Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 9
10 NCHRP Synthesis 357: Use of Geophysics for Trans- portation Projects (Sirles 2006) provides a comprehensive overview of the topic and additional survey data relevant to this study. Table 1 identifies the primary and secondary methods used to investigate selected subsurface objectives. The table is an abridged version from the Sirles report (2006) in which only objectives pertaining to foundation investiga- tions are included. The survey of transportation agencies for this project identified "seismic" as the most widely used geophysical methods and "mapping rock" as the most widely used application of geophysics. Mapping karst or other voids was also identified as a major objective. FIGURE 4 Bridge site with surface exposures of foundation Results of the survey for this study are consistent with rock. those of Sirles (2006). The most frequently applied method is seismic refraction, which is based on measuring the travel time of compressional waves through the subsurface. Upon FIELD INVESTIGATIONS striking a boundary between two media of different proper- ties the direction of travel is changed (refraction). This Field methods for characterization of rock include geophys- change in direction is used to deduce the subsurface profile. ical methods, rock core drilling, and in situ testing. These Figure 5a illustrates the basic idea for a simple two-layer activities normally are carried out during the Preliminary profile in which soil of lower seismic velocity (Vp1) overlies Foundation Design phase of the design process as described rock of higher seismic velocity (Vp2). A plot of distance from in chapter one, and would be used to provide a description the source versus travel time (Figure 5b) exhibits a clear of subsurface conditions and a preliminary subsurface pro- change in slope corresponding to the depth of the interface. file. The detailed results of field investigations, including The equipment consists of a shock wave source (typically a detailed boring logs, in situ testing results, and interpreta- hammer striking a steel plate), a series of geophones to mea- tion, would be included in the final geotechnical report pre- sure seismic wave arrival, and a seismograph with oscillo- pared during the Final Foundation Design phase of Figure 2. scope. The seismograph records the impact and geophone signals in a timed sequence and stores the data digitally. The Geophysical Methods technique is rapid, accurate, and relatively economical when applied correctly. The interpretation theory is relatively Geophysical methods, in conjunction with borings, can pro- straightforward and equipment is readily available. The most vide useful information in areas underlain by rock. The most significant limitations are that it is incapable of detecting common application of geophysics is to determine depth to material of lower velocity (lower density) underlying higher bedrock. When correlated with data from borings, geophys- velocity (higher density) and that thin layers sometimes are ical methods provide depth to bedrock information over a not detectable. For these reasons, it is important not to rely large area, eliminating some of the uncertainty associated exclusively on seismic refraction, but to verify depth to rock with interpolations of bedrock depths for locations between in several borings and correlate the seismic refraction signals borings. to the boring results. Seismic velocity, as determined from seismic refraction measurements, can be correlated to small- Geophysical methods are based on measuring the trans- strain dynamic modulus of soil and rock by the following mission of electromagnetic or mechanical waves through the relationships: ground. Signal transmission is affected by differences in the physical properties of geomaterials. By transmitting electro- Ed = 2 (1 + vd ) Vs2 (1) magnetic or seismic signals and measuring their arrival at other locations, changes in material properties can be located. (1 - 2 v ) (1 + vd ) 2 Ed = Vp (2) In some cases, the material properties can also be quantified. (1 - vd ) For foundation site characterization, geophysical methods can be placed into two general categories, those conducted from in which Ed = small-strain dynamic modulus, vd = small- the ground surface (noninvasive) and those conducted in strain dynamic Poisson's ratio, = mass density, Vs = shear boreholes (invasive). When grouped according to method, the wave velocity, and Vp = compressional wave velocity. Eqs. 1 six major categories are: seismic, electromagnetic, electrical, and 2 are based on the assumption that the rock mass is a magnetic, radar, and gravity. Basic descriptions of geophysi- homogeneous, isotropic, elastic solid. Because most rock cal methods and their application to geotechnical engineering masses depart significantly from this assumption, elastic are given by the U.S. Army Corps of Engineers ("Geophysi- modulus values calculated from seismic wave velocities are cal Exploration . . . ." 1995) and Mayne et al. (2001). normally larger than values measured in static field load
OCR for page 10
