RETAINING WALLS

Retaining walls are structures constructed for the purpose of retaining earth or other materials like coal, ore, water etc.

• It may also be defined as a wall provided to maintain ground at two different levels. Provisions of retaining walls become necessary in the construction of hill roads, embankments, bridge abutment, basement in buildings, water reservoir, in preventive measures against soil erosion, in landscaping etc.

• The material retained by the wall is generally known as backfill. The backfill may be horizontal i.e., levelled with the top of wall or it may be inclined at certain angle to the top.

• The inclined fill is also known as surcharge. Besides loads due to retained material, the retaining wall may also be subjected to surcharged load (due to automobile, rail road etc.) acting directly on the wall as well as on the backfill.

• The retaining wall should be stable enough to resist all type of forces acting on it.

Retaining walls have primary function of retaining soils at an angle in excess of the soil’s nature angle of repose.Walls within the design height range are designed to provide the necessary resistance by either their own mass or by the principles of leverage.

Design consideration:

1. Overturning of the wall does not occur2. Forward sliding does not occur3. Materials used are suitable4. The subsoil is not overloaded

• Back fill: The soil placed behind a wall.

• Back fill slope: Often the back fill slopes upward from the back face of the wall. The slope is usually expressed as a ratio of horizontal to vertical (e.g. 2:1).

• Footing (or foundation): That part of the structure below the stem that supports and transmits vertical and horizontal forces into the soil below.

• Grade: The surface of the soil or paving; can refer to either side of the wall.

• Heel: That portion of the footing extending behind the wall (under the retained soil).

• Retained height: The height of the earth to be retained, generally measured upward from the top of the footing.

• Stem: The vertical wall above the foundation.

• Surcharge: Any load placed in or on top of the soil, either in front or behind the wall.

• Toe: That portion of footing which extends in front of the front face of the stem (away from the retained earth).

• Weep holes: Holes provided at the base of the stem for drainage. Weep holes usually have gravel or crushed rock behind the openings to act as a sieve and prevent clogging. Poor drainage of weep holes is the result of weep holes becoming clogged with weeds, thereby increasing the lateral pressure against the wall

Factors which designer need to take account:

• Nature and characteristics of the subsoil's• Height of water table – the presence of water can create

hydrostatic pressure, affect bearing capacity of the subsoil together with its shear strength, reduce the frictional resistance between the underside of the foundation

• Type of wall• Materials to be used in the construction

Types of walls:

• Mass retaining walls• Cantilever walls• Counterfort retaining walls• Precast concrete retaining walls• Precast concrete crib-retaining walls

Mass retaining walls:

• Sometimes called gravity walls and rely upon their own mass together with the friction on the underside of the base to overcome the tendency to slide or overturn

• Generally only economic up to 1.8 m• Mass walls can be constructed of semi-engineering quality bricks

bedded in a 1:3 cement mortar or of mass concrete• Natural stone is suitable for small walls up to 1m high but

generally it is used as a facing material for walls over 1 m• The wall is so proportioned that the dead weight of the wall

provides required stability against the thrust exerted by the backfill including surcharge.

BRICK MASS RETAINING WALL

MASS CONCRETE RETAINING WALL WITH STONE FACINGS

Cantilever Walls:• Usually of reinforced concrete and work on the principle of

leverage where the stem is designed as a cantilever fixed at the base and the base is designed as a cantilever fixed at the stem

• Economic height range of 1.2 m to 6 m using pre-stressing techniques

• Any durable facing material can be applied to the surface to improve appearance of the wall.

TWO BASIC FORMS:

• A base with a large heel• A cantilever with a large toe

CANTILEVER T CANTILEVER L

Counterfort Retaining Walls:

• Can be constructed of reinforced or Prestressed concrete• Suitable for over 4.5 m• In this type of wall the base slab as well as the stem of the wall

span horizontally as continuous slabs between vertical brackets known as counterforts.

• The counterforts are provided behind the wall (on the backfill side) and are subjected to tensile forces. The spacing of the counterforts may vary from of the height of wall. The more the height of the wall, the closer should be the spacing of counterforts

Precast concrete retaining wall:

• Manufactured from high-grade pre cast concrete on the cantilever principle.

• Can be erected on a foundation as permanent retaining wall or be free standing to act as dividing wall between heaped materials which it can increase three times the storage volume for any given area

• Other advantages- reduction in time by eliminating curing period, cost of formwork, time to erect and dismantle the temporary forms

• Lifting holes are provided which can be utilized for fixing if required

Pre cast concrete crib-retaining walls:• Designed on the principle of mass retaining walls• A system of pre cast concrete or treated timber components

comprising headers and stretchers which interlock to form a 3 dimensional framework or crib of pre cast concrete timber units within which soil is retained

• Constructed with a face batter between 1:6 and 1:8• Subsoil drainage is not required since the open face provides

adequate drainage.