The Retaining Wall and Its Concepts - Report Example

Retaining wall Name Date Course Retaining Wall Introduction The topography between Gilwern and Brynmawr where the road construction project will be carried out has a hilly topography. The terrain is quite unstable as it is muddy with loose soil. This has negative impact on the project. The loose muddy soil may fall over and block the road. This may result to road accidents and it will also render the road impassable. To avoid this problem, a retaining wall is required along several parts of the roads. The design of the retaining wall has to take into consideration various factors so as to ensure stability. The retaining walls can be used effectively in dealing with the problem of landslides in the hilly areas through stabilizing, filling and cutting the slopes (Khajehzadeh, Taha, El-Shafie, & Eslami, 2011). A lot of excavations have to be carried out and this may have negative impacts on the environment if it is not carried out well. The environmental aspects therefore need to be considered during the design of the retaining wall. Several factors have to be considered during the design of the retaining wall including the different type of pressure that the wall is likely to experience. Different types of material will be required during the construction of the retaining wall. Various engineering details will also be required for the purposes of ensuring that the retaining wall is stable. The transportation of the construction materials as well as the material that has been excavated needs to be considered. The paper is a report for consideration for the team with design details for the retaining wall. Discussion Purpose of the retaining wall and its concepts The retaining wall will be for resisting the lateral pressure from the soil as a result of changes in the ground elevation due to the road construction process. The angle of response of the soil is likely to be affected as a result of the road construction process. The retaining wall must also resist the water pressure resulting from seepage from rainfall as well the ground water. A wedge of soil has to be supported by the retaining wall. The wedge of soil is an engineering term that is used to define the soil which extends beyond the failure plane of the soil (Yu, Tan, Wang & Ding, 2012). There is always a tendency of the loose soil to move downwards due to gravity. This usually results to lateral earth pressure which needs to be countered by the retaining wall. The angle of internal friction and the cohesive strength of the soil of the materials being retained is an important factor that determines the movement that the retaining structure is likely to undergo. At the top of the retaining wall, the lateral earth pressure is zero. The lateral pressure increases down the slope. The pressure is thus high at the base of the slope and this requires the retaining wall to have a strong foundation to avoid overturning. The retaining wall is also likely to face hydrostatic pressure due to the groundwater. The hydrostatic pressure can be reduced in the presence of drainage or drainage materials. Factors to be considered during the design Earth and water pressure in the retaining walls During the design process, the lateral earth pressures that are expected to act on the wall has to be considered. This mainly involves the at-rest earth pressure as well as the active earth pressure. The passive soil resistance also needs to be considered during the design. Since the road expansion will lead to excavations, the stability of the slopes may be affected and this may also have impacts on the earth pressure (Huang & Lin, 2013). Water pressure has to be considered during the design process. Since the soil is muddy, the water table may be high at the hills and hence creating water pressure. The design should also consider how the water can be drained in order to reduce the water pressure. Each of the pressure has to be multiplied with the factor of safety in order to ensure that the retaining wall can sufficiently deal with the pressure. Stability checks The stability checks have to be considered in order to ensure that the retaining wall does not collapse as a result of pressure. The external stability checks have to be carried out on the walls with the assumption that the whole structure is a single body (Dong, Ma & Zhu, 2013). The sliding stability of the retaining wall has to be considered during the design. This mainly involves the horizontal components of the forces. The horizontal shear resistance at the base of the retaining wall has to be calculated using the following formula: Base Shear Resistance= Sum of vertical forces X Tan Ө +Base Length x Soil wall Adhesion During the calculations, the factor of safety is at least 1.5. The overturning stability of the retaining wall has to be considered during the design process. The overturning stability considers the overall stability of the wall against toppling (Wu & Yan, 2012). The stability is usually calculated using the following formula: Factor of safety overturning= Resisting moments/Driving moments Bearing stability in the retaining wall The foundation of the retaining wall is usually on a base material which may be a rock or soil. The bearing stability of the soil is critical when the base of the retaining wall is placed on a soil (Brown, et al, 2011). The bearing stresses have to be computed on the toe and heel of the wall. This calculation is important in ensuring that the retaining wall does not overturn. The permissible bearing stresses have to be considered and the minimum factor of safety has to be 3.0. This is because the terrain is hilly and the soil is loose and muddy. A high factor of safety has to be considered to ensure that the wall settlement is kept within the acceptable levels. Detailed settlement calculation may however be required if the settlement control is considered critical. Global stability and structural checks of the retaining wall The risk of rotational failure is also present after the other factors have been considered. Such failures are common in hillsides where the soil zones are weak. The presence of geo-materials on the base of the wall can also lead to this failure. This has to be considered during the design as the soil in the region is weak. The structural checks have to be carried out during the design after the stability checks are satisfactory. The longitudinal sizing and shear reinforcement is required to ensure