Designing And Testing For Retaining Walls, Anchors, And Bracing Systems

1. For a given soil type and its properties, what are the differences between Rankine and Coulomb earth pressure coefficients and why?

2. What types of soil testing should be used to determine the needed soil properties for designing earth retaining walls and tieback anchors?

3. What is the purpose of installing louver blocks between adjacent lagging boards?

4. Describe raker braces and heel blocks and how they are installed.

5. What was the purpose of Terzaghi & Peck’s apparent earth pressure diagrams for braced or anchored retaining walls?

6. Why do different types of loads on walls (e.g. earth, water, wind, surcharges, etc.) and different types of structural materials (e.g. steel, wood, soil, etc.) have different load factors and resistance factors?

7. What type of soldier beam is most appropriate for use in bouldery or Karst ground conditions and why?

8. What is the main difference between Wayne C. Teng’s conventional and simplified methods for designing cantilevered, non-gravity walls?

9. Define “redundancy” in anchored, non-gravity walls and name several ways that redundancy may be designed into a permanent, anchored retaining wall.

10. Describe the differences between a tieback anchor and a soil nail.

11. When designing a braced or anchored wall, what is the main difference that results from assuming a hinge at subgrade as opposed to solving directly for the toe embedment, D, by calculating for moment equilibrium about the lower brace or tieback level?

12. How does cohesion affect lateral earth pressure on retaining walls and when, if ever, is it appropriate to use cohesion in wall design?

13. What types of materials should and should not be used to backfill a drilled or augered hole after the soldier beam has been placed into the hole and why?

14. What conclusion can be made when comparing different apparent earth pressure diagrams that have been developed by the various geotechnical “legends?”

15. Describe potentially significant problems with installing a permanent, cast-in-place, concrete facing on a soil nail wall.

16. Name at least two different methods for developing an apparent earth pressure diagram for a braced or anchored wall in a stratified soil profile.

17. What is a roll chock and what is its purpose?

18. What costs should be considered when weighing the economics of using wales and braces as opposed to using tieback anchors to support a retaining wall?

19. What are several factors that may determine whether a wide flange beam, H-pile beam, or pipe pile is appropriate to be used as a soldier beam?

20. Explain the differences between Allowable Stress Design (ASD) method and the Load and Resistance Factor Design (LRFD) method.

21. Describe several problems associated with using precast concrete lagging between soldier beams.

22. Name different types of soils or ground conditions into which it is difficult to install steel sheet piling.

23. How do sloping ground surfaces behind and in front of a retaining wall influence the active and passive, lateral earth pressures?

24. What is an independent reference point and what is its purpose when testing tieback anchors?

25. Explain “top down” and “bottom up” wall construction methods. Which wall types are most appropriate for each construction method and why?

Rankine vs. Coulomb Earth Pressure Coefficients

Rankine earth pressure coefficients (active and passive pressure coefficients) are determined on the following assumptions: there is zero wall friction (frictionless wall), the soil is cohesionless, failure and ground surfaces are straight planes, the interface between the soil and wall is vertical, and there is no friction acting between the backfill and the soil (i.e. resultant force is acting parallel to the slope of backfill). On the other hand, Coulomb earth pressure coefficients are determined for any wall slope, wall friction and backfill slope centered on the assumption that development of shear resistance of soil occurs along the failure plane and wall. The differences between Rankine and Coulomb earth pressure coefficients are important because retaining systems are of different types and have varied properties, which determine the theory or coefficient to be used in analyzing or designing them.  

The required types of soil testing for design of retaining walls and tieback anchors are: moisture content test (used to determine the percentage of water present in a given soil sample to the dry mass of that particular soil. This values influences several other properties of soil including particle size, permeability and compaction), Atterberg limits tests (used to determine plastic limit, liquid limit and shrinkage limit of the soil), , specific gravity test (used to determine the ratio of unit weight of soil to the weight of distilled water at a standard temperature), dry density test (used to determine the weight of soil particles in a given volume of soil sample) and proctor or compaction test (used for determining compaction properties of soil with varying moisture content) (The Constructor, 2017).