11 TABLE 1 GEOPHYSICAL METHODS AND APPLICATIONS (after Sirles 2006) Techniques Methods Seismic Electromagnetic Electrical Other EM61--TimeDomain Metal Detector Electrical Resistivity Tomography/P Surface Wave (SASW, MASW, & TimeDomain EM Soundings EM31--Terrain Conductivity EM34--Terrain Conductivity Ground Penetrating Radar Electrical Resistivity/P Seismic Tomography Seismic Refraction Seismic Reflection Shear Wave Magnetics Passive) Gravity Investigation Objectives Bedrock Depth P P P P S S S Rippability P P P Lateral & Vertical Variation in Rock or Soil P P P P Strength Location of Faults and P P P P S S S S S S S Fracture Zones Karst Features P P S S P P Notes: P = primary; S = secondary; blank = techniques should not be used; EM = electromagnetic; SASW = spectral analysis of surface waves; MASW = multi-channel analysis of surface waves. tests, such as plate bearing or pressure chamber tests. Alter- material properties with depth (layering) can be determined. natively, a method that correlates rock mass modulus to shear The second method is a profiling survey in which the elec- wave frequency has been shown to provide a reasonable first- trode spacing is fixed but the electrode group is moved order estimate of modulus. Figure 6 shows the relationship horizontally along a line (profile) between measurements. between in situ modulus and shear wave frequency using a Changes in measured apparent resistivity are used to deduce hammer seismograph, as described by Bieniawski (1978). lateral variations in material type. Electrical resistivity The data can be fit to a straight line by methods are inexpensive and best used to complement seis- mic refraction surveys and borings. The technique has ad- EM = 0.054f 9.2 (3) vantages for identifying soft materials in between borings. Limitations are that lateral changes in apparent resistivity where EM = rock mass static modulus (GPa) and f = shear can be interpreted incorrectly as depth related. For this and wave frequency (hertz) from the hammer blow received at other reasons, depth determinations can be in error, which distances of up to 30 m on a rock surface. is why it is important to use resistivity surveys in conjunc- tion with other methods. Resistivity is a fundamental electrical property of geo- materials that varies with material type and water content. To The use of multi-electrode resistivity arrays shows promise measure resistivity from the ground surface (Figure 7), elec- for detecting detailed subsurface profiles in karst terranes, one trical current is induced through two current electrodes (C1 of the most difficult geologic environments for rock-socketed and C2), while change in voltage is measured by two poten- foundations. Dunscomb and Rehwoldt (1999) showed that tial electrodes (P1 and P2). Apparent electrical resistivity is two-dimensional (2-D) profiling using multi-electrode arrays then calculated as a function of the measured voltage differ- provides reasonable resolution for imaging features such as ence, the induced current, and spacing between electrodes. pinnacled bedrock surfaces, overhanging rock ledges, frac- Two techniques are used. In a sounding survey, the center- ture zones, and voids within the rock mass and in the soil line of the electrodes is fixed while the spacing of the elec- overburden. Hiltunen and Roth (2004) present the results of trodes is increased for successive measurements. The depth multiple-electrode resistivity surveys at two bridge sites on of material subjected to current increases with increasing I-99 in Pennsylvania. The resistivity profiles were com- electrode spacing. Therefore, changes in measured apparent pared with data from geotechnical borings. Both sites are resistivity with increasing electrode spacing are indicative located in karst underlain by either dolomite or limestone. of a change in material at depth. In this way, variations in The resistivity profiles provided a very good match to the
OCR for page 11
12 (a) FIGURE 6 Rock mass modulus versus shear wave frequency by Bieniawski (Goodman 1980). at the ground surface. Crosshole seismic involves measur- ing travel times of seismic waves between boreholes. Both methods provide depth to rock, and s-wave velocities, dynamic shear modulus, small-strain Young's modulus, and Poisson's ratio. Crosshole tomography is based on computer analysis of crosshole seismic or resistivity data to produce a 3-dimensional (3-D) representation of subsurface conditions. These techniques are more expensive and require specialized (b) expertise for data interpretation, but may be cost-effective for large structures where the detailed information enables a FIGURE 5 Seismic refraction method (Mayne et al. 2001): more cost-effective design or eliminates uncertainty that may (a) field setup and procedures; (b) data reduction for depth to hard layer. otherwise lead to construction cost overruns. All geophysical methods have limitations associated with stratigraphy observed in borings, particularly for top-of- the underlying physics, the equipment, and the individuals rock profile. Figure 8 shows a resistivity tomogram at one running the test and providing interpretation of the data. The of the bridge pier sites, in which the top-of-rock profile is study by Sirles (2006) includes several informative case his- well-defined by the dark layer. Inclusions of rock in the tories from state DOTs of both successful and unsuccessful overlying soil are also clearly defined. This technology projects. The single case history related to a bridge founda- should be considered for any site where a rock surface pro- tion investigation is one of a failure to provide accurate file is required and would provide valuable information for both design and construction of rock-socketed founda- tions. Table 1 identifies electrical resistivity tomography Battery Current meter profiling as a primary method for investigating karstic con- ditions and as a secondary method for measuring depth to bedrock. Volt meter Other geophysical methods have potential for rock sites, P1 P2 but have yet to be exploited specifically for applications to V foundations in rock. These include downhole and crosshole C1 Spacing, A Spacing, A Spacing, A C2 seismic methods. Downhole seismic p-wave is based on mea- suring arrival times in boreholes of seismic waves generated FIGURE 7 Field configuration for resistivity test.
