The Improvement of the A465 Heads of the Valleys Road between Gilwern and Brynmawr - Coursework Example

SECTION 2: RETAINING WALL Client Insert Name Client Insert Institution Client Insert Date Section 2: Retaining Wall Introduction The improvement of the A465 Heads of the Valleys Road between Gilwern and Brynmawr from a single 3-lane carriageway to dual e carriageway is dependent on different engineering constructions that strengthen the road as well as making it well equipped for its function. Given the topography and the environmental constraints on the road, the use of extensive retaining structures is paramount to try and reduce land and take impact on sensitive areas of the road. It is estimated that over 115km of these structures are required for to support the road structure required (Crosbie & Watson 2005). The section where the retaining structures are to be constructed are shown in figure 1a highlighted with bold black rectangles. This paper provides a detailed description of the suitable design of the retaining structure that can be considered for this project giving the exact attributes of the structure that is required. Fig. 1a: Section of the Road showing where the Retaining Structures are to be built The Project’s Retaining Wall Design Overview Before starting the design of a retaining wall to support a road, it is important to determine the purpose for which the wall is designed. For this case, the wall will be designed to hold soil on one side of the road and will be free standing on the other side. This will help accommodate changing grades in landscape especially around the place of the road where it has been identified with an uneven topography (Crosbie & Watson 2005). Because of these specific attributes of the wall, it is able to allow a steeper cut to a slope so as to be usable space on a cut-and-fill hillside of a road and this makes them very vital in leveling a driveway as is the case in this project. The most important attribute that makes retaining walls such as this one being designed for this case is the amount of lateral earth pressure (load) that they carry with the traffic that continuously uses the roads which they support. In order to make a good design, all the constituents of lateral earth pressure including the weight of the soil, water that builds up hydrostatic pressure and the weight of the traffic flow, have to be adequately determined since inability to determine these constituents of the lateral earth pressure could be the reason for the failure of the retaining wall eventually (Ching, Faia & Winkel 2006). This design seeks to design a concrete safety barrier to support the road. These structure as well as its accompanying Mechanically Stabilized Earth (MSE) is designed using a pseudo-static design method which recommends the design of a moment slab measuring 350mm thick by 1250mm wide by 6000mm long at the very least as shown in figure 1b below. This design is informed by the provisions of The American Association of State Highway and Transportation Officials (AASHTO) as regards the design and construction of MSE (Ching, Faia & Winkel 2006). Fig. 1b: Precast Concrete Barrier and Moment Slab for the Retaining Wall The retaining wall designed as a barrier system is supposed to contain and safely redirect a vehicle during impact event that may occur. This impact should be contained sufficiently so as not to be transfer the impact caused to other parts of the structure which means that the design has to consider the parapet shape, overall mass stability, and internal strength in its calculations and development (Ching, Faia & Winkel 2006). According to Bowles (2011), the design requires a 20mm gap between the throat of the precast barrier and the backside of the panels facing it to provide capacity for the retaining wall to absorb load impacts created to the MSE wall panels. When developing this barrier, it is required that there be a 20mm thick compressible foam material be placed on the hind side of the facing panels before the moment slab is poured into place. This arrangement does not allow contact between the barrier and the panel and this is good for the design as it ensures that all the horizontal impact force is successfully transferred through shear stresses developing under the barrier slap to the reinforced soil (retaining wall). There are three main attributes of influence depth of this share force that is transferred and these are: i. The soil shear strength ii. The width of the barrier slap iii. The stiffness of the reinforced soil structure Bowles (2011) underlines that when the structure is stiff, the shear forces distributed will be deeper and this helps in lessening the strain of the forces at the top of the wall. When the load is applied, the coefficient of friction between the reinforcements and the soils increases exponentially tending towards infinity because of the nature of the impact loading which is instantaneous (Bowles, J. 2011). In this design based on t he full scale crash testing shows that there is no time for pullout of the reinforcement to happen before the impact loading ends (Burger 2012). There is a situation where the pullout during impact can be safely ignored when the tensile stress in MSE soil reinforcement is well appropriated and gauged. Cantilever Retaining Walls Design A cantilever wall is a type of restraining structure that would be very appropriate for this road design because of its critical attributes that provide solutions required for the road construction. By description, cantilever walls are designed to lie on a wide T or L shape footing to support stabilization or at times others may be stabilized by vertical poles that are embedded