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5- Special Purpose Projects In recent years there has been an increase in the number of underground projects carried out for special purposes that were previously less com- mon. Case histories of such projects as underwater taps, steep inclines, and deep underground chambers are so few and the data so limited that ~ data were obtained tor shafts. However, the geotechn~cal factors are equally important to site selection, design, and construction of these projects. Construction methods have been changing in recent years, and there is now a distinct trend to mechanizing the excavation process. The use of mechanized excavators requires data that may not have been needed for older excavation processes. Mechanized construction requires a large capital investment by a contractor, and delays become costly. Thus, avoiding delays by means of a site investigation can easily recoup the cost of the investigation. It must be recognized that the specific parameters to be investi- gated vary with both site and use. It must also be recognized that a project comprises several phases--site selection, facility design, con- struction, and operation and maintenance--and appropriate data should be collected and analyzed to support the requirements of each of the indi- vidual phases. they were not collected for this study; by the same token, limited access to them; the DEEP UNDERGROUND CH - BED ID SINS These two categories are jointly discussed for two reasons. The first is that most underground chambers, as later defined, require some type of shaft for access to them; the second is that (as just noted) very little of the case history data obtained by the subcommittee deals with either ~ ~~ forts to obtain such data. Consequently, most of the conclusions and recommendations are extrapolations from data obtained on shallower exca- vations, tempered by the personal knowledge of the subcommittee members. In general it may be stated that the geological, geotechnical, and hydrological information that can be obtained with existing techniques in the course of siting a deep underground chamber will be sufficient for the supporting shafts. However, that information must be used second is that (as just deep shafts or seep underground chambers, Respite vigorous er- 35

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properly to achieve effective design, appropriate bidding, and successful construction of the supporting shafts. Deep Underground Chambers Deep underground projects that require large chambers usually serve one or more of the following purposes: to extract a resource, to store a resource, to dispose of waste, to house equipment, or to protect against hazards. Whether the pr incipal use for the excavation is civil, mili- tary, or commercial, there will be a requirement to optimize its loca- tion in light of the intended use and required performance. Site selec- tion is controlled by the required performance and use, with limited latitude available to the user for other considerations. Factors in Site Selection Chambers excavated for resource recovery are sited on the basis of ex- ploration that has defined the dimensions of the resource body and the hazards and exigencies associated with extraction. Chambers excavated for resource storage or waste disposal are sited, insofar as possible, to maximize their ability to provide containment and isolation and to avoid or minimize conditions adverse to safe performance. Chambers for protection are usually sited according to the nature of the threat and what is to be protected. Waste disposal in particular has required the development, still evolving at present, of a closely constrained siting process. This pro- cess favors explicit delineation of disqualifying conditions, adverse conditions, and favorable conditions, each of which is def ined in terms of the user's ability to demonstrate the likelihood of safe performance as mandated by regulatory agencies. A waste-disposal chamber must meet the containment and isolation stipulations during both its operational (i.e., filling) and functional (i.e., disposal) lifetimes. Given the long-term toxicity of many of the wastes produced by our society, the abi 1 ity to s i te, design, and eons true t f ac il i t ies that mus t pe r f orm safely for many generations poses a signif icant challenge. Chambers excavated to house equipment derive their site selection criteria from the operational conditions. Some chambers must withstand vibration alone; some, vibration and heat; others, vibration and mois- ture. Uniform (predictable) rock bodies and groundwater systems, long-term tectonic stability, and a benign geochemical environment are important considerations for all storage cavities. In all three categories, the disqualifying criteria must be specified prior to exploration activities so that the drilling and logging activities may be planned properly. Exploration activities must also provide information required for design and construction, allowing input to updated designs which incorporate realistic cost and schedule estimates. Site-Specific Considerations After choice of a site, for which the controlling criteria should be in- fluenced by geotechnical data, the specific considerations that must be addressed in a chamber for resource recovery are similar to those for 36

