Introduction
Accurate and complete subsurface information is necessary for all types of civil engineering projects, for without this information it is not possible to arrive at a rational design for structure and proper construction procedures. Structures have failed because of inadequate or misleading subsurface data, and many so called successful structures could probably have been completed at much less cost because proper consideration have been given to obtaining more complete subsurface information.
Boring and Sampling Methods
Auguring is a simple method of putting down holes a few inches in diameter to depths up to 20ft in soft sediments. Trial pits and trenches in addition to providing samples of the deposits excavated, allow inspection of the rocks in the walls and floor over an area. Boreholes allow much greater depths to be penetrated. Boring methods are broadly of two kinds:
1. Percussion Drilling
In percussion drilling the bit is suspended from the rods (or a cable) and is jumped up and down so that the rock is broken up by repeated rows. Water is added to the hole to keep the bit cool and make slurry and debris removed by means of a bailer. The pounded rock mixed with water from slurry, from which chips may be recovered for identification. The rate of progress of drilling, and the cost varies according to the hardness encountered.
2. Rotary Drilling
Rotary drilling methods include mud rotary drilling and core drilling. In the former the bit is rotated, and is attached to hollow drilling rods through which a fluid mud is pumped continuously. The mud is returned to the surface through the annular space between the rods and the hole, bringing with it small fragments of rock which can be screened out and examined. The rods are usually in 20 ft lengths, and successively added to the assembly as the hole is lowered.
3. Core Drilling
Core drilling uses a tabular bit with a lower cutting edge, which is rotated in the hole; many forms of bits are available, some containing diamonds and other hard abrasives for penetrating hard rocks. Diamond drills with a diameter of 2 to 4 inches are commonly used, for sampling, or in exploratory bore from underground workings and holes can be drilled at any angle.
As the bit is rotated, a core is cut out and enters a barrel mounted above the bit. The length of the barrel controls the length of the core which can be obtained at any time. The core is recovered by drawing the drilling rod and barrel. The cost of core drilling is greater than for any other method and increase greatly with increasing depth of the hole; but the recovery of the core is a great advantage, as it yields information from great depth and is a sample of rocks drilled through.
4. Core Barrels
The aim of structural drilling is to recover undisturbed core upon which measurements of structural features can be made. This can be achieved either by the multiple-tube core barrels or by the use of large diameter barrels.
In the multiple-tube core barrel, the inner tube or tubes are mounted on a bearing so that they remain stationary, while the outer barrel, which carries the diamond bit, rotates. The core, cut out by bit, is transferred into the non-rotating inner barrel where it remains undisturbed until the barrel is removed from the hole. Removing the core from the barrel is the most critical part of the operation. The most satisfactory system is to use a split inner barrel which is removed from the assembly with the core inside it and then split to reveal undisturbed core.
5. Geophysical Methods
Geophysical methods allow subsurface features to be located, mapped, and characterized by making measurements at the surface that respond to a physical, electrical or chemical property. These non-invasive measurements can be effectively used to provide reconnaissance to detailed geological information guide, subsurface sampling and excavation, and provide continuous monitoring.
If all sites were simple (horizontally stratified geology with uniform properties), site characterization would be easy. Data from just one boring would be sufficient to characterize the site. However in most situations, this will not be the case. Even at sites where geology appears to be uniform, one must be alert to often-subtle variations that can cause significant changes in structural or hydrological properties.
Traditional approaches to subsurface field investigations commonly rely only on the use of direct sampling methods such as boring of rock and soil samples, monitoring wells for gathering hydro geologic data points. Soil and rock sample programs and the placement of boring and wells are done mainly by educated guess work. Numerous pitfalls are associated with this approach that can result in an incomplete or even erroneous understanding of site conditions. These oversights are the cause of many structural and environmental failures.
In many cases direct sampling alone is not sufficient to accurately characterize site conditions. This is the primary reason for the application of surface geophysical methods. Surface geophysical measurements can be made relatively quickly; they provide means to significantly increase data density. Because of the greater sample density, anomalous conditions are more likely to be detected, resulting in an accurate characterization of subsurface conditions.
5.1. Application
There are four major areas where surface geophysical methods may be applied to environmental and engineering problems: 1) Assessment of natural geologic and hydro geologic conditions. 2) Detection and mapping of contaminant plumes, spills and leaks. 3) Detection and mapping of landfills, trenches or other underground structures and utilities. 4) Evaluation of soil and rock properties and man-made structures.
The following is a summary of various geophysical methods:
6. Seismic Methods
Seismic measurements involve the measurement of seismic waves travelling through the subsurface. Stratigraphy, structure, and material properties can be assessed with seismic methods. All applications of seismic methods are based on the fact that the elastic properties of soils and rocks determine the velocities of wave propagation through them. The higher the elastic modulus, for example, higher the velocity is. In rocks the dominant factors that influence the velocity are the crystallinity and porosity. Rocks having crystalline textures and low porosity have higher shock wave velocities just as they have high elastic moduli and compressive strengths.