deep within the ground (Burger 2012). The support for the wall is derived both from the footings or vertical poles as well as the tensile strengths and compressive strengths of the materials used to construct it. The design taken for this case comprises of wide-footing cantilever retaining walls design that is built in situ reinforced concrete with horizontal rails of H-5 treated logs and precast planks which are installed at the back of the poles for support (see figure 2). Fig. 2: Cantilever Concrete Block Retaining Wall with L-Shaped Footing Advantages and Disadvantages of Cantilever Wall Designs The following are the main advantages of cantilever wall design that is pertinent to this project description: This design offers an unobstructed open excavation when designing and constructing it Using this design, it is not necessary to install tiebacks under the adjacent properties The construction procedure used for the construction of this cantilever retaining wall is simple and its staging is equally simple and easier to install Despite these advantages, there are also a number of shortcomings that have to be appreciated when using cantilever retaining walls suggested herein: The maximum depth of excavation that can be attained with cantilevers is limited to 6m which is about 18ft Cantilever retaining walls are not appropriate for places where there are many adjacent buildings in the vicinity There is a challenge with the control of the lateral wall displacements with cantilever retaining walls as it is dependent on how much passive earth resistance can be mobilized When the excavation for the cantilever retaining wall is deeper than the recommended levels there may be increased wall stiffness which limits the available space within the excavation as well as posing challenges with stability of the whole structure (Vesic 2010). In this design (see figure 3), there are four different types of failures that are pertinent to the proper functioning of cantilever retaining walls and it is important for all designs developed to seek to address these modes of failures as applicable: Fig. 3: Design of the Cantilever Retaining Wall for the Project Structural Failure of the wall – this refers to the failure where the wall bends at the base of the wall where there is a connection of the stem and the foundation (Vesic 2010). Structural failure of the foundation – this refers to the bearing failure of the soil under the foundation toe as well as the forwards rotation of the wall. Structural failure of the wall sliding – this refers to the failure that happens within non-cohesive soils leading to the movement of the wall outwards when there is a passive failure of the soil in front of the foundation and there is an active soil failure behind the wall (Vesic 2010). Structural rotational failure that is deep seated – this happens often within cohesive soils where the factor of safety is determined by the increasing heel length and depth of key. Design Specifications for the Cantilever Retaining Wall In order to design this wall, there are a number of assumptions that are made to determine the way confines within which the wall can be designed and developed. The soil type where the wall is to be constructed is assumed to be made of Gilwern and Brynmawr loess. The strength of the soil parameters are defined by the following conditions: In this case, the Gilwern and Brynmawr loess is further assumed for the long term, gravity only load case and for the earthquake load case, the foundation loess is designed based on an assumed un-drained strength, . For the wall situation itself, this design assumes that the wall is going to down-slope to support the road foundation for the construction of the road as required. The road surcharge is assumed to be, 5kN/m2 averaged across the active soil wedge for the gravity and 4kN/m2 for the case of the earthquake. The calculation of the surcharge is obtained using the following relationships: It is noted that using Wd = 0.5 implies that the retaining wall and the ground that is retained for the wall has a high chance of yielding and accumulating permanent displacement during the situation for the designing for earthquake (Pender 2015). This means that the wall elements have to be maintained well to have a good resilience and ductility so as to take care of the displacement. In the event of an earthquake, it should be expected that there will be some settlements behind the wall which has to be taken into account during design. There are about seven steps that are involved in the design of cantilever retaining wall for this project which involve complex calculations and differential equations in the design. This section presents a simple description of these steps and the pictorial representations of what the design entails. Geometry Trial at the Initial Design Stage For this design, the main variables affecting the geometry of the retaining wall are the toe length, the heel length, and the key depth. The structural design should provide a way of refining the stem thickness and footing (Pender 2015). It is recommended that the location of the key be placed at the end of the heel as shown in figure 5 below which also provides a detailed analytical model used for gravity design. In this regard, the moments for this model are taken about point O. Fig. 5: Cantilever Wall Design for Geometry Trial Analysis Foundation Bearing Design for the Retaining Wall This design specification addresses the issue of foundation bearing capacity which guides the rest of the design specifications and should therefore be done at the very beginning. This is important in determining the resistance of the soil holding the toe of the foundation