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shafts (see next section). The exploration phase for a storage chamber is somewhat different. For a storage chamber, the information acquired must confirm the use of a volume of rock-surrounded space for containment; yet the explora- tion activities to obtain that information must not significantly reduce that containment capability. Storage caverns must not encounter faults, breccia pipes, Elastic dikes, or karst features that can permit the transmission of contained material within or near their confines. Con- sequently, the exploratory drilling program for a storage chamber must thoroughly evaluate the candidate sitets) without creating migration pathways that cannot be properly sealed. The Ethology of the rock mass surrounding a storage cavern must be compatible with the material to be stored. The geochemistry of the host rock must be such that the rock does not interact adversely with the material to be stored under the conditions of storage. The strength of the host rock must be sufficient to withstand the maximum potential adverse conditions. Such adverse conditions include the potential maximum stresses caused by seismic ac- tivity, thermal load, hydrogeologic differential, in-situ stresses, and stress differential. AS a part of the geologic site investigation, all of these conditions must be evaluated through careful laboratory and in- situ tests prior to actual emplacement. Depending on the depth and geo- logic complexity of the proposed facility, an exploratory shaft and test chamber may be the only practical method of obtaining the quality of in- situ data needed for final design. Storage chambers for a single-phase, nontoxic material that gener- ates no heat require relatively simple preliminary test programs. Chambers for the emplacement of equipment involve similar site- specific considerations as those that apply to sinking shafts. These chambers may also serve as underground civil or military structures and, as a result, their design and support measures tend to be conservative compared to those used solely in mining practice. This is because the chambers are intended for both continuous and "permanent" use rather than the more limited periods of use customary in mining. Geology and Construction The methods of construction for an underground chamber must be chosen so as to yield a functional product while being compatible with the mate- rial properties of the host rock. Depending on the material to be exca- vated, a chamber can be mined either by conventional drill-and-blast methods or by mechanical mining, or both. The excavation can proceed in either an axial or radial direction, or in stages in which the two di- rections interchange or alternate. Because of the large size and shape of most chambers, excavation usually proceeds from the top down in sev- eral stages. For the purpose of economy, the handling of broken or fragmented material should be minimized. Both drill-and-blast and me- chanical methods of mining should take care not to produce a combination of natural and induced fractures yielding unstable roof conditions in the host rock. Blasting operations must be strictly evaluated to ensure that structural integrity and isolation capability are not adversely affected.

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Shafts Shafts may be either vertical or inclined from the vertical. The degree of inclination is, by definition, generally steeper than 45 degrees from the horizontal. (Slopes of less than 45 degrees define the "steep in- cline," discussed in the next section.) Cross sections may be rectan- gular or circular, or variations thereof; the vertical length is great compared to the cross-sectional dimension. Shafts may be lined with a variety of materials or left unlined in highly competent ground. Shafts may be constructed for access or egress, material hoisting or lowering, ventilation, mucking, or water conveyance. Many shafts ful- fill a combination of several purposes; design is governed by projected end users) and geotechnical considerations. Factors in Site Selection In most cases' geotechnical factors dictate the location of an under- ground facility such as a nuclear-waste repository or a powerhouse. However, shaft site may be prescribed by the position of such an accom- panying underground facility, rather than from an optimum geotechnical standpoint. Nevertheless, every attempt should be made to site the shaft so as to reduce the impact of undesirable geotechnical features, within the constraints imposed by the location of the accompanying fa- cility. As noted in the previous section, the voluminous data essential to proper siting of a large underground chamber (or repository complex) are generally adequate for appropriate shaft design and construction, If properly used. In cases where the shaft location is essentially prede- termined by the accompanying facility, there may be no low-cost approach. For example, the data may reveal the presence of a number of obstruc- tions to low-cost shaft sinking and may dictate approaches such as blind