Retaining Wall between Gilwern and Brynmawr - Coursework Example

Name Course Date Title: Retaining wall Table of Contents Table of Contents 2 Abstract 3 Introduction 4 Objectives of the Project 5 Design constraints 5 Locations of retaining walls 6 Design Methodology 7 Testing 8 Soil Analysis 8 Methods of Soil Test 9 Estimating Of Safe Bearing Capacity 12 Geotechnical Conditions 12 Durability & Maintenance 13 Cost Estimates 13 Conclusion 13 References 15 Appendix 16 Abstract Our project is to design a suitable retaining wall for A465 Heads of the Valleys Road between Gilwern and Brynmawr. There is a need to extend Valleys Road from a single 3-lane carriageway to dual 2-lane carriageway. The topography and the environmental constraints would require extensive use of retaining structures to reduce land take and impact on sensitive areas. Therefore, this project focus on the design of retaining wall for Valleys Road section that extends for over 11.5km. The significance of retaining walls is to hold soil in places that have undesirable slopes. They are also constructed in areas whose landscape needs to be engineered and shaped for more uses such as building bridges and roadway overpass. The design of retaining walls structures requires information on earth pressure acting on the retaining wall due to soil backfill. Thus this project will include estimation of safe bearing capacity of the soil. Introduction A soil mass is not stable on a slope that is steeper than the safe slope. Such a slope requires the construction of retaining structures to provide a lateral support to soil mass. Retaining structures are rigid walls that support the soil mass literally, such that the soil mass is retained in vertical elevations of different levels on both sides of the wall (Wales, 2008). If the retaining wall is not constructed, then the soil on high level will slide down the slope. However, a roadway can be constructed without retaining walls on less sloping places. The project focuses on designing of a retaining wall along the Valleys Road between Gilwern and Brynmawr. The existing road is a 3-ane carriageway of varied alignment and width. The road will be extended to a dual 2-lane carriageway to accommodate more traffic. It passes through Clydach Gorge where which has caves and other sub-surface uncertainties. This makes necessary to construct retaining structures along the edges of the road extending for over 11.5 km. Other barriers that will be constructed include sidewalks, gutters, curbs and sheer rock cutting. Retaining structures are structures that hold soil mass back (Ranjan, 2007). Such structures include retaining walls, basement walls, crib walls and others. Retaining wall assists in maintaining the ground level on both sides of the wall on a steep slope. The soil on a steep slope with no retaining structures tends to move down the slope until it a stable configuration. The soil retained by the wall (also called backfill) exerts a force on the wall due to earth pressure. Examples of retaining wall include gravity retaining wall, counter-fort walls and cantilever (Wales, 2008; Ranjan, 2007). The degree of earth pressure is dependent on the nature and magnitude and relative movement of soil mass. The choices of retaining structures are determined based on the line of action and the magnitude of the earth pressure acting laterally. This depends on other factors such as the wall flexibility, drainage conditions, wall movement and soil properties (Wales, 2008). The design is accomplished after performing a number of tests. The site has soil strata which is rockier. This means that the soil has safe bearing capacity. This report provides the definition of retaining wall, types, tests and safe bearing capacity of the soil. Objectives of the Project This project focuses on designing a retaining wall along the Valleys Road between Gilwern and Brynmawr. This is because the road will be widened to allow more traffic as part of the main project. In the construction, the road will be extended from a single 3-lane carriageway to dual 2-lane carriageway to accommodate more traffic. The idea behind this project is to design retaining structures at various locations along a distance of 11.5 km. The type of wall would be chosen based on the local environment. Soil properties such as optimum moisture content, shear strength parameters, dry density, are evaluated by performing appropriate tests. The results obtained are used to calculate safe bearing capacity. The ultimate goal is to produce a sustainable structure that will have minimum future maintenance and disruption to the road used. Design constraints The existing road runs through a very steep terrain and the Clydah Gorge, means that a number of retaining walls as well as strengthened earthworks would be constructed along the road under consideration (Ranjan, 2007). The purpose of designing the retaining walls and earthworks is to minimize intrusion on the natural environment in the area. Although the new road will have some impact on the local environment, the road construction is design so that their will constructed safely, but at the same time to ensure that it is well adapted to the surface features and with complex underground conditions created by mining work around Gorge area and the existence of caves and limestone in the gorge. The road pass through steep slopes and rivers, its magnificence is maintained by designing and constructing the road such that it blends into the topography and the flora of the land. In order to preserve the natural environment, green slope measures would be adopted together with linear and regular slopes. The rocks will be cut as natural as possible, and at the same time ensuring that the safety of the road users is maintained by creating slack slopes and using