Earthworks of an Underground Structure - Coursework Example

Earthworks of an Underground Structure Name: Unit: Course: Supervisor: Date of submission: Introduction Earthworks entail engineering activities that are created by excavating soils or unformed rocks in the earth service in order to put up underground structures. The underground structures may include roads, beds for railways, canals and fortifications for military use (Boscardin & Cording, 2001). In the earthworks of underground structures there are major concerns that engineers need to concern themselves with especially in relation to safety. The architectural designs of the underground structure should incorporate such concerns. For instance, the density of the soil covering the structure in relation to the strength of the structure needs to be incorporated in the initial design plan. This paper is an overview of important considerations in earthworks of an underground structure. The overview uses a case study of a tunnel (TURL2) for laying railway line in Tabriz a crowded town in Northern Iran Features of Underground Structures Underground structures have common characteristics in that that they are enclosed in soil or under a rock, therefore in the design of the structures, key attention of all the underground structures stem from the consideration of them being not in the surface space. According to Ishiara (2006) underground structures are distinct from the surface structures because they do not resonate, are enclosed in soil or under rock and they experience very high damping because of the radiation of energy coming from the structure to the immediate environment which is the surrounding ground. In the construction of structures both on surface and underground possible risks are assessed, considerations made and measures taken to mitigate the risk (Hashash & Hook, 2002), for instance the risk of earthquake. Earthworks of an underground structure In the construction of underground earth structures, attention is given to the surrounding environment, which includes the surface structures. In the urban centers the underground structures can be close to other buildings. For instance, the Tabriz tunnel passes under foundations of commercial buildings. This implies that support is crucial for the other structures in case an analysis points to eminent impact of the underground structure on the surrounding buildings (Wolf, 2005). Before the onset of the earthwork of any underground structure, the following factors are considered: The control of underground water Ground treatment Ground movement control Water tightness of the structure and the durability. Control of underground water In the construction of an underground structure, the control of the underground water is very important consideration. Serby (2008) noted that the controlling of the underground water is a critical issue which should be incorporated in the initial designs of the structure and measures should be taken during the real work. The design should include strategies that ensure wall retaining. Due to the environmental considerations, the issue of underground water has to be minimally abstracted during the earthworks. Wolf (2005) noted that the underground water regime is an important aspect in earthworks and should be analysed. In addition an underground structure needs to have a wall that will withstand the stress over time. For the tunnel on the corridor TURL2 the water table is found at depths of 8.8 m, this implies that the tunnel design should incorporate measures to deal with this shallow water table. Ground treatment Earthworks of underground structures is a very crucial type of work in which safety of the structure is key. The grounds selected to hold the underground structure must provide the foundation that ensures durability and ability to withstand the stress from different forces. Treatment of the ground is serves as one of the issues which help in boosting the durability of the structure. According to Hashash and Hook (2002) ground treating reduces the ground permeability and is one of the strategies that are used to control the ground water. In addition, the treatment ensures that the strength and stiffness of the earthwork area is improved which in turn reduces the movement of the ground. There are major ways in which the treatment of the ground is carried, the commonly applied methods are the permeating grouting, the freezing of the ground, jet grouting and mixing of soil. Depending on the type of treatment, different earthworks equipment is used (Hashash & Hook, 2002). It is worth to note that not all earthworks require ground treatment; the treatment is depended on the structure and the condition of the ground. Ground movement control In many instances, earthworks will result to deformation of the ground and can affect structures that are adjacent. Therefore, before the onset of the earthworks it is advisable that risk assessment be carried in order to establish the possibility of the movements of the ground. For the underground structures in the urban centers, this assessment ensures that measures are taken that are incorporated in retaining the wall solution and curbing possible ground movements which can deform foundations of nearby structures (Attewell and Taylor, 2004). The deformations are critical in the categorisation of risk. Burland (2007) provided the categorisation that is based on cracks that may be present in an underground structure. The earthworks of an underground structure can result to deformations modes to the nearby building. It is therefore advisable for engineers to evaluate the condition of a building before the commencements of earthworks of new underground structures. This helps in reducing the vulnerability of the