shaft drilling or raise boring (vs "conventional" sinking) . However, the geologic, hydrologic, and geotechnical data should be defined well enough so that unknowns are minimized, if not totally avoided. When the end use of a shaft is not specific to a chamber or reposi- tory, as in the case of resource recovery, the required design and con- struction data are less likely to be available. In those cases the trial site selection should be made after an analysis of geotechnical, environmental, legal, economic, and utility factors. Where geotechnical knowledge is lacking, surface mapping, seismic surveys, and data from boreholes should be employed to pinpoint the trial site. Site-Specific Considerations At the point that verification of the trial site commences, the geotech- nical {including hydrologic) considerations become paramount. The scope of the geotechnical investigation is determined by several factors: o End use of the shaft Typefs) of geology anticipated Water conditioned anticipated Nature and depth of overburden Shaft depth Sinking method (and alternatives) under consideration Information available from other nearby boreholes or openings. 38

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In order of priority, the most important data to be obtained pertain to the water conditions that will be encountered, and to the character- istics of the soil and rock that the shaft must penetrate. This is because water, or water in combination with unconsolidated ground, has historically presented the greatest impediment to conventional shaft sinking. The shaft exploration should include, as a minimum, a cored borehole or holes near the shaft site or within its area,, because geo- logic and hydrologic conditions may vary considerably over a short dis- tance, even in flat-lying sedimentary formations or strata. With nonhorizontal geologic features, and especially in areas where potentially near-vertical features occur, more than a vertical hole is required. It is also important to drill an angled hole or holes {depending on depth) in an effort to detect unfavorable features such as parallel faults, dikes, or shear zones. Deep shaft exploratory holes drilled without directional techniques may eventually deviate from the vertical enough to make their informa- tion ambiguous, and hence less relevant. Although this may not be a problem in uncomplicated geologic settings, it is recommended that straight-hole drilling techniques be used where difficult conditions are expected and specific data are required. In all cases, the core should be carefully logged and a geologic section prepared. (Electric logging may be helpful in preparing useful information.) Tests to be conducted are tailored to the specific situa- tion, but some of the more essential determinations are the following: Potential water inflow and groutability (or freezability) at various elevations, when drilling fluid is lost or water is sus- pected. Swelling ground, when the potential is expected. Standing water level (which should be recorded). Chemical composition and pH of the water. Orientation of the exploratory holes. Rock and water temperatures, in deep shafts or in shafts where freezing or extra ventilation may be required (or where personnel may subsequently have to work). When the foregoing have been determined (and their importance cannot be overemphasized), it may appear time- and cost-effective to change the approach from a "conventionally" sunk (drill-and-blast) shaft to a drilled shaft or shafts. One example of an environment where a drilled shaft presents an attractive alternative is in the more heavily water- bearing areas of the Grants (New Mexico) uranium belt. Geology and Construction A thorough and systematic geologic/hydrologic/geotechnical investigation is fundamental to time- and cost-effective shaft construction. Proper exploration will predefine the problems and will permit an engineer's estimate to accurately reflect the actual construction costs. The determinations noted above will confirm whether certain alterna- tives may work. Additional tests should then be made, depending on the desired or anticipated construction methods. For example, if dewatering or Repressurization is contemplated, additional boreholets) and related 39

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tests should be carried out. The relationship between appropriate geo- logical information and selection of shaft construction methods is am- plified in the following discussion of freezing, grouting, ventilation, mechanical mining, and drilling. When freezing is contemplated, groundwater velocity and direction of movement and in-situ water content are important; laboratory tests to determine the properties of the frozen material should be conducted. Gas content and the potential weakening of a freeze wall by dissolution of gas should be addressed. In cases where freezing is to be used, ultrasonic readings should be taken between freeze holes before ground freezing starts, in order to provide a comparative background for later ultrasonic testing to detect windows in the freeze wall. When grouting is anticipated, laboratory tests should be conducted to determine the ground characteristics related to