1.0 Excavation and Ground characteristics (Tunnels & TBM) 1.1 General As the excavation and the support of the ground are often one operation, appropriate cross-references are made and other relevant clauses. 1.2 Basic principles The essence of safe excavation is to maintain the stability of the ground at all times. This is achieved by either: — maintaining continuous ground support during excavation (e.g. closed-face and closed-mode); or — opening up only as much ground as is safely self-supporting (e.g. open-face and open-mode) until temporary or permanent support can be provided. The amount of ground opened up varies from a small excavation in soft clay or running sand, which needs immediate support, to the total excavated surface of a tunnel in sound hard rock, which can safely remain unlined for many weeks or indeed permanently. Following a full risk assessment, a safe system of work should be instituted and maintained at all times. The time intervals between excavation, immediate support and final lining are critical to the whole construction procedure and its cost. As soon as a volume of ground is excavated, there is a redistribution of stresses in the remaining ground. This can initiate movement in the ground, which can cease quickly in sound rock, or can continue to develop slowly in firm and stiff clays or rapidly in soft to very soft clays, sand or gravel. If the effects are to be localized, immediate support should be provided to prevent progressive deterioration. The stand-up times of unsupported ground range from seconds to days or weeks. The nature of the necessary ground support varies from mere containment to support for full overburden. Major factors that should be taken into account when determining the loads to be taken in the Ground-support system includes: a) The size and depth of the tunnel; b) The shape of the tunnel; c) The method and speed of excavation and lining; d) The stiffness and water tightness of the lining system; e) The groundwater regime; f) The structural geology; g) The proximity of other underground structures; h) The construction of adjacent tunnels; i) Vibration. Depending on the influence of these factors, and in particular groundwater, a small face can sometimes be safe to leave almost indefinitely, whereas extensive and continuous support can be required for a larger face in similar material. Experience and sound judgment are essential in assessing how much ground can safely be opened up and for how long it can remain unsupported. In some types of closed-face machine operating in open mode, it is not always possible to see the excavation face upon which a decision is to be made. In this case, alternative methods of assessing the face support required are essential. These include inspection of the excavated material, forward probing, and ground radar and excavation volume assessment. Where sudden changes in ground condition are encountered, immediate temporary support and containment can be vital. In loose ground where silt and sand start to run, mechanically closing down the face or alternatively stuffing quickly with bags of straw or similar material can be effective in minimizing ground loss and maintaining the stability of the face. In methods of tunneling which involve incremental excavation and support, continuous observation of both the ground and the ground support is necessary. 1.3 Ground characteristics 1.3.1 General Soils and rocks are not homogeneous. The following subclasses (1.3.2, 1.3.3, 1.3.4, 1.3.5 and 1.3.6) offer guidance on the general characteristics of various soil types; but ground conditions can change continuously as the tunnel proceeds, and the material being excavated is unlikely to match any single category described below. 1.3.2 Granular soil 1.3.2.1 General A granular non-cohesive soil has little or no stand-up ability and will only lie at its angle of repose. Failure in granular soils can occur as: a) Slow or fast raveling, when material begins to dry out or to loosen due to overstress; b) Running, in dry granular materials lacking cohesion; c) Flowing as a viscous mixture, when a granular material becomes fluid. 1.3.2.2 Sand and gravel In sand and gravel there is no plasticity and little yielding under stress, but a small fall of loose material can quickly destroy any arching action which is carrying the load and result in sudden progressive collapse. It is vital to provide support immediately to prevent initiation of movement. If water is present, restraint is of even greater importance, and further precautions against the washing out of fine material, resulting in loosening of the whole, may be necessary. A closed-face machine with pressurized heavy slurry, such as bentonite, will not only support the sand and gravel but also negate the effects of the groundwater by equalizing the hydrostatic head. Open faces should be carefully boxed up or closed down, and all voids grouted, when progress is interrupted. 1.3.3 Cohesive soil 1.3.3.1 General In soils having some degree of cohesion and plasticity, ranging from silts to clays, failure can occur as: a) Raveling, where the ground dries out or is overstressed, but fractures rather than flows; b) Squeezing, e.g. where a clay is overstressed and slowly extrudes without visible fractures; c) Flowing, by vibration or liquefaction in moist or saturated silts, and by vibration in clays; d) Swelling, of clay absorbing water, possibly from the atmosphere, and increasing in volume. Some fluvial and marine clay are particularly sensitive. Soft sensitive clays sometimes appear safe, but are subject to loss of strength and progressive collapse. (Sensitivity relates to the rate of loss of strength on disturbance.) 