to sliding shear, vertical bearing loads, and to ensure that they is enough passive resistance to sliding (Pender 2015). Wall Sliding Design Critique After determining the foundation bearing, it is important to then conduct a sliding analysis. For this case based on the design model in figure 5 above includes the weight of the block of the soil under the footing as well as the key in this calculation for base friction, Vs. In this brief analysis, the self-weight components are factored down (to α = 0.9) as a way of catering for the uncertainty as they provide a basis for stabilizing in this case (Brinch-Hansen 2011). This analysis does not however factor in the vertical component of active thrust as well as the vertical component of passive resistance since it is considered de-stabilizing for this case according to Brinch-Hansen (2011). This means that the α = 1. To check wall sliding on base; From this analysis, it is found that the factored resistance is greater than the factored load and therefore the retaining wall is structurally OK. Bending strength of the Wall Stem As has been identified above under modes of failure that may occur within the structure, it was indicated that the stem of the wall may fail through bending where the maximum bending moment is felt at the base of the stem and figure 6 shows the analytical model designed for this analysis. This analysis has to be done in accordance with the relevant materials used in the construction and as guided by the various guidelines provided for this kinds of structures (Brinch-Hansen 2011). In order to calculate the maximum bending moment of the stem of the wall, it is assumed for this case that the wall has a membrane that is waterproof which means that is interface friction can is neglected. Fig. 6: Model for Analysis for Earthquake Case The bearing strength of the Foundation Under this design consideration, the bearing capacity of the foundation is determined in the same way geometry bearing was developed in the first step. The earthquake inertial load from the weight of the wall as well as the weight of the soil above the heel of the foundation is computed based on the model shown in figure 7 below. Wall Sliding Analysis and Design Consideration The model shown in figure 5 provides the analytical model of the design used for this analysis. In this analysis, the cohesive component of the passive soil resistance of the toe of the reinforcement wall is neglected as there is a chance for there being disturbance and desiccation (Bowles 2010). Methods of analysis of the Cantilever Retaining Walls From the design specification provided for this cantilever retaining wall, it has been done with minimal equilibrium methods where it has been achieved through calculating active and passive earth pressures (Bowles 2010). The two methods of Limit Equilibrium analysis are as follows: Free earth Method for Cantilever Walls – this approach requires the balancing of the overturning and resisting moments assuming active earth pressures are considered on the retained side and the passive pressures are considered on the excavation side (Bowles 2010). At the point of zero moment, the safety factor of 1 is then determined but this method has the challenge of not balancing shear forces and this requires a correctional factor of 1.2 to be multiplied by the obtained embedment for FS=1. Fixed earth Method of analysis of cantilever walls – this approach is more wholistic and leads to effective and adequate balancing of both the overturning moments and shears. Despite this sufficiency, this method is highly complex and therefore has its application limited to very special conditions where it first finds the critical pivot point about which rotational moments as well as share forces usually balance. In order to achieve this, the active earth pressures on one side (the excavation side) and the passive earth pressures on the other side (retained side under the point of pivot) are compared and analyzed together (Bowles 2010). Recommendations and Conclusion The foregoing design description has been detailed and extensive in the presentation of the different elements of the cantilever retaining wall designed for the A465 Heads of the Valleys Road between Gilwern and Brynmawr. In this regard, in order to improve the quality and safety of the wall designed, it is recommended that the embedment in the cantilever wall stand at least 1.5m times the height excavated (Bowles 2010). As it has been indicated, the cantilever retaining wall has different advantages main of which is that which allows it to have easy excavation especially when it is within the 4.5m recommended depth as is the case for this design recommended herein. Bibliography Bowles, J. 2010. Foundation Analysis and Design. (5th edn.). McGraw-Hill, New York. Bowles, J. 2011. Foundation Analysis and Design. New York: McGraw-Hill Book Company. Brinch-Hansen, J. 2011. “A revised and extended formula for bearing capacity”. Bulletin vol. 28, no.1, pp. 23 – 27. Burger, J. 2012. Retaining Walls Industry The Retaining Walls Industry web portal Italiantrivelle is the number one source of informations regarding the building of retaining walls. New York: Sage. Ching, D., Faia, R. & Winkel, P. 2006. Building Codes Illustrated: A Guide to Understanding the 2006 International. London: Sage Crosbie, M. & Watson, D. 2005. Time-Saver Standards for Architectural Design. New York, NY: McGraw-Hill. Pender, M. 2015. “Moment and Shear Capacity of Shallow Foundations at Fixed Vertical Load”. Proc., 12th Australia New Zealand Conference on Geomechanics, Wellington. Vesic, A. 2010. Foundation Engineering Handbook. 1st edn. New York: Van Nostrand Reinhold. Read More

