Retaining walls and basements.

In commercial developments occupying congested city centre sites it has become common to utilize deep basements to provide accommodation for plant room, car parking and other areas. The depth of these basements requires careful consideration of the aspects of design and construction in order to achieve a satisfactory engineering solution. For the engineer requiring a full explanation of  the approach to design and the methods of construction  of deep basements, reference should be made to the IStructE publication where this topic is dealt with in comprehensive detail.

Retaining walls and peripheral walls to basements are subject to lateral (i.e. horizontal) pressure from retained earth, liquids or a combination of soil and water. They are normally made, in structural work, of concrete or brick (plain, reinforced or prestressed).

Basements are relatively expensive to construct (the cost per square metre is higher than for normal floor construction) so the client should be advised to carry out a cost  evaluation of, say, adding a further storey to the structure and eliminating the basement. However, basements can  be made cost-effective when they are used as cellular buoyancy rafts or where increased height is restricted by planning.

The walls are basically vertical cantilevers, either free or propped (at the top by a floor slab). Where the ground floor slab can be made continuous with the top of the wall (and not merely be propped) the basement can be designed as a continuous box. The walls can be constructed with either a base slab extending under the retained earth (see Fig. 15.1 (a)), which is generally the more economical form for  cuttings, or projecting forward (see Fig. 15.1 (b)), the more  economical form for basements.

Fig. 15.1 Typical retaining walls.

While propped cantilevers (e.g. basement wall propped by ground floor slab) have a maximum bending moment (for a udl) of  pH^2 /8, compared to that of a free cantilever of  pH^2 /2, they are not frequently used in building structures.

This is because the wall must either be temporarily propped, or not backfilled, until the ground floor can act as the prop.

However, in the authors’ experience it can be worth considering the use of the more economical propped cantilever, especially for design-and-build contracts, where a close relationship is developed with the contractor from an early stage, and construction methods can be programmed into the design. It is important to provide a clear route for  the propping force through the substructure and to take account of any out-of-balance lateral forces, such as those resulting from sloping backfill on one side of the structure.

The bending moment diagrams for triangular pressure (i.e. no surcharge) for the three cases: free cantilever, propped cantilever and cellular (fixed), are shown in Fig. 15.2.

Fig. 15.2 Bending moment diagrams for retaining walls.

It can be seen that partially filling a basement with water can equalize the external earth pressure on the basement wall. The authors’ practice has used this method of temporary propping, raising the water level as backfill is placed. Where the basement is con- structed in waterlogged ground, filling the basement in this way can also be utilized to avoid flotation before the weight of the rest of the building is added.

Walls to swimming pools are a special case since they can be subject to reversal of stress. With the pool empty, the wall is subject to earth/water pressure on its earth face and with the pool full and earth pressure absent (either due  to shrinkage of backfill or water testing for leaks, before backfilling), the wall is subject to water pressure alone on its water face (see Fig. 15.3).

Walls to culverts can similarly be subject to reversal of stress under the two conditions of earth pressure acting alone or when the water pressure is acting alone. Service ducts, boiler houses, inspection chambers and similar excavated substructures can unwittingly be subject to internal water pressure acting alone, which needs to be designed for. This has happened when heavy rainfall during con- struction has flooded and filled the substructures with water before the backfill has been placed.

Fig. 15.3 Pressures acting on swimming pool walls.

Retaining Walls.

Retaining walls in relation to structural foundations are dealt with here in general terms. Such walls are a necessary part of many foundations where changes in level occur.

They may be used to retain earth or other material within, below or around the foundation and can be constructed in numerous forms from a number of materials. Common materials used are plain, reinforced and prestressed masonry, and plain, reinforced and prestressed concrete (see Fig. 9.50 for some examples).

The walls may be acting as pure cantilevers, propped cantilevers, tied cantilevers, simply supported or continuous spanning slabs, etc. (see Fig. 9.51). They may be stiffened by shaping into fins, counterforts, diaphragms, zig-zag, and many other profiles (see Fig. 9.52). They can be mass filled, reinforced or post-tensioned (see Fig. 9.53). The engineer should apply skill and ability in arriving at the most suitable and economic form for each individual situation.