1.3.3.2 Clay In soft clays, and in stiff clays where discontinuities (i.e. slicken sides or “greasy backs”) are present, it may be necessary to provide, as soon as practicable, support that restrains movement. The plastic characteristics of clays can result in a gradual development of ground disturbance. Immediate support keeps disturbance to a minimum. Changes of moisture content in clays due to exposure of the surface and load changes can, over a long period, result in swelling or shrinkage, with a consequent increase in the load on the supports. In hand excavations where timber supports are used, these should be tightly wedged into position with a proper system of timber or steel struts. In open-faced shields, hydraulic struts may be used. These allow the shield to move forward whilst keeping a controlled support on the face by maintaining hydraulic pressure. In soft plastic clays, a closed-face machine may be appropriate. In a tunnel, in clay or any yielding ground, additional temporary support may be needed to seal the clay surface if a normal cycle of operations is interrupted. All voids should be boxed up and grouted, or the face should be sealed with sprayed concrete. 1.3.3.3 Silt The cohesion of silt can suddenly be lost due to changes in water content. The face should be fully supported unless cohesion can be relied upon. When a tunnel in silt is below the water table, compressed air, freezing, or other stabilization measures may be used as an alternative to a closed-face TBM. 1.3.4 Chalk Chalk is a fine-grained limestone. Its nature varies widely, depending on its mineral content and the degree of weathering. It can be soft, and therefore unstable when excavated; or it can be hard, requiring significant break-out effort. Soft chalk can be putty-like. Fracturing, block size, permeability and the frequency of bedding joints are also variable. Chalk can be relatively homogeneous, or discontinuous and blocky. It can be a significant aquifer containing open and water-bearing fissures, or it can be dense and of low permeability. These conditions can be encountered in varying combinations, affecting the safety of the excavation, the working conditions within the tunnel and the viability of the chosen methods of excavation and support. Of particular importance is the presence, or otherwise, of flints within the chalk mass. Excavation, particularly mechanized excavation, can be adversely affected by the presence of flints, which can result in rapid and excessive wear of cutters. 1.3.5 Rock After excavation or blasting in rock, there is normally an arching action across the tunnel in the newly exposed roof, with some support from the undisturbed rock ahead of the face. The value of this support depends greatly on the direction and slope of the planes of weakness in the immediate vicinity. The dip and strike of the bedding planes, the spacing and pattern of joints, and the presence of faults and any consequent fracturing and crushing should therefore be taken into consideration when assessing the stability of the excavation, and in the design of the supports. In particular, the designer should consider the thickness of rock cover beneath superficial deposits as it could be appropriate to increase the depth of the tunnel. The designer should make every effort to minimize tunneling through a mixed face where possible. The arching action is accompanied by redistribution of stresses. There can be movement of blocks of rock and ingress of water acting as a lubricant. Continued redistribution of the stresses can result in rock falls and repetition of the process at higher levels. This can lead to formation of a void above the tunnel profile. Slabs of rock present in the crown can fail in bending. Rock tunnels that break with a square crown should always be treated with caution. The risk of breaking into faults, fissures or cavities should be recognized and assessed. Faults can contain water or gas under high pressure. At faults, changes in the strata can be encountered and there will possibly be zones of extensive fracture, often with slicken sides and fissures filled with gouge or pug. In fault and shear zones, rock can be altered so much as to acquire the characteristics of a soil. This alteration, when combined with the presence of groundwater and higher cover or in-situ stresses, results in some of the most difficult tunneling conditions found. In such fault zones, full-face excavation may be possible if pre-drainage, pre-grouting and fore poling are employed. Advance exploration (e.g. forward probing or drilling), followed by grouting or other special precautions should be considered, particularly in subaqueous tunneling. 1.3.6 Made-ground and contaminated ground Made-ground, mostly encountered in shaft sinking, is usually unstable and requires full support. Contaminated land should be treated before other construction work proceeds. Removal of the made-ground or contaminated material is a possible solution. A latent hazard arises if contamination from the surface seeps below ground to the level of a tunnel. The investigation and information gathering process should establish the likelihood of hydrocarbons, other solvents and bacteria, as well as of hazard gases. 1.4 Indicative methods of tunneling A risk assessment as outlined should be carried out to determine the methods of ground support to be considered. Principal methods of tunneling and ground support that is appropriate for various ground conditions. These methods are indicative only. The initial support may be the first stage in the ground-support system or, as in some uses of sprayed concrete, may be the only functional lining of the completed tunnel. 