Site selection4.1
Investigations for foundations follow the usual procedure for geotechnical projects comprising a staged program with the objective of progressively refining the information required for final design.
Typically, the four stages of a complete investigation are as follows:
1. Reconnaissance—examination of published geological maps and reports, study of air photographs, gathering of local experience on foundation performance, field visits;
2. site selection—test pits, outcrop mapping, geophysics, index tests of rock properties, limited diamond drilling at alternative sites;
3. preliminary site investigation—diamond drilling of selected site, detailed mapping of out-crops and exploration adits, laboratory testing;
4. detailed investigations—drilling of selected geological features critical to foundation performance, in situ testing, laboratory testing.
A distinguishing feature of investigations for rock foundations is that it is particularly important to focus on the details of the structural geology. For example, the.orientation of one clay-filled discontinuity can make the difference between stability and instability, or a compressible seam may cause settlement of the structure. This condition means that it is usually necessary to carry out a drilling program to investigate sub-surface conditions, and in some cases drive exploration
adits to examine in situ conditions. Figure 4.1 shows a diamond drill on a platform on a steep cliff investigating the foundations for a bridge abutment.
However, drilling may not be required in circumstances where the applied loads are significantly less than the bearing capacity of the rock, where there is no possibility of a sliding type failure, or where there are extensive outcrops and the sub-surface conditions can be confidently established by interpretation.
This chapter describes investigation methods for rock foundations, with emphasis on in situ testing methods and detailed structural geology studies. In situ testing is one of the particular features of investigation programs for major structures founded on rock because of the difficulty in sampling and testing large samples representative of the rock mass. Samples that are representative of both the intact rock and the discontinuities may be as large as 1 m (3 ft) in diameter. Samples this large are very difficult to recover undisturbed, and the required testing equipment would have to exert extremely high forces even to deform the rock mass.
At the early stages of most projects there may be some choice available in the site of the structure.
Under these circumstances, one of the first tasks in the geotechnical program is to evaluate alternative sites and to recommend which site is the most favorable. In this reconnaissance stage of the project the objective of the investigation would be to concentrate on large scale geological features that would influence the overall stability of the structure. These features include landslides, contacts between rock types with significantly
different engineering properties, fault zones and persistent sets of discontinuities sets that dip out of any face in which a steep cut is to be made.
Geological information of this nature would form part of the input for the overall site selection study that would include alignment studies in the case of bridges on transportation routes. In the case of dam projects, the site selection should take into consideration the foundations of the dam itself, the spillway and the powerhouse. Geological conditions that could justify moving a structure would be a very significant hazard such as a major landslide, movement of which could destroy the structure, or karstic terrain which contains substantial cavities.
For other geological features such as faults or continuous bedding planes that would only cause local instability, remedial measures such as rock reinforcement could be carried out during construction (see Chapter 10).
The following is a discussion on some of the
reconnaissance techniques that may be used early in a project, mainly for the purpose of site selection. It is very rare that the information gathered at this stage of a project would be adequate for use in final design, so these studies would have to be followed by more detailed investigations such as surface mapping and drilling.
4.1.1
Aerial and terrestrial photography
The study of stereo pairs of vertical aerial photographs and oblique terrestrial photographs provides much useful information on the larger scale geological conditions at a site (Peterson et al., 1982). Often these large features will be difficult to identify in surface mapping because they are obscured by vegetation, rock falls or more closely spaced discontinuities. Photographs most commonly used in geotechnical engineering are Figure 4.1 Photograph of diamond drill investigating rock conditions for bridge abutment (photograph by Tony Rice).
black and white, vertical photographs taken at heights of between 500 and 3000 m (1500 and 10 000 ft) with scales ranging from 1:10 000 to 1:30 000. On some projects it is necessary to have both high and low level photographs, with the high level photographs being used to identify landslides, for example, while the low level photographs provide more detailed information on geological structure.
One of the most important uses in foundation engineering of aerial photographs is the identification of landslides which have the potential for causing movement, or even destruction of facilities on which they are constructed. Landslides features that are often readily apparent on vertical aerial photographs are tension cracks and scarps along the crest of the slide, hummocky terrain in the body of the slide and areas of fresh disturbance in the toe, including sudden changes in river directions. Figure 4.2 shows a landslide area in the side of a steep glacial valley in the Coastal Range of western Canada. Area a, which is a talus slope, is an ancient slide, while in area b, which is a potential slide of similar proportions, there are a number of tension cracks with widths up to 15 m (50 ft). The cause of these slides are sets of orthogonal joints, one of which dips out of the valley wall at an angle of about 50°, and a second vertical set striking at right angles to the valley that forms side release surfaces. By comparing photographs taken over a number of years it may be possible to determine the rate of movement of a slide, and whether it is growing in size.
Related to landslides are debris flows which occur in mountainous terrain with high precipitation levels such as occurs on the north west coast of North America, in Japan, the Alps and Himalayas.
Potential sites of debris flows are evident on aerial photographs as areas of erosion in steep banks in the upper reaches of the creeks, as well as fans of accumulated debris at the toe of the slope.