grout-curtain design and grout selection, such as grain size and distribution, porosity, and permeability. The feasibility of pregrouting should also be studied. Pregrouting is becoming increasingly important as down-the-hole mining machines become more commonly employed in shaft sinking, considering the relatively greater capital cost involved in stand-by time. In exploration for deep shafts, temperature measurements are essen- tial to the selection of mining method, need for and design of freeze walls, and design of ventilation during both construction and operation. When mining by mechanical methods is considered, physical specimens should be tested to provide data for estimating rates of penetration and cutter costs. Ground having a potential for balling of the bit or cut- ters should be characterized carefully in cases where surface-based or down-the-hole boring is contemplated. When drilling in the form of raise bore and slash or full face is employed, the angles of formation intersection and behavior of the core holes will indicate potential accuracy problems. For drilling, the ability to control potential sloughing or wall failure with mud becomes important. The ability to provide appropriate control should be ad- dressed at the geotechnical investigation stage in consultation with mud experts. The potential for sloughing in a raise bore should be assessed geologically, because the raise bore and slash method can be very ad- versely affected by sloughing. The potential for high in-situ stress in the rock should be eval- uated for effect on construction methods. In extreme cases, prestres- sing and benching may be indicated instead of full-face blasting. STEEP INCLINES A steep incline is a tunnel or shaft constructed on a slope; roughly de- fined, it includes angles between about 15 degrees and 45 degrees from the horizontal. The method of construction is almost always drill- and-blast, although the use of TBMs is becoming more common. The actual construction can proceed either upward ("positive angle" ~ or downward ~ "negative angle". Incline is an acceptable term for both, but downward construction is sometimes referred to as a decline. 40

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As is true for shaft sinking by drill-and-blast, inclines are among the most difficult, hazardous, and costly underground excavations. Geo- logical conditions will affect water handling, roof support, and the shape of the opening. Particular attention to roof conditions is re- quired, as inclines frequently intersect the bedding planes of a strati- fied rock formation at low angles, which creates a situation prone to roof falls. In weakly bonded sediments, a rectangular opening may be more desirable than a circular cross section because a circular configu- ration may encourage the cusps, or corners, to fall out. Water inflows create difficult problems for incline construction, and efforts to stem flows by grouting, freezing, or pumping down the water table are frequently required. However, if construction proceeds upward, some inflow of water may be considered an advantage if it can be employed to remove the muck. Factors in Site Selection l An incline has inherent advantages and disadvantages in site selection that are similar to those that affect a tunnel. During site investiga- tions there is usually some discretion permitted in routing the incline to avoid the worst ground conditions. A thorough geological profile of the immediate area is therefore warranted. Particular attention to water flow and the dip of the strata will minimize two of the major haz- ards of slope construction. Site-Specific Considerations In most cases, once the course of an incline has been determined from general site geological studies, a limited site-specific study has been the normal approach. Coring of additional, closely spaced boreholes along the specific route is unusual, unless perhaps a particularly crit- ical zone is suspected. Boring an inclined hole along the proposed axis of the incline is a highly desirable procedure, once the location and bearing have been determined. Such a boring eliminates the need for probe holes during construction. The feasibility of this approach, how- ever, is limited by the length of the incline. A more usual approach is to prepare contingency clauses in the con- struction contract and handle special problems if and when they arise. Without an inclined exploratory boring, the progress of the incline can be delayed as the excavation approaches an anticipated fault zone. In this case, a probe drill can be bored ahead of the face to determine the exact nature of the zone; then grouting and/or stabilization methods can be employed if necessary. Roof support methods are probably the most frequent deviation from the construction specifications. The bonding of formations is perhaps the most critical rock characteristic in determining proper roof support in an incline. This characteristic is also one of the more difficult to predict on the basis of core samples. As construction progresses, instrumentation to assess roof and rib creep or floor heave is war- ranted, unless similar excavations have determined otherwise. Geology and Construction The direction, geology, purpose, and shape of the completed incline (or decline) influence the methods of construction. Methods include drill- 41

