Retaining Wall Archives - Civil Engineering Tips

Concrete Batter Protection

Concrete slabs are frequently used for concrete batter protection, in which case the concrete is normally reinforces with mesh to control cracking. Typical slabs thicknesses may range from 100 to 150 mm.

When a floodway is inundated the slabs may be subjected to water pressure, which tends to lift them. Consequently, for stability, batters are normally made 1:1.5 or flatter. Pressure relief holes may be provided in the concrete slabs in order to reduce the uplift pressures.

In order to minimize scour adjacent to the protection work, protection on the downstream batter may be shaped so as to defect the tail-water jet upwards away from the stream bed.

In practice it has been found that despite the upward deflection of the tailwater jet by the curved apron some scour does occur beyond the protection. However, generally it does not reach serious proportions.

Alternate treatments of downstream batters are:-

(a) Provide a cut off wall in lieu of an apron, accepting some scour downstream of the batter but isolating it from the embankment;

(b) Provide a flexible stone filled wire mattress apron to protect the end of the concrete slab.

The principal requirement of concrete for floodway batter protection is durability. Mixes with an aggregate/cement ratio of between 8:1 and 10:1 (28 day compressive strength of 10 to 20 MPa) have been found to be sufficiently durable.

The concrete is normally cast in alternate bays, the first bays cast being screeded from forms set to the profile of the batter. The intermediate bays are screeded from the previously cast bays. A stiff mix is used because of the sloping surface. A kerbing machine has been found to be effective for casting the concrete margins adjoining the pavement.

The principal advantage of concrete slope paving is that concrete aggregate is generally readily available, whereas stone for rip rap (or stone filled wire mattress protection) is not always available on economic leads.

The principal disadvantage of concrete slope paving is its rigidity. Also it cannot adapt to scoured conditions and is subject to greater uplift forces than free draining rip rap or stone filled wire mattress protection. Please leave a comment about concrete batter protection

Gabions Wall & Mattresses

Gabions and mattresses are a useful form of batter protection where the only rock available is too small for dumped rip rap.

The gabion is basically a rectangular box made of galvanised (or sometimes PVC coated) steel wire mesh, which after assembly on site is filled with stone. The boxes are normally supplied in sizes from 2 to 4 m in length, 1 m in width and from 0.3 to 1 m in height.

Mattresses are similar to gabions except that they are wider and flatter and are usually more suitable for floodway batter and apron protection than are gabions, Sizes of proprietary mattresses available in Australia are 6 m long x 2 m wide x 170 mm, 230 mm or 300 mm deep.

Gabion Mattresses have been successfully made on site using 1.8 m wide chain wire mesh. The chain wire mesh is rolled out on the prepared batter or apron and a layer of stone 150 mm – 200 mm deep is placed on it. The layer of stone is then covered with a top layer of mesh and the edges of the sheets wired together. Intermediate wire ties between the sheets are provided at 1 m centres.

Factory made gabions and mattresses arrive flat in bundles, and are assembled by raising the sides, ends and diaphragms which come laced with wire to the bottom section. Any kinks in the mesh must be removed before assembly.

After assembly, the boxes or mattresses are placed in position. On a slope, the longer dimension of the mattress should be up and down the slope. Empty gabions must be attached to stakes (or completed gabions) and stretched with a wire strainer or which of at least 1 tonne capacity before lacing to adjoining gabions and filling.

Internal wire braces are required in the 1 m deep gabions. These are inserted during filing. Filling is accomplished with the aid of a small loader or excavator. The stone size
should be not less than the greater mesh dimension, and not greater than 2/3 of the thickness of the basket or mattress.

The usual stone sizes for gabions range from 125 mm to 200 mm and those for mattresses range from 90 mm to 150 mm.

After filing, the lids are stretched tight using bars and wired down to the sides, ends and diaphragms.

Typical figures for working times per m3 are as follows:-

• For 4m x 1m x 1m gabions – 0.7 man hours and 0.2 plant hours
• For 2 m x 1m x 0.5m gabions – 2.2 man hours and 0.2 plant hours

Tools required include fencing pliers, bars for closing down lids, steel bars for use in anchoring and tensioning and a wire strainer or winch.

The mattresses need to be anchored at the top of the batter (if used as batter protection, or to the toe of the batter 9 if used as an apron. One means of doing this is to
attach the mattresses to driven steel star pickets. The pickets may be driven by means of a loader bucket or grader blade, using the machines’ hydraulic system to apply a downwards force.