Page 1 CONCRETE INSTITUTE of AUSTRALIA Queensland Branch. CONCRETE RETAINING SOLUTIONS. SEMINAR – BRISBANE. Wednesday 23rd August, 2000 CONCRETE SLEEPER and CRIB WALL SYSTEMS Dr. Peter Mullins Mullins Consulting 1. INTRODUCTION Several retaining wall options are available to the designer/builder. These include: timber sleeper, concrete sleeper, boulder, gabion, stone pitched, timber crib, concrete crib, reinforced blockwork and concrete cantilever walls and reinforced soil. The type of wall selected is dependent upon issues such as: available space, wall height, soil parameters, surcharge loads, durability, aesthetics and cost. It is not uncommon for several retaining solutions to be used on a project. This paper will discuss two concrete retaining systems, which are commonly used in South East Queensland. A concrete sleeper and a concrete crib wall system. The specific systems discussed are manufactured and constructed by Concrib Retaining Walls Pty Ltd. The discussion is however general and applies to many of the retaining systems. The Concrib sleeper and crib systems are described in the next section. Following this is a general discussion of the factors which affect the loads applied to the sleeper and crib walls and the capacities of these walls to resist such loads. It is the writer’s observation that some of the subtleties are often over looked or misunderstood by many designers and contractors. It is concluded that the concrete retaining systems presented are effective methods of retaining with the sleeper system suiting low walls where space is restricted and the crib system applicable to higher walls where space is not as restricted. 2. RETAINING WALL SYSTEMS 2.1 Concrete Sleeper Retaining Wall System. Sleeper retaining wall system consists of posts, which cantilever vertically or near vertically from the footing. Sleeper units are stacked behind the post and normally are butt jointed behind the post. A slotted pipe is placed at the base, behind the wall and free draining material (gravel) is separated from the retained embankment by geofabric. A soil plug about 300mm thick is placed on top of the gravel is also
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CONCRETE INSTITUTE of AUSTRALIAQueensland Branch.
CONCRETE RETAINING SOLUTIONS.SEMINAR – BRISBANE.Wednesday 23rd August, 2000
CONCRETE SLEEPER and CRIB WALL SYSTEMS
Dr. Peter MullinsMullins Consulting
1. INTRODUCTION
Several retaining wall options are available to the designer/builder. These include:timber sleeper, concrete sleeper, boulder, gabion, stone pitched, timber crib, concretecrib, reinforced blockwork and concrete cantilever walls and reinforced soil. Thetype of wall selected is dependent upon issues such as: available space, wall height,soil parameters, surcharge loads, durability, aesthetics and cost. It is not uncommonfor several retaining solutions to be used on a project.
This paper will discuss two concrete retaining systems, which are commonly used inSouth East Queensland. A concrete sleeper and a concrete crib wall system. Thespecific systems discussed are manufactured and constructed by Concrib RetainingWalls Pty Ltd. The discussion is however general and applies to many of theretaining systems.
The Concrib sleeper and crib systems are described in the next section. Followingthis is a general discussion of the factors which affect the loads applied to the sleeperand crib walls and the capacities of these walls to resist such loads. It is the writer’sobservation that some of the subtleties are often over looked or misunderstood bymany designers and contractors.
It is concluded that the concrete retaining systems presented are effective methods ofretaining with the sleeper system suiting low walls where space is restricted and thecrib system applicable to higher walls where space is not as restricted.
2. RETAINING WALL SYSTEMS2.1 Concrete Sleeper Retaining Wall System.Sleeper retaining wall system consists of posts, which cantilever vertically or nearvertically from the footing. Sleeper units are stacked behind the post and normallyare butt jointed behind the post. A slotted pipe is placed at the base, behind the walland free draining material (gravel) is separated from the retained embankment bygeofabric. A soil plug about 300mm thick is placed on top of the gravel is also
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separated from the gravel by geofabric. the soil plug is used to prevent surfacestormwater flowing into the drainage gravel behind the wall.In the Concrib system the concrete post are spaced at 2cm centres. The posts arenormally founded in a bored hole 450 diameter with a depth equal to the wall height.The depth is varied depending on the nature of the foundation soils. Where thefoundation material is rock or boulders are present a cantilever footing may beadopted. Sleeper units 150mm wide and 80mm thick are stacked behind the post.The maximum wall height is 1.95m. The posts are normally installed vertically with200mm width of drainage material behind. Except on expansive soil sites where theposts are leaned into the embankment. A face slope of 1 on 20 is normally adoptedon moderately expansive sites (class M – AS2870) on 1 on 10 for highly expansivesites (class H – AS2870). The width of the gravel drainage layer is also increased onexpansive soil sites. A typical section is shown in Figure 1.
Figure 1: Concrete Sleeper Retaining Wall – Typical Section.
2.2 Concrete Crib Retaining Wall System.Several concrete crib systems are available in Australia. Crib systems consist ofstretcher and header units. The stretcher units are placed in the front and rear of thewall parallel to the face of the wall. The header is placed between the stretcher unitsand are perpendicular to the face of the wall. The unites stack to form a 3-D gridstructure, which is filled with gravel for most of the height of the wall. Typically, thetop section of the wall is filled with soil to prevent surface stormwater flowing intothe free draining gravel. The combination of header and stretcher units together withthe wall fill material act as a gravity retaining wall. Typically, there is no bondbetween the header and stretcher unit.