OCR for page 12
13 Resistivity Test #7 rotated and forced downward to form an annular ring while East West preserving a central rock core. Standard core barrel lengths are 1.5 m and 3 m (5 ft and 10 ft). Fluid, usually water but Depth (feet) -10 possibly drilling mud, is circulated for cooling at the cutting interface and removal of cuttings. Selecting the proper tools -20 and equipment to match the conditions and the expertise of 10 20 30 40 50 60 70 80 90 100 an experienced drill crew are essential elements of a suc- Distance (feet) cessful core drilling operation. Once rock is encountered, coring normally is continuous to the bottom of the hole. 0 12 25 50 100 200 Where the rock being sampled is deep, wire line drilling, Resistivity (Ohm-feet) in which the core barrel is retrieved through the drill stem, FIGURE 8 Resistivity tomogram at Pennsylvania bridge site in eliminates the need to remove and reinsert the entire drill karst (Hiltunen and Roth 2004). stem and can save considerable time. If sampling is not con- tinuous, drilling in between core samples can be accom- plished using solid bits. depths to bedrock in a river channel using both seismic refraction and an electrical resistivity sounding survey. Rea- Rock coring bits and barrels are available in standardized sons cited for the failure include loss of geophones owing sizes and notations. Important considerations in core barrel to running water and ice, instrumentation malfunctions, ex- selection are: (1) core recovery and (2) the ability to deter- cessive background noise, differences of opinion between mine the orientation of rock mass structural features relative consultants on data interpretation, and discrepancies between to the core. Core recovery is most important in highly frac- top of rock from geophysical results and borings. Although tured and weak rock layers, because these zones are typically this is not believed to be a typical case, it demonstrates some critical for evaluation of foundationrock load transfer. real world lessons. For sampling of competent rock, bits and core barrels that provide a minimum of 50-mm-diameter (nominal) core are Additional findings by Sirles (2006) are that "in-house adequate for providing samples required for index tests, rock geoscientists and engineers do not understand the value, the quality designation (RQD), laboratory specimens for benefit, or the science of geophysics for their projects." How- strength testing, and evaluating the conditions of discontinu- ever, several factors point to geophysics becoming more ities. For example, NWM (formerly NX) diamond bit and widely accepted and implemented as a tool in the transporta- rock core equipment drills a 76-mm (3-in.) diameter hole and tion industry. These include a manual published by FHWA provides a 54-mm (2.125-in.) diameter rock core. When weak, and available on-line (http://www.cflhd.gov/geotechnical), soft, or highly fractured rock is present, it may be necessary additional programs aimed at training of agency personnel, to use larger diameter bits and core barrels to improve core and increasing levels of experience. recovery and to obtain samples from which laboratory strength specimens can be prepared. Coring tools up to 150 mm Borings (6 in.) in diameter are used. A highly recommended practice for best core recovery is to use triple-tube core barrels. The Borings provide the most direct evidence of subsurface con- inner sampling tube does not rotate during drilling and is ditions at a specific site. They furnish detailed information removed by pushing instead of hammering; features that on stratigraphy and samples of soil and rock from which minimize disturbance. Thorough descriptions of coring engineering properties are determined. Borings also provide equipment and techniques are given in Acker (1974), the means for conducting in situ tests, installation of instru- AASHTO Manual on Subsurface Investigations (1988), mentation, and observing groundwater conditions. Conven- Mayne et al. (2001), and U.S. Army Corps of Engineers tional soil boring and testing equipment is used to drill ("Geotechnical Investigations" 2001). through overlying soil deposits and to determine depth to bedrock. Once encountered, the most widely used technique Steeply dipping or near-vertical bedding or jointing may for investigating rock for the purpose of foundation design go undetected in holes drilled vertically (Terzaghi 1965). is core drilling. Samples are obtained for rock classification Such features can significantly influence the strength and de- and determining rock properties important to both design and formability of rock foundations. Inclined (nonvertical) construction. A core sample can be examined physically and drilling provides the opportunity to detect the orientation and tested, providing information that is hard to obtain by any characteristics of near-vertical features. Oriented core refers other methods. to any method that provides a way to determine the geomet- rical orientation of planar structural features, such as bed- Rock core drilling is accomplished using rotary drill ding, joints, fractures, etc., with respect to the geometrical equipment, usually the same truck- or skid-mounted rigs orientation of the core. One approach is to mark the core with used for soil drilling and sampling. A hollow coring tube a special engraving tool so that the orientation of the discon- equipped with a diamond or tungstencarbide cutting bit is tinuity relative to the core is preserved and the orientation of
