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4- Geologic Problems and Consequences in Construction In the most general and simplified sense, the major problem during con- struction is "ground" (i.e. rock or soil) behaving differently than anticipated. The nature of the ground has significantly different im- plications for hand mining, drill-and-blast, and shield and/or tunnel boring machine operations. Equally important to the basic identifica- tion of "hard" or "soft" ground is the determination of the zone of transition from "hard" to "soft" or vice versa, as well as the potential for both extremes to exist in the same place, i.e. "mixed face." In identifying characteristics important for construction, consideration must also be given to factors that can affect ground behavior, such as the presence of water or the construction process itself. Classifications and predictions based on inaccurate or insufficient information can easily result in a partial to total difference between expected and encountered conditions. Especially from the construction point of view, the geotechnical site investigation should provide the basis for anticipating in a reliable and specific way the behavior of the ground. Using the abbreviated list of "problems encountered" appearing in the project abstracts (Volume 2), Table 4.1 was prepared to identify some of the construction consequences. The conditions noted became construction problems, or escalated to greater and more troublesome importance, mostly because the contractor was not prepared for them. This circumstance raises the question of whether these problems could have been avoided, or their impacts minimized, with a more thor- ough or different preconstruction geotechnical site investigation. It is possible that some problems could have been eliminated by making soil or rock borings at closer spacings, and/or by utilizing geophysical seismic surveys to gain information, or by applying other investigative techniques. Still, it is essential to note that not all of the problems could have been anticipated by additional investigation. 22

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TABLE 4. 1 Ef feet of Geologic Conditions on Construction Major Problem Areas Ground behavior blocky or slabby running flowing squeezing swelling spelling (bursts) stand-up time rock loads in-situ stress Groundwater operating nuisance large quantity high pressure corrosive or insoluble salts Existing conditions noxious wastes utilities and structures obstructions (boulders, piles, concrete, etc.) gas mixed face Mechanical problems in rock hard, abrasive mucking soft bottom face fall-out gripper instability (TBMs) roof stabbing drilling and blasting necessary for line drilling pressure binding Soft-ground problems in machine mining surface subsidence face instability water inflow (significant) material hardness steering high rock or intrusions Compressed air blowouts fire other safety restrictions contingency dewatering and grouting Consequences/Requirements excavation method, special equipment, immediate support time loss, special method of control time loss, special method of control immediate support intermediate support progress shut-down, safety immediate support extra steel, long-term support cave-ins, time loss, special pro- cedures inefficiencies, slow-downs, extra pumping progress shut-down, handling pro- cedures progress shut-down, handling pro- cedures damage to excavation equipment, temporary supports, concrete safety, inefficiencies, time loss progress shut-down progress shut-down, equipment damage safety, progress shut-down special procedures, techniques, equipment progress rate, tool life downtime progress rate, grade and alignment, special design progress shut-down progress rate, alignment cave-ins, progress shut-down, addi- tional support progress rate progress shut-down, immediate support damage at surface progress slow- or shut-down, immedi- ate support or compaction grouting progress slow- or shut-down machine binding, progress shut-down time loss, grade and alignment cor- rection machine damage, blasting, excavation method safety, progress shut-down safety, progress shut-down productivity progress rate, safety 23

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MAJOR PROBLEMS FOR CONSTRUCTION The project case histories developed during this study probed the nature of the geotechnical site investigations and subsequent conditions en- countered during construction. The discussion that follows centers on the conditions and resulting problems which, based on the case histories and on the experience of the subcommittee, have been shown to be impor- tant either because of frequency of occurrence or magnitude of impact. Stand-Up Time A stand-up time problem occurs when the ground (rock or soft earth) will not support itself for a time sufficient to accommodate the construction. Stand-up time affects five major areas of concern: type of ground sup- port; equipment selection (e.g., shield); manpower requirements; produc- tion and schedule; and cost. Construction Impact Stand-up time, particularly in blocky ground, dictates in large part whether ground support systems such as rock bolts and mesh are required, or other systems such as steel supports or shotcrete. In extreme cases, a combined system of rock bolts, steel sets, and shotcrete may be re- quired. Stand-up time, or lack thereof, dictates when ground support must be installed, which in turn may have a pronounced effect on rate of prog- ress and costs. The type and timing of ground support installation will affect progress either directly or indirectly. In the case of the re- quirements to install ground support immediately after blasting or me- chanical excavation, the work involved will be "in the cycle," and thus will directly extend the round time and the schedule. The work involved is "unit" production and no short-cuts can be taken. The method and timing of ground support installation will dictate the type of equipment required to accomplish the work effectively. The equipment selected must permit rapid and proper installation of the sup- port elements and yet not be so cumbersome as to delay other operations that must be accomplished in the tunnel. Highly specialized equipment that is capable of installing only a specific ground support system will be totally costed against the project in question, as opposed to partial amortization. As stand-up time strongly influences the type of ground support system to be used--and consequently the equipment necessary to install it--manpower will also be affected. Some types of ground support sys- tems are more labor intensive than others. For example, a simple system of widely spaced rock bolts will no doubt require much less labor than a very elaborate system of rock bolts, steel sets, and shotcrete. Because significant elements of the construction plan are dictated by the assessment of stand-up time, an unanticipated adverse condition is disruptive--and sometimes dramatically so. Delayed performance, im- plementation of a different ground support plan, and increased cost are the results. 24