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and-blast, tunnel borers, road headers, raise drills, or box-hole drills. Each method has benefits and limitations. On a relatively short incline, raise drills or box-hole drills may be considered. Although modern raise drills are capable of pulling raises as far as 3,000 ft. it becomes very difficult and hazardous to trip the reamer head at the slopes considered here (15 to 45 degrees from the horizontal). The raise drill is therefore limited to what can be achieved on a single pull. If machine boring or excavating is con- templated, site investigations require an assessment of both rock strength and abrasiveness. A box-hole drill operates in the same excavation mode as a raise drill, except that the head is pushed up from below rather than pulled down from above. This process is slower and more costly than raise drilling, and is limited to a length of about 600 ft. It is, however, a mechanical method of achieving a steep, blind incline. Tunnel boring machines have found limited use in downward construc- tion but have been successful in upward construction. On a downward slope, these machines have difficulty removing the cuttings from the face. Full-face borers have been used in upward slopes, including one at 45 degrees. Muck can be removed by flushing with the aid of gravity in such cases. However, compared to traditional horizontal rates, prog- ress of the machine is slow and costs are higher. Drill-and-blast is still the most common method of incline construc- tion. Because the rates of mechanized methods are slow, the lower capi- tal investment of the dr ill-and-blast method is attractive . Muck remov- al is also simplified with free access to the face by loading equipment. Inclines driven by the drill-and-blast method typically use specialized suspended-track climbers which provide drilling and ground support in- stallation platforms. Depending on the slope angle, the inclines can be self-mucking and are free-draining. Drill-and-blast also offers versa- tility in shape of the tunnel (i.e., the cross-sectional shape can vary with depth). A method which in some cases competes with drill-and-blast is a roadheader. These mobile excavators offer the flexibility and access to the face equivalent to drill-and-blast techniques, while providing the safety and smooth surfaces of a mechanical excavation. The major weak- ness to date has been the inability of the roadheaders to economically cut hard or abrasive rocks. Recent developments in water-jet-enhanced picks and a mobile unit employing rolling cutters may increase the pro- portion of mechanically excavated inclines. The purpose of the incline also affects the construction method and is similarly related to geological conditions. Tubular shapes are de- sirable for aqueducts, whereas a flat floor is required for operating vehicles. In certain applications, smooth walls and minimum disturbance of surrounding rock can be compelling requirements. Therefore, the geological studies of slope sites will also have a profound influence on the choice of construction method within the constraints imposed by end use. 42

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UNDERWATER TAPS An underwater tap is a subsurface connection between a body of water and an onshore installation. Taps may be required for either the withdrawal or discharge of water by the installation. The applications for underwater taps are growing more numerous, and will continue to increase within the United States as water and power resources are developed. Hydroelectric power will be extracted with low- head generating technology from dams and sites previously ignored. Pow- er plants require large quantities of water for cooling. The trend is for waste water from urban sources to be discharged into the sea by con- trolled diffusers rather than using only rivers or open drainage. Because of the expanding need and its implications for the future, site selection procedures and planning of underwater taps based on geological study are increasingly important. Frequently the tap is constructed between a vertical, or steeply in- clined shaft and a horizontal tunnel. A direct lateral tap into a body of water is avoided if possible because of the difficulty in judging the breakthrough cut. The final cut must be taken with utmost care because there is minimal opportunity to correct this tap after the workings are flooded. The tap should be made as nearly normal to the bottom of the water body as possible to assure uniform thickness of the final plug. Factors in Site Selection In a project involving an underwater tap, location of the tap must be considered in relation to the other project facilities. In some instan- ces, such as an offshore intake or discharge, considerable latitude in location is possible. In others, such as a powerhouse intake, the tap location may be dictated largely by adjacent facilities. In either case, as much geological data as possible should be ob- tained, including at least the use of overt lights, photography, and a study of the surrounding formations. Cores