The purpose of installing louver blocks (also known as spacers) between adjacent lagging boards is to create gaps that facilitate drainage of anchored walls used for stabilizing slopes and excavations. The width of the gaps should be sufficient to allow drainage and at the same time prevent retained soil falling out from behind the adjacent lagging boards. Additionally, the louver blocks are installed so as to provide space for checking whether there is adequate earth/soil behind the lagging or if there is need for adding some fill so as to fill voids, or to determine the need for fill some filter fabric, straw or hay so as to regulate water seeping from behind the lagging.

Raker braces are temporary diagonal/inclined systems or kickstands that are used for supporting vertically installed sheeting (shoring walls) against earth walls that are created by excavation (Pendley, 2009). The raker braces sit inside the excavation and can make construction activities inside the excavation area difficult. Heel blocks are temporary concrete footings that are placed at the bottom of the excavation on which bracings supporting the shoring systems walls of the excavation are attached. Therefore heel blocks are concrete blocks that support raker braces, which in turn support temporary excavation walls. Installation of raker braces and heel blocks start by cutting the desired sizes of raker braces depending on the length and height of the shoring walls. The angle of the raker braces is then decided, which determines where to place the heel blocks. The raker braces are fixed on the shoring wall using wall plates. The raker braces are then fixed into the heel blocks placed on the ground using sole plate anchors. The fixing is usually done by nailing.

Required Soil Testing for Design of Retaining Walls and Tieback Anchors

The purpose of Terzaghi & Peck’s apparent earth pressure diagrams for braced or anchored retaining walls was to provide loads and envelopes of pressure for conventional designs of struts or braces in internally braced excavations. These diagrams represented drained loadings taking place in sand soils, undrained loadings taking place in soft to medium clay soils, and undrained loadings taking place in stiff to hard clay soils (Sabatini, Pass, & Bachus, 1999).

Different types of loads on walls have different load factors because the manner in which these loads act on the walls is influenced by various factors such as magnitude of the load, angle at which the load is acting on the whole, predictability or variance of the load, geographic location, height of the wall, shape of the wall and weather conditions, among other factors. On the other hand, different types of structural materials have different resistance factors because materials have unique properties that affect how they resist loads or behave when subjected to loadings.

The most appropriate type of soldier beam for use in bouldery or Karst ground conditions is reinforced concrete beams. This is because reinforced concrete soldier beams are highly stable on their own, can be constructed faster, are cheaper to construct than most of the other retaining systems, they are versatile, and their construction do not require highly advanced construction methods.

The main difference between conventional and simplified methods of Wayne C. Teng for designing cantilevered, non-gravity walls is that the latter is much easier and more accurate than the former because simplified methods usually use computer simulations while conventional methods are done manually. However, simplified methods have more limitations than conventional methods because of the numerous assumptions made.

Redundancy refers to the capacity of the anchored, non-gravity walls to resist collapse progressively (Liu, Li, Liu, & Cai, 2017). Some of the ways in which redundancy may be designed into permanent, anchored retaining walls include: ensuring proper drainage of the retaining wall (by drilling weep holes through the wall, installing a drainage pipe behind the retaining wall, using cohesionless, granular soil as backfill and ensuring proper grading behind the wall), ensuring that the hand-compaction zone is properly filled and compacted, ensuring that reinforcement is properly placed and tensioned, and ensuring that the retaining wall is protected from water permanently.

Tieback anchors are active structural members that support much higher design loads whereas soil nails are passive structural elements that support much lower design loads. The pressure imposed by the tieback anchors on the retained soil mass is greater than surcharge and earth pressures but soil nails only provide support when the soil is starting to mobilize (they gain their pullout resistance from the retained soil mass) (RST Environmental Solutions, 2018). Also, soil nails are shorter and spaced closer to each other while tieback anchors are usually longer and spaced far apart. Before installation, tieback anchors must undergo performance or proof tests while this is not mandatory for soil nails.

Installation of Louver Blocks and Raker Braces

Designing a braced or anchored wall by assuming a hinge at subgrade is a more generalized approach that does not consider the specific or unique conditions or requirements of the retaining wall. However, designing by solving directly for the toe embedment, D, by calculating for moment equilibrium about the lower brace or tieback level is a more improved method because it considers the unique and specific parameters that may affect the strength and stability of the retaining wall.