Debris flows are highly fluid mixtures of water, solid particles and organic matter. This mixture has a consistency of wet concrete and consists of about 70–80% water, and solid material ranging from clay and silt sizes up to boulders several meters in
diameter. The organic matter can include bark mulch as well as large trees and logs swept from the sides of the channel. Debris flows usually occur during periods of intense rainfall or rapid snow melt and a possible triggering event can be the failure of a temporary dam, formed by slope failure or a log-jam, that releases a surge of water and solid material. Where such flows originate in streams with gradients steeper than about 20°– 30°, they move at velocities of approximately 3–5 m/s (10–16 ft/s), with pulses as great as 30 m/s (100 ft/s). At this speed, material is scoured from the base and sides of the channel so the volume of the flow increases as it descends. This combination of high density and high velocity can cause devastation to any structure built in their path. Bridges constructed over creeks which are susceptible to debris flows must be adequately sized to accommodate the likely flow volume, and footings should not be located in the creek bed unless they are designed to withstand the considerable impact loads (Skermer, 1984;
VanDine and Lister, 1983).
Other features that may be evident on aerial photographs are major geological structures such as faults, bedding planes and continuous joint sets. The photographs will give some information on the position, length and continuity of these features.
However, to establish the orientation (dip and dip direction) of a discontinuity, it is necessary to fix the positions of a minimum of three, and preferably four, points on the same surface. This technique has been used predominately with terrestrial photographs where individual discontinuities with large exposures can be clearly identified; even on low level aerial photographs it is rare to be able to see exposures of a single discontinuity surface, except perhaps in the case of a fault scarp. Structural mapping from aerial photographs is normally only carried out when there is no access to the face; direct surface mapping which allows the characteristics of each discontinuity to be examined in detail is preferable.
Other information that can be obtained from aerial photographs is the location of gravel deposits, rock outcrops and the study of river hydraulics for siting
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dams and bridges.
4.1.2 Geophysics
Geophysical methods are often used in the preliminary stages of a site investigation to provide such information as the depth of weathering, the bedrock profile, contacts between rock types of significantly different density, the location of major
faults and solution cavities, and the degree of fracturing of the rock (Griffiths and King, 1988).
The results obtained from geophysical measurements are usually not sufficiently accurate to be used in final design and they should always be calibrated by putting down a number of test pits or drill holes to spot check actual properties and contact elevations. However, geophysical surveys provide a continuous profile of subsurface condi tions and this information can be used as a Figure 4.2 Vertical aerial photograph stereo pair showing typical features of a major rock slide in a glaciated valley: (a) slide scarp; (b) slide debris; (c) tension cracks; (d) valley floor; and (e) talus slope.
fill-in between drill holes. Most geophysical investigations for engineering purposes consist of seismic, resistivity or ground penetrating radar surveys carried out on the ground surface as described in this section. Downhole techniques are also available to measure the properties of materials in the walls of the drill hole, or alternatively between adjacent holes. Downhole geophysics may be used in percussion drill holes, which are less expensive and faster to drill than diamond drill holes, as part of the preliminary investigation of a site.
(a) Seismic surveys
The primary purposes of seismic surveys are to determine the approximate location and density of layers of soil and rock, a well defined water table, or the degree of fracturing, porosity and saturation of the rock. Seismic velocity can also be related to the rippability of the rock mass (see Section 10.5.3).
The seismic method is effective to depths in the range of tens of meters to a maximum of a few hundred meters. Discontinuities within the rock such as joints and shears will not be detected by seismic methods unless there is shear displacement and a distinct elevation change of a layer with a particular density as a result of fault movement.
However, continuous overwater seismic profiling using a repeating shock source called a ‘sparker’
may recognize discontinuity zones.
Seismic surveys measure the relative arrival times, and thus the velocity of propagation of elastic waves traveling between a shallow energy source and a number of transducers set out in a straight line along the required profile. The energy source may be a hammer blow, an explosion of a propane-oxygen mixture in a heavy chamber (gasgun), or a light explosive charge. In elastically homogeneous ground subject to a sudden stress near its surface, three elastic pulses travel outward at different speeds. Two are body waves that are propagated as spherical fronts affected to only a minor extent by the free surface of the ground, and the third is a surface wave which is confined to the region near the surface, its amplitude falling off rapidly with depth. The two body waves, namely the primary or
‘P’ wave and the secondary or ‘S’ wave, differ in both their direction of motion and speed. The P wave is a longitudinal compressive wave in the direction of propagation, while the S wave induces shear stresses in the medium. The velocities of the primary (Vp) and secondary (Vs) waves are related to the elastic constants and density of the medium by the equations
(4.1)
(4.2) where K is the bulk modulus, G is the shear modulus and ? is the density. The velocity of the S wave in most rocks is about one half that of the velocity of the P wave. The S wave is not propagated at all in fluids. The value of the ratio Vp/ Vs depends only on the Poisson’s ratio of the medium. Figure 4.3 shows typical P wave velocities for a range of different materials.