meshes and bolt where it is appropriate (Ranjan, 2007). Due to the nature of the gorge and the terrains along the route, the construction of the road structures is complex, as it results in large cuts in the soil and on rocks, high retaining walls, embankments, depending on the ground geometry. The angle and thickness of retaining structures depends on the existing conditions, for example, some are embedded in the soil and some in the rock. This complexity necessitates a lot of flexibility when designing structures and earthworks, as well as the use of the proven construction systems in order to accommodate such changes. The design that is most likely to be used in this construction is reinforced soil together with the use of soil nails to construct very steep slopes. Reinforced soil retaining walls are the most common type of structure used for holding back the soil mass. A common example is mechanically stabilized earth retaining wall and can be constructed to a very large height. They preferred to be used in this project because they are more economical in roads construction especially in routes such as this. The inherent flexibility and strength of the reinforced soil wall has produced robust and economical designs that have been used in the past to solve stability issues that exist in the route. The system can used around curves, sharp change in direction, making it to be useful in different parts of the project. The main objective in this project is to reduce the use of hard faced structures as much as possible through the use of reinforced soil slopes with grass face with an angle of less than 600. In case a vertical wall with hard facing is needed, a precast panel is used as a facing in order to minimize visual effect. Locations of retaining walls Some sections of the road will be supported by retaining wall with steep natural slope. The walls will be up to 15 m high above the ground surface. Due to its possibility of being unstable, the earth retaining wall will be used, with foundations that have the following characteristics. The outer edge of the wall will be supported with reinforced concrete bored into the bedrock such that it cannot be affected by the possible soil failures. Excavation into the bedrock and construct a stable foundation that will not be affected by any instability. An alternative way would be to replace wall sections with 450 reinforced soil embankments. Especially in locations where there is enough space. The different types of walls that would be used in this project and their locations are listed below. Table 1: Retaining Wall Types Type of Structure Description of the Process Type 1 Vertical precast paneled anchored/nailed wall Type 2 Vertical precast paneled wall anchored at top only Type 3 Vertical reinforced earth retaining wall clad in precast panels Type 4a Reinforced earth slope (less than 45 degrees) Type 4b Reinforced earth slope (greater than 45 degrees) Type 5 Vertical reinforced concrete retaining wall Type 6 Back to back vertical reinforced earth retaining walls Type 7 Soil nailed slope Type 8 Mechanically stabilized vertical reinforced earth Type 9 Near vertical fully faced rock cut (>700) Type 9 Steep natural rock cut (>450) stabilized using dentition and bolts Type 10 King post retaining wall Type 11 Contiguous bored pile wall The design drawings for these types of retaining will are shown in the appendix Design Methodology The geotechnical design of retaining wall and earthworks would be done in accordance with the appropriate Eurocodes, Highway Work, and the Highways Agency Design procedure for roads. There is also flexibility where necessary, due to the complexity of environmental factors and ground conditions Testing Soil Analysis A soil mass consists of broken rocks that has gone through physical, chemical or biological process. Soil strength is significant in the construction of retaining wall as it holds back the vertical face from sliding on a steep slope. Some factors which can be tested in soil analysis are the bearing capacity, soil properties, shear and compressive strength, and dry density (Aysen, 2003). a) Dry Density, γd –This test assisting in getting the dry density of the soil which is produce a design of a retaining wall that is economical and safe. It can be obtained from a standard compaction test. The property affects its bearing capacity. A soil mass with good dry density can bear weights. Soil is stabilized by compacting it to increase its dry density. This is done by packing solid particles in order to reduce the amount of air in it (Aysen, 2003). b) Bearing Capacity - This is the capacity of soil mass to bear a load. In this test, a large amount of pressure is applied on the soil mass. A soil mass with sufficient bearing capacity does not have shear failure (Aysen, 2003). c) Moisture Content - This is the amount of moisture content through which the maximum soil density is obtained. It is obtained through a standard compact test. Through compaction, the soil moisture is reduced and the soil weight is increased. The maximum dry weight is obtained at optimum water content, which is proportional to the number of impacts on the soil and the weight of the roller. d) Water and Earth Pressure - This is an important factor in designing the retaining structures. It is the pressure exerted by soil and water on the side of the retaining wall in horizontal direction. e) Shear Strength – The magnitude of shear stress that can be determined from direct shear test technique. Factors such as the sizes of soil particles, its friction, and the bonding of the particles affect the shear strength of the soil. This soil property is determined by the interaction between the cohesive angle and internal friction angle. f) Compression Strength - This is the compressive stress at which