According to Bowles (2011), the design requires a 20mm gap between the throat of the precast barrier and the backside of the panels facing it to provide capacity for the retaining wall to absorb load impacts created to the MSE wall panels. When developing this barrier, it is required that there be a 20mm thick compressible foam material be placed on the hind side of the facing panels before the moment slab is poured into place. This arrangement does not allow contact between the barrier and the panel and this is good for the design as it ensures that all the horizontal impact force is successfully transferred through shear stresses developing under the barrier slap to the reinforced soil (retaining wall).

 Bowles (2011) underlines that when the structure is stiff, the shear forces distributed will be deeper and this helps in lessening the strain of the forces at the top of the wall. When the load is applied, the coefficient of friction between the reinforcements and the soils increases exponentially tending towards infinity because of the nature of the impact loading which is instantaneous (Bowles, J. 2011). This design based on the full-scale crash testing shows that there is no time for the pullout of the reinforcement to happen before the impact loading ends (Burger 2012).

There is a situation where the pullout during impact can be safely ignored when the tensile stress in MSE soil reinforcement is well appropriated and gauged.Cantilever Retaining Walls DesignA cantilever wall is a type of restraining structure that would be very appropriate for this road design because of its critical attributes that provide solutions required for road construction. By description, cantilever walls are designed to lie on a wide T or L shape footing to support stabilization or at times others may be stabilized by vertical poles that are embedded deep within the ground (Burger 2012).

The support for the wall is derived both from the footings or vertical poles as well as the tensile strengths and compressive strengths of the materials used to construct it. The design taken for this case comprises of wide-footing cantilever retaining walls design that is built in situ reinforced concrete with horizontal rails of H-5 treated logs and precast planks which are installed at the back of the poles for support (see figure 2).Advantages and Disadvantages of Cantilever Wall DesignsThe following are the main advantages of cantilever wall design that is pertinent to this project description:This design offers an unobstructed open excavation when designing and constructing it,Using this design, it is not necessary to install tiebacks under the adjacent properties,The construction procedure used for the construction of this cantilever retaining wall is simple and its staging is equally simple and easier to install.

Despite these advantages, there are also several shortcomings that have to be appreciated when using cantilever retaining walls suggested herein:The maximum depth of excavation that can be attained with cantilevers is limited to 6m which is about 18ftCantilever retaining walls are not appropriate for places where there are many adjacent buildings in the vicinity There is a challenge with the control of the lateral wall displacements with cantilever retaining walls as it is dependent on how much passive earth resistance can be mobilized When the excavation for the cantilever retaining wall is deeper than the recommended levels there may be increased wall stiffness which limits the available space within the excavation as well as posing challenges with the stability of the whole structure (Vesic 2010).

Structural Failure of the wall – this refers to the failure where the wall bends at the base of the wall where there is a connection of the stem and the foundation (Vesic 2010).Structural failure of the foundation – this refers to the bearing failure of the soil under the foundation toe as well as the forward's rotation of the wall. Structural failure of the wall sliding – this refers to the failure that happens within non-cohesive soils leading to the movement of the wall outwards when there is a passive failure of the soil in front of the foundation and there is an active soil failure behind the wall (Vesic 2010).

Structural rotational failure that is deep-seated – this happens often within cohesive soils where the factor of safety is determined by the increasing heel length and depth of key.

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