OCR for page 13
14 the discontinuity (strike and dip) can be determined accu- For shafts supported on or extending into rock, a minimum of rately (Goodman 1976). A method used with wire line 3 m of rock core, or a length of rock core equal to at least three times the shaft diameter for isolated shafts or two times the max- drilling involves making an impression of the core in clay. imum shaft group dimension, whichever is greater, shall be The combination of inclined and oriented coring techniques extended below the anticipated shaft tip elevation to determine can provide an effective tool for characterizing orientation of the physical characteristics of rock within the zone of foundation discontinuities in complexly fractured rock masses. Rock influence. core orienting methods are covered in more detail in the AASHTO Manual on Subsurface Investigations (1988) and If the tip elevation changes at some point during the project, are also reviewed and compared with borehole televiewer additional drilling may be required to meet this recommenda- methods by Eliassen et al. (2005). tion. O'Neill and Reese (1999) provide the following guidance on boring depth. When the RQD is less than 50%, extend bor- ing depths to at least 125% of the expected depths of the drilled Depth and Spacing of Boreholes shaft bases plus two base diameters. If RQD values are greater than approximately 50% at the planned base elevation, borings O'Neill and Reese (1999) recommend the number of borings only need be extended to the expected base elevation plus two to be made per drilled shaft location at bridge sites when the base diameters as long as the RQD remains above 50%. The material to be excavated is unclassified (Table 2). Unclassi- rationale is that it is not likely the shafts will need to be deep- fied means the contractor is paid by the unit of excavation ened once the actual strata are exposed. This approach requires depth (meters or feet) regardless of the material encountered. that foundation diameters and depths be estimated before the For rock sites, these recommendations should be considered boring program and that RQD be determined during drilling. a minimum. If possible, it is recommended to locate one The approach described is only a general suggestion and local boring at every rock-socketed shaft. In practice, this is not geologic conditions may dictate other criteria for boring depths. always possible and factors such as experience, site access, If in the course of design or construction it becomes necessary degree of subsurface variability, geology, and importance to deepen the shafts, supplementary borings should be taken. of the structure will be considered. If materials are classi- fied for payment purposes, it becomes more important to An available, but not widely used tool for subsurface inves- locate a boring at every drilled shaft location for the purpose tigation is to drill one or more large-diameter borings or to have of making accurate cost estimates and for contractors to a drilled shaft contractor install a full-sized test excavation. base their bids on knowledge of the materials to be exca- Large-diameter borings can be made with augers in soft rock vated. Where subsurface conditions exhibit extreme varia- and with core barrels in hard rock. The sidewalls of the boring tions over short distances, multiple borings at each shaft or shaft can be examined directly (with appropriate safety mea- location can reduce the risk of founding a shaft on soil in- sures) or with downhole cameras. Observations can then be stead of rock. For example, large-diameter, nonredundant made of rock mass features, including degree of roughness and shafts in karstic limestone may require multiple borings at general quality of the drilled surfaces, and fracture patterns. each shaft location to determine that the entire base will be Large-diameter holes provide access for obtaining high-quality founded in rock and to identify voids or zones of soil undisturbed samples and may be used for performing in situ beneath the base that may affect load-settlement behavior plate load tests to measure rock mass modulus. If a full-size of each shaft. excavation is made by a drilled shaft contractor, information of value to both engineers and contractors is obtained. In Fig- The draft 2006 Interim AASHTO LRFD Bridge Design ure 3, "constructability" is one of the items to be determined Specifications recommends the following for depth of borings during the site characterization. A full-sized excavation is the below anticipated tip elevations: most direct method for obtaining this information. TABLE 