The effect of cohesion on lateral earth pressure varies depending on other factors such as friction angle and active and passive coefficients of earth pressure. But generally, an increase in cohesion of the soil reduces the lateral earth pressure on retaining walls (Yazdani, Azad, Farshi, & Talatahari, 2013). This is because more cohesive soils do not crumble easily thus they are able to resist greater lateral earth pressure. Cohesion should be used in wall design if it will enhance the strength and stability of the retaining wall.

Some of the materials that are suitable for use as backfill of a drilled or augured hole include: coarse-grained soils, fine-grained soils, marginal materials, culvert gravel, subsurface drainage fill, etc. On the other hand, some of the unsuitable materials for use as backfill include rocks and shale. The backfilling materials should be well drained, dewatered, easy to compact and have other desirable properties.

14: Conclusion from comparing different apparent earth pressure diagrams

From comparing different apparent earth pressure diagrams that have been developed by the various geotechnical professionals, it can be concluded that each method has its own advantages and disadvantages or limitations. Another important conclusion is that each apparent earth pressure diagram is most appropriate for specific applications and conditions.

First, the reinforcement used in soil nail wall has to be passive and should be encased inside grout so as to prevent corrosion. Second, extra deformation control measures must be taken because soil nailing is not adequate for strict control of deformation. Third, for a shotcrete skin to be established, the soil nail wall must have a dewatered face. Fourth, there are numerous challenges associated with installing soil nail in clean gravels and sands. Fifth, there are several failures associated with it, including facing failure, pull out failure, and nail tendon failure. Fifth, utilities in the area must be identified and considered because they affect the location, length and inclination of soil nail wall components. Last but not list, installation must be done by specialized workers.  

Terzaghi and Peck's Apparent Earth Pressure Diagrams

The two most common method used to develop apparent earth pressure diagrams for a braced or anchored wall are Terzaghi apparent earth pressure method and Peck apparent earth pressure method. These methods were developed several years back and have been improved progressively to meet current design requirements for retaining walls (Song & Hong, 2008).

A rolling chock is a type of fastener fixed to the center of a retaining wall to limit rolling movement of members of the retaining wall. A retaining wall is prone to various factors that can cause rolling movement. It is therefore important to fasten rolling chocks behind the retaining wall so as to restrict this movement and maintain stability of the retaining wall.

Some of the costs to be considered when weighing the economics of using wales and braces as opposed to using tieback anchors to support a retaining wall include: type and magnitude of loads to be supported, cost, ease of construction, use of the retaining wall, available construction space, transportation requirements, complexity of installation, prevailing environmental conditions, durability of the retaining wall, and the experts that are required to install these components.

Some of the factors that may determine whether a wide flange beam, H-pile beam, or pipe pile is appropriate to be used as a soldier beam include: type and size of total load acting on the structure, ease or limitations of construction, available construction methods or equipment, predominant ground or soil conditions, cost, availability and level of expertise of the contractors.

One of the differences is that ASD method compares allowable and actual stresses while LRFD method compares actual and required strengths. Another difference is that LRFD uses load factors to predict applied loads and resistance factors to predict material and construction variability, whereas ASD combines these two factors to form a single factor of safety (Quimby, 2014). Other differences include: LRFD method is more economical than ASD method when dead load is greater than live load, ASD cannot predict the behavior of the structure when collapse but LRFD can, LRFD method can apply plastic design concept but ASD method cannot, and LRFD method produces safer structures than ASD method. It is also argues that LRFD method is an improved version of ASD method.

One of the problems of using precast concrete lagging between soldier beams is that they make it expensive and difficult to install tieback anchors (Akmilah, Ong, & Choong, 2014). Other problems are: it can only be constructed using bottom up method, the piles have to be protected against corrosion by galvanizing or using any other appropriate measures, the spacing of piles have to be at the lagging’s dimensions or else the structural soundness of the entire system shall be compromised, and there is a possibility of fill erosion through the vertical joints between panels and piles and horizontal joints between panels.   