that the structure is able resist the external pressures (Xu-jun, 2012). Reinforcement is also required for the purposes of ensuring that the retaining wall does not bend as a result of the forces and shrinkage effect. This factor is important and it must be considered during the design of the retaining wall. Design Criteria During the design of the retaining wall, the British Standards (BS) 8002 and the International Standards Organization (IS ) should be use in order to ensure that the retaining wall is effective. The design has to take into account two main features which involve the analysis of loads and pressures that may act on the structure as well as the design of the retaining wall that may withstand the loads and pressures. The earth pressure resulting from surcharge has to be considered as it affects the stability of the structure. A factor of safety of at least 2.0 should be used to guard the wall against overturning. The factor of safety against sliding should be at least 1.5.The factor of safety against floatation should be at least 1.25 (Anderson, Gladstone, & Sankey, 2012). Since the retaining wall will be located in some areas with steep hills, the factor of safety for the slip surface below foundation has to be greater than 1.5 and 1.0 during the static as well as seismic conditions. In order to ensure that the minimum factor of safety is achieved, the base area has to be increased. This can be achieved through the addition of a concrete key monolithic supported with a foundation slab. The allowable bearing capacity should be directly related to the maximum pressure. The self weight cannot be determined for the entire wall but it should be carried out per meter. The retaining wall should slope back into the retained slope in order to ensure that it is more stable and structurally secure. This factor has to be considered during the design as it will influence the structural design of the retaining wall. The foundation of the wall should be constructed on a well compacted base material. Gravel foil is required at the site since the soil is loose and muddy. The base width has to be wide enough in order to ensure that it is stable. The material selected for the construction of the retaining wall must be able to withstand the expected pressure (Camp & Akin, 2011). Reinforced Concrete wall should be used as the expected pressure on the wall is likely to be high. Drainage at the retaining wall is an important factor that must be considered during the design process. Pressure from water or soil moisture against the wall is usually one the main contributing factors of the failure of the wall. Drainage pipes, drainage blankets and gravel backfill can be used for the purposes of draining the water. Planning ahead is also important and it should be carried out during the design process. Contingency plan s should be put in place in order to ensure any uncertainties can be dealt with effectively without affecting the construction process. The environmental consideration should also be planned during the construction process. Design details The depth of the wall is an important factor that has to be considered during the design. The depth of the wall below the ground is dependent on the availability of firm ground. In rocky areas, the depth of the wall is dependent on the depth of the firm rock below the grounds. However, on other soil conditions, the depth must be a minimum of 500mm. The dip of the base of the wall is also important in preventing sliding (Tang, Zheng & Jie, 2014). The dip of the wall towards the hillside should be at the ration of 3:1. This is economic and it also increases the factor of safety against sliding. A negative plays an important role in stabilizing the wall and its recommended ration is 1:3. The wall geometry should have an inclined face and the base should also be inclined with the hill. During the selection of the dimensions of the wall, the allowable bearing capacity has to be greater than the foundation pressure. Since the location is muddy, it is likely that the water table is high. This means that the flow of water may end up affecting the road stability of the wall. Common territory Retaining wall for hilly areas A drainage plan should be put in place in order to ensure that the water can be drained. An inverted filter is required to drain off groundwater or the rainwater seepage. The weep holes have to be provided at a spacing of 1.5m center to center in either direction. It is recommended that the size of the weep holes should be 100mm or 150mm PVC pipes which has to be embedded at 10 degrees from the horizontal towards the valet side in order to drain water from the ground. Impervious silty soil layer has to be provided on the top in order to prevent the seepage of rainwater. Self draining materials such as coarse sand, boulders or gravel can also be used. Prevention of erosion is also important as it may affect the stability of the wall (Yu, Tan, Wang & Ding, 2012). This can be prevented through planting of grass and any natural gullies must be diverted away from the building site. The flow of rainwater may also result to the erosion of soft rocks at the foundation of the wall. This can be reduced through dry stone pitching. This may be achieved by laying stones of 150mm in size below the toes of the retaining wall at a distance of 1meter. A soil test must be carried out in order to provide more information that can be used during the design process. Safe design factors to be considered In order to obtain a safe design, no sliding requirement has to be fulfilled . This can be calculated using the following formulas: =>1.5 (For Stability) iii).  Where: = Coefficient of friction between the base of the retaining wall and the soil.  