The surface wave, which travels about 10% slower than the S waves, causes a surface disturbance in homogeneous ground called the Raleigh wave. The Raleigh wave has both vertical and horizontal components, with the horizontal motion being of rather smaller amplitude than the vertical, and 90°
out of phase with it. The resultant path of an element of the medium during passage of a Raleigh wave cycle follows an ellipse lying on the plane of propagation. The magnitude of the ground motion becomes negligibly small within a distance below the free surface of the same order of magnitude as the wavelength of the disturbance.
The amplitude of the waves decreases with distance from their source as a result of spreading of the wave energy over the increasing wave front area. Earth materials are imperfectly elastic leading to energy loss and attenuation of the seismic waves that is greater than would be expected from geometric spreading alone. This reduction in amplitude is more pronounced for less consolidated rocks. Also, the reduction in amplitude is greater for higher frequencies resulting in selective loss of the higher
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frequencies as the pulse propagates.
The sonic velocity of the elastic wave will be greater in higher density material, and in more massive rock compared with closely fractured rock.
Where a layer of denser material underlies a less dense layer, such as soil overlying bedrock, then the elastic wave velocity will be greater in the bedrock and the contact between the layers will act as a refracting surface. In a specific range of distances from the shot point, the times of first arrival at different distances from the shot point will represent waves traveling along this surface. This information can be used to plot the profile of the contact between the two layers.
(b) Resistivity surveys
At locations where the rock types have similar densities and seismic surveys would be ineffective,
resistivity surveys can provide information on variations in the geological structure and material type. Since most rocks are themselves nonconductive, the electrical resistivity of a rock derives mainly from the salinity of the ground water occupying pores and discontinuities. Accordingly, rock formations will differ in resistivity because of porosity and jointing differences, with the resistivity decreasing with greater discontinuity frequency. For example, in faults and shears the water content may be higher than the country rock and anomalously low resistivity will be measured. Conversely, in porous country rock, a discontinuity may act as a drain and appear as an anomaly of high resistivity (Stahl, 1973).
Resistivity surveys may also be used to detect such structures as clay-filled sinkholes in limestone Figure 4.3 Approximate ranges of P wave velocities Vp for some common geological materials (Griffiths and King, 1988).
because the clay will tend to have a relatively low resistivity compared with the surrounding rock and will show up as an anomaly. The conductivity of clay takes place by way of weakly bonded surface ions whereas rocks are themselves nonconducting.
In general, the resistivities of formations vary widely not only from formation to formation but also within a particular deposit, this being particularly true for near surface unconsolidated materials (Griffiths and King, 1988). Consequently there is no precise correlation of lithology with resistivity and it is preferable that the results of resistivity surveys be calibrated with boreholes or test pits.
(c) Ground penetrating radar
Ground penetrating radar (GPR) is a technique for mapping bedrock depth, changes in rock type, discontinuities in bedrock, soil strata and the water table in course grained soils, as well as voids, pipes and solution cavities (Inkster et al., 1989). The technique has been used to detect features with thickness of a few tens of millimeters at a range of several meters, and to map geological structures at depths of up to 50 m.
GPR systems for geological investigations usually comprise a sled equipped with transmitting, receiving and recording equipment that is towed along the survey line at a fixed distance above the ground surface to produce a continuous subsurface profile. The transmitter introduces a short pulse of high frequency (10–1000 MHz) electromagnetic energy into the ground that is reflected by layers with differing electrical properties and detected at the receiver.
The propagation characteristics of the GPR signal depend largely on the electrical properties of the materials being probed, with the two parameters of concern being the conductivity which controls the attenuation of the signal through the ground, and the dielectric constant which controls the signal velocity. The most important factor is the conductivity: higher conductivity materials attenuate the radar signal more quickly, giving rise to radar reflections. The electrical properties of geological materials are primarily controlled by the
water content with the conductivity of soils being related to the volumetric water content. In rocks, the radar is sensitive to changes in rock type and water-filled or dry discontinuities. GPR is of limited use when the conductivity is greater than about 10–15 mS/m. Clays, for example, are relatively conductive and opaque so the depth of penetration in these materials may be limited to about 1 m (3 ft), while in sands and gravels it is possible to achieve penetrations of as much as 10 m (30 ft). GPR is also used to map discontinuities in rock, with reflections being generated as a result of the dielectric constant of the infilling material being different from that of the host rock, or where the discontinuities are filled with water. Discontinuities in granite have been detected at depths up to 50 m (Davis and Annan, 1989).
Geological mapping4.2
Geological mapping of surface outcrops or exploration adits usually furnishes the fundamental information on site conditions, and is often the basis for many subsequent engineering decisions such as relocation of the structure, type of structure that will be built, or the need for rock reinforcement. While mapping is a vital part of the investigation program, it is also an inexact process because a certain amount of judgment is usually required to extrapolate the small amount of information available from surface outcrops and drill core to the overall foundation. This section describes mapping techniques that have been developed to assist in producing both consistent results, and information that can be used directly in design.
4.2.1
Standard geology descriptions
In order to produce geological maps and descriptions of the engineering properties of the rock mass that can be used with confidence in design, is it
In order to produce geological maps and descriptions of the engineering properties of the rock mass that can be used with confidence in design, is it