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Data (and Interpretations) Available Prior to Bid There is no known specific laboratory or field test that can be con- ducted prior to construction that will accurately predict stand-up time. However, the use of RQD and core recovery, close inspection of the joint conditions, consideration of depth of overburden, and observed behavior of the ground in road cuts, outcrops, or any existing projects adjacent to the one contemplated, may provide valuable insight into the stand-up time to be expected. To be useful, collections of observed behavior need to be accurately and completely described. For example, the amount of induced ground vibration caused by blasting should be addressed, along with factors which relate to slaking or squeezing, such as circu- lating air, water saturation, and water flow. In cohesive soil, stand-up time is fairly well indicated by the re- lation of overburden load to undrained shear strength (sometimes called the "overburden factory. If the overburden factor is 5 or 6, the soil is only marginally stable. However, if the factor is 3 or 4, stand-up time will be good. In cohesionless soil, the stand-up time is less easy to quantify. The acting water pressure, gradation, and relative density are important. For example, marginal stability and short stand-up time with a tendency to running conditions would be indicated by a cohesionless soil having a uniformity coefficient of less than 3, with less than 5 percent fines (passing a No. 200 sieve) and a relative density of less than 40 to 50 percent. To be constructive in terms of potential stand-up time problems, the geotechnical investigation should include as relevant information sur- veys of adjacent projects or projects in similar ground conditions, in addition to site specific data. Complete and accurate logging of rock core and description of joint spacing, orientation, and roughness would be useful. In-Situ Stresses In-situ stresses in rock are induced by geologic loading, such as may have occurred dur ing glaciation, or by tectonic activity. The excava- tion of a tunnel opening creates a change in stress cordite on that can result in excess movement and, possibly, local failures of the rock sur- face created by the tunnel. Construction Impact The ratio of the in-situ stress to the strength of the rock determines the scope and degree of deformation or failure that can occur. If the stress is much higher than the strength, local to large failures can occur; in the extreme, the rock may behave in a plastic manner, or in a way similar to soil. The excavation cycle can be adversely affected by local failure, such as fall-out at the heading, popping or squeezing rock, and other movements of rock that directly interfere with the excavation process and necessitate the installation of special supports. Support through rock reinforcement (e.g., rock bolts and spires or sets) increases the excavation cycle time. The selected equipment may then be inappropriate. 25

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Data (and Interpretations) Available Prior to Bid The geotechnical site investigation must be structured toward identifi- cat~on of zones of high stress, especially in known areas of high pre- load or tectonic activity. Overcoring and/or hydraulic fracturing at greater depths are methods useful for investigating the in-situ stresses. Swelling and Squeezing Ground Swelling occurs when the ground expands in volume by absorbing or ad- sorbing water and then tends to move into an available opening or to exert pressure. Squeezing occurs when weak material (generally clayey) behaves plastically under the weight of overlying ground and slowly ad- vances inward without perceptible volume increase. Either condition can be serious, mainly affecting support requirements and excavation equip- ment, particularly shields. Construction Impact Steel sets and lagging can become distorted and out-of-shape with appre- ciable swelling or squeezing pressures. Distortion may be great enough to preclude the placement of permanent support, such as concrete lining. Excavation can be affected if swelling or squeezing ground impedes equipment. In (admittedly) the most dramatic case, the skin of a shield may seize due to friction that cannot be overcome by the hydraulic thrust system. Also, a heaving bottom can ruin tramming equipment and mucking procedures by inducing severe shock loads. Data (and Interpretations) Available Prior to Bid The geotechnical investigation should include analyses both for clay minerals indicative of swelling (i.e., montmorillonite) and for wet con- ditions that could induce physical swelling. The boring program should be structured to provide appropriate samples for laboratory tests and analyses. Groundwater A groundwater problem is the presence in higher volumes than predicted-- or worse, the unanticipated presence--of water during tunneling. The presence and movement of water strongly influences ground behavior, cre- ates a requirement for handling, and affects labor and equipment produc- tivity, and thus cost. Construction Impact In the case of rock, as water moves throughout the medium, particularly in areas having fault and gouge zones,~the water may carry loosened par- ticles of material into the excavated opening. As this material mi- grates out of the host medium, voids are created and the matrix becomes loosened, in turn causing instability in the rock mass. Water can change physical properties of the ground such as cohesion, plasticity, and tendency to swell. In the case of soft ground, water under pressure {even low pressure) may bring smaller particles with it 26

