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

3 Construction Considerations in Design

3.5 Temporary Works - General Aspects

3.5.1 Responsibility for Temporary Works Design

It is important that the responsibility for the design of temporary works is clearly identified. Where temporary works are to be designed by the contractor, the amount of control which should be exercised to ensure the safety of the temporary works needs to be carefully considered, particularly where the ground conditions are complex and the consequences of failure are high.

3.5.2 Standard of Temporary Works Design

Temporary works associated with the construction of a retaining wall should be designed to the standard which would be adopted if the temporary works were to be permanent. However, allowance may be made in the design of temporary works for the shorter design life, and for less severe loading conditions which are likely to occur during the life of the temporary works, e.g. groundwater conditions.

3.5.3 Influence of Temporary Works on Retaining Wall Design

The influence which the temporary works may have on the design of a retaining wall should be fully considered. For example, the slope angle selected for the temporary excavation to be formed for the construction of a gravity or reinforced concrete retaining wall could influence the earth pressure that will act on the wall, depending on the relative strength of the insitu soil and the backfill. If weak insitu material exists, it may be economical to replace this so that the most likely potential failure surfaces lie wholly within the stronger compacted fill.

Where the temporary works are to form part of the permanent works, the earth and water pressures which act on the partially-completed retaining wall may be different from those assumed in the design for the permanent state. Therefore, the loadings for the temporary and permanent conditions should be individually assessed.

Excavation and foundation works are often necessary in the construction of retaining walls. These aspects are discussed in more detail in the next Section.

3.6 Temporary Excavation and Foundation Works

3.6.1 Excavation Works

Excavations required for the construction of a retaining wall should be designed to have adequate stability. Also, the works should not lead to unacceptable movements in nearby structures, services and land.

Guidance on the design of slopes which form a part of temporary works, together with advice on the excavation programme and methods appropriate for Hong Kong conditions, are given in Chapter 9 of the Geotechnical Manual for Slopes (GCO, 1984). Particular care should be taken where the groundwater level is high. The surface of temporary slopes should also be adequately protected against erosion. Where the temporary slope is higher than say 7.5 m, the slope surfaces should be benched. Any weak material at the insitu soil interface should be removed prior to backfilling.

Temporary excavations with lateral support should be properly designed and the sequence of construction carefully planned. Stability should be ensured for each construction stage and movements should be estimated. Limits on allowable movements should be clearly stated on the drawings. A review of methods for assessing movements around excavations is given in GCO Publication No. 1/90 (GCO, 1990).

Where lateral support to an excavation is to be removed, this should be done progressively as placement and compaction of fill material proceeds. Lateral supports should be removed in such a way as to ensure that the stability of the adjacent ground is maintained and to avoid disturbance to compacted backfill, filter and drainage materials. Where lateral support is to be left within the permanent structure, the effect of local transfer of stresses onto the retaining wall should be carefully evaluated.

3.6.2 Foundation Works

The extent of excavation for the foundation of a gravity or reinforced concrete retaining wall should be clearly shown on the drawings. The excavation should be protected from the effects of traffic, exposure to weather (rain and drying conditions) and the action of water (flow or ponding of surface water or the removal of groundwater). Foundations in clays or weak rocks, and foundations in soils derived from insitu weathered rocks (e.g. completely decomposed granites) below groundwater level, are particularly susceptible to deterioration. In such cases, surface protection such as a layer of 'blinding' concrete should be placed immediately after excavation to the foundation level in order to protect the foundation and to keep the new construction clean. Blinding concrete is usually not placed for rock- or earth-filled gravity retaining walls such as reinforced fill structures. In these cases, backfilling of foundations that are susceptible to deterioration should take place as soon as possible. In all cases, any loose debris or slurry at the foundation level must be removed prior to placement of blinding concrete or backfilling.