Geotechnical Site Investigations for Underground Projects Volume 1 (1984) / Chapter Skim
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4. Geologic Problems and Consequences in Construction
4. Geologic Problems and Consequences in Construction
Pages 22-34
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From page 22... ...
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 thorough or different preconstruction geotechnical site investigation.
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From page 23... ...
progress shut-down, equipilent damage gas safety, progress shut-down mixed face special procedures, techniques, equipaent Mechanical probleaas in rock hard, abrasive progress rate, tool life mucking downtime soft bottom progress rate, grade and alignment, special design face fall-out progress abut-down gripper instability (TBMs) progress rate, alignment roof slabbing cave-ins, progress shut-down, addi tional support drilling and blasting necessary for line drilling progress rate pressure binding progress shut-down, i.-ediate support Soft-ground probleaas in machine mining surface subsidence damage at surface face instability progress slow- or shut-down, iamedi ate support or ca.paction grouting water inflow (significant)
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From page 24... ...
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 required.
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From page 25... ...
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 spiles or sets)
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From page 26... ...
Either condition can be serious, mainly affecting support requirements and excavation equipment, particularly shields. Construction Impact Steel sets and lagging can become distorted and out-of-shape with appreciable swelling or squeezing pressures.
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From page 27... ...
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.
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From page 28... ...
Available Prior to Bid Data relevant to the presence and physical condition of existing structures should be obtained and provided. Site investigation methods and laboratory tests should be planned to provide clear information and conclusions regarding (a)
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From page 29... ...
The contractor's excavation method, equipment selection, labor estimates, and round-cycle times are based on the preconatruction assessment of rock hardness and strength. Inadequate information on these factors introduces risks which may lead to undesirable contingency pricing or to later disputes.
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From page 30... ...
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 intrudes into a rock tunnel, it is usually a serious problem. The adverse effects of high rock in a soft-ground tunnel will vary in degree depending 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.
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From page 31... ...
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.
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From page 32... ...
This consideration requires cautious construction procedures, eliminating the use of many common materials such as wood lagging for initial support. Certainly it should severely limit the use of compressed air in gassy ground where explosions are already a potential hazard.
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From page 33... ...
COST CONSEQUENCES An important consideration in devising an exploration program is the need to correlate the actual investigations to the problem areas of construction, 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.
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From page 34... ...
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.
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