should be taken and exam- ined. Seismic examination may also be considered, as tap construction depth is generally shallow. A complete geological profile should be de- veloped, with particular attention to faults, weathered rock, blocky ground, and parameters for determining water inflow {e.g., permeability, discontinuity spacing, and hydrostatic head). Usually a tap will in- volve drill-and-blast excavation; therefore, RQD, abrasiveness, and com- pressive strength are significant factors. It is important to remember that construction of a tap often crosses two interfaces and involves two or three dissimilar construction methods. The geological interfaces are the water-sediment boundary and the sedi- ment-underlying rock boundary. Construction through the water-sediment boundary may involve piling construction with excavation by dredge, pump, or clam. At the rock layer, a blind drill using reverse circula- tion for muck removal may be involved. From the opposite direction, a tunnel or incline construction is probable. Geological considerations will probably dictate the method and type of equipment selected for tap construction, and will probably affect its location as well. 43

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Site-Specific Considerations Once a specific site is selected, the complexities of design and con- struction of an underwater tap demand that a thorough site investigation be conducted prior to initiating final design or construction. The on- shore site must be investigated by geological mapping, and core drilling is needed to eliminate as many uncertainties as possible. Thorough off- shore studies are required as well. Cored boreholes are essential; cores can be drilled in the subsurface rock using barges or platforms. Inspection by divers, underwater photography, and soil sampling are use- ful for determining the water-sediment and sediment-rock interfaces.' It is particularly important to establish the occurrence and charac- ter of faults and intercepting joints or fractures that could conceiv- ably conduct water. The strength of the rock and definition of the strata are needed to ensure structural integrity of the completed in- stallation. Determination of the applicable rock properties will en- hance the selection of construction equipment and control of its opera- ting cost. In the case of an underwater tap, a second opportunity for specific site investigation occurs when construction nears the point of break- through. Given the penalty for error if the crucial tap is performed carelessly, this opportunity should not be ignored. From a tunnel, probe drills, with packers or operating through a seal, can safely drill horizontally for about 200 ft or less. If a drilling operation is in- volved, a probe drill along the axis of the excavation is possible. Seismic and electromagnetic pulse examination should prove effective; density changes are among the most readily detectable characteristics recognized by seismic tests. Electromagnetic (radar) means are some- times successfully used for identifying aquifers at close range. The objective of this final study is to accurately identify the rock bounda- ries and confirm evaluations of the integrity of the rock. Geology and Construction Consideration must be given to the methods of geological analysis and potential treatment in relation to the type of excavation to be em- ployed. A fissure intersecting the floor of a water body may not be a deterrent to blind drilling, but it could be calamitous in a hand- excavated tunnel. Treatment such as grouting or ground stabilization must be compatible with the planned construction method. Methods of creating the final tap vary with end use, but generally f all into four categories. The vertical structure is built on bed rock offshore. A caisson may be built to surface. Bed rock below the structure is drilled down to a point where it will intersect the approaching tunnel. This cut can be "wets or "dry." If the offshore structure permits, the vertical column can be pumped out and the f inal cut is dry. The fa- cility is then flooded by removing a portion of the vertical structure. o A reverse procedure can be employed, in which the tunnel por- tion is completed first and the connection is made by drilling into it. An underwater caisson is built on bed rock above the tunnel and the con- necting hole is drilled, sometimes through the caisson and into the tun- nel. This procedure is used where a diffuser outflow or manifold tap is employed. 44

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The connection may be made by dr illing upward from the tun- nel. A raise drill can be employed from a drilling platform. Again, wet or dry connections are pass ible . The caisson built offshore on bed rock may contain a plug. After contact with the caisson is made from the tunnel, either below or horizontally, the plug is removed by explos Ives. Such a hydraulic bulk- head can even be dewatered by pumping so that a liquid explos ive can be poured in and detonated remotely. When blasting the final plug wet, a sump or trap may be built downstream to catch and permanently store the rock from the final blast or spread it evenly along the floor of the tunnel. 45 -