1.5 Geotechnical processes for ground improvement 1.5.1Ground injection Cementitious or chemical ground treatment in advance of tunneling can usefully enhance safety Particularly in open-face excavation. It does this directly by improving the characteristics of the ground to be excavated and indirectly by sealing off water, or strengthening the overlying or surrounding ground. The design of a suitable grouting system and pattern for particular circumstances is very specialized and expert advice is essential. If sand or gravel is expected in the tunnel face, particularly if it is water-bearing, its permeability can be greatly reduced by the injection of suitable grout mixtures; cohesive strength can be added, and if the tunnel is being driven under compressed air, the air requirements can be greatly reduced. The choice of grout mixture and the spacing and pattern of injection holes will be determined largely by the grain size of each stratum. For example, cement grout will not travel far or prove effective except in very coarse open gravel. Progressively finer grouts of lower viscosity are required in progressively finer soils. Most difficult of all are silts, which can normally be treated only by claque. Clays cannot be treated by permeation grouting. The permeability of the ground is rarely consistent and the possibility of meeting untreated pockets should never be ignored. Groundwater quality should be taken into account when designing the grout. Where excessive fines content in the ground prevents permeation grouting, or where silts and peat occur, jet grouting is an alternative. Jet grouting should only be considered where ground heave is not a problem. Excessive pressures can cause ground heave and damage to surface structures. Where fissured rock is to be treated, it can be difficult to locate and treat all significant fissures, especially if some are filled with gouge or soft clay. Advance ground treatment by grouting should be considered. This can be carried out from the surface, from a pilot tunnel or through the tunnel face during construction. When compressed air is being used in a tunnel it can result in the grout being blown aside during injection. The toxic and environmental risks from chemical grouts should be assessed. In all cases, the chemical properties of the grout used should be taken into account and the risks arising from the handling of the materials during and after mixing, including risks from burst pipes and hoses, should be assessed. The toxicity of some grouts makes stringent precautions necessary, including the provision of protective clothing and full washing facilities. Grouting from within the confined space of the tunnel, particularly in a small pilot tunnel, can increase the risk. Care should be taken to avoid pollution and damage at the surface from spillage and waste discharge. Arrangements for collection and disposal of waste should be made in advance and should be approved by all relevant authorities. Similar arrangements should be made with authorities responsible for the prevention of pollution of underground water. Excavation in ground previously impregnated with cementitious or chemical grouts can release toxic gases or vapours. 1.5.2 Dewatering For shallow tunnels in water-bearing sands and gravels, it may be possible to stabilize the ground by means of well-point dewatering. Care should be taken to prevent loss of fines from the ground. By using deep wells and “down the hole” pumps, greater drawdown of the water table can be achieved. The main hazards of these systems are settlement at the surface due to lowering of the water table, and failure of the dewatering system whilst the tunnel is being driven. The latter causes the ground to revert to its former unstable condition and rapid and massive ground loss can follow. It is essential that the dewatering system be set up with the best available equipment and plant, that systems be duplicated where possible and that surface pipe work be protected from accidental damage. Settlement due to dewatering can affect third-party property and cause damage and injury. Consideration should be given to the following precautions: — The use of a piezometer to monitor the groundwater level; — The construction of recharge wells to maintain groundwater level adjacent to sensitive structures; — The installation of cut-off walls to protect sensitive foundations. 1.6 Instructions to TBM Operators ---Knowing about the Geological fault zones through survey. ---Watch out for the parameters while TBM is advancing especially Survey parameters. ---Cutter head rpm and thrust force to be in correlation. ---Single or Double shield mode operation depending on rock or soil condition. ---Before pushing TBM, operator should notice about Ring building, pea gravel filling, Cement Grout, and other stages ongoing. ---Proper communication between TBM operator and Shift Engineer to be maintained. ---It is important to notice dewatering system is running properly. ---Time to time inspection of cutters to be done and assess about the penetration and thrust force. ---‘Human errors are common’ but not to repeat the same mistake again. ---Operator should see the alarm systems are properly functioning like Grease lubrication, Water level, Hydraulic Oil, Gear oil, Belt conveyor, Gas alarm etc. ---Hot seat exchange should be followed strictly. Prepared as per the instructions of Vice President (tech) during the meeting with HKT & UTS.
Posted on: Wed, 20 Nov 2013 07:29:03 +0000
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