The design of retaining walls in relationship to foundations does mean that the normal design to retain earth can become secondary to or parallel to the overall foundation behaviour. For example, where the building is constructed on a raft foundation and the retaining wall becomes part  of the raft, then continuity of raft stiffening ribs are most critical to the design and detail (see Fig. 9.54).

The location of settlement or other movement joints through foundations which embrace the retaining walls can be critical to or dictate the structural behaviour of the wall, for example, by effectively removing the prop/tying action of the upper floor slab of a change in level (see Fig. 9.55).

In mining areas the need to relieve horizontal ground  stress by allowing the foundation to move relative to the sub-strata can conflict with the need to resist lateral loads in a retaining situation. On sloping sites this conflict can often be overcome by the detail shown in Fig. 9.56.

Where a basement is required on a flat mining site the conflict is more difficult and much greater forces have to be resisted by the building foundations (see Fig. 9.57).

Fig. 9.50 Basement retaining walls.

 Fig. 9.51 Retaining walls – design approach.

Fig. 9.52 Masonry retaining walls/plan forms.

Fig. 9.53 Masonry retaining walls – structural forms.

 Fig. 9.54 Retaining wall/raft slab.

 Fig. 9.55 Retaining wall/movement joint.

Fig. 9.56 Mining raft slab/retaining wall.

Fig. 9.57 Mining (basement) raft /retaining wall.

DESIGN CONSIDERATIONS FOR A MECHANICALLY STABILIZED EARTH WALL.

The design of a MSE wall involves the following steps:

1. Check for internal stability, addressing reinforcement spacing and length.
2. Check for external stability of the wall against overturning, sliding, and foundation failure.

The general considerations for the design are:

1. Selection of backfill material: granular, freely draining material is normally specified.
However, with the advent of geogrids, the use of cohesive soil is gaining ground.
 
2. Backfill should be compacted with care in order to avoid damage to the reinforcing material.
 
3. Rankine's theory for the active state is assumed to be valid.
 
4. The wall should be sufficiently flexible for the development of active conditions.
 
5. Tension stresses are considered for the reinforcement outside the assumed failure zone.
 
6. Wall failure will occur in one of three ways

a. tension in reinforcements
b. bearing capacity failure
c. sliding of the whole wall soil system.
 
7. Surcharges are allowed on the backfill. The surcharges may be permanent (such as a roadway) or temporary.
 
a. Temporary surcharges within the reinforcement zone will increase the lateral pressure on the facing unit which in turn increases the tension in the reinforcements, but does not contribute to reinforcement stability.
b. Permanent surcharges within the reinforcement zone will increase the lateral pressure
and tension in the reinforcement and will contribute additional vertical pressure for the
reinforcement friction.
c. Temporary or permanent surcharges outside the reinforcement zone contribute lateral
pressure which tends to overturn the wall.
 
8. The total length L of the reinforcement goes beyond the failure plane AC by a length Le.
Only length Le (effective length) is considered for computing frictional resistance. The length LR lying within the failure zone will not contribute for frictional resistance (Fig. 19.15a).

9. For the propose of design the total length L remains the same for the entire height of wall H.
Designers, however, may use their discretion to curtail the length at lower levels. Typical ranges in reinforcement spacing are given in Fig. 19.16.

Figure 19.15 Principles of MSE wall design

Figure 19.16 Typical range in strip reinforcement spacing for reinforced earth
walls (Bowles, 1996)

CONSTRUCTION DETAILS - RETAINING WALLS.

The method of construction of MSE walls depends upon the type effacing unit and reinforcing material used in the system. The facing unit which is also called the skin can be either flexible or stiff, but must be strong enough to retain the backfill and allow fastenings for the reinforcement to be attached. The facing units require only a small foundation from which they can be built, generally consisting of a trench filled with mass concrete giving a footing similar to those used in domestic housing. The segmental retaining wall sections of dry-laid masonry blocks, are shown in Fig. 19.12(a). The block system with openings for vegetation is shown in Fig. 19.12(b).