Shock waves in earth materials follow multiple paths from source to receiver. In the near surface, waves take a direct path from source to receiver and the measurements of elapsed travel time for measured distance results in the wave velocity through that material. Waves moving downward into the earth may be refracted and reflected at velocity interfaces. The ray-path diagram is a convenient method of showing wave propagation by direct, refracted and reflected paths. Unless otherwise noted, all velocities refer to compressional waves.
Seismic data are obtained by recording shock-wave travel time between a source and a receiver, or geophone, for various chosen distances. Two general types of seismographs are used in engineering applications. One type permits the simultaneous, multichannel recording of shock-wave arrivals at a number of geophone locations from a single energy source such as a buried explosive charge or a weight-dropping system. The output from the geophones may be recorded in analog and digital forms in a variety of ways, all having a time base to permit extraction of elapsed travel times to each geophone location. Two advantages of multi-channel recording are; (1) single energy source for all geophone channels, and (2) more sophisticated filtering, recording, data processing, and printout capabilities than simpler single-channel seismographs. Disadvantages include higher firs cost, greater operating cost, large size, and often the need for greater peripheral support such as computer hardware and software.
The second type of seismographs is a single-channel instrument that records shock-wave travel time from a source to a single geophone location. As a result, the operation must be repeated for different geophone distances until a suitable number of travel times have been obtained. Single-channel seismographs may record geophone output in several ways such as on an oscilloscope; on a chart, as in multichannel unit; or only as a first-arrival pulse time digitally (or by some other means). Timing systems are inherently a part of each system. Single e-channel seismographs are low initial cost, small size and provision for tailoring of geophone recording distances to the given site as work progresses. Disadvantages include time require for obtaining data, lack of single energy source for the entire geophone spread, and usually a restriction of use to refraction seismic surveys.
6.1. Seismic Refraction
It is a method used to determine the P-wave velocity structure of the subsurface. Seismic P-waves are generated on the surface, propagate through the soil and rock, and are recorded by geo-phones at known distances from the source. When seismic waves encounter interfaces separating materials of different seismic velocities, the waves are refracted according to Snell’s Law. At the critical angle for each interface (energy refracted 90 degrees) the seismic wave will travel along the interface with velocity of underlaying layer. A seismograph is used to record the travel times of these first arrivals, after which seismic velocities can be derived. Depths to the refracting layers can also be determined.
Primary applications include determination of depth to bed rock and thickness of geologic strata, rippability and dredge-ability, and in-situ elastic modulus of soil and rock.
It can provide data to depth of 100 ft or more and resolves up to 2 to 3 layers. It also provides a 2D cross-section of P-wave velocity. The source of seismic energy can be as simple as 8-pound sledge hammer. Deep measurements may require explosive as an energy source.If a velocity interface is not parallel with the surface, the velocities recorded at the surface are apparent rather than true velocities. The velocities recorded or plotted will be less than or greater than true velocity when energy is travelling down-dip or up-dip, respectively. The shallower down-dip end will exhibit a lower intercept time and critical distance compared to the deeper or up-dip end.
6.2. Seismic Reflection
Seismic reflection measures the travel time of seismic waves from the surface downwards to geologic contacts where part of the seismic energy is reflected back to hydrophones at the surface. A reflection will occur from geologic strata when the reflection coefficient (derived from density and seismic velocity contrasts) between strata is sufficient. After raw data are processed, a cross-sectional picture of subsurface strata and anomalous conditions can be developed.
Primary application is for determination of depth and thickness of geologic strata, structural and anomalous condition. The depth ranges from as shallow as 30 ft to greater than 100 ft.
6.3. Down-hole seismic surveys
These are the simplest and cheapest method as they required only a single borehole. Seismic energy is generated on surface at a fixed distance from the top of the borehole. The travel times of the first-arrival seismic waves measured at regular intervals down the hole using string of hydrophones or in case of S-wave surveys, a single clamped tri-axial geophone that is gradually moved down the hole. P and S wave arrival time for each receiver location are combined to produce travel time versus depth curves for the complete hole. These are then used to produce total velocity profiles from which interval velocities and various elastic moduli can be calculated (in conjunction with density data from geophysical logging of borehole).
6.4. Cross-hole Seismic Surveys
This involves measurement of the travel time of seismic energy transmitted between two or more boreholes in order to derive information on the elastic properties of intervening materials. One hole is used to deploy the source while the other hole(s) are used to detect the arrival of the seismic energy. The travel time of the seismic waves are derived from the first arrivals identified on the seismic trace for each short-receiver position and are used with the known distances between the short/receiver boreholes to calculate the apparent velocities (P & S) for each depth interval. This data is then used to derive a vertical profile of the various elastic moduli. The relationship between the velocity of seismic waves and the density and elastic properties of the materials through which they are travelling means that seismic techniques can be utilized to provide information on various geotechnical properties of the subsurface, such as Poisson’s ration and shear modulus. The most common method of measuring these properties in engineering studies through the use of cross-hole seismic surveys.
