CONCRETE RETAINING SOLUTIONS. CONCRETE …

1 Page 1 CONCRETE INSTITUTE of AUSTRALIAQ ueensland RETAINING 23rd August, 2000 CONCRETE SLEEPER and CRIB WALL SYSTEMSDr. Peter MullinsMullins Consulting1. INTRODUCTIONS everal 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.

2 It is not uncommonfor several RETAINING solutions to be used on a 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 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.

3 It is the writer sobservation that some of the subtleties are often over looked or misunderstood bymany designers and 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 RETAINING WALL CONCRETE Sleeper RETAINING Wall 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.

4 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 alsoPage 2separated from the gravel by geofabric. the soil plug is used to prevent surfacestormwater flowing into the drainage gravel behind the 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 depth is varied depending on the nature of the foundation soils.

5 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 maximum wall height is 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.

6 A typical section is shown in Figure 1: CONCRETE Sleeper RETAINING Wall Typical CONCRETE Crib RETAINING Wall 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.

7 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 3In 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.

8 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 2: CONCRETE Crib Wall Typical Section Single Header the crib wall is filled with gravel and has an open face it is well drained.

9 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 4 Figure 3: CONCRETE Crib Wall Typical Section Double Header LOADS on RETAINING WALLSThe following comments and plots are based upon a Coulomb analysis to determinethe active earth pressure of a cohesionless : Effect of Soil Strength and Ground soil effective internal angle of friction (phi) is possibly the most significantfactor effecting the load on the wall.

10 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 Effect of Wall HeightThe bending moment at the base of a wall is a function of the cub of the increasing the wall height by one quarter doubles the base moment 5 Figure 4: Active earth pressure for various phi angles and ground Effect of Surcharge LoadsUniform surcharge loads increase the load on the wall, in a similar manner toincreasing the wall height.