(after Webber & Gowans, 1996).
Borehole viewing and photography equipment include borescopes, photographic cameras, TV cameras, sonic imagery loggers, caliper loggers, and alignment survey devices.
Borehole Periscopes can be used in small holes, but due to distortion of the optical path the depth is usually limited to about 30 m (Goodman, 1976; ISRM, 1978).
Borehole cameras can be used to take photos of the borehole wall and the orientation and spacing data can be obtained by interpreting the photos (Goodman, 1976; ISRM, 1978).
Closed Circuit Television (CCTV) provides a means of directly inspecting a borehole wall and if the direction of view of the camera can be orientated, it is possible to determine discontinuity orientation and spacing. In addition aperture, infilling and water seepage may also be assessed. Successful CCTV surveys can be conducted in either dry or water-filled holes. For best results in either case the borehole wall should be clean and stable. If the borehole is full of water then measures should be taken to ensure that the water is clear enough to give a side view image of the borehole wall (Gunning, 1992; McMillan et al., 1996).
Acoustic Televiewer and Dipmeter offer great potential for analyzing discontinuities.
Advances in technology and digital instrumentation mean that such methods can provide effective data acquisition systems. Acoustic televiewers only operate in fluid filled boreholes (Gunning, 1992).
5.3.3 Large-diameter borings
Large-diameter borings, 2 feet or more in diameter (e.g., the borings of large drilled shafts), permit direct examination of the sidewalls and bottoms of the boring and provides access for obtaining high-quality undisturbed samples. Direct inspection of the sidewalls and bottoms may reveal details, such as thin weak layers or old shear planes, that may not be detected by continuous undisturbed sampling. However, direct measurements may not always be possible because of water in the borehole and concerns for the safety of personnel.
It is very often that no outcrop can be used at a site to obtain trace lengths of discontinuities. In this case, trace length data can be obtained from the sidewalls and bottoms of large-diameter borings such as the borings of large drilled shafts. If only several traces or even no trace is present at a bottom, one can use bottoms at different depths during the drilling process to collect the trace length data and then use the entire data set (see Fig. 5.6).
5.4 EXPLORATORY EXCAVATIONS
Test pits, test trenches, and exploratory tunnels provide access for larger-scaled observations of rock mass conditions, for determining top of rock profile in highly weathered rock/soil interfaces, and for some in situ tests which cannot be executed in a smaller borehole.
5.4.1 Test pits and trenches
In weak or highly fractured rock, test pits and trenches can be constructed quickly and economically by surface-type excavation equipment. Final excavation to grade where samples are to be obtained or in situ tests performed must be done carefully. Test pits and trenches are generally used only above the groundwater level. Exploratory trench excavations are often used in fault evaluation studies. An extension of a bedrock fault into much younger overburden materials exposed by trenching is usually considered proof of recent fault activity.
Fig. 5.6 Sampling at the bottom of the hole, during the drilling process at different depths, to obtain trace length data (L is the depth of the drilled shaft).
5.4.2 Exploratory tunnels
Exploratory tunnels/adits permit detailed examination of the composition and geometry of rock structures such as joints, fractures, faults, shear zones, and solution channels.
They are commonly used to explore conditions at the locations of large underground excavations and the foundations and abutments of large dam projects. They are particularly appropriate in defining the extent of marginal strength rock or adverse rock structure suspected from surface mapping and boring information. For major projects where high-intensity loads will be transmitted to foundations or abutments, tunnels/adits afford the only practical means for testing in-place rock at locations and in directions corresponding to the structure loading. The detailed geology of exploratory tunnels, regardless of their purpose, should be mapped carefully. The cost of obtaining an accurate and reliable geologic map of a tunnel is usually insignificant compared with the
cost of the tunnel. The geologic information gained from such mapping provides a very useful additional dimension to interpretations of rock structure deduced from other sources. A complete picture of the site geology can be achieved only when the geologic data and interpretations from surface mapping, borings, and pilot tunnels are combined and well correlated. When exploratory tunnels are strategically located, they can often be incorporated into the permanent structure. Exploratory tunnels can be used for drainage and post-construction observations to determine seepage quantities and to confirm certain design assumptions. On some projects, exploratory tunnels may be used for permanent access or for utility conduits.
5.5 GEOPHYSICAL EXPLORATIONS
5.5.1 General description
Geophysical techniques consist of making indirect measurements on the ground surface, or in boreholes, to obtain generalized subsurface information. Geologic information is obtained through analysis or interpretation of these measurements. Boreholes or other subsurface explorations are needed for reference and control when geophysical methods are used. Geophysical explorations are of greatest value when performed early in the field exploration program in combination with limited subsurface explorations. Geophysical explorations are appropriate for a rapid, though approximate, location and correlation of geologic features such as stratigraphy, lithology, discontinuities, ground water, and for the in situ measurement of dynamic elastic moduli and densities. The cost of geophysical explorations is generally low compared with the cost of core borings or test pits, and considerable savings may be realized by judicious use of these methods.
Geophysical methods can be classified as active or passive techniques. Active techniques impart some energy or effect into the earth and measure the earth materials’
response. Passive measurements record the strengths of various natural fields which are continuous in existence. Active techniques generally produce more accurate results or more detailed solutions due to the ability to control the size and location of the active source.
5.5.2 Seismic methods
Seismic methods are the most commonly conducted geophysical surveys for engineering investigations. Seismic surveys measure the relative arrival times, and thus the velocity of seismic waves traveling between an energy source and a number of geophones. The energy source may be a hammer blow, an explosion of a propaneoxygen mixture in a heavy chamber (gas-gun), or a light explosive charge. There are two major classes of seismic waves: body waves which pass through the volume of a material and surface waves that exist only near a boundary. The body waves consist of the compressional or pressure or primary wave (P-wave) and the secondary or transverse or shear wave (S-wave). P-waves travel through all media that support seismic waves. P-waves in fluids, e.g. water and air, are commonly referred to as acoustic waves. S-waves travel slower than P-waves and can only transit material that has shear strength. S-waves do not exist in
liquids and gases, as these media have no shear strength. The velocities of the P- and S- waves are related to the elastic properties and density of a medium by the following equations: S-wave; G is the shear modulus; ρ is the density; and E is the Young’s or elastic modulus. It should be noted that these are not independent equations. Knowing two velocities uniquely determines only two unknowns of ρ, ν and E. Shear modulus is dependent on two other values. Usually the possible range of ρ is approximated and ν is estimated. The typical density values of intact rocks have been presented in Table 2.7. Table 5.1 provides some typical values of Vp and ν. The velocity of the S-wave in most rocks is about half the velocity of the P-wave.
Surface waves are produced by surface impacts, explosions and wave form changes at boundaries. One of the surface waves is the Rayleigh wave which travels about 10%
slower than the S-wave. The Rayleigh wave exhibits vertical and horizontal displacement in the vertical plane of the ray path. A point in the path of a Rayleigh wave moves back, down, forward, and up repetitively in an ellipse like ocean waves.
The equipment used for seismic surveys includes the following components:
1) Seismic sources. The seismic source may be a hammer repetitively striking an aluminum plate or weighted plank, drop weights of varying sizes, a rifle shot, a harmonic oscillator, waterborne mechanisms, or explosives. The energy disturbance for seismic work is most often called the “shot,” an archaic term