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

3 Construction Considerations in Design

3.6.3 Groundwater Control

In Hong Kong, there have been cases where inadequate groundwater control has led to failure of temporary slopes and weakening of the foundations of retaining walls, resulting in expensive remedial works. Therefore, adequate groundwater control measures should be implemented where appropriate.

Where excavations are required in permeable material below groundwater level, provisions should be made for adequate drainage control measures to prevent piping or a general weakening of the foundations due to the flow of water. This is particularly important where there are stringent requirements on the allowable movements of the retaining wall or on the differential movements between adjacent structures. Similar measures may also be needed for foundations in layered deposits of variable permeability. Excavations in compressible soils may result in heaving of the base of the excavation. This may give rise to problems of differential settlement where the retaining wall is required to span between excavated and unexcavated ground.

Groundwater levels may be controlled by dewatering. The dewatering method chosen should ensure the stability of the excavation and the safety of nearby structures. Reference may be made to Terzaghi & Peck (1967), NAVFAC (l982b), BS 8004 (BSI, 1986a) and Somerville (1986) for guidance on techniques of dewatering.

Groundwater control measures which result in a lowering of groundwater levels (e.g. pumping by means of dewatering wells) may cause settlement. The effects of dewatering settlement on structures, services and land in the vicinity of the site should be properly assessed.

3.7 Placement and Compaction of Backfill

All backfill materials placed behind retaining walls, including granular filter and drainage materials, should be compacted. The methods of placement and compaction of such materials are similar to those for any earthworks involving filling. General guidance on the compaction of fill materials is given in Chapter 9 of the Geotechnical Manual for Slopes (GCO, 1984).

In specifying the degree of compaction for backfill, granular filter and drainage materials, consideration should be given to the functions that the material is required to perform. The higher the degree of compaction, the higher are the shear strength and stiffness properties of the fill, and the lower is its permeability. It is recommended that the degree of compaction specified should be at least 95% of the maximum dry density of the fill obtained using the appropriate test method given in Geoguide 6 (GEO, 2017b). For level backfill, the top 1.5 m should be composed of fine soil compacted to at least 98% of its maximum dry density, in order to minimize infiltration through the surface. Such a compacted, low permeability layer should be provided irrespective of whether the surface is paved or not. Sloping backfill should also be compacted to the same standard for a vertical thickness of at least 3 m (GCO, 1984). The shear strength and other properties of the fill used in the design should be consistent with the degree of compaction specified.

Compaction can induce pressure on a retaining wall greater than that given by classical earth pressure theories. Therefore, unless a wall is very massive, heavy compaction equipment should not be used close to the back of the wall. The effect of compaction should be taken into account in the design, and the allowable loading due to compaction equipment should be clearly specified (see Section 6.8).

3.8 Services

Where it seems possible that services will be placed in front of the retaining wall in the future, the effect of the trench excavation should be taken into account in the design. For example, it is prudent to design roadside retaining walls in built-up areas assuming the presence of a trench at least 1 m deep at the toe. Where appropriate, relevant authorities should be consulted on requirements to cater for future provisions. If a gravity or reinforced concrete retaining wall is used, the foundation should be set at a level such that the retaining wall will not be undermined during future trench excavation. If a cantilevered retaining wall is selected, the passive resistance should be reduced accordingly in the design.

3.9 Use of Prestressed Ground Anchors

Prestressed ground anchors may be required solely for temporary support; they may form part of the permanent structure, or they may be designed to perform a dual function.

The use of permanent ground anchors in a project imposes a long-term commitment on the maintenance authority or owner, which usually involves appreciable recurrent cost. Therefore, permanent ground anchors should be used only when other methods of providing the required support are not practicable, and the agreement of the maintenance authority or owner should be obtained prior to their use.

Where ground anchors are to be used, an appraisal should be made of the suitability of the proposed anchor installation technique in relation to the ground conditions, the possible effects on adjoining structures and services, and the rights of adjoining owners whose structure or land may be affected. Corrosion protection and creep are two aspects which must be given special consideration. The standards set out in Geospec 1 : Model Specification for Prestressed Ground Anchors (GCO, 1989b) should be followed.

A review of the design methods for single-level and multi-level anchored or tied-back retaining walls is given in GCO Publication No. 1/90 (GCO, 1990).