building and the new underground structure. The Tabriz tunnel runs under foundations of commercial building, a distance of 13.1m. This tunnel excavation was through a mechanised method that applied the EPB-TBM and has a diameter of 9.2 m Watertightness Another peculiar feature of underground structures is that of the exposure to the external pressure of water. This means that the underground structures are prone to being infiltrated by surface water if the engineers do not take considerable measures to address the issue. Engineers are thus tasked with ensuring that the structures have joints that are watertight and considerations should be made to ensure that materials are used to avoid the pressure coming from the external water. In ensuring watertightness, the engineers can employ the use of concrete that is water tight, ensuring that the joints are water tight and application of waterproof materials. These undertakings are depended on the type of the structure (ZawZaw, Dhiraj and Christian, 2006) Durability Just like in any other works, durability is core in all engineering projects. The underground structures construction and designing should thus put into consideration the issue of durability. This means that designs that resist attack coming from different chemicals in the soil and the general pressure should be incorporated. The attacks normally emanate from chemicals found in the underground water and the soil such as the chlorides and the sulphates. The durability component of underground structures brings different dimensions of the possible external forces that may affect the structure. One aspect is the effect of earthquake on the structure. According to Hashash & Hook, (2008) the impact of the earthquake is normally not influenced by force or stress of the structure on the ground, the main influence is the deformation caused to the ground by the earthworks. Different design approaches are applied in ensuring the aspect of durability. Examples of the approaches include the dynamic earth pressure approaches in which the effects of the earth movement are evaluated by incorporating the pseudo-static analysis, the free field deformation, soil structure interaction analysis and the simplified analysis. The applicability of these approaches are depended on the nature of the structure. For instance, a tunnel that has minimal thickness of soil cover can apply the dynamic earth pressure method while soil structure interaction approach can be applied in all conditions of the structure. Stratification of soil The stratification of the soil is very important aspect of the earthworks of underground structures. Mechanically, the layers of the soil are treated as homogenous, the thickness of the soil layers are taken constant in the area of the earthworks. In the depths of over 65 M the stress variations and deformation of the soil are considered to disappear at such depths. This means that dimensions of the structure should be integrated to the soil stratification depending on the depth i.e. design optimisation. The depths of the excavation of an underground tunnel, the depth of the trenches are considered based on the soil stratification, the design and the zone of influence. The geotechnical aspects in earthworks of the tunnel included the geological analysis of the structure and materials of the earth and the possible influence they may have on the construction. According to Guglielmetti, Grasso, Mahtab and Xu (2008) the soil stratification and segmentation can be achieved through beam element. In the case of the tunnel in Tabriz, the regions soil is mainly silt and has low plasticity. Conclusion The earthworks of underground structures are a great engineering undertaking in which precautions and holistic safety consideration are considered. The considerations revolve around the safety, durability and the general geological factors in the area. In the earthworks of an underground structure, the deformations, the soil stratification and the risk factor of the structure and impacts on the environment are crucial. In the construction of the tunnel, the main consideration is effects to the adjacent building in relation to the ground movement and the impact of the excavation on the adjacent buildings. References Attewell, P.B. and Taylor, R.K. (2004). Ground Movements and their Effects on Structures. NewYork: Chapman and Hall. Boscardin, M.D. and Cording, E.G. (2001). Building response to excavation-induced settlement. Journal of Geotechnical Engineering, 115 (1), pp. 1-21 Burland, J.B. (2007). Behavior of foundations and structures on soft ground. International Conference on Soil Mechanics and Foundation Engineering (SMFE), pp. 495-546. Guglielmetti, V. Grasso, P. Mahtab, A. and Xu, S. (2008). Mechanized Tunneling on Urban Areas – Design methodology and construction control. London: Taylor & Francis Group. Hashash, Y. and Hook, J. (2002). Seismic design and analysis of underground structures. Tunnelling Underground Space Technology, 16(1), 247-293. Ishiara, K. (2006). Soil behaviour in earth movement geotechnics. Oxford: Clarendon Rankin, W. (2008). Ground movements resulting from urban tunnelling: Predictions and effects. Engineering Geology of Underground Movements, 5 (1), pp.79- 92. Selby, A. R. (2008). Surface movements caused by tunnelling in two-layer soil. Engineering Geology of Underground Movements, 5 (1), pp. 71-77. Wolf, J.P. (2005). Dynamic Soil Structure interaction. New Jersey. Prentice Hall. ZawZaw, A., Dhiraj, K. and Christian, S. (2006). Ground movement prediction and building damage risk-assessment for the deep excavations and tunnelling works in Bangkok subsoil: Proceedings of the International Symposium on Underground Excavation and Tunneling, Bangkok, Thailand. Read More

