Retaining Walls - Soil Mechanics - Civil Engineering (CE) PDF Download

Introduction

Retaining walls are structures used to retain soil, water or other materials (for example, coal or ore) where natural slope or grading is not possible or permitted. The material retained behind the wall is commonly called the backfill. The principal functions of retaining walls are to stabilise slopes, control erosion and permit the construction of roadways or structures where the available land or site geometry prevents natural slopes.

Purpose and applications

• Stabilise hillsides and prevent slope failures.
• Control erosion caused by surface runoff and protect adjacent structures or roadways.
• Allow roadway construction across steep or confined terrain without excessive grading.
• Reduce vegetation removal and sediment runoff compared with extensive grading, thereby improving water quality by filtering sediments and pollutants.
• Provide bank protection (for rivers and canals) and retaining capacity for terraces, basements and embankments.

Topics covered in this section

• Gravity walls
• Semi-gravity retaining walls
• Flexible walls
• Special types (for example, gabion walls)

Different types of retaining structures

On the basis of how they attain stability, retaining structures may be classified as follows.

1. Gravity walls - stability derives primarily from their self-weight.
2. Semi-gravity walls - reinforced or partially reinforced; stability from weight plus structural action (includes cantilever and counterfort walls).
3. Flexible walls - generally thin elements that depend on embedment or anchorage in the ground (sheet-pile and diaphragm walls).
4. Special types - e.g., gabion walls, mechanically stabilised earth (MSE) walls (not covered in detail here), etc.

1. Gravity walls

Gravity walls are stabilised by their mass. They are usually built from dense, heavy materials such as masonry, mass concrete or similar. Some traditional gravity walls are dry masonry (no mortar) and rely entirely on weight and interlock. Gravity walls are economical and suitable for small heights where the required base width and weight remain practical.

2. Semi-gravity retaining walls

Semi-gravity walls are generally trapezoidal in section and constructed in concrete. They derive most of their stability from weight, but a small amount of reinforcement is provided to reduce concrete mass and control cracking. Semi-gravity walls are often subdivided into:

• Cantilever retaining walls
• Counterfort retaining walls

Fig 6.3. Semi-gravity retaining wall

Cantilever retaining wall

A cantilever retaining wall is a reinforced concrete structure that uses cantilever action of a thin vertical stem connected to a base slab to retain the backfill. It is commonly used for moderate heights (typical range 4 m to 7 m). In section the stem and base slab often resemble an inverted "T" or an "L". The base is constructed on adequate foundation and is heavily reinforced to resist bending and shear. The base slab transmits the overturning moment to the soil and resists uplift near the heel. Cantilever walls may be faced with stone, brick or concrete finishes and sometimes have a slight batter (a backward inclination) on the face for stability and appearance.

Where foundation soils are weak, variants such as reinforced block cantilever walls or earth tieback walls (where horizontal anchors or deadmen extend into the slope and are loaded by the backfill) are used to reduce required base dimensions.

Counterfort retaining wall

When cantilever walls exceed about 7 m in height, it is often economical to provide vertical transverse braces called counterforts. Counterforts are slender vertical slabs on the inside face that tie the vertical stem to the base at regular spacings so that the stem and base span between counterforts. This reduces bending in the vertical stem and the base slab, making the wall more economical for larger heights.

Fig. 6.5 Counterfort retaining wall

3. Flexible walls

Flexible retaining walls are typically thin structural elements that resist lateral earth pressures mainly by the passive resistance and anchorage provided by the surrounding ground. Two principal classes are:

• Sheet pile walls
• Diaphragm walls

Sheet pile walls

Sheet piles are commonly manufactured in steel, timber or precast concrete. Timber sheet piles are generally suitable only for temporary works or shallow embedment (typically up to about 3 m). Steel sheet piles are most suitable for permanent structures and deeper embedment; they are relatively water-tight and can be extracted and reused when appropriate. Reinforced concrete piles or precast concrete sections are used where jetted or cast-in-place piles are required in soft soils, though they may break in tough driving conditions.

