The sides of the shaft can be supported by walling in concrete. Construction is carried out using a variety of sheet pile sections, piling hammers and methods depends on application, environmental factors and plant availability. The sheet piles are supported by prestressing bar behind the sheet piles, as shown in Fig 4. Sheet piling can also be used as groundwater exclusion as a mean of groundwater control (Fletcher & Lavan, 1987) & (Woodward, 2005).
However, this method is very expensive but would be more cost-effective if it is used as permanent structure. Also any major disruption can cause poor standards of wall construction which may be difficult and expensive to resolve when it happens. Besides that, the quality of the work is directly dependent on the subsoil strata since the soil forms a temporary shutter against which the in-situ concrete is cast (Puller, 1998).
Another method of groundwater control is by using Ammonium/brine refrigeration (freezing). It is a temporary cut-off barrier to exclude groundwater in most saturated and near-saturated soils by lowering the ground temperature to form a wall with contiguous ice cylinders, as shown in Fig 8. The formation of ice in the voids will stop the water flow into the site of work, as shown in Fig 9 (Woodward, 2005).
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The bad side of this method is that the treatment takes time to develop; 2-3 months for system installation, 1.2-2.5 months to form freeze, and 4-7 months to maintain freeze during construction (Bickel, Kuesel & King, 2002). Besides that, the initial installation costs are extremely high and refrigeration plant is expensive and it only economic at greater depth. Strict site control is required and hence leads to higher cost as well. Moreover, the ground may heave during freezing which will affect the adjacent structures, as well as thawing may leave voids in the soil which cause settlement under load and self-weight (Puller, 1998).
After the completion of shaft, tunnel boring will subsequently starts. During the boring of tunnel, groundwater control is fundamental to prevent structural failure as construction of tunnel will change the groundwater regime of locality as tunnel usually acts as a drain (Bell, 2004).
NATM is flexible to adopt different excavation geometries and able to create very large cross sections. This is indeed a lower cost for creating the tunnel and low cost requirement for TBM equipment at the beginning of the project. NATM is easy to install a waterproof membrane. Besides that, it is flexible to monitor deformation and stress redistribution so that necessary precautions can be taken. NATM also gives less overall support cost as sufficient support for the loadings and ground conditions without being excessive can be identified and the primary support such as shotcrete can be installed easily by providing a good contact surface between support and ground. NATM offers short lead-in time before start of works which is very much cost-effective as time cost money (Karakus & Fowell, 2004) & (Harris, 1996).
The construction of the bridge pier will be firstly constructed. Cofferdams and caissons are used for excavations either in waterlogged or unstable ground that requires lateral support, or when excavations are made below water level. They enclosed an area down to foundation level from which water is excluded sufficiently to permit excavation and placement of a foundation structure to take place. The principal factors influencing the choice between cofferdams and caissons are ground conditions and proposed depth of the foundation level (Bell, 2004).
Whether created in-situ or using precast segments, the balanced cantilever or increment launching method is one of the most dramatic ways of building a bridge. Work starts with the construction of the abutments and piers. Then, from each pier, the bridge is constructed in both directions simultaneously. In this way, each pier remains stableand hence ââ‚¬Ëœbalancedââ‚¬â„¢ until the individual structural elements are finally meet and are connected together. In addition, the segments are progressively tied back to the piers by prestressing tendons or bars threaded through each unit (Fletcher & Lavan, 1987).
It is undeniable that this method minimises the disruption to existing traffic networks under the bridge (waterways) and provides clear, unobstructed access to all work fronts.ÂÂ Also, support falsework which is expensive and hazardous is also reduced due to temporary works being relocated by gantry or lifting frame and there is no risk of differential settlement of falsework under the water or ground. The temporary loads of gantry or lifting frame are introduced directly into piers. Besides that, faster rates of erection are possible and hence lead to shorter construction duration and save the total project cost. Also, the optimization of crew size and reduction in crane demand reduce the labour and machinery costs as well. This minimization of costs will have low cost impact on the total project cost (VSL, n.d.), (Rosignoli & Ing, n.d.).
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In conclusion, in order to have low cost impact of the tunnel construction cost, Earth pressure balance (EPB) method of Tunnel Boring is suggested where the typical costs is lower than Slurry Pressure Balance (SPB). In contrast with conventional method, the temporary ground support can be reduced or eliminated where significant cost and time can be saved. As for bridge, the segmental method of construction is favourable as labour and falsework or formwork can be minimized to the minimum. Also, significant period of time can be saved, which is also one of the cost-incentives criteria.