2 Downhole devices are available for borehole viewing and RECOMMENDED FREQUENCY OF BORINGS, photography, including borescopes, photographic cameras, and DRILLED SHAFT FOUNDATIONS FOR television cameras. A visual image of rock in the sidewalls of BRIDGES, UNCLASSIFIED EXCAVATION a boring provides information on structural features that may Shaft add significantly to the overall picture of subsurface geology. Redundancy Diameter Condition (m) Guideline Advantages and disadvantages of some remote viewing Single-column, single One boring per devices are discussed in "Geotechnical Investigations" (2001); All shaft however, the technologies for borehole imaging are advancing shaft foundations Redundant, multiple- >1.8 m One boring per rapidly and the user should consult commercial providers for shaft foundations (6 ft) shaft the most up-to-date information. These devices are effective Redundant, multiple- 1.21.8 m One boring per for examining soft zones for which core may not have been shaft foundations (46 ft) two shafts <1.2 m recovered, determination of dip and strike of important struc- Redundant, multiple- One boring per shaft foundations (4 ft) four shafts tural features, and viewing of cavities such as solution voids, Source: OíNeill and Reese 1999. open joints, and lava tunnels in volcanic rocks.
OCR for page 14
15 Borehole televiewers provide high-resolution images showing rock mass structural and textural features and ac- curate measurement of dip and dip direction of structural features without the use of oriented core. Optical teleview- ers (OTV) generate a high-resolution digital color image of the inside of the borehole wall and are capable of resolving fractures as narrow as 0.1 mm with a radial resolution of 1 degree (Eliassen et al. 2005). The OTV can be operated in air- or fluid-filled boreholes; however, fluid requires thor- ough flushing before image acquisition is undertaken. Acoustic televiewers (ATV) produce images of the borehole wall based on the amplitude and travel time of acoustic signals reflected from the borehole wall. A portion of the reflected energy is lost in voids or fractures, producing dark bands on the amplitude log. Travel time measurements allow recon- struction of the borehole shape, making it possible to generate a 3-D representation of a borehole. Both types of televiewers orient their image data using a three-component fluxgate magnetometer and a three- component tilt meter incorporated into the tool. Before inter- pretation, the image is rotated to a common reference direction, either magnetic north or the high side of the borehole. Planar features that intersect the borehole wall produce sinusoidal traces in the "unwrapped," or 2-D, televiewer image. Using the reference direction recorded during logging, sinusoids can be analyzed to produce dip and dip directions of structural features. Figure 9 shows OTV and ATV images of the same FIGURE 10 An acoustic television log (Caltrans 2005). borehole and illustrates some advantages of each device. The OTV is able to provide a color image of the dike and excel- lent imaging of the texture of the granite. The ATV highlights According to Eliassen et al. (2005), use of optical and fracturing within the diorite. The California DOT (Caltrans) acoustic televiewer equipment is gaining popularity over reports using the ATV to provide very-high-resolution sonic oriented coring techniques because it is generally less labor images in the format of a 3-D "pseudo-core," as illustrated in intensive and is particularly useful where access or ability to Figure 10. drill inclined holes is limited or where local drilling compa- nies lack the equipment necessary to collect oriented cores. However, to date, this technology is being applied to site char- acterization for rock slope engineering and underground openings, and is not being used in foundation investigations. Eliassen et al. (2005) note further that televiewer logs are best used to supplement data obtained from quality rock coring, which provides samples for laboratory testing, assessment of joint and discontinuity planes, and correlation of lithologic and geologic boundaries with geophysical data. The authors suggest that drilling time and costs can be optimized with ap- propriate combinations of coring and less expensive air rotary boreholes logged with OTV and ATV equipment. Borehole televiewing may be most useful in rock-socket applications at sites where the structural orientation of discontinuities is a significant factor in foundation stability. For example, some modes of bearing capacity failure (described in chapter three) depend on the orientation of discontinuities in the rock mass below the socket base. LaFronz et al. (2003) describe use of OTV as part of the subsurface investigation for the Colorado River Bridge at Hoover Dam. The primary purpose was to FIGURE 9 Optical and acoustic televiewer images of a 50-cm obtain structural data to develop recommendations for exca- diorite dike in granite (Eliassen et al. 2005). vation of cut slopes at the abutment foundations.