Sheet pile walls are classified by structural behaviour into:

• Cantilever sheet pile walls
• Anchored sheet pile walls

Cantilever sheet pile walls

Cantilever sheet piles derive stability from embedment of the pile tip into the ground beneath the dredge or retained level. Two common forms are:

• Free cantilever sheet pile - a sheet pile subjected to a concentrated horizontal load at its top and without backfill above the dredge level; stability comes entirely from lateral passive resistance of soil below the dredge level.
• Cantilever sheet pile with backfill - a sheet pile retaining higher backfill on one side; stability similarly derives from passive resistance of the soil below the embedded tip.

Fig. 6.6 Cantilever sheet pile wall

Anchored sheet pile walls

Anchored sheet pile walls are provided with anchors (tiebacks) at appropriate levels that resist lateral translation of the pile above the anchor line. The anchors supplement the lateral passive resistance of the embedded length. Anchored sheet pile walls are used where shallow embedment alone would be insufficient.

Fig. 6.7 Anchored sheet pile wall

Anchored piles are described by the support condition of the embedded tip:

• Free earth support piles - depth of embedment is small; the pile rotates at its tip and a point of contraflexure exists along the embedded length.
• Fixed earth support piles - depth of embedment is large; the pile tip is effectively fixed against rotation and an inflection point occurs in the curvature.

Diaphragm walls

Diaphragm walls are cast-in-place reinforced concrete walls constructed in narrow excavated panels. They are widely used in congested urban sites for permanent foundation walls, deep retaining structures and groundwater cut-off walls. Diaphragm walls can be built close to existing structures with minimal loss of support to neighbouring foundations, and they often eliminate the need for extensive dewatering, reducing risk of subsidence.

Diaphragm walls are normally constructed by the slurry trench technique. The common sequence is:

• Excavate a narrow panel while supporting the trench with an engineered slurry (for example, bentonite slurry) so the trench walls do not collapse.
• Install stop-end pipes vertically at each end of the panel to form joints with adjacent panels.
• Place a steel reinforcement cage centrally in the panel once excavation is complete.
• Place concrete by tremie through pipes reaching the bottom of the trench; as concrete fills the panel the tremie is withdrawn while remaining embedded in fresh concrete.
• Recover and reuse the displaced slurry for subsequent panels.
• Withdraw stop-end pipes after concrete has gained sufficient strength and construct secondary panels between primary panels to form a continuous wall.

Panels are typically 8 to 20 ft long and 2 to 5 ft wide (typical values; chosen according to site and contractor practice). The completed diaphragm wall may act as a cantilever or may require anchors or props for lateral support.

Fig. 6.8 Construction stages of a diaphragm wall using the slurry trench technique

4. Special types of retaining walls

Gabion walls

Gabion walls are constructed by stacking wire mesh baskets (gabions) filled with rock or stone. Gabions are flexible structures that can be stepped or built with a battered face. They are permeable and therefore allow substantial flow of water through the structure, which makes them effective for applications such as riverbank stabilisation and drainage-critical retaining situations.

Important design and construction points for gabion walls:

• Use of an appropriate filter fabric behind the gabion face is necessary to prevent adjacent soil from washing into the gabion and clogging voids.
• Gabions are well-suited where some deformation is anticipated because their flexibility tolerates movement without catastrophic failure.
• Vegetation can be re-established within or around gabions to improve appearance and provide additional surface stabilisation.
• For local roads and low-cost retaining needs, gabions are often a preferred economical option.

Fig. 6.9 (i) Gabion wall

Design considerations (summary)

• Assess ground conditions, groundwater level and drainage; water pressures on the wall significantly increase lateral loads.
• Choose the type of wall by comparing height, available space, foundation capacity and economic considerations.
• Provide adequate drainage (weeps, granular backfill, filters) to reduce hydrostatic pressure behind the wall.
• Check stability for overturning, sliding and bearing capacity failure; include factors of safety per relevant codes.
• Detail reinforcement and joints for durability and crack control; consider freeze-thaw and other environmental effects where relevant.
• Consider constructability factors: site access, excavation methods, dewatering and proximity to existing structures.

Recommended further reading: standard soil mechanics and foundation engineering texts covering retaining wall design, codes of practice and practical construction methods. Practical design requires geotechnical parameters of backfill and foundation soils, relevant code provisions and detailed structural checks.