Sum of all the vertical forces The design must also ensure that the wall is safe against overturning at the toe. The following formula can be used for calculating against overturning:  ≥ 1.5 = The design also needs to consider bearing capacity failure and tension. The first step is to calculate the resultant force (e) using the following formula:  = Therefore the resultant force (e) =-≤ During the design, it is important to ensure that the pressure at the toe of the wall should not exceed the allowable bearing capacity of the soil. An assumption has to be made regarding the pressure at the base and it has to be assumed to be linear. The maximum and the minimum pressure can be calculated using the following formula: = {1+} ={1-} The design should also ensure that the maximum pressure is less than the safe bearing capacity. This can be confirmed using the following formula: =   Design loads Various loads are expected and this should be considered during the design. The dead loads are mainly comprised of the unit weight of the building materials together with any other load that is not part of the retaining wall. Imposed loads are also likely to be encountered and this type of load is mainly artificial. The wind load is expected as winds are common at the slopes (Khajehzadeh, Taha, El-Shafie, & Eslami, 2011). This has to be factored in during the design process as such loads may contribute the collapse of the structure. Although the area is not prone to earthquakes, it is important for the design to ensure that the foundation can resist earthquakes. During the design process, the analysis of the loads should be carried out in accordance with the British Standards Code. A lot of flexibility is also required during the design process in order to accommodate the different loads that are likely to be encountered. Environmental considerations The construction process of the retaining wall is likely to have a lot of negative environmental impacts if control measures are not put in place. Excavations are expected along the stretch of the road and this is likely to have negative impacts on the environment. A lot of dusts as well as noise are also expected during the construction process. The site must be enclosed in order to ensure that the dust from the construction site does not spread, to the other parts. Soil erosion is common at the constriction sites during the construction process (Xu-jun, 2012). This can be attributed to the hilly nature of the terrain along the road. Gabions have to be constructed at some of the selected points in order to prevent the soil erosion and block the natural gullies. All the materials from the excavation should be collected and backfilled at the correct dumping sites. This means that the dumping sites have to be identified in advance. Dumping at the site should not be carried out as it is prohibited by the environmental laws as it contributes to the environmental degradation. A lot of noise is also expected at the site during the construction process. Noise reduction strategies should be put in place for ensuring that the impacts on the environment are reduced. The environmental conservation should be prioritized during the construction process. Temporary roads should also be developed for the transportation of the materials to and from the site. Conclusion In Conclusion, it is evident that the design of the retaining wall during the expansion of the road has to consider various factors. The stability of the retaining wall is the most important factor that has to be considered during the design process. Various calculations for determining the stability of the retaining wall has to be carried out. It is evident that the earth pressure and the other forces are likely to affect the stability of the retaining wall. It is evident that different formula can be used for the calculation of stability. The design has to ensure that the retaining wall is able to counter sliding, slipping, bearing capacity and overturning. It is evident that the soil tests have to be carried out in order to aid in the design process. The forces have to be analyzed with a lot of accuracy to avoid the collapse of the wall. The minimum and maximum capacities have to be established during the design process. It is evident that the drainage should be part of the design in order to avoid the collapse of the wall as a result of the pressure from water or soil moisture. It is evident that the design has to consider the factor of safety in order to prevent the collapse of the wall. It is evident that the design also needs to consider the environmental aspects. The materials excavated from the site should be backfilled at the appropriate areas in order to ensure that environment is not affected during the process. Bibliography Khajehzadeh, M, Taha, M, R, El-Shafie, A, & Eslami, M, 2011, Modified particle swarm optimization for optimum design of spread footing and retaining wall, Journal of Zhejiang University SCIENCE A, 12(6), 415-427. Yu, B, Tan, Y, Z, Wang, N, J, & Ding, P, L, 2012, Strip type rebar of the rear sets of spherical bodies anchorage reinforced earth retaining wall design model, In Applied Mechanics and Materials (Vol. 170, pp. 361-365). Huang, Y, P, & Lin, J, W, 2013, Interactive remote computations for retaining wall design and risk assessment, Natural hazards, 66(2), 985-993. Jeldes, I, A, Drumm, E, C, Bennett, R, M., & Zisi, N. (2014). Piling Framed Concrete Retaining Wall: Design Pressures and Stability Evaluation. Practice Periodical on Structural Design and Construction, 04014041. Dong, J, H, Ma, W, & Zhu, Y, P, 2013, Seismic analysis and design method for soil nailing retaining wall, Zhongguo Gonglu Xuebao(China Journal of Highway and Transport), 26(2), 34-41. Sander, A. C., Fox, P. J., & Elgamal, A. (2014). Full-scale seismic test of MSE retaining wall at UCSD. In Geo-Congress 2014. Talatahari, S., Sheikholeslami, R., Shadfaran, M., & Pourbaba, M. (2012). Optimum design of gravity retaining walls using charged system search algorithm. Mathematical Problems in Engineering, 2012. Wu, W, & Yan, D, 2012, Analysis on the Collapse Reasons of a Multi-used Building’s Retaining Wall in Kangding, Physical and Numerical Simulation of Geotechnical Engineering, (6). Brown, A, C, et al, 2011, Long-term monitoring of a drilled shaft retaining wall in expansive clay: behavior before and during excavation, Proc Geo Front Adv Geotech Eng, Mar, 13-16. Xu-jun, W, 2012, Analysis of retaining wall deformation for deep and big foundation pits podium in Shanghai Tower, Chinese Journal of Rock Mechanics and Engineering, 31(2), 421-431. Anderson, P, L, Gladstone, R, A, & Sankey, J, E, 2012, State of the practice of MSE wall design for highway structures, Proceedings of the Geocongress 2012, state of the art and practice in geotechnical engineering, Oakland, March 25, 29. Camp, C, V, & Akin, A, 2011, Design of retaining walls using Big Bang–Big Crunch optimization, Journal of Structural Engineering, 138(3), 438-448. Tang, X, S, Zheng, Y, R, & Jie, L, 2014, Application of FEM Strength Reduction Dynamic Analysis in the Seismic Design of Reinforced Earth-Retaining Wall with Geo-Grid, In Applied Mechanics and Materials (Vol. 580, pp. 1419-1425). Read More