Redundancy in Retaining Walls

Some of the types of soils or ground conditions into which it is difficult to install steel sheet piling are soils or grounds with cobbles or boulders, expansive soils, hard clays, coarse, medium and fine gravels and sands, and medium to soft rock strata. It is difficult to install steel sheet piling in these soils because they can develop deep cracks, absorb water resulting to a change in their properties, shrink significantly when they dry out  or they make it difficult to drive sections of the sheet pile walls into the ground because of their hardness (Eskandari & Kalantari, 2011).

Sloping ground surfaces behind and in front of a retaining wall influences the active and passive, lateral earth pressures by exerting vertical and horizontal components of force on the retaining wall. When the ground surface is sloped, it means that the soil is acting on the retaining wall at an angle, which produces vertical and horizontal components of force (an in turn horizontal and vertical earth pressures). Therefore the retaining walls have to be designed with the capability to resist both vertical and horizontal earth pressures.

Independent reference point is a point from which measurements of various parameters are taken when testing tieback anchors. The purpose of using an independent reference point is to ensure consistency and enhance accuracy during testing. When an independent reference point is used, it means that all measurements taken during testing of the tieback anchors are consistent because they are evaluated, assessed or compared from one location/point. Accuracy and consistency are very important during design of tieback anchors, and they are improved by use of an independent reference point. For instance, the reference point can be used to measure, record and monitor movement of the tieback anchor at different loadings.

Top down wall construction method is a method where a retaining wall is constructed from the top downwards. This method is most suitable when constructing tall or underground structures in a limited space. Here, the first shaft is dug using machine’s cutter. It is in this shaft where one wall panel will be installed. The reinforcement of the wall is formed by lowering the rebar cage into the dug shaft. The machine is moved to another point to construct a parallel wall shaft, which is also installed with rebar cage once it has been excavated. Concrete is then poured into the panels to make the concrete walls. The most appropriate wall types for top down construction method are: retaining walls, precast concrete walls, panelized load bearing walls and shear walls. This is because these wall types can be easily and safely constructed from the top downwards.

Bottom up wall construction method is a method where a retaining wall is constructed from the bottom upwards. Here, the site has to be excavated first before the wall can be constructed. Once the site is excavated, a wall is constructed on the edges from the bottom upwards (in a similar way of constructing a wall of a typical building). The most appropriate wall types for bottom up construction method are: masonry walls, stone walls, engineering brick walls and cavity walls. This is because these wall types can only be constructed from the bottom upwards.

References

Akmilah, N., Ong, C., & Choong, K. (2014). Precast concrete soldier pile system with corrugated section post for riverbank protection. Applied Mechanics and Materials, 567, 457-462.

Eskandari, L., & Kalantari, B. (2011). Basic Types of Sheet Pile Walls and Their Application in the Construction Industry - a Review. Electronic Journal of Geotechnical Engineering, 16, 1533-1541.

Liu, J., Li, B., Liu, Y., & Cai, S. (2017). Design method of redundancy of brace-anchor sharing supporting based on cooperative deformation. IOP Conference Series: Earth and Environmental Science, 94, 1-6.

Pendley, T. (2009, May 1). How to Construct Raker Shores. Retrieved from Fire Rescue Magazine: https://www.firerescuemagazine.com/articles/print/volume-4/issue-5/special-operations/how-to-construct-raker-shores.html

Quimby, T. (2014, November 4). Basic Design Concepts. Retrieved from Beginner's Guide to Structural Engineering: https://www.bgstructuralengineering.com/BGDesign/BGDesign05.htm

RST Environmental Solutions. (2018). Anchors. Retrieved from RST Environmental Solution Limited: https://www.rst.co.nz/anchors.html

Sabatini, P., Pass, D., Bachus, R. (1999). Geotechnical Engineering Circular No. 4: Ground Anchors and Anchored Systems. Atlanta, Georgia: GeoSyntec Consultants.

Song, Y., & Hong, W. (2008). Earth pressure diagram and field measurement of an anchored retention wall on a cut slope. Landslides, 5(2), 203-211.

The Constructor. (2017). Types of Soil Tests for Building Construction. Retrieved from https://theconstructor.org/geotechnical/types-of-soil-tests-construction/12679/

Yazdani, M., Azad, A., Farshi, A., & Talatahari, S. (2013). Extended "Mononobe-Okabe" Method for Seismic Design of Retaining Walls. Journal of Applied Mathematics, 2013, 1-10.