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as it moves into the opening, thus creating voids. As these voids are created, the ground will move inward, often resulting in subsidence above or adjacent to the excavation. Subsidence in soft-ground excava- tions may be particularly critical in urban areas because of the poten- tial for damage to adjacent structures. - Water also creates requirements for handling. The primary handling necessitates the use of collecting pumps, conveyance lines, and in many cases a centralized pumping system. The installation and maintenance of this primary handling system may adversely affect production and sched- ule. Moreover, a secondary handling system may be required to satisfy environmental concerns. Features of a secondary system may include elaborate settling basins and chemical treatment equipment. In addi- tion, effluent testing is a likely requirement. The presence of groundwater raises problems with respect to labor and equipment productivity. In the case of labor-intensive operations that are to be accomplished in a wet environment, human effectiveness will be reduced by these less-than-optimal conditions. This reduction of effectiveness translates directly into production losses. However, less labor-intensive operations are also subject to the effects of water. For instance, when excavated material must be trammed over long distances, abrasion of rotating parts occurs as the equipment travels through rock slurry covering the floor of the tunnel. In addition, sa- line water can be detrimental to mechanical equipment because of its corrosive effects on electrical components. TBMs are particularly sus- ceptible to corrosion; however, the problem usually occurs only when a project is located under a saltwater body. Problems and resulting costs are magnified when the groundwater vol- umes and pressure are not anticipated. The result is that the primary and secondary handling systems, ground support systems, excavation equipment, and general method of attack are usually inadequate. This impedes production and increases costs. Data (and Interpretations) Available Prior to Bid Pump tests to determine groundwater levels and behavior should be per- formed, because groundwater will often be the key element in the con- tractor's work plans. Multiple piezometers to measure perched water tables should be installed and monitored over a considerable period of time. These measurements will enable the contractor to evaluate methods of handling excess groundwater. In a fractured rock medium, pump tests are often advantageous. In the case of soft ground, chemical analyses and testing for effects of exposure to air should be performed, in addition to standard pumping tests taken to equilibrium and drawdown studies. For rock, there are no highly reliable groundwater prediction mecha- nisms which can be used and still maintain cost-effective construction. Nevertheless, a statement concerning estimated volumes and pressures, based on engineering judgments, should be presented. Particular atten- tion should be given to the jointing system and its degree of openness. Finally, data from any adjacent projects should be canvassed carefully for pertinent information. (Note: A variety of techniques exist that can be combined for reliability, but the cost is usually far beyond the resources of general underground construction.) 27

