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TheStructuralEngineer October 2013
Technical Technical Guidance Note
Retaining wall construction Introduction
Retaining walls have been used for thousands of years; whether in the construction of terraced fields on a steep slope, or a railway cutting through a hill side, a retaining wall is used in some form or another. This Technical Guidance Note is a description to the various forms of retaining walls that are currently used. It is primarily concerned with structures that retain soil; although many of the aspects described can be translated to retaining other materials such as grain, sand and liquids. The water table will have design implications on the wall, the pressure of which can be alleviated by providing drainage and weep holes, depending on the soil conditions. The aim of this note is to provide you with sufficient familiarity of the various types of retaining wall so that when you encounter them or are developing a retaining wall solution, you will know what options are available.
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ICON LEGEND
W Applied practice
W Further reading
W Web resources
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Figure 1 Concept diagram of gravity based retaining wall
W Retaining walls
Figure 3 Concept diagram of hybrid retaining wall
Figure 2 Concept diagram of embedded retaining wall with pivot point
Active Passive
Passive
Retaining walls Retaining walls can be grouped into three categories: gravity, embedded and hybrid. They all perform the function of supporting a material, which is typically soil, at an angle that exceeds the angle of repose. Each type has unique properties making them ideally suited to specific situations. Gravity retaining walls A gravity retaining wall relies upon the friction between it and the founding material, together with the wall’s self-weight, to resist sliding and overturning. They can be formed from masonry, concrete, or reinforced soil and are typically used when a shallower depth of excavation needs to be retained. They are normally between 1-3m and are constructed to make way for level ground, such as a railway siding (Figure 1).
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Active
Embedded retaining walls Embedded retaining walls rely on being installed into the soil to below the depth of excavation. Once the wall is in place within the ground, soil is excavated from its front to a defined depth based on the bending capacity of the retaining wall and the depth of its embedment into the soil. Embedded retaining walls rely on the passive resistance of the soil in front of them, along with the shear and bending capacity of the wall itself, to keep it in place. They are most often used for basements and other excavations cut into previously level ground. They are suitable for very deep excavations i.e. in excess of 10m and/or where the site is restricted e.g. adjacent site boundaries or existing sub-structures (Figure 2). For excavations in excess of 4-6m deep, propping to the embedded wall is usually necessary to control wall deflections and prevent excessive depths of embedment.
Hybrid retaining walls Hybrid retaining walls (Figure 3) combine the features of both gravity and embedded forms. They rely on shear and bending resistance of the wall and its base, which may or may not be fixed into position by piles. They are typically used where ground levels need to be raised e.g. where there are poor soil conditions, to the extent that a gravity retaining wall solution is not viable.
Gravity retaining walls Gravity walls come in many different forms, all of which follow the principles described in Fig. 1 i.e. supporting lateral forces via their resistance to sliding and overturning. They are often constructed using a backfill method, where the wall is built with the soil it is to retain, being placed against it. Gravity retaining walls can be split into three subgroups: masonry, reinforced concrete and soil reinforcement.
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Figure 4 Masonry retaining wall
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Figure 5 Dry masonry retaining wall
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Figure 6 Gabion retaining wall
Masonry retaining walls Masonry retaining walls (Figure 4) can be anything from a 500mm high garden wall built from clay bricks, to a 2m high dry stacked stone wall. They typically sit on a mass or reinforced concrete footing and require vertical movement joints at approximately 10-15m centres to alleviate thermal expansion and contraction effects. Consideration should be given to waterproofing and drainage to prevent frost damage and to ensure durability. Dry masonry retaining walls Dry masonry retaining walls (Figure 5) consist of interlocking masonry units that are placed on top of one another without any mortar or cement. Gravity is the primary force that holds dry masonry walls together as well as the shear key between each masonry unit. This shear key is a formed protrusion from the soffit of the masonry unit that laterally locks the units into place. Dry masonry retaining walls can be drained either by letting water pass through the wall into a drain in front or by installing a granular fill layer behind the wall to divert the water. They can also be placed at an angle creating a battered face to the retained soil. Gabion retaining walls Gabion retaining walls (Figure 6) are a subset of the dry masonry wall, in that they are formed from modular units that are not bonded together with mortar or cement. Rather, the modular units are steel cages each filled with large stones/rocks. Gabion walls are free draining as they allow the passage of water to flow through them. The steel cages are typically galvanised to prevent corrosion. Gabion walls are used in poor ground conditions as they are less susceptible to the effects of subsidence due to their flexibility. They are often placed at an angle and can have a stepped or flat outer surface. Crib retaining walls Crib retaining walls (Figure 7) consist of a series of struts made from reinforced concrete or timber that are bound together to form a cage. This cage is filled with sand and gravel or mass concrete to create the retaining wall. They are usually placed at an angle, in a similar manner to gabion retaining walls and can be placed in multiple layers depending on their height. They can be drained either by allowing passage of water through the wall or, if it is filled with concrete, by creating a barrier behind the wall with granular fill that provides a path of low resistance of water passage, allowing it to drain elsewhere.