unconfined soil specimen fails in compressive test. It is determined from the load per unit area (Aysen, 2003). Methods of Soil Test The soil can be analyzed using various tests. This include direct shear test, standard compaction test and unconfined compression test a) Standard Compact Test This one of the most widely used and effective method for soil stabilization. This test is performed in order to determine the relationship between the soil dry density and the moisture content of the compacted soil sample. It is used by geotechnical engineers to perform an analysis of field control test to ensure that the compacted fills meets the design specifications that include the water content and the required density. By increasing the soil density, most of the engineering properties like stiffness and strength are improved (Chen et al., 2008). A standard compact test is carried out to determine the optimum amount of water in the soil. If the water content is higher than the prescribed optimum water content, the soil structure will be weak, softer, susceptible to shrinking and more ductile. If it is compacted lower than the optimum water content, it results in a more flocculated soil structure (Chen et al., 2008). Compaction Rammer and Mould The soil that is to be compacted is placed in the standard cylindrical mould in a number of layers. The layers are compacted through a number of blows as shown below. Compacting soil sample b) Direct Shear Test Direct shear test is the oldest soil strength test. Consolidated-drained shear strength of a soil sample is determined through this method. Shearing strength determines the soil shearing resistance which affects the stability of the slopes. Using this technique, the bearing capacity for the foundation is determined. It is also used in the calculation of the pressure exerted by the soil on a retaining wall (Laloui & Ferrari, 2013). The apparatus used for shearing test is shown below. Apparatus used for direct shearing test An angle of internal friction and the optimum shear stress are determined. c) Unconfined Compressive Strength Test The unconfined compressive strength is the compressive stress at which unconfined cylindrical soil mass fails in a compression test. It is used to calculate the unconsolidated undrained shear strength of the sample in unconfined state. The unconsolidated undrained shear strength is used to calculate the bearing capacity of the foundations (Laloui & Ferrari, 2013). The load is applied as shown below. The soil sample specimen under a load and after a failure Estimating Of Safe Bearing Capacity The foundation transfers the forces and moments of a structure to the ground. It provides stability against overturning and sliding. The structure is stable if the stresses in the soil are within the permissible limits. The tests are carried out to ensure that soil and the foundation are safe from failure and reduce excessive settlement. The bearing capacity is the ability of the soil to hold the forces from structure without failure. The soil in contact with the foundation is exposed to stresses when the structure is constructed on the ground (Das, 2009). Geotechnical Conditions Geotechnical conditions of the route are taken into consideration. Specific investigation will be undertaken and a geotechnical report of the route is produced. Generally, the ground profile contain top soil layer that overlay a colluvium layer with varying depth, and is followed by greywacke bed rock. Retaining walls requires shallow soil undercut and replacing them with compacted material. The retaining walls which are not related with the bridge will generally be designed to withstand seismic occurrence. A design displacement of approximately 150mm is generally acceptable. Durability & Maintenance The type of design adopted for the locations near the bridge stabilized earth retaining walls with concrete facing panels and galvanized steel straps. Steel - Highly durable structural steel has been adopted due to its durability. The system is expected to take upto 50 years to the first maintenance. Special care will be used to minimize corrosion. Concrete – the concrete element in the retaining walls is designed in harmony with 100 years design life requirements. It is expected that there will be little or no maintenance over the structural life. Cost Estimates The costs for the type of wall were compiled as preliminary estimates, but do not include all the costs related with the project. The costs are for 2015 dollars. The crib block wall is estimated to cost $40/foot2. It was assumed that soil nailing was used on 75% of each wall with approximate cost of $75 per square foot. The masonry was assumed to cost $ 40 per square foot. The average height for the walls was assumed to be 10 feet. Conclusion The construction of Valleys Road between Gilwern and Brynmawr (section 2 of the project), will involve widening of the road. This will affect the slopes on both sides of the road. This current project is focused on designing of retaining wall along Valleys Road. The project has been laid out on the project plan. These include the location of the retaining walls. Geotechnical investigation can be carried out to confirm all assumptions made in the report. Widening of the road will run through Valleys Road for A465 Heads from a single 3-lane carriageway to dual 2-lane carriageway. The topography and the environmental constraints would require extensive use of retaining structures to reduce land take and impact on sensitive areas. New retaining structures that will be constructed along edges of the roadway include sidewalks, gutters, curbs and retaining walls will hold back the soil mass. We explored different methods that can be used in designing of the retaining walls. This includes tests such as direct shear