These are similar in logic to anchored walls but here the difference is, instead of a wall that is anchored to the soil, here the earth itself is reinforced by soil reinforcement strips, which are then each connected to a vertical facing, each of which makes a small fraction of the entire wall and when these facings combined they make up the whole wall.
The reinforcements are flat and long strips that gather strength along their whole length by friction (except inside the “soil wedge” as we described in paragraphs before). These reinforcements take the tension force in the soil, which is something earth is weak at. Another big difference of MSE walls from anchor walls is that, in order to place the reinforcements, the earth behind the wall must be excavated first, in order to lay those strips, which is unlike anchored walls, where we drill holes into the soil horizontally (or at least not very far from horizontally) and insert anhors, without excavating that soil.
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The reinforcement materials can include steel strips, geotextile sheets, steel or polymeric grids, steel nails or tendons between anchorage elements. The facings can be made of precast concrete panels, modular blocks, metal sheet and plates, gabions (we will show gabion walls in the following pages), welded wire mesh, shotcrete, wood lagging and panels, sheets of wrapped geosynthetics.
Advantages of MSE Walls:
- Because the reinforcings take the tension in the earth, at the facings, little outward push load exist and essentially the soil is self supporting. This is unlike rigid retaining walls without reinforcement, where all the pushing action of soil must be countered by the rigid wall itself, or the anchor walls, where all load of soil still pushes the wall and therefore must be resisted by anchors. In some cases, in addition to tension, the reinforcements can also support bending, resulting from shear of soil, such as in slopes.
- Through MSE walls, and reinforced soil slopes, very large heights can be maintained, which can be very costly or even practically impossible to do with rigid retaining walls, such as gravity or cantilever walls.
- MSE walls can tolerate much larger settlements than rigid retaining walls, because here the reinforced soil is simply supporting itself. In addition to larger tolerance for total settlement, differential settlement is also much better tolerated with these systems because here the soil is moving itself and it keeps its reinforcing and the facings connected to it. To support large settlements with traditional rigid walls, pile foundations could be necessary, significantly adding to the cost and schedule.
- For similar reasons of tolerating large settlements, seismic forces are also better resisted by MSE walls, than rigid walls.
- Construction of MSE wall systems are less complicated than anchor walls or even rigid retaining walls. It doesn’t require large construction equipment, require less site preparation.
Disadvantages of MSE walls:
Needs only granular (coarse grained soil) backfill.
Design must address problems of corrosion of steel or degradation of geosynthetics material
In the next post of this series, we will discuss “Various types of retaining walls”