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Figure 7 Crib retaining wall
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Figure 8 Reinforced soil retaining wall
"Gravity is the primary force that holds dry masonry walls together" Reinforced soil retaining walls The concept of reinforcing soil to allow it to act as a retaining wall can be traced back to pre-historic times where sticks were used to strengthen the soil, increasing its angle of repose. This creates a bank or wall against which soil can be retained. Today the method of reinforcement is a form of mesh that is placed in layers in the soil. It is made from a material that has high tensile stiffness, which enhances the soil’s shear and tensile capacity. These meshes have been developed into geotextiles or ‘geogrids’ and are either a synthetic polymer or a steel fabric (Figure 8).
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TheStructuralEngineer October 2013
Technical Technical Guidance Note
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Figure 10 Contiguous bored piled wall
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Figure 11 Secant bored piled wall
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Figure 9 Sheet pile walls
Embedded retaining walls Embedded retaining walls come in many different forms, which can be split into two sub-groups: driven precast and prefabricated elements, and bored/cast in situ elements. They usually rely entirely on the passive resistance of the soil they are installed into, but anchors or props can be used to increase the maximum excavation depth achievable. Embedded retaining walls are commonly used for temporary works and for the construction of caisson foundations as well as permanent installations. They require special equipment to build, which is a key differentiator between them and gravity based options and such tools add to the cost of their construction due to this requirement. However, the amount of material required to construct them is much less than gravity retaining walls and their speed of construction is typically greater. They can also retain larger volumes of soil vs. amount of material contained within them, compared to gravity walls. Sheet pile walls Sheet pile walls are some of the most common types of embedded retaining wall systems currently in use. They consist of profiled steel sheets linked together via an interlock that runs along the edge of each sheet. They may be driven into the ground by a mechanical hammer, which can cause a
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"Embedded retaining walls are commonly used for temporary works and for the construction of S caisson foundations"
Figure 12 King post/Soldier piled wall
significant amount of noise and vibration as the sheets are installed. This makes creating a sheet pile wall in urban areas problematic due to the disturbance it causes as they are being driven into the soil. Alternatively, there are several different techniques available to push sheet piles into the ground, reducing the noise and vibration during installation. They are generally used in clay, sand and silt soils with no hard inclusions such as existing foundations or boulders. There are many variants of the sheet pile retaining wall, all with different section profiles, some having integrated I beams and circular hollow sections within them (see Figure 9 for examples). Sheet piled walls can, with care, be sealed to prevent water penetration, which allows them to retain liquid.
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Contiguous bored piled walls Contiguous bored piled walls, also referred to as ‘contig’ are dug and cast into the ground with a small gap of 50-150mm between each pile. The presence of this gap makes contiguous piled walls unsuitable to retain granular soils like sand and gravel, as such soil types fall through the gaps between the piles (Figure 10). Similarly, they cannot retain water and can only be placed in areas above the water table or where de-watering is being employed. Where they are used in permanent conditions, a facing wall is needed in front of contiguous piled walls due to the gaps between each pile.
