13.6 Practical considerations

13.6.1 General

There are various practical considerations relating to excavations which should be considered before and during the excavation work.

13.6.2 Methods of excavation and types of support

COMMENTARY ON 13.6.2

No matter what method of excavation is used, ground displacements occur both within and immediately surrounding an excavation. These ground displacements depend partly on the geological structure and are principally due to elastic strains. In cohesive soils, volumetric strains due to changes in moisture content also take place. If the method of excavation and the type of support are unsuitable for the particular ground conditions, then shear deformations or shear failures of the soil or failures due to hydrostatic pressures can occur. Vibrations from construction equipment can cause consolidation of cohesionless soils or have a detrimental effect on existing structures in a weak condition. The sequence of excavation and installation of lateral supports has a significant effect on the stresses and strains induced in the ground.

It may not be practicable to prevent significant vertical and lateral ground displacements immediately beyond the limits of an excavation, although careful design of the support system will help to minimize displacements. The effects of the inevitable movements on any adjoining structures should be considered. It may be necessary to underpin adjoining structures before commencing an excavation, in order to protect them from the ground displacements. Alternatively, and with the agreement of their owners, damage to the adjoining structures may be accepted which should be repaired after completion of the permanent work. However, the damage should not be such as to cause any danger to the occupants of these structures, or to the general public. A simple construction procedure is desirable since alterations to a complex construction sequence, when unexpected variations in ground conditions are encountered, is often difficult. The work should not be undertaken without experienced supervision, and inspection should be made several times each day to ensure that stable conditions are being maintained.

Narrow trenches may sometimes be excavated with unsupported vertical faces, depending on rate of construction, soil type and strength and depth of trench excavation. It is essential that trenches are supported where people are required to enter them. Stability conditions should be regarded as unfavourable even in firm and stiff clays and in fissured and closely jointed rocks.

It may be economical to incorporate support systems such as steel sheet piling, concrete diaphragm walls, or contiguous bored pile walls in the permanent construction.

No lateral supports for any part of an excavation should be altered or dismantled except under the direction of the designer or a competent person possessing adequate knowledge and experience. Where an observational method is being used, a store of suitable materials should be kept on site to provide immediate strengthening, if found necessary.

13.6.3 Existing buildings, buried structures and services

The age, types of construction and the type and depth of foundations of existing buildings which would be affected by the excavation should be ascertained before commencing work on site. An appraisal of the dead and superimposed loads from the foundations of existing buildings should be made since the stability of the excavation can depend on an accurate prediction of imposed loading on the selected system of retention for the excavation. All buildings and buried structures which are likely to be affected by the excavation work should be surveyed with the representatives of the owners, and a report on the condition of the structures prepared. The report should contain photographs of any building defects. Significant structural cracks should be instrumented. Appropriate instrumentation should be installed to permit any building movements to be monitored; however, it is advisable to include simple strain gauge devices and levelling points.

Where ground anchors used for excavation supports pass beneath existing buildings and infrastructure, the effects of the drilling and grouting processes used to install the anchors should be considered. The permission of the land owners should be obtained for the installation of anchors beneath their property.

The design and construction of excavations and their retention systems should also take into consideration the prior location and safe support of all services such as water and gas mains, and buried structures such as underground tunnels and sewers.

13.6.4 Disposal of spoil

13.6.4.1 General

General recommendations and guidance on waste management is given in 5.4.2. Arisings from trench excavation should be dealt with in the same way as other earthworks spoil.

13.6.4.2 Temporary spoil and material heaps

Temporary spoil and material heaps should be sited to interfere as little as possible with the work to be carried out. Whilst, for convenience in handling, it might be necessary to place them near excavations, the following points should be borne in mind:

• a) they should not interfere with free access to the excavation (in trench work it is desirable to place the material which is to be used for backfilling on one side of the trench only);
• b) they should be so constructed that there is no danger of the spoil slumping in wet weather and entering the excavation;
• c) spoil heaps should not be placed in such a position as to endanger the stability of existing works above or below ground or of the excavation, the sides or side supports of which should be so designed as to be capable of withstanding the additional stresses due to any superimposed load.

Spoil heaps should be graded to safe slopes taking into consideration the nature of the material and the effects of wet weather. With coarse sand or clean gravel the natural angle of repose of the tipped materials should remain substantially unaltered in wet weather, but with materials that soften and slump, e.g. clays, silts, mudstones, etc., a substantial reduction in slope should be anticipated and an adequate distance maintained between the periphery of the spoil heap and the edge of excavation.

The clearance between the toe of the spoil heap and the edge of the excavation should give sufficient working space at all times, and for this purpose the clearance should be a distance equal to the depth of the excavation with a minimum width of 1.50 m.