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Temporary works are used during tunnel and bridge construction as support structures for scaffolding, removal of debris, movement of construction material, guides and ways of boring and concreting machines. Temporary work types include formwork, falsework, temporary retaining structures, Cofferdams, cribs and shoring for foundation construction, foundation seal, Wood Form, Metal ties, anchorages, Metal Forms, etc. These structures are of temporary nature and may often have to be dismantled when one section has been done and again erected for a construction of a new section of the bridge or tunnel. Some engineers would regard these structures as a waste of resources and time since they would not be a part of the final bridge or tunnel. However, for some engineers the temporary works are inevitable; it would be impossible to construct the bridge, and tunnel without these temporary works (Illingworth, 2007). This report examines the role of temporary works and equipment in large scale Tunnel and Bridge construction projects.
2. Role of Temporary Works for Tunnels and Bridges
The BS 5975: 2008 standards specify the code of practice for temporary works procedures. It also provides the allowed stress design of false works and the parts of the structure that would allow construction of structures. The standards identified such structures as those used for the support of equipment, plants, and machinery used in the construction. Temporary and false work also includes structures that are built to provide access to men, material and machinery for different areas of the structure being built. The standard indicates that the temporary works may or may not be retained in situ after the work is completed. However, just because a structure is temporary, it does not mean that it can be weak or dangerous. All such structures are strictly monitored by the Construction Design and Management Regulations 2007 (CDM 2007) and the associated Approved Code of Practice ACOP. Project owners, contractors, vendors and those responsible for erecting and making people work on such works are held responsible for the health and safety of employees and visitors to the site (HSE, February 2010).
2.1. Importance and Role of temporary works
Temporary works are classified into two categories. These are major temporary works, and temporary works. Major temporary works often involve parallel construction of a service tunnel or a bridge and in many cases; this works is used for the purpose of laying concrete to the adjacent main line. It would also carry material, equipment and other infrastructure equipment. The role and importance of the temporary work would depend on the type of tunnel and bridge being constructed, nearby access areas and the restrictions on the structure being built. A tunnel for only rail traffic in an open country would have sufficient access areas and the temporary work can be larger, meaning it can take more loads and the rate of work is increased. The nature of soil in the tunnel and the surrounding area also becomes a very important factor. A loose alluvial soil can mean that the temporary structure would have to be very strongly reinforced with concrete structure. If the soil is hard and has a rock substrate, then the temporary works can be smaller. The Medway Tunnel under the River Medway in the Kent County is a good example of how temporary works were used to construct a tunnel (Mainwaring, 2000). Please refer to the following diagram of the tunnel and temporary works for the Medway Tunnel.
Figure 2.1. Temporary Works for Medway Tunnel (Mainwaring, 2000)
The Medway Tunnel under the Medway River was to be constructed under a very soft loamy soil. There were dangers of not only the equipment being stuck in the soft soil but also that the tunnel structure itself would settle into the riverbed. It was decided that immersed tube tunnels would be used. Temporary works would be required for the float out and immersion for different elements of the tunnel. After the tunnel was constructed, it was decided to retain the temporary works since the tunnel was built in a wilderness area and there would not be issues of space restriction. The temporary works that were built included cut and cover tunnels made of permanent diaphragm walls, cast in place concrete, combi-pile wall arrangement, gravity structures, piled slabs and different methods for placing sand on the embankment (Mainwaring, 2000). Following figure gives a schematic of how the tunnel, river and road were placed.
Figure 2.2. Schematic of temporary structures and tunnel (Mainwaring, 2000)
In the above case, it can be seen that temporary works mainly helped in constructing the immersed tunnel. Else, it would not have been possible to construct the tunnel. The temporary works not only helped the immersed tube tunnel to be laid bit also served as support structures.
2.2. Some type of Temporary works
There are many types of temporary works and each would be used for certain specific structures. For tunnels, most of the structures are below ground. For bridges, the temporary works would be adjacent to the bridge and they would have to be removed after the bridge is ready. One very important point is that supporting temporary structures should always share the load of the main structure. They should always act as load bearing members. They should never force the main structure to bear the load. This possibility can happen when to cut costs, the temporary works is not given sufficient foundation and then it starts leaning on the main structure. Some examples of temporary works for bridge and tunnel structures are given. Items such as scaffolding are also temporary works but these are not covered in this paper since they would be used for all types of structures. Given below are some examples of components used to make temporary works. These components are building blocks and they would be cast in place, welded, grouted, bolted and other joining mechanisms used to create the structures (Barr, 2006).
Figure 2.3. Some examples of Temporary Works Components (Barr, 2006)
In many cases, reinforced masonry is used to construct the temporary works. This is done in the case of bridges and tunnels where the temporary work is not expected to carry the load of large structures such as the bridge. In any case, the term temporary does not mean filmy, weak or unstable. The foundation of the temporary works is buried underground and cannot be seen. Temporary works walls can be constructed from solid and hollow masonry units and irrespective of the construction method, masonry is mandatory. There are different types of masonry walls. The types are concrete or clay masonry walls; load bearing and non load bearing walls; single and multiple Wythe walls (Zhao, 2009). Other types are solid or partially grouted walls, reinforced concrete walls, composite walls; cavity walls, curtain walls/ panel walls; and screen walls; veneer walls, engineered walls and other types. Based on the load carrying capacity required, the temporary works are classified into two main types of walls and these are load bearing and non-load bearing. Load bearing works help to hold up and distribute the weight of the structure and they are mainly subjected to compressive and shear loads. Non-load bearing walls are like partitions that may extend from floor to ceiling and they do not bear and distribute any loads. As per the MSJC-08 Section 1.6, load-bearing walls are the ones that have to carry at least 200 pounds per linear foot of vertical load in addition to their own weight (Minaie, 2009).