test, standard compaction test and unconfined compression test. The methods enable estimation of the standard dimensions of the retaining wall. The most economical designs that provide optimum strength are adopted. References Aysen A., (2003). Problem Solving in Soil Mechanics, CRC Press Chen, Y., Tang, X. Li, G., & Geosynthetics Asia '08, (2008). Geosynthetics in civil and environmental engineering: Proceedings of the 4th Asian Regional Conference Geosynthetics Asia 2008 in Shanghai, China. Berlin: Springer. Das, B. M. (2009). Shallow foundations: Bearing capacity and settlement. Boca Raton: CRC Press. Laloui, L., & Ferrari, A. (2013). Multiphysical testing of soils and shales. Berlin: Springer. Ranjan, G. (2007). Basic and applied soil mechanics. Place of publication not identified: publisher not identified. Wales. (2008). A465 Heads of the Valleys dualling Abergavenny to Gilwern. Cardiff: Welsh Assembly Government. Appendix Type 1: Vertical precast paneled anchored/nailed wall Type 2: Vertical precast paneled wall anchored at top only Type 3: vertical reinforced earth retaining wall clad in precast panels Type 4a: reinforced earth slope (less than 45 degrees) Type 4b; reinforced earth slope (greater than 45 degrees) Type 5: vertical reinforced concrete retaining wall Type 5: back to back vertical reinforced earth retaining walls Type 7a: soil nailed slope (600) Type 8: Mechanically stabilized vertical reinforced earth retaining wall Type 9a: near vertical fully faced rock cut (>700) Type 9b: steep natural rock cut (>450) stabilized locally using dowels, bolts, dentition Type 10: king post retaining wall Type 11: contiguous bored pile wall Read More

The idea behind this project is to design retaining structures at various locations along with a distance of 11.5 km. The type of wall would be chosen based on the local environment. Soil properties such as optimum moisture content, shear strength parameters, dry density, are evaluated by performing appropriate tests. The results obtained are used to calculate safe bearing capacity. The ultimate goal is to produce a sustainable structure that will have minimum future maintenance and disruption to the road used.

Design constraints
The existing road runs through very steep terrain and the Clydach Gorge, which means that several retaining walls, as well as strengthened earthworks, would be constructed along the road under consideration (Ranjan, 2007). The purpose of designing the retaining walls and earthworks is to minimize intrusion on the natural environment in the area. Although the new road will have some impact on the local environment, the road construction is design so that their will constructed safely, but at the same time to ensure that it is well adapted to the surface features and with complex underground conditions created by mining workaround Gorge area and the existence of caves and limestone in the gorge.
The road passes through steep slopes and rivers, its magnificence is maintained by designing and constructing the road such that it blends into the topography and the flora of the land. To preserve the natural environment, green slope measures would be adopted together with linear and regular slopes. The rocks will be cut as naturally as possible, and at the same time ensuring that the safety of the road users is maintained by creating slack slopes and using meshes and bolts where it is appropriate (Ranjan, 2007).
Due to the nature of the gorge and the terrains along the route, the construction of the road structures is complex, as it results in large cuts in the soil and on rocks, high retaining walls, embankments, depending on the ground geometry. The angle and thickness of retaining structures depend on the existing conditions, for example, some are embedded in the soil and some in the rock. This complexity necessitates a lot of flexibility when designing structures and earthworks, as well as the use of the proven construction systems to accommodate such changes. The design that is most likely to be used in this construction is reinforced soil together with the use of soil nails to construct very steep slopes.
Reinforced soil retaining walls are the most common type of structure used for holding back the soil mass. A common example is a mechanically stabilized earth retaining wall and can be constructed to a very large height. They preferred to be used in this project because they are more economical in road construction especially in routes such as this. The inherent flexibility and strength of the reinforced soil wall have produced robust and economical designs that have been used in the past to solve stability issues that exist in the route. The system can use curves, sharp changes in direction, making it to be useful in different parts of the project.

The main objective of this project is to reduce the use of hard-faced structures as much as possible through the use of reinforced soil slopes with grass face with an angle of less than 600. In case a vertical wall with a hard facing is needed, a precast panel is used as a facing to minimize the visual effect. Some sections of the road will be supported by a retaining wall with a steep natural slope. The walls will be up to 15 m high above the ground surface. Due to its possibility of being unstable, the earth retaining wall will be used, with foundations that have the following characteristics:

• The outer edge of the wall will be supported with reinforced concrete bored into the bedrock such that it cannot be affected by the possible soil failures.
• Excavation into the bedrock and construct a stable foundation that will not be affected by any instability.
• An alternative way would be to replace wall sections with 450 reinforced soil embankments. Especially in locations where there is enough space.

Read More