soil. There are many different types of cone penetration devices, such as the mechanical cone, mechanical-friction cone, electric cone, and piezocone. The simplest type of cone is shown in Fig. 6.2. The cone is first pushed into the soil to the desired depth (initial position) and then a force is applied to the inner rods that moves the cone downward into the extended position. The force required to move the cone into the extended position (Fig. 6.2) divided by the horizontally projected area of the cone is defined as the cone resistance (qc). By continual repetition of the two-step process shown in Fig. 6.2, the cone resistance data is obtained at increments of depth. A continuous record of the cone resistance versus depth can be obtained by using the electric cone, where the cone is pushed into the soil at a rate of 10 to 20 mm/sec (2 to 4 ft /min). Figure 6.3 presents four simplified examples of cone resistance (qc) versus depth profiles and the possible interpretation of the soil types and conditions.
A major advantage of the Cone Penetration Test is that by use of the electric cone, a continuous subsurface record of the cone resistance (qc) can be obtained.
This is in contrast to the Standard Penetration Test, which obtains data at intervals in the soil deposit. Disadvantages of the Cone Penetration Test are that soil samples can not be recovered and special equipment is required to produce a steady and slow penetration of the cone. Unlike the SPT, the ability to obtain a steady and slow penetration of the cone is not included as part of conventional drilling rigs.
Because of these factors, in the United States the CPT is used less frequently than
Vane Shear Test (VST). The SPT and CPT are used to correlate the resistance of driving a sampler (N value) or pushing a cone (qc) with the engineering properties (such as density condition) of the soil. In contrast, the Vane Test is a different insitu field test because it directly measures a specific soil property, the undrained shear strength (su) of clay. Shear strength will be further discussed in Art. 6.3.6.
The Vane Test consists of inserting a four-bladed vane, such as shown in Fig. 6.4, into the borehole and then pushing the vane into the clay deposit located at the bottom of the borehole. Once the vane is inserted into the clay, the maximum torque (Tmax) required to rotate the vane and shear the clay is measured. The undrained shear strength (su) of the clay can then be calculated by using the following equation, which assumes uniform end shear for a rectangular vane:
The vane can provide an undrained shear strength (su) that is too high if the vane is rotated too rapidly. The vane test also gives unreliable results for clay strata that contains sand layers or lenses, varved clay, or if the clay contains gravel or gravelsize shell fragments.
A log is defined as a written record, prepared during the subsurface excavation of borings, test pits, or trenches, that documents the observed conditions. Although logs are often prepared by technicians or even the driller, the most appropriate individuals to log the subsurface conditions are geotechnical engineers or engineering geologists who have considerable experience and judgment acquired by many years of field practice. It is especially important that the subsurface conditions likely to have the most impact on the proposed project be adequately described.
Figure 6.5 presents an example of a boring log.
The final part of Art. 6.2 presents an example of a subsoil profile. As shown in Figure 6.6, the subsoil profile summarizes the results of the subsurface exploration.
The results of field and laboratory tests are often included on the subsoil profile.
The development of a subsoil profile is often a required element for geotechnical and foundation engineering analyses. For example, subsoil profiles are used to de termine the foundation type (shallow versus deep foundation), calculate the amount of settlement of the structure, evaluate the effect of groundwater on the project and develop recommendations for dewatering of underground structures, perform slope stability analyses for projects having sloping topography, and prepare site development recommendations.