Stability checks The stability checks have to be considered in order to ensure that the retaining wall does not collapse as a result of pressure. The external stability checks have to be carried out on the walls with the assumption that the whole structure is a single body (Dong, Ma & Zhu, 2013). The sliding stability of the retaining wall has to be considered during the design. This mainly involves the horizontal components of the forces. The horizontal shear resistance at the base of the retaining wall has to be calculated using the following formula: Base Shear Resistance= Sum of vertical forces X Tan Ө +Base Length x Soil wall Adhesion During the calculations, the factor of safety is at least 1.5. The overturning stability of the retaining wall has to be considered during the design process.

The overturning stability considers the overall stability of the wall against toppling (Wu & Yan, 2012). The stability is usually calculated using the following formula: Factor of safety overturning= Resisting moments/Driving moments Bearing stability in the retaining wall The foundation of the retaining wall is usually on a base material which may be a rock or soil. The bearing stability of the soil is critical when the base of the retaining wall is placed on a soil (Brown, et al, 2011).

The bearing stresses have to be computed on the toe and heel of the wall. This calculation is important in ensuring that the retaining wall does not overturn. The permissible bearing stresses have to be considered and the minimum factor of safety has to be 3.0. This is because the terrain is hilly and the soil is loose and muddy. A high factor of safety has to be considered to ensure that the wall settlement is kept within the acceptable levels. Detailed settlement calculation may however be required if the settlement control is considered critical.

Global stability and structural checks of the retaining wall The risk of rotational failure is also present after the other factors have been considered. Such failures are common in hillsides where the soil zones are weak. The presence of geo-materials on the base of the wall can also lead to this failure. This has to be considered during the design as the soil in the region is weak. The structural checks have to be carried out during the design after the stability checks are satisfactory. The longitudinal sizing and shear reinforcement is required to ensure that the structure is able resist the external pressures (Xu-jun, 2012).

Reinforcement is also required for the purposes of ensuring that the retaining wall does not bend as a result of the forces and shrinkage effect. This factor is important and it must be considered during the design of the retaining wall. Design Criteria During the design of the retaining wall, the British Standards (BS) 8002 and the International Standards Organization (IS ) should be use in order to ensure that the retaining wall is effective. The design has to take into account two main features which involve the analysis of loads and pressures that may act on the structure as well as the design of the retaining wall that may withstand the loads and pressures.

The earth pressure resulting from surcharge has to be considered as it affects the stability of the structure. A factor of safety of at least 2.0 should be used to guard the wall against overturning. The factor of safety against sliding should be at least 1.5.The factor of safety against floatation should be at least 1.25 (Anderson, Gladstone, & Sankey, 2012). Since the retaining wall will be located in some areas with steep hills, the factor of safety for the slip surface below foundation has to be greater than 1.5 and 1.0 during the static as well as seismic conditions.

In order to ensure that the minimum factor of safety is achieved, the base area has to be increased. This can be achieved through the addition of a concrete key monolithic supported with a foundation slab. The allowable bearing capacity should be directly related to the maximum pressure.

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