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Existing Structures Existing surface and underground structures can be sensitive to the con- struction operations of dewatering, excavation, and support as well as to blasting and equipment vibration. If construction operations are undertaken without adequate forewarning or preparation, these structures can be damaged. In addition, performance of the contract may be dis- rupted while emergency protection measures are instituted. Construction Impact In earth tunnels, if the soils are compressible, dewatering can cause subsidence and damage to adjacent surface structures. To correct these problems requires underpinning of structures, recharging, continuous sheeting or slurry wall, or combinations of these construction proce- dures. Depending on the permeability and gradation of the soil, compac- tion grouting may also be used. Settlements caused by "loss of ground" at the tunnel face and sup- ported perimeter, as well as elastic yielding into the excavated space, may be the determining factors in selecting the methods of excavation and support, as well as the support plans for structures within the area of inf luence . Data (and Interpretations) Available Pr for to Bid Data relevant to the presence and physical condition of existing struc- tures should be obtained and provided. Site investigation methods and laboratory tests should be planned to provide clear information and con- clusions regarding (a) the behavior of the water table and (b) the com- pressibility, permeability and gradation (important in recharge and grouting considerations), density, and strength of the soil. Precon- struction monitoring of area elevations and detailed inspection of pre- existing structure distress is desirable. In a rock profile where drill- and-blast procedures are applicable and sensitive structures exist at the site or nearby, preconstruction blast/vibration/noise/sensitivity measurements should be made for comparison with later construction effects and for use in establishing a public relations program. Gases Gases encountered underground can be noxious, toxic, and hazardous, pos- ing significant problems in construction. If unanticipated, such gases can obviously be dangerous; if anticipated and planned for, the danger may be reduced or eliminated. Construction Impact When gas is encountered, its properties as well as its chemical and physical reactions to moisture, air (chemical constituents), high tem- perature, pressure, etc., must be evaluated. The gas and/or its prod- ucts must be studied for effect on personnel, corrosive action on mate- rial and equipment, and potential for explosion. In addition, the potential for release of gas f rom groundwater should be investigated. 28

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The tunneling equipment may have to be ~spark-proofed" if explosive gases are contemplated. A special ventilation and absorption system may have to be installed for noxious and corrosive gases. Personnel may need training in the detection and handling of unexpected gases, and safety equipment may have to be specif fed or issued. Moreover, the entire excavation procedure may have to be controlled by strict mining standards. Data (and Interpretations) Avail able Prior to Bid Adjacent soil and rock should be evaluated as potential sources of ex- plosive gases and other noxious or corrosive gases. Chemical testing should be conducted according to strict standards, especially in sus- pected problem areas. Groundwater samples must be checked as a source of gas as well as for reaction with gas. All exploratory boreholes should be checked for the presence of gas, and it may be advisable to install special probes within some boreholes to permit recurring checks of gas type, concentration, and pressure. Rock Hardness and Strength* Problems can occur when the rock to be excavated is (a) harder and more difficult to penetrate than anticipated, or (b) less competent than an- ticipated. Information about rock hardness and strength is important, and may be critical, to the success of the tunneling operation. Construction Impact Rock hardness and strength affect drill or cutter penetration rates and equipment wear. At least to some degree, strength affects stand-up time. The contractor's excavation method, equipment selection, labor esti- mates, and round-cycle times are based on the preconstruction assessment of rock hardness and strength. Inadequate information on these factors introduces risks which may lead to undesirable contingency pricing or to later disputes. Construction will be significantly affected when rock hardness and strength turn out to be materially different from those an- t~cipated. A revised excavation approach and changes in equipment are required, leading to delays and attendant cost increases. The adequacy of available information on these parameters affects the bidder's ability to determine the competency of the rock to be self- supporting. For TBM operations it is important in determining advance rates, cutter costs, and types of cutters to use; for drill-and-blast operations it affects round cycle time and costs of labor and equipment. Data (and Interpretations) Available Prior to Bid In any one type of rock formation the strengths may vary by several thousand psi, or even tens of thousands of psi. Hence, although uncon- fined compressive strength provides excellent data overall with respect In this report the term "rock strength" generally refers to the uncon- fined compressive strength of intact rock cores. 29