6.5. Cross-hole Seismic Tomography
Borehole seismic tomography involves the measurement of the travel times of seismic ray paths between two or more boreholes in order to derive an image of seismic velocity in the intervening ground. Data is collected using one hole for the seismic source (normally a speaker) and measuring one arrival time using strings of hydrophones in the other. Travel times are collected at regular intervals (usually 0.5 m to 2 m) all the way down the hole(s) for each short position. This results in a network of overlapping ray paths that can be used to model the velocity profile. The resulting velocity image is termed a tomogram and enables identification of anomalous velocity zones laying between the boreholes as well as imaging individual velocity layers.
The primary applications of borehole seismic tomography is in engineering studies for the identification of features such as fault zones and voids when combined with S-wave survey, the data can additionally be used to provide information in material stiffness properties.
7. Electrical Resistivity
Electrical resistivity measurements are made by placing four electrodes in contact with soil or rock. A current is caused to flow in the earth between on pair of electrodes while the voltage across the other pair of electrodes is measured. The depth measurement is related to the electrode spacing. The resistivity measurement represents the apparent resistivity averaged over a volume of the earth determined by the soil, rock, and pore fluid resistivity, along with the electrode geometry and spacing.
In contrast to wave velocities in refraction seismology, resistivity values are not representative of specific physical properties of earth materials no do they remain constant over time. The flow of current through soil and rock is by ion conduction, which is dependent on a combination of the conductivity of the fluid present, porosity, and percentage of saturation. Dissolved salts in water provide for ion conductance of electrical current. The conductance that is the reciprocal of resistance is directly proportional to amount of dissolved salt in water, or salinity. The amount of fluid regardless of its salinity that can be present is controlled by the porosity of material. The more interconnected the pore spaces, the greater the ease of ion migration through the material. In addition, the degree of saturation that varies with season, in turn affects conductance (or in the context of this discussion resistivity). Seasonal fluctuations in resistivity of as much as 200% have been reported.
The rock forming minerals normally are highly resistive to current flow. An exception, which complicates resistivity work, is the presence of clay minerals. The exchangeable ions in the clays may separate from the lattice and make the pore water conductive even though the formation water may not be saline. As a result, clays have low resistivity – whether occurring as clay-rich soils or as shales.
If we are certain that the groundwater in an area is fresh, low resistivity is representative of clay. Conversely, freshwater (a poor conductor) will cause high resistivity when present in the poor spaces of a clean or clay-free soil or in the pores or joints of a porous or dense, relatively clay-free rock. Note, however, that there is nothing distinctive about the kind of material that has high or low resistivity values, as is the case with seismic velocities. For instance, it would be possible on the basis of high resistivity to drill expecting to encounter porous, freshwater-bearing sand and instead encounter tight sandstone. Also saline pore water in sand or porous or highly fractured rock gives low resistivity values that are also indicative of clay or shale.
Because of these perplexing problems, there is the need for subsurface control of materials and thickness from either exposures or boreholes.
8. Magnetic
There are two primary applications for magnetic measurements; 1) locating and mapping buried ferrous metals, and 2) mapping geologic structures. The presence of buried ferrous metals creates a local variation in the strength of the earth’s magnetic field permitting the detection and mapping of buried ferrous metal. Total field measurements made with one magnetometer, and gradient measurements made with two magnetometers are commonly used. Magnetic gradient measurements are made by a gradiometer, which is simple two magnetic sensors separated by a constant offset.
Magnetic measurements can be used for geologic mapping by responding to the magnetic susceptibility of soil and rock. Generally total field measurements are used for geologic mapping. The primary application is for mineral exploration and for characterizing geologic structures such as faults.
9. Microgravity
A microgravity survey provides a measure of change in subsurface density. Natural variations in subsurface density include lateral changes in soil or rock density, buried channels, large fractures, faults, dissolution-enlarged joints and cavities. A microgravity survey consists of making sensitive gravity measurements at the micro Gal (µGal) level (1/1000 of a milliGal or 10E-9 of the earth’s gravitational field) with a gravimeter. Gravity measurements are acquired at discrete points along a profile line or within a grid, and are corrected for instrument drift, tidal effects, elevation changes, and latitude. Gravity anomalies are directly related to lateral variations in subsurface density.
This method is used to identify caves, voids, skin-holes and weathered zones. It is also used to map top of rock. It can also be used to identify man-made structures such as tunnels and mines.
10. Ground Penetrating Radar
Ground penetrating radar (GPR) uses high frequency electro-magnetic waves to acquire subsurface information. Energy is radiated downward into the ground from a transmitter and is reflected back to receiving antenna. The reflected signals are recorded and produce a continuous cross-sectional profile of shallow subsurface conditions. Reflections of the radar wave occur where there is a change in the di-electric constant or electric conductivity between two materials. These changes are associated with natural hydro-geologic conditions such as bedding, cementation, moisture, clay content, voids and fractures. Large changes in dielectric properties often exist between geologic materials and man-made structures such as buried utilities or tanks. This technique is also used for the location of rebar in concrete nondestructive testing of man-made structures. Ground penetrating radar is also used to map void space behind concrete tunnel lining.
