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BASEMENT OR RETAINING WALLS

A Retaining Wall is designed to resist the lateral pressure of soil when there is a desired change in ground elevation that exceeds the angle of deposited soil.

A basement wall usually refers to a supported retaining wall, which is a freestanding structure without lateral support at the top. These are supported from a footing and rise above the grade on one side to retain a higher level grade on the opposite. The walls must resist the lateral pressures generated by unrestrained soil or water pressure.

Before and after

Every retaining wall supports a wedge of soil. The wedge is a section of soil which extends beyond the failure plane of the soil type, and can be calculated once the soil friction angle is established. As the setback of the wall increases, the size of the wedge is reduced. This reduction lowers pressure on the retaining wall.

The most important consideration in proper design and installation of retaining walls is to recognize and counteract the inclination of the retained material to move downslope due to gravity. This creates ground pressure behind the wall which depends on the angle of internal friction and the cohesive strength of the retained material, as well as the direction and magnitude of movement the retaining structure may experience.

Lateral earth pressures are zero at the top of the wall and, in stable ground, increase proportionally to a maximum value at the base. Earth pressure will push the wall forward or overturn it if not properly addressed. Also, any groundwater behind the wall may cause hydrostatic pressure on the wall.

Unless the wall is designed to retain water, It is important to have proper drainage behind the wall in order to limit the pressure against the wall. Proper drainage materials will reduce or eliminate the pressure and improve the stability of the material behind the wall. Drystone retaining walls are normally self-draining.

The International Building Code requires retaining walls to be designed for a safety factor of 1.5 to ensure stability against overturning, sliding, excessive foundation pressure and water uplift.

TYPES OF RETAINING WALLS

GRAVITY WALLS

Gravity walls depend on their mass to resist pressure from behind. For short landscaping walls, they are often made from mortarless stone, segmental concrete, or masonry units. Homeowners who build larger gravity walls that require a rigid concrete footing, can make use of a professional excavator in order to dig a trench for the base of the gravity wall.

CANTILEVERED WALLS

Cantilevered retaining walls are made from an internal stem of steel-reinforced, cast-in-place concrete or mortared masonry. These walls cantilever loads, like a beam, to a large, structural footing, converting horizontal pressures from behind the wall to vertical pressures on the ground below. Occasionally, cantilevered walls are buttressed on the front, or include a counterfort 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 supplying additional reinforcement. These walls require rigid concrete footings below frost depth. The cantilevered wall uses much less material than traditional gravity walls.

SHEET PILING WALLS

Sheet pile retaining walls are typically used in soft soils and tight spaces. Sheet pile walls are made out of steel, vinyl or wood planks which are driven directly into the ground. The material is usually driven 1/3 above ground, and 2/3 below ground, but this may be altered depending on the environment. Taller 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 the wall, usually by a cable or a rod. Anchors are placed behind the potential failure plane in the soil as a precaution.

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 fixed into the rock or soil behind it. Usually driven into the material with boring, anchors are then expanded at the end of the cable, either by mechanical means or often by injecting pressurized concrete, which expands to form a protuberance in the soil. Although, technically complex, this method is very useful where high loads are expected, or where the wall itself has to be slender and would otherwise be too weak.

ALTERNATIVE RETAINING TECHNIQUES

SOIL NAILING

Soil nailing is a technique in which soil slopes, and excavations or retaining walls are reinforced by the insertion of steel reinforcing bars. These bars are usually installed into a pre-drilled hole and then grouted into place or drilled and grouted simultaneously. These bars are installed untensioned at a slight downward inclination. A rigid or flexible facing, often sprayed concrete, or isolated soil nail heads may be used at the surface.

SOIL-STRENGTHENED

A number of systems exist that reduce the earth pressure acting on the wall itself. These are usually used in combination with one of the other wall types, though some may only use it as facing, for aesthetic purposes.

GABION MESHES

This type of soil strengthening consists of wire mesh boxes, where roughly cut stone or other material is filled. The rigid mesh cages reduce some internal movement/forces, and also reduce erosive forces due to their strength and weight.

MECHANICAL STABILIZATION

Mechanically stabilized earth is soil constructed with artificial reinforcing, via layered horizontal mats, or geosynthetics, fixed at their ends. These mats provide added internal shear resistance beyond that of simple gravity walls. This type of soil strengthening usually needs outer facing walls, or Segmental Retaining Walls, to affix the layers to. The wall face is often composed of precast concrete units that can tolerate some differential movement. The reinforced soil mass, along with the facing, then acts as an improved gravity wall. This reinforced soil mass must be built large enough to retain the pressures from the soil behind it.

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