Flash floods are with us again and, even though they present a big headache to urban dwellers, the greatest danger lies in rural, on-slope settlements.
However, builders can avoid the risk of collapsing structures by building retaining walls, which are designed to resist the lateral pressure of soil when there is a desired change in ground elevation that exceeds the angle of repose of the soil.
Retaining walls are built in order to hold back ground, which would otherwise move downwards.
Their purpose is to stabilise slopes and provide useful areas at different elevations, for example terraces for agriculture, buildings, roads and railways, dams, flower boxes, and basement walls.
Due to the constant lateral pressure that the retained soil applies on the wall, coupled with the ever-dynamic surcharge and water pressures, retaining walls are always under assault by these forces and, if not properly constructed or designed, they may fail, putting the lives of anyone in the vicinity at risk.
Basement walls may collapse, burying people alive, or dams may fail, sweeping entire populations under a massive wave of sudden floods in river towns downstream.
The main types of retaining walls are:
Gravity walls: These are the most basic retaining walls available in the design range. They depend on the weight of their mass to resist pressure from behind and often have the slight setback of improving stability by leaning back into the retained soil. For short landscaping walls, they are often made from mortarless stone or segmental concrete units. Dry-stacked gravity walls are somewhat flexible and do not require a rigid footing.
Cantilevered walls: Cantilevered retaining walls are made from an internal stem of steel-reinforced, cast-in-place concrete or mortared masonry, often in the shape of an inverted. These walls cantilever loads like a beam to a large, structural footing, converting horizontal pressure from behind the wall to vertical pressure on the ground below. Sometimes cantilevered walls are buttressed on the front, or include a counter fort on the back, to improve their strength, resisting high loads. Buttresses are short wing walls at right angles to the main trend of the wall. These walls require rigid concrete footings below seasonal frost depth. This type of wall uses much less material than a traditional gravity wall.
Sheet piling: Sheet pile retaining walls are usually used in soft soils and tight spaces. They are made out of steel, vinyl, or wood planks which are driven into the ground. Tall sheet pile walls will need a tie-back anchor, placed in the soil a distance behind the face of the wall that is tied to it, usually by a cable or a rod. Anchors are placed behind the potential failure plane in the soil.
Anchored walls: An anchored retaining wall can be constructed in any of the aforementioned styles but also includes additional strength using cables or other stays anchored in the rock or soil behind it. Anchors, usually driven into the material with boring, are then expanded at the end of the cable, either by mechanical means or often by injecting pressurised concrete, which expands to form a bulb in the soil. Technically complex, this method is useful where high loads are expected, or where the wall itself has to be slender and would otherwise be too weak.
There are basically two major ways through which retaining walls fail. These are:
Slip cycle failure: This is when the entire soil mass behind and under the structure becomes saturated with water, making the wall to slide in a circular manner as it gives way to the push of the lateral pressures behind it.
Failure by overturning: The wall is forced to overturn when the lateral pressure is more than the counteracting forces. This mainly happens when surcharge forces are added. This may affect basement walls that are subjected to access surcharge forces from vehicles parked too close to the wall, hence exerting too much pressure.
For maximum safety, the following points must be considered when installing a retaining wall:
When choosing materials, select the type that is best suited for the desired result. Many long-lasting materials are available in the market.
The type of wall you choose should be determined by need. Decide if you need a mass concrete wall or a less expensive pre-cast wall.
Most retaining walls fail because of pressure against the wall caused by water or soil-moisture build up behind the wall. All walls should provide for the back-of-wall water to freely drain down and away from the wall.
This is accomplished with gravel backfill or manufactured drainage blankets and drain pipes. Structural walls require “weep” holes to allow water to drain from behind the wall.
A wall is only as good as its foundation and all retaining walls should be built on structurally sound, compacted foundation sub-base material.
Levelled and compacted earth or gravel fills are acceptable. The foundation material should extend at least one foot beyond the front and back of the base width of the wall.
When building dry-laid stone walls, place the largest, most stable stones at the bottom of the wall and be aware that the base width may need to be as wide as the wall is high.
Walls are more stable and structurally secure if they slope back into the retained slope.
Timber walls and other walls of solid horizontal materials usually have “T” anchors extending back into the slope of undisturbed earth.
This helps walls to resist pressures that force them forward or cause them to pivot on footing material.
A good rule of thumb is to provide at least one anchor per 16 square feet of exposed wall face.
Source: Daily Nation