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to a single sample of rock, it has been found that a minimum of 50 to 60 samples per litholog ic unit at tunnel depth are needed to obtain a good statistical average or range of strength for making j udgments as to equipment and methods of excavation. For example, compressive strength has been used for at least 28 years by a major machine manufacturer to predict TBM performance. With- in a given rock type, compressive strength is useful (although perhaps not definitive) for predicting TBM progress. When the rock cutter's normal force load exceeds the compressive strength of the intact rock, TBM performance is good. There is no agreed test for abrasiveness, but the percentage of min- erals with Mobs hardness of 5.5 or greater is commonly reviewed to esti- mate cutter wear. Silica (quartz) is among the most abrasive minerals. Percentage by volume of silica in rocks is best determined by thin sec- tions, but in practice an approximation based on standard petrology textbooks is often used. Indications are that, at present, an insuffi- cient number of silicate content determinations are being made of the material to be excavated (e.g., granite and sandstone). Knowledge of silica content permits more realistic estimates of the abrasion to which excavation equipment will be subjected. The subsurface exploration program, therefore, should be set up to obtain a level of information that enables the bidder to make reasonable assumptions in regard to strength and silica content, thereby reducing uncertainties and lowering project costs. Deviations in Rock or Soil Elevation When bed rock is unexpectedly found to protrude to a point within the excavation limits of an underground opening, or when soft ground in- trudes into a rock tunnel, it is usually a ser ious problem. The adverse effects of high rock in a soft-ground tunnel will vary in degree depend- ing on the extent of the rock, on the elevation of the top of rock, and also on whether or not what results is a mixed hard-rock/soft-rock face. For a mixed-face tunnel from inception, the height or top of rock above the invert will affect both the top heading and bench operations. Construction Impact In the case where the rock is higher than expected, the top heading or the steel support will not have the proper configuration to fit the tun- nel and will have to be modified. Also, the excavation and mucking equipment may well prove to be too large for efficient use. Conversely, in the bench operation the drilling equipment may have booms which are too high to accommodate vertical drilling. Thus, a change to horizontal drilling may be required. Because the volume of material to be moved will be larger than anticipated, the original muckers may be too small. In a situation where the rock is lower than expected, the top heading steel will be "too short" and will require expensive splice welding. If the top-of-rock variance is large enough, multiple top drifting may be required. In a case of lower top of rock, however, the bench operation may escape without major adverse impact. 30

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In the case of a full-face soft ground tunnel from inception, a rock intrusion, particularly if continuous over any distance, may be disas- trous. The shield will have been designed to excavate soft ground, not rock. Blasting will be required and the shield skin, doors, and hydrau- lics may be subject to critical damage because blast-proofing may not have been incorporated in the design of the shield. Additionally, the excavating tool, or "digger," may be too light to excavate even loosened rock on a sustained basis. In the extreme case of high rock, the shield may have to be abandoned, necessitating a top heading and bench oper- ation. The scheduling impact of a change in construction method may be great because it will have been unexpected; support steel and other top heading excavation equipment will not be readily available. A similar problem will exist with regard to equipment needed for the benching operation. Data (and Interpretations) Available Prior to Bid Core boring is the most reliable indicator of top of rock and of the weathered transition zone between top of rock and overlying soil. How- ever, uncertainty will persist for the area between borings because that area is unexplored; only inferences can be drawn from borings. Seismic surveys may yield more continuous data, although the quality of those data is inferior to that gained by coring. Both core borings and seismic data should be obtained for use in lo- cating the top of rock. In the case where rock is thought to be close to an otherwise full-face soft-ground tunnel, or where soft-ground is thought to be close to an otherwise full-face rock tunnel, extra data should be collected in order to rule out the (potentially disastrous possibility of a significant intrusion. Compressed Air Compressed-air techniques are confined mainly to earth tunnels driven through pervious, water-bearing soils or driven under or adjacent to bodies of water. In shallow tunnels, such as those frequently found in or near urban centers, excessive water inflow is generally the primary concern, with face instability being an exacerbating factor. The use of compressed air to combat face instability alone is more often encoun- tered in tunnels at great depth or in very weak soils. Compressed air may be the most reliable or only feasible construction technique for completing a length of tunnel in certain soils below the water table, especially when water inflow or face instability, or both, are expected to be severe. Construction Imoact Compressed-air techniques are expensive and can impose risks to con- struction and to personnel that are not encountered otherwise. There- fore, serious effort should be devoted to finding an alternative align- ment. If that process fails, the second most desirable approach is for the owner to undertake a sufficiently detailed investigation program to permit dictating procedures. If compressed air is not specified, then 31