The construction procedure with the use of geotextiles is explained in Fig. 19. 14(a). Here, the
geotextile serve both as a reinforcement and also as a facing unit. The procedure is described below
(Koerner, 1985) with reference to Fig. 19.14(a).

1. Start with an adequate working surface and staging area (Fig. 19.14a).
2. Lay a geotextile sheet of proper width on the ground surface with 4 to 7 ft at the wall face draped over a temporary wooden form (b).
3. Backfill over this sheet with soil. Granular soils or soils containing a maximum 30 percent silt and /or 5 percent clay are customary (c).
4. Construction equipment must work from the soil backfill and be kept off the unprotected geotextile. The spreading equipment should be a wide-tracked bulldozer that exerts little pressure against the ground on which it rests. Rolling equipment likewise should be of relatively light weight.
5. When the first layer has been folded over the process should be repeated for the second layer with the temporary facing form being extended from the original ground surface or the wall being stepped back about 6 inches so that the form can be supported from the first layer. In the latter case, the support stakes must penetrate the fabric.
6. This process is continued until the wall reaches its intended height.
7. For protection against ultraviolet light and safety against vandalism the faces of such walls must be protected. Both shotcrete and gunite have been used for this purpose.

Figure 19.14(b) shows complete geotextile walls (Koerner, 1999).

Figure 19.12 Geosynthetic use for reinforced walls and bulkheads (Koerner, 2000)

Figure 19.14(a) General construction procedures for using geotextiles in fabric
wall construction (Koerner, 1985)

Figure 19.14(b) Geotextile walls (Koerner, 1999)

BACKFILL AND REINFORCING MATERIALS - RETAINING WALLS.

Backfill
The backfill, is limited to cohesionless, free draining material (such as sand), and thus the key properties are the density and the angle of internal friction.

Reinforcing material
The reinforcements may be strips or rods of metal or sheets of geotextile, wire grids or geogrids (grids made from plastic).

Geotextile is a permeable geosynthetic comprised solely of textiles. Geotextiles are used with foundation soil, rock, earth or any other geotechnical engineering-related material as an integral part of a human made project, structure, or system (Koerner, 1999). AASHTO (M288-96) provides (Table 19.2) geotextile strength requirements (Koerner, 1999). The tensile strength of geotextile varies with the geotextile designation as per the design requirements. For example, a woven slit-film polypropylene (weighing 240 g/m2) has a range of 30 to 50 kN/m. The friction angle between soil and geotextiles varies with the type of geotextile and the soil. Table 19.3 gives values of geotextile friction angles (Koerner, 1999).

The test properties represent an idealized condition and therefore result in the maximum possible numerical values when used directly in design. Most laboratory test values cannot generally be used directly and must be suitably modified for in-situ conditions. For problems dealing with geotextiles the ultimate strength TU should be reduced by applying certain reduction factors to obtain the allowable strength Ta as follows (Koerner, 1999).

Geogrid
A geogrid is defined as a geosynthetic material consisting of connected parallel sets of tensile ribs
with apertures of sufficient size to allow strike-through of surrounding soil, stone, or other
geotechnical material (Koerner, 1999).

Geogrids are matrix like materials with large open spaces called apertures, which are typically 10 to 100 mm between the ribs, called longitudinal and transverse respectively. The primary function of geogrids is clearly reinforcement. The mass of geogrids ranges from 200 to 1000 g/m2 and the open area varies from 40 to 95 %. It is not practicable to give specific values for the tensile strength of geogrids because of its wide variation in density. In such cases one has to consult manufacturer's literature for the strength characteristics of their products. The allowable tensile strength, Ta, may be determined by applying certain reduction factors to the ultimate strength TU as in the case of geotextiles. The equation is

The definition of the various terms in Eq (19.10) is the same as in Eq. (19.9). However, the reduction factors are different. These values are given in Table 19.5 (Koerner, 1999).

Metal Strips
Metal reinforcement strips are available in widths ranging from 75 to 100 mm and thickness on the order of 3 to 5 mm, with 1 mm on each face excluded for corrosion (Bowles, 1996). The yield strength of steel may be taken as equal to about 35000 lb/in2 (240 MPa) or as per any code of practice.

