SOILS INVESTIGATION
3. A clear, concise report describing the conditions of the ground, soil stratigraphy, soil parameters, and any potential construction problems must be prepared for the client
3.5.1 Soils Exploration Methods
The soils at a site can be explored using one or more of the following methods.
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Geophysical methods—nondestructive techniques used to provide spatial information on soils, rocks, and hydrological and environmental conditions. Popular methods are:1. Ground-penetrating radar (GPR)
GPR, also called georadar, is a high-resolution, high-frequency (10 MHz to 1000 MHz) electromagnetic wave technique for imaging soils and ground structures. An antenna is used to transmit and recover radar pulses generated by a pulse generator. The returned pulse is then processed to produce images of the soil profi le. The key geotechnical uses are soil profi le imaging and location of buried objects. GPR produces continuous-resolution images of the soil profi le with very little soil disturbance. GPR is not suitable for highly conductive (.15 milliohms/m) wet clays and silts. GPR resolution decreases with depth.
2. Seismic surveys
Seismic investigations utilize the fact that surface waves travel with different velocities through different materials. The subsurface interfaces are determined by recording the magnitude and
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travel time of the seismic waves, essentially compression waves (P waves), at a point some distance from the source of the wave. The velocity of propagation is the most important parameter in the application of seismic methods. The densities and elastic properties of the geological materials control the velocity of propagation. When a seismic wave encounters a boundary between two elastic media, the wave energy is transmitted by refl ection, refraction, and diffraction. Seismic refl ection and refraction are used in geotechnical site characterization.
In seismic refl ection tests, the travel times of waves refl ected from subsurface interfaces are measured by geophones. Geophones are motion-sensitive transducers that convert ground motion to electric signals. The travel times are correlated to depth, size, and shape of the interfaces. The angle of refl ection of the waves is a function of the material density contrast. Seismic refl ection is used when high resolution of soil profi le is required, especially at large depths (.50 m).
Seismic refraction surveys are very similar to seismic refl ection surveys except that refraction waves are measured and the source geophone is placed at a greater distance. The latter enables the recording of seismic waves that are primarily horizontal rather than vertical. In most refraction surveys, only the initial P waves are recorded. The seismic refraction method is used to determine the depth and thickness of the soil profi le and the existence of buried structures.
For shallow depths of investigation, the ground surface is pounded by a sledgehammer to generate the seismic waves; for large depths, a small explosive charge is used. Seismic methods are sensitive to noise and vibration. Various fi ltering techniques are used to reduce background noise and vibration. Multichannel analysis of surface waves (MASW) is used to map spatial changes in low-velocity materials. A soil profi le interpreted from MASW is shown in Figure 3.2.
To get information on the stiffnesses of soil layers, crosshole seismic tests are used. The seismic source is located in one borehole and the geophone is located in an adjacent borehole.
The P and S (shear) wave velocities are calculated from the arrival times and the geophone distances. These are then used to calculate the soil stiffnesses.
Downhole seismic tests are used to detect layering and the strength of the layers. The seismic source is located on the surface and geophones are located in a borehole.
FIGURE 3.2 Soil profi le from a multichannel analysis of surface waves from seismic tests.
(Source: Courtesy of Lynn Yuhr, Technos, Inc.)
East
H-1 H-2 H-3 H-4 H-5 H-6 H-7 H-8 H-9H-10 H-11 H-12
West Former sinkhole
0
20
40
Depth (feet)
60
60
Shallow boring
Low-velocity zones possibly indicating soil raveling and/or cavities in weathered limestone
Westing (feet)
Sand Soft
300 500 700 900 1100 1300 1700 1900 2100 2300 2500 2700 2900
Hard Limestone Weathered limestone
Shear-wave velocity (ft/s)
1500ⴙ
ⴙ ⴙ
ⴙⴙ ⴙ ⴙ ⴙ ⴙ
Top of weathered limestone in boring Cavity in boring
80 100 120 140 160 180 200 220 240 260 280 300
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3. Electrical resistivity
Electrical resistivity measurements can be used for identifi cation and quantifi cation of depth of groundwater, detection of clays, and measurement of groundwater conductivity. Soil resistivity, measured in ohm-centimeters (ohm-cm), varies with moisture content and temperature changes.
In general, an increase in soil moisture results in a reduction in soil resistivity. The pore fl uid provides the only electrical path in sands, while both the pore fl uid and the surface charged particles provide electrical paths in clays. Resistivities of wet fi ne-grained soils are generally much lower than those of wet coarse-grained soils. The difference in resistivity between a soil in a dry and in a saturated condition may be several orders of magnitude.
The method of measuring subsurface resistivity involves placing four electrodes in the ground in a line at equal spacing, applying a measured AC current to the outer two electrodes, and measur- ing the AC voltage between the inner two electrodes. A measured resistance is calculated by divid- ing the measured voltage by the measured current. This resistance is then multiplied by a geometric factor that includes the spacing between each electrode to determine the apparent resistivity.
Electrode spacings of 0.75, 1.5, 3.0, 6.0, and 12.0 m are typically used for shallow depths (,10 m) of investigations. Greater electrode spacings of 1.5, 3.0, 6.0, 15.0, 30.0, 100.0, and 150.0 m are typically used for deeper investigations. The depth of investigation is typically less than the maximum electrode spacing. Water is introduced to the electrode holes as the electrodes are driven into the ground to improve electrical contact. A subsurface resistivity profi le is typically performed by making successive measurements at several electrode spacings at one location. A soil profi le from resistivity measurements is shown in Figure 3.3.
4. Other geophysical methods of geotechnical engineering interests
(a) Gamma density, or gamma-gamma, measures electron density and can be used to estimate the total soil density or porosity.
(b) Neutron porosity measures hydrogen density. It is used for porosity estimation below the groundwater level.
(c) Sonic-VDL measures the seismic velocity. It is useful to measure soil stiffnesses and to detect bedrock elevation.
(d) Microgravity is used to detect changes in subsurface densities and is particularly good at detect- ing cavities. A gravimeter is used at discrete points on the earth’s surface to detect small changes in gravity. These changes are called gravity anomalies and are related to density changes.
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FIGURE 3.3 Soil profi le from electrical resistivity tests. (Courtesy of Lynn Yuhr, Technos, Inc.)
630 620 610 600 590 580 570
0 100 200 300 400
Distance in feet
Elevation (feet)
500 600 700 800
1000 900 800 700 600 500 400 300 200 100 50 0
More competent rock Low moisture content
Weathered rock Resistivity (ohm-meters)
Clay Highly weathered rock Higher moisture content
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Trial pits or test pits. A pit is dug by hand using shovels or with a machine such as a backhoe.This method can provide excellent shallow-depth soil stratigraphy.
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Hand or power augers. These are tools used to quickly create a hole about 100 mm to 250 mm in diameter in the ground. You can inspect the soil and take undisturbed samples for lab tests.•
Wash boring. Water is pumped though a hollow rod that may or may not be equipped with a drill bit to remove soil from a borehole. The washings can be used to estimate the soil types.•
Rotary rigs. These are mechanical devices used to drill boreholes, extract soil samples, and facili- tate in situ tests.The advantages and disadvantages of each of these methods are shown in Table 3.1.