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the low bidder may be the one who is gambling on success with a less conservative {hence, less expensive) technique. That type of risk- taking can have serious consequences; failure of the construction method will shut down and delay not only the particular tunnel section involved but also (usually) the entire system. The third approach is to select the alignment according to a comprehensive subsurface exploration, within the siting constraints for the project. The decision to use compressed air is not one to be made lightly; although the technique overcomes some potentially serious construction problems, it introduces unique problems of its own. Any compressed-air operation requires the mobilization of expensive equipment in the form of compressors and air locks. Personnel costs rise because workers must be medically certified for fitness to work in the environment, and medi- cal personnel must be available to handle emergencies. In addition, operational complications and slow-downs are introduced, the most obvi- ous being that personnel, equipment, and muck must all use air locks to enter or exit the compressed-air working environment. At moderate levels of pressure (generally 12 psig or less), the pen- alty for compressed-air tunneling is not significant in comparison with more conventional methods. For example, compressor size and capacity are relatively modest and working shifts are shortened only slightly. With increasing pressure, compressor expenses mount, but the largest cost increase is caused by shortening shifts to protect worker health. When working shifts are limited to one-half or one-third the length of the usual eight hours, several crews may be required to perform work normally accomplished by one. Moreover, increasing pressures require additional time for gradual compression and decompression to protect against the bends. Thus, the contractor is paying full salary to per- sonnel who must spend hours of the shift in relatively unproductive work while confined to the air locks. In addition to inconvenience and expense, compressed-air tunneling is subject to unique and self-induced hazards, especially at higher pressures. For example, the sudden loss of air (i.e., a blow-out) through an anomaly such as unexpectedly thin overburden can result in crippling decompression or extensive flooding. Fire is also a major concern, because a blaze that would be minor in free air can be a con- flagration in a compressed-air environment. This consideration requires cautious construction procedures, eliminating the use of many common ma- terials such as wood lagging for initial support. Certainly it should severely limit the use of compressed air in gassy ground where explo- sions are already a potential hazard. Data (and Interpretations) Available Prior to Bid In evaluating the feasibility of using compressed air, it is essential to determine the density and shear strength of the soil, the grain size distribution, the cohesion, and the hydrostatic and overburden pressures. Applied air pressure must be given careful analysis because it affects the safety of crews, adjacent structures, and equipment. Groundwater levels, flow rates, soil classifications, and soil prop- erties (particularly density and permeability) are data that must be de- termined to arrive at a decision about the need for compressed air to control hydraulic pressure and water inflow. Hydrostatic pressure, 32

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porosity, and shear strength must be well defined in order to permit a reasonable analysis of required air pressure, and thus of production and cost. When compressed air is being contemplated for a project, considera- tion should be given to offsetting the exploratory borings from the tun- nel alignment to minimize piercing of the tunnel opening. Special care should always be exercised in sealing exploratory borings in soil and rock. Geologic and hydrologic investigations may reveal that the proposed tunnel will encounter a poorly graded gravel, rock, or old timbers as well as high head and low shear strength, etc. If so, the use of compressed-air methods could be severely restricted or inappropriate. As an alternative in such cases, pipe jacking or preconstructed tubes sunk in place in pre-excavated underwater trenches may be solutions. COST CONSEQUENCES An important consideration in devising an exploration program is the need to correlate the actual investigations to the problem areas of con- struction, as well as to provide the basis for design assumptions and engineering cost estimates. The investigation data and interpretations should be available to the designers during the design stage, as well as to the bidders who need answers for the problem areas, or an opinion, or a statement that "we do not know." The design assumptions and criteria for temporary structures should be clearly stated, with explanations and qualifications included. Possible or alternative solutions to problems identified as a result of the investigations should be detailed in the contract documents, with provisions for approval of procedures and pay- ment (which should be accomplished in a prompt and ongoing manner during prosecution of the contract). In addition, if the data are presented with "unknowns," an equitable solution should be offered in the contract documents for the possible costs due to lack of information. The equi- table solution should be a clear statement of the assumed expected behavior of the soil and/or rock with regard to the anticipated con- struction. Thus, bids can be appropriately prepared as a function of production estimates based on a common anticipation of support criteria, soil/rock behavior, and acceptance of risk by owner and contractor. Common to all of the problem areas identified in Table 1 at the be- g inning of this chapter is the fact that had they been identif fed pr for to construction, the disruption of construction caused by the unexpected would have been eliminated. If disruptions do not occur, the non- productive cost of delays and inefficient ies can be avoided. Performance of effective geotechnical investigations can minimize these costs by g iving both designers and contractors a better under- standing of the conditions to be encountered. If problem areas have been detected and the possible consequences recognized at an early stage in the design/construct process, all necessary design and construction activities can be geared to overcome them. In a sense, then, they are no longer "problem" areas. The cost of overcoming these conditions will 33

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have been included in the proj ect budget, rather than appearing later as cost overruns, magnified in amount by disruption costs and, too often, litigation costs as well. However, subsurface investigation is something less than an exact science, and not all problems can be predicted. Therefore, provision should be made for clear definition and allocation of risk and asso- ciated cost. Establishing a baseline of information and assumptions concerning subsurf ace conditions will benefit both owner and contractor. The owner will receive a reliable cost estimate from the designer, and the contractor can be accorded appropriate adjustments in the contract and price for conditions varying materially from those assumed. 34