Table 19.2 AASHTO M288-96 Geotextile strength property requirements

Table 19.3 Peak soil-to-geotextile friction angles and efficiencies in selected
cohesionless soils*

Table 19.5 Recommended reduction factor values for use in Eq. (19.10) for
determining allowable tensile strength of geogrids

MECHANICALLY STABILIZED EARTH RETAINING WALLS.

GENERAL CONSIDERATIONS
Reinforced earth is a construction material composed of soil fill strengthened by the inclusion of rods, bars, fibers or nets which interact with the soil by means of frictional resistance. The concept of strengthening soil with rods or fibers is not new. Throughout the ages attempts have been made to improve the quality of adobe brick by adding straw. The present practice is to use thin metal strips, geotextiles, and geogrids as reinforcing materials for the construction of reinforced earth retaining walls.

A new era of retaining walls with reinforced earth was introduced by Vidal (1969). Metal strips were used as reinforcing material as shown in Fig. 19.11 (a). Here the metal strips extend from the panel back into the soil to serve the dual role of anchoring the facing units and being restrained through the frictional stresses mobilized between the strips and the backfill soil. The backfill soil creates the lateral pressure and interacts with the strips to resist it. The walls are relatively flexible compared to massive gravity structures. These flexible walls offer many advantages including significant lower cost per square meter of exposed surface.

The variations in the types effacing units, subsequent to Vidal's introduction of the reinforced earth walls, are many. A few of the types that are currently in use are (Koerner, 1999)

1. Facing panels with metal strip reinforcement
2. Facing panels with wire mesh reinforcement
3. Solid panels with tie back anchors
4. Anchored gabion walls
5. Anchored crib walls
6. Geotextile reinforced walls
7. Geogrid reinforced walls

In all cases, the soil behind the wall facing is said to be mechanically stabilized earth (MSE) and the wall system is generally called an MSE wall.

The three components of a MSE wall are the facing unit, the backfill and the reinforcing material. Figure 19.11(b) shows a side view of a wall with metal strip reinforcement and Fig. 19.1 l(c) the front face of a wall under construction (Bowles, 1996).

Modular concrete blocks, currently called segmental retaining walls (SRWS, Fig. 19.12(a)) are most common as facing units. Some of the facing units are shown in Fig. 19.12. Most interesting in regard to SRWS are the emerging block systems with openings, pouches, or planting areas within them. These openings are soil-filled and planted with vegetation that is indigenous to the area (Fig. 19.12(b)). Further possibilities in the area of reinforced wall systems could be in the use of polymer rope, straps, or anchor ties to the facing in units or to geosynthetic layers, and extending them into the retained earth zone as shown in Fig. 19.12(c).

A recent study (Koerner 2000) has indicated that geosynthetic reinforced walls are the least expensive of any wall type and for all wall height categories (Fig. 19.13).

Figure 19.11 (a) Component parts and key dimensions of reinforced earth wall
(Vidal, 1969)

(c) Front face of a reinforced earth wall under construction for a bridge approach fill using patented precast
concrete wall face units

Figure 19.11 b) and (c) Reinforced earth walls (Bowles, 1996)

Figure 19.12 Geosynthetic use for reinforced walls and bulkheads (Koerner, 2000)

 Figure 19.13 Mean values of various categories of retaining wall costs
(Koerner, 2000)

EARTH PRESSURE CHARTS FOR RETAINING WALLS.

Charts have been developed for estimating lateral earth pressures on retaining walls based on certain assumed soil properties of the backfill materials. These semi empirical methods represent a body of valuable experience and summarize much useful information. The charts given in Fig. 19.4 are meant to produce a design of retaining walls of heights not greater than 6 m. The charts have been developed for five types of backfill materials given in Table 19.1. The charts are applicable to the following categories of backfill surfaces. They are

1. The surface of the backfill is plane and carries no surcharge
2. The surface of the backfill rises on a slope from the crest of the wall to a level at some elevation above the crest.

