Hong Kong Geoguide 1: Guide to Retaining Wall Design (2020 Edition)

2 Design Considerations

2.4 Selection of Type of Retaining Wall

The following factors should be considered in selecting a suitable type of retaining wall :

(a) location of the proposed retaining wall, its position relative to other structures and the amount of space available, including the necessity or otherwise to confine the support system within the site boundaries,

(b) height of the proposed retaining wall and the topography of the ground to be formed,

(c) ground and groundwater conditions,

(d) extent of ground movement acceptable during construction and in service, and the effect of movement of the retaining wall on nearby structures and services,

(e) availability of materials,

(f) time available for construction,

(g) appearance, and

(h) design life and maintenance.

A wide variety of different types of retaining walls is available (Figure 2). Their relative advantages and disadvantages are given in Table 1. Where several alternatives are suitable, an economic comparison should be made based on their initial construction and subsequent maintenance costs.

2.5 Durability and Maintenance

Inadequate durability may result in disproportionate maintenance costs. It may also cause a retaining wall to reach either serviceability or ultimate limit states. Therefore, the durability of the wall during its design life and the maintenance requirements must be considered in the selection and specification of materials for construction and in the evaluation of design parameters. The local climate and environment under working conditions should also be taken into account.

Some guidance on how concrete, steel and timber can be expected to deteriorate in different circumstances is given in BS 8004 (BSI, 1986a), which also indicates some of the steps which may be taken to prolong the life of these materials when used in foundations.

2.6 Aesthetics

Retaining walls can be very dominant features on the urban and rural landscape. Careful design can make a considerable improvement to their appearance without leading to a significant increase in cost.

Apart from having to satisfy the functional requirements, a retaining wall should, as far as possible, be made to blend in with its surrounding environment and to be aesthetically pleasing. The aspects of a retaining wall that are important to its aesthetic impact are :

(a) height of the wall and inclination of its front face,

(b) curvature of the wall on plan (poor design can give the appearance of a 'kink' in the longitudinal elevation of the wall),

(c) gradient and surface treatment of the adjacent ground,

(d) surface textures of the front facing, and the expression and position of vertical and horizontal construction joints, and

(e) the coping of the wall.

Aesthetic improvement can be achieved through the choice of a suitable structural form. Often, suitable types of vegetation can be incorporated to improve the appearance of the retaining wall. Guidance on the planting of grass, trees and shrubs in Hong Kong is given in the Geotechnical Manual for Slopes (GCO, 1984). Care should be taken to ensure that the proposed planting will not cause any damage to the wall in the long term. The appearance of a retaining wall may also be improved by providing features in the finished face or decorative facings. The advice of a landscape architect should be sought whenever appropriate.

2.7 Patents

Certain proprietary systems and components of retaining walls are covered by patents. Tender and contract documents should contain suitable clauses to ensure that no unforeseen liabilities are incurred by the employer with respect to the use of patented products.