Table 1: Types of primary piles in secant bored piled wall Secant primary pile type
Description
Characteristic compression strength (N/mm2)
Reinforcement present
Soft
Bentonite cement
1-3
No
Firm
Concrete with an admixture that delays the development of strength in the pile during drilling
10-20
No
Hard
Full strength concrete, often with an admixture that delays the development of strength in the pile during drilling
25-60
Can be installed in primary pile*
* There is a risk of encountering any reinforcement in the primary pile when drilling Secant bored piled walls Secant bored piled walls are a variant of the contiguous piled wall, with the major difference being that there is no gap between piles. Instead the piles are divided into primary and secondary (historically referred to as female and male respectively). The primary piles are installed first and the bored secondary piles overlap their primary neighbours (shown in green in Figure 11). This creates a continuous wall that can resist water penetration when carefully detailed and executed. Secant bored piled walls come in three varieties that are dependent on the characteristic compressive strength of the primary bored pile (Table 1). Secant bored piled walls can be installed into almost any type of soil and can be placed where there is a high water table. King post/Soldier piled walls King post or Soldier piled walls consist of a series of steel posts that are either driven into the ground or cast into place within an excavated hole. The posts are installed 1-3m apart and the gap between them is filled with spanning panels, which are typically precast concrete segments (Figure 12). These are installed as the soil in front of the retaining wall is removed. They are typically used in clay and granular soils, provided the water table is below the extent of the excavation. Diaphragm walls Diaphragm walls are a series of interlocking reinforced concrete panels that have been cast in situ by excavating a segment of soil and replacing it with a fluid (typically bentonite). A reinforcement cage is then inserted and concrete is poured in via a tremie pipe. The concrete displaces the bentonite as it is poured. Diaphragm walls can be 600-1500mm thick with each panel being typically 2.2-8m wide. They can retain significantly more soil than most other walls, with a depth of 120m being possible due to
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the secondary piles, so it is preferable to avoid reinforcing primary piles.
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Figure 13 Diaphragm wall
their ability to withstand very high lateral loads. They can be installed into almost any type of soil and can resist water pressure, provided water bar seals are installed across each segment (Figure 13).
Hybrid retaining walls The archetypal hybrid retaining wall is the unpropped cantilever that is founded on pile foundations. Hybrid walls can be installed in most soils and can retain water pressures, provided appropriate drainage and water proofing measures are installed (Figure 14).
Criteria for retaining wall selection
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Figure 14 Hybrid retaining wall
The primary criterion for the selection of a retaining wall is the height required. Of secondary concern are space restrictions as they will impact on the preferred construction method and can preclude certain types of walls. As an example, a 2m high retaining wall adjacent to a rural highway would likely be a gravity or hybrid wall as it is not very high and space restrictions are not likely to be a concern. A 4m deep basement construction within a city centre, however, would most likely require an embedded wall due to its height and limited site access. Other criteria that should be considered, include stiffness of the wall to prevent movement during its design life, and required water resistance, which is of particular relevance in basement construction. Water proofing, construction method and materials all have an impact on cost and programme and should therefore be taken into consideration. For further advice on selecting an appropriate retaining wall solution, see Table 8.2 in the Manual for the geotechnical design of structures to Eurocode 7.
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› Note 33 Level 1
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Technical Technical Guidance Note
TheStructuralEngineer October 2013
Eurocode 0.
Glossary and further reading
Applied practice BS EN 1992-1-1 Eurocode 2: Design of Concrete Structures - Part 1-1: General Rules for Buildings BS EN 1992-1-1 UK National Annex to Eurocode 2: Design of Concrete Structures - Part 1-1: General Rules for Buildings BS EN 1997-1 Eurocode 7: Geotechnical Design - Part 1-1 General Rules BS EN 1997-1 UK National Annex to Eurocode 7: Geotechnical Design - Part 1-1 General Rules BS EN 1536:2010 - Execution of special geotechnical work – bored piles
Angle of repose - The deepest angle of slope relative to a horizontal surface a granular material, such as soil, is resting upon.