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In the Concrib system the header units are recessed to accommodate the stretcherunits. Hence, the forward movement of the stretcher unit relative to the header unit isprevented not only by friction between the units but by a mechanical interlock. Theheader and stretcher units combined thickness is 250mm. Hence the wall height has aslope wall height increment of 250mm. The header unit lengths are such that thedistance between the rear of the rear stretcher and the front of the front stretcher is500, 700, 1000 or 1300. Walls with the thicknesses of 1300 or less are referred to assingle header construction (refer Figure 2). Greater wall thickness can be achievedby lapping header units. Wall thicknesses of 240mm can be achieved with doubleheader construction (refer Figure 2) and 3500mm with triple thickness. Walls aretypically constructed with a face slope of 4 vertically to 1 horizontally. This faceslope may be varied. Steeper walls may be used where space is restricted, but greaterwall thickness may be required. Wall slopes as flat a 3 vertical to 1 horizontal areoften used when space is not restricted to achieve a more economic wall solution.
Figure 2: Concrete Crib Wall – Typical Section – Single Header Construction.
As the crib wall is filled with gravel and has an open face it is well drained. Thewidth of gravel drainage layer behind the wall can therefore be reduced comparedwith conventional solid retaining walls. A geofabric is used to envelope the graveland is placed between header units to separate the soil plug at the top of the wallfrom the gravel. A subsoil drain is provided at the rear of the wall to ensure thefooting region is drained. As the footing is on the slope equal to the wall face slope,the rear of the footing is below the ground level at the front. On flat sites the sub-soildrain needs to be connected to the stormwater drainage system to effect drainage ofthe base of the wall.
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Figure 3: Concrete Crib Wall – Typical Section – Double Header Construction.
3. LOADS on RETAINING WALLSThe following comments and plots are based upon a Coulomb analysis to determinethe active earth pressure of a cohesionless material.
3.1: Effect of Soil Strength and Ground Slope.The soil effective internal angle of friction (phi) is possibly the most significantfactor effecting the load on the wall. As the phi angle decreases the active pressureon the wall increases. The slope of the ground above the wall, also has a significanteffect on the pressure exerted on the wall. Figure 4 is a plot of the active earthpressure coefficient, as a function of phi, for various ground slopes above the wall,for a wall with a face slope of 1 horizontal to 4 vertical.
3.2 Effect of Wall HeightThe bending moment at the base of a wall is a function of the cub of the height.Hence increasing the wall height by one quarter doubles the base moment capacityrequired.
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Figure 4: Active earth pressure for various phi angles and ground slopes.
3.3 Effect of Surcharge LoadsUniform surcharge loads increase the load on the wall, in a similar manner toincreasing the wall height. A 5KPa surcharge load is about equal to an additional0.25m of wall height.
Large point load surcharges can increase the load on a wall significantly.
3.4 Effect of Wall Face SlopeAs the wall face slope decreases from 90 degrees (vertical), the load on the wall alsodecreases. Figure 5 presents a plot of the active pressure coefficient for various wallface slopes and soil friction angles (phi).
Figure 5: Active earth pressure for various phi angles and wall slopes
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3.5 Effect of Water PressureIf a wall is not effectively drained the increase in load on the wall due to waterpressure is very significant. The crib wall is effectively drained due to the nature ofthe construction. Gaps between sleeper wall units, provide additional drainage to thedrainage system behind the wall, and tend to ensure the sleeper wall is also welldrained. However, exclusion of surface stormwater from the drainage system behindthe wall by means of the clay surface plug is essential.
3.6 Effect of Wind Loads including Wind Loads on FencesMany sleeper walls are constructed on the boundary and they therefore have fenceserected immediately above the wall. Often the fence is solid timber paneling. Whenthe wind load applied to the fence is considered in the design of the sleeper post theincrease in load can be significant. Figure 6 is a plot of the ultimate bending momentas a function of wall height for a sleeper wall retaining soil with a phi of 25 degrees,a live load surcharge a 3KPa and a fence height of 15m exposed to a W4I wind.
Figure 6: Bending moment in sleeper wall post
4. CAPACITY of RETAINING WALLS
4.1 Crib WallsIncreasing the mass of the wall and the effective width of the wall increases thecapacity of gravity walls. Increasing the thickness and height of the wall can mosteffectively increase the mass. Increasing the width of the wall, the face slope of thewall and the height of the wall can increase the effective width of the wall, as thecentre of gravity is moved away from the front toe of the wall.
Reducing the cross section of the wall from the rear, has the disadvantages ofreducing the capacity of the wall as the mass is reduced and the effective width isreduced. The added disadvantage is that the slope of the retained embankment isgreater than the wall face slope, hence the load on the wall is increased. Reducingthe cross section from the front has similar but less significant reduction of wall
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capacity. The load on the wall is however, not increased.
A crib wall of uniform cross section is often the most economical solution.
4.2 Sleeper WallsThe capacity of sleeper walls is limited by the capacity of the post to cantilever fromthe footing and the capacity of the sleeper to span between posts.
5. ConclusionsConcrete crib and sleeper retaining wall systems are effective methods of retaining.The sleeper wall system suits low walls where space is restricted and the crib systemis applicable to higher walls where space is not as restricted. It has been shown thatthe loads on retaining walls can vary significantly depending upon the soil type, wallgeometry, ground slopes and surcharge loads.
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