The chart is drawn to represent a concrete wall but it may also be used for a reinforced soil wall. All the dimensions of the retaining walls are given in Fig. 19.4. The total horizontal and vertical pressures on the vertical section of A B of height H are expressed as

Values of Kh and Kv are plotted against slope angle Β in Fig. 19.4 and the ratio H1/H in Fig. 19.5.

Table 19.1 Types of backfill for retaining walls

Figure 19.4 Chart for estimating pressure of backfill against retaining walls
supporting backfills with a plane surface. (Terzaghi, Peck, and Mesri, 1996)

Figure 19.5 Chart for estimating pressure of backfill against retaining walls supporting backfills with a surface that slopesupward from the crest of the wall for limited distance and then becomes horizontal. (Terzaghi et al., 1996)

PROPORTIONING OF RETAINING WALLS.

Based on practical experience, retaining walls can be proportioned initially which may be checked for stability subsequently. The common dimensions used for the various types of retaining walls are given below.

Gravity Walls
A gravity walls may be proportioned in terms of its height given in Fig. 19.3(a). The minimum top width suggested is 0.30 m. The tentative dimensions for a cantilever wall are given in Fig. 19.3(b) and those for a counterfort wall are given in Fig. 19.3(c).

Fig. 19.3 Tentative dimensions for retaining walls

CONDITIONS UNDER WHICH RANKINE AND COULOMB FORMULAS ARE APPLICABLE TO RETAINING WALLS UNDER ACTIVE STATE.

Conjugate Failure Planes Under Active State
When a backfill of cohesionless soil is under an active state of plastic equilibrium due to the stretching of the soil mass at every point in the mass, two failure planes called conjugate rupture planes are formed. These are further designated as the inner failure plane and the outer failure plane as shown in Fig. 19.1. These failure planes make angles of αi and α0 with the vertical. The equations for these angles may be written as (for a sloping backfill)

Conditions for the Use of Rankine's Formula
1. Wall should be vertical with a smooth pressure face.
2. When walls are inclined, it should not come in the way of the formation of the outer failure

plane. Figure 19.1 shows the formation of failure planes. Since the sloping face AB' of the retaining wall makes an angle αw greater than αo, the wall does not interfere with the formation of the outer failure plane. The plastic state exists within wedge ACC'.

The method of calculating the lateral pressure on AB' is as follows.

1. Apply Rankine's formula for the vertical section AB.
2. Combine Pa with Ws , the weight of soil within the wedge ABB', to give the resultant PR.

Let the resultant PR in this case make an angle δr with the normal to the face of the wall. Let
the maximum angle of wall friction be δm. If δr > δm, the soil slides along the face AB'of the wall.

Figure 19.1 Application of Rankine's active condition to gravity walls

Figure 19.2 Lateral earth pressure on cantilever walls under active condition

In such an eventuality, the Rankine formula is not recommended but the Coulomb formula may be
used.

Conditions for the Use of Coulomb's Formula

1. The back of the wall must be plane or nearly plane.
2. Coulomb's formula may be applied under all other conditions where the surface of the wall is not smooth and where the soil slides along the surface.

In general the following recommendations may be made for the application of the Rankine or

Coulomb formula without the introduction of significant errors:

1. Use the Rankine formula for cantilever and counterfort walls.
2. Use the Coulomb formula for solid and semisolid gravity walls.

In the case of cantilever walls (Fig. 19.2), Pa is the active pressure acting on the vertical section AB passing through the heel of the wall. The pressure is parallel to the backfill surface and acts at a height H/3 from the base of the wall where H is the height of the section AB. The resultant pressure PR is obtained by combining the lateral pressure Pa with the weight of the soil Ws between the section AB and the wall.

CONCRETE RETAINING WALLS.

INTRODUCTION
The lateral pressure theories and the methods of calculating the lateral earth pressures were described in detail in the same chapter. The two classical earth pressure theories that have been considered are those of Rankine and Coulomb. In this chapter we are interested in the following:

1. Conditions under which the theories of Rankine and Coulomb are applicable to cantilever and gravity retaining walls under the active state.
2. The common minimum dimensions used for the two types of retaining walls mentioned above.
3. Use of charts for the computation of active earth pressure.
4. Stability of retaining walls.
5. Drainage provisions for retaining walls.