Bentonite - A form of absorbent clay that is used during the construction of some forms of retaining wall. Caisson - A water-tight retaining substructure, typically used for the construction of marine engineering works such as piers. Tremie pipe - Method of placing concrete within a liquid or highly viscous material that relies on the delivery pipe remaining in the freshly placed concrete so that it displaces the liquid. Unsupported height - The extent of a
BS EN 1538:2010 - Execution of special geotechnical work – diaphragm walls
retaining wall that has no propping.
BS EN 12063:1999 - Execution of special geotechnical work – sheet pile walls
Water table - The depth at which ground water is located.
Pai Lin Li Lecture 4HE0AI,IN,I4RAVEL!WARDISPRESENTEDTO Institution members wishing to spend 4 – 6 weeks outside their own country studying CURRENTPRACTISESORTRENDS4HISPROVIDESAN unrivalled opportunity to sample the technical, economic, social and political conditions in another country and to examine how these various factors affect the practise of structural engineering. 4HISYEAR WEAREDELIGHTEDTOANNOUNCETWO winners, John Orr and Katie Symons who will BOTHPRESENTTHEIRlNDINGSATTHISLECTURE
$ATE \ Thursday 17 October 4IME \ Registration from 17:30 Lecture at 18:00 6ENUE\ International HQ
Annual Institution Events
Further Reading The Institution of Structural Engineers (2013) Manual for the geotechnical design of structures to Eurocode 7 London: The Institution of Structural Engineers The Institution of Civil Engineers (2012) ICE manual of geotechnical engineering London: Thomas Telford The Institution of Structural Engineers (2004) Design and construction of deep basements including cut and cover structures London: The Institution of Structural Engineers CIRIA (2000) Publication C516: Modular gravity retaining walls: design guidance London: CIRIA CIRIA (2003) Publication C580: Embedded Retaining Walls London: CIRIA Eurocode 0.
Web resources The British Geotechnical Association: http://bga.city.ac.uk
Form: an adventure in concrete and brick
International perspectives on Life Cycle Assessment of structural materials, particularly timber: lessons to be learned from Australasia
Form active design can facilitate architecturally INTERESTING STRUCTURALLYOPTIMISED MATERIALLYEFlCIENT CONSTRUCTION4HISPRESENTATIONWILLCONSIDERHOW PRESTRESSEDBRICK ANDmEXIBLYFORMEDCONCRETE CAN INmUENCEFUTUREPOSSIBILITIESFORGLOBALLYSUSTAINABLE construction.
4HISPAPEROUTLINESEXPERIENCEGAINEDFROMTRAVELIN Australia and New Zealand in 2013, examining the STATUSOFLIFECYCLEASSESSMENT,#! OFCONSTRUCTION materials, particularly with respect to embodied ENERGYANDCARBON4HEINNOVATIVEUSEOFLOWCARBON materials, particularly timber, is also explored.
John Orr PhD MEng(Hons) John Orr is a lecturer in sustainable construction at the University of Bath, UK. He RECEIVEDHIS-%NGAND0H$ degrees from the same institution in 2009 and 2012 respectively. His research interests include the design of concrete structures using fabric formwork and the shear behaviour and computational modelling of concrete.
Conferences & Seminars
Katie Symons MEng MA (Cantab) MIStructE MICE CEng
Katie Symons has a keen interest in the sustainability of the built environment, especially the embodied energy and carbon of buildings. She presented her research, undertaken during a secondment to the University of #AMBRIDGE ATTHE!USTRALIAN,IFE#YCLE!SSESSMENT Society conference in Sydney in July 2012. She has had journal papers published on the subject of evaluating embodied energy and carbon.
Special Interest Series
Technical Lecture Series
A series of lectures organised in partnership by the Institution and other leading organisations.
Registration is required in advance by visiting the events section of the Institution website, www.istructe.org, and following the instructions provided. Registration will close Friday 11 October. Space is limited and latecomers will only be admitted to the OVERmOWFACILITY NOTTHEMAINLECTURETHEATRE)FYOUHAVEANYQUESTIONSPLEASECONTACTTHE%VENTS4EAMAT
[email protected]
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