Figure 2.4. Temporary works for tunnel boring (Burland, 2004)
In the case of tunnel boring, temporary works provide access to the operational areas of the bore. Much depends on the soil structure of the tunnel. Soft loamy sandstone is the easiest to bore but it would give away very easily. Hence, it is important that there should be sufficient shoring up and support given for the walls. This would have to be done quickly, just as the boring is completed, so that the walls do not collapse. Temporary works reinforcements can be vertical or horizontal or a combination of both. When hollow units are used, they are positioned so that their cores align vertically and allow the placement of reinforcement bars in the cells of the grout. Structural requirements may mean that only the cells that contain reinforcement would be grouted. Vertical reinforcement in tunnel and bridge walls is carried out so that there is sufficient flexural strength in the vertically spanning walls. The vertically reinforcement however does not carry any compressive loads since the vertical reinforcement bars cannot be restrained laterally by means of ties (Burland, 2004).
This concept used in the Channel Tunnel construction. Please refer to the figure shown below.
Figure 2.5. Channel Tunnel Schematic and Temporary Works (Zhao, 2009)
The channel tunnel was one of the most challenging projects since there was the danger that the inner walls could buckle and lose their compressive strength. The walls in the tunnel are higher and horizontal reinforcement is given to provide shear reinforcement in shear walls and to resist flexure in the horizontally spanning walls. It must be noted that designers provide vertical and horizontal reinforcement based on the structural specifications and needs. In some cases, the reinforcements are spaced at greater distances and such structures are called as partially reinforced or lightly reinforced walls. In such structures, the walls are considered to have strips of reinforced and un-reinforced masonry that spans the structure. Reinforced strips can be horizontal or vertical and this would depend on the position of the reinforcing bars ((Zhao, 2009). Please refer to the following figure.
Figure 2.6. Extended reinforced masonry temporary works (Taly, 2005)
Vertical reinforcement is usually grouted in vertically aligned cells and the reinforcement that runs on the grouted masonry units serves as a vertical beam. It should be noted that un-reinforced masonry spans horizontally between the strips and helps to resist the buckling vertical loads. Bond beams that are provided at the wall bottom and top areas serve as horizontal beams. In this case, the un-reinforced masonry strip that runs between the beams helps to resist lateral loads. As per the MSJC code, composite walls and non-composite walls can built as multiple Wythe walls, Composite masonry would have multiple components in the masonry members that act with a composite forces. Wythes that are built in non-composite walls would be acting independently to resist the loads. Composite action refers to the internal mechanism in the unit that allows the stresses to be transferred between the components. This feature allows the combined component to act as an integral unit and thus heavier loads can be resisted. The term, bonded wall is also important and in this type, two Wythes are bonded with others to create structural units (Taly, 2005).
The Incheon Bridge project in Korea is a good example of how temporary works have proved to be indispensible for the structure. The bridge is 12.3 kilometres long and the cost is about 1.4 billion USD. The longest span would be 800 meters long and the bridge is designed to withstand tsunami, seismic activity and even the impact of a 100,000 tons dead weight ship. For the bridge, the launching gantry was regarded as a major temporary works since the structure would be used to lift the heavy pre stressed concrete structure and place them in position. Among other temporary works was the temporary access jetty of two kilometres that was meant to provide access for low-level viaducts. There were also temporary back spun piers for supporting the cable stayed bridge deck when the construction was proceeding. There were also temporary struts that were designed to prop the inclined pylon legs when jump forming was taking place, In addition there was a temporary overhead gantry needed for the viaduct construction. It is important to understand about composite temporary walls and the reinforcements since modern constructions use these types of structures. These walls would be made of block-to-block, brick to brick or even brick to block Wythes. The collar joint is filled with grout or mortars and reinforcement is provided in the form of metal ties that connect the Wythes. The collar joints can have a width that varies from 4 inches to 3/8 inches and these joints can be reinforced either vertically or horizontally. In some cases, the Wythes can be placed in the brick or the block Wythe or two Wythes can be connected by means of headers. Composite walls have the property of being able to resist high loads and they are more cost effective. Such walls are used as exterior walls where the exterior facing Wythe would have concrete brick or split block. The backing Wythe can be made of solid concrete or hollow blocks. It would not be possible to construct such structures without using temporary structures (Marengo, 2006).
2.3. Cost impact of Temporary works
According to Illingworth (1987), the cost of construction of the temporary works often is as much as 15 to 20% of the structure costs for bridges and tunnels. However, these costs are supposed to be included and have to be considered by the designer and people involved in costing when making the bid. Generally, the bid for the structure includes costs of temporary works also and separate bids are not accepted for them. Even the removal of temporary works is not paid for by the project owner who has to make internal provisions. In some cases, designers attempt to reduce the cost impact by using the temporary works as extensions or support for the main structure. In other cases as in bridge and tunnels, the temporary works are used for service and maintenance.
The report has examined the role of temporary works and equipment in large scale Tunnel and Bridge construction projects. It was seen that in many cases, temporary works were crucial to building the bridge and tunnel. Temporary works provide support and access to the main structure. Construction crews use the temporary works to carry out the work such as concrete casting, steel reinforcement beam placement, material movement, and crucial support during the initial stages where concrete is poured and so on. The cost impact was examined and it was seen that temporary costs typically have a cost burden of 15-20% of the project cost. Therefore, the conclusion is that temporary works are an integral and indispensible part of construction. They cost a substantial part of the project and the costs have to be adjusted in the bid price since separate bids for temporary works are not accepted.