Due to the environmental considerations, the issue of underground water has to be minimally abstracted during the earthworks. Wolf (2005) noted that the underground water regime is an important aspect in earthworks and should be analysed. In addition an underground structure needs to have a wall that will withstand the stress over time. For the tunnel on the corridor TURL2 the water table is found at depths of 8.8 m, this implies that the tunnel design should incorporate measures to deal with this shallow water table.

Ground treatment Earthworks of underground structures is a very crucial type of work in which safety of the structure is key. The grounds selected to hold the underground structure must provide the foundation that ensures durability and ability to withstand the stress from different forces. Treatment of the ground is serves as one of the issues which help in boosting the durability of the structure. According to Hashash and Hook (2002) ground treating reduces the ground permeability and is one of the strategies that are used to control the ground water.

In addition, the treatment ensures that the strength and stiffness of the earthwork area is improved which in turn reduces the movement of the ground. There are major ways in which the treatment of the ground is carried, the commonly applied methods are the permeating grouting, the freezing of the ground, jet grouting and mixing of soil. Depending on the type of treatment, different earthworks equipment is used (Hashash & Hook, 2002). It is worth to note that not all earthworks require ground treatment; the treatment is depended on the structure and the condition of the ground.

Ground movement control In many instances, earthworks will result to deformation of the ground and can affect structures that are adjacent. Therefore, before the onset of the earthworks it is advisable that risk assessment be carried in order to establish the possibility of the movements of the ground. For the underground structures in the urban centers, this assessment ensures that measures are taken that are incorporated in retaining the wall solution and curbing possible ground movements which can deform foundations of nearby structures (Attewell and Taylor, 2004).

The deformations are critical in the categorisation of risk. Burland (2007) provided the categorisation that is based on cracks that may be present in an underground structure. The earthworks of an underground structure can result to deformations modes to the nearby building. It is therefore advisable for engineers to evaluate the condition of a building before the commencements of earthworks of new underground structures. This helps in reducing the vulnerability of the building and the new underground structure.

The Tabriz tunnel runs under foundations of commercial building, a distance of 13.1m. This tunnel excavation was through a mechanised method that applied the EPB-TBM and has a diameter of 9.2 m Watertightness Another peculiar feature of underground structures is that of the exposure to the external pressure of water. This means that the underground structures are prone to being infiltrated by surface water if the engineers do not take considerable measures to address the issue. Engineers are thus tasked with ensuring that the structures have joints that are watertight and considerations should be made to ensure that materials are used to avoid the pressure coming from the external water.

In ensuring watertightness, the engineers can employ the use of concrete that is water tight, ensuring that the joints are water tight and application of waterproof materials. These undertakings are depended on the type of the structure (ZawZaw, Dhiraj and Christian, 2006) Durability Just like in any other works, durability is core in all engineering projects. The underground structures construction and designing should thus put into consideration the issue of durability. This means that designs that resist attack coming from different chemicals in the soil and the general pressure should be incorporated.

Read More