Una anàlisi descriptiva de la relació entre les variables. Pregunta 6

The following narrative is confined to trench excavations to lay water pipelines

H.1.1 General

Trenching works are steep temporary cuttings that are backfilled once a utility such as a pipeline has been laid.

 The design of trench excavation is normally the responsibility of the Contractor appointed for the works. The Contractor shall engage a Competent Person to undertake a safe and proper shoring design. No temporary works deemed unsafe shall be allowed.

 For excavations not exceeding 1.2m, shoring support may not be required if the ground is found to be self-supporting. If external loads are likely to be present, or if there is doubt as to the stability of the trench sides due to weak ground and/or high ground water table, especially in rainy weather, the trench sides should be supported even if the excavation depth is less than 1.2m.

 For excavations exceeding 1.2m, adequate support must be installed in a timely manner, and as far as practicable, ahead of any further excavation.

 The typical shoring details given in Figure H.1 may be acceptable for depths up to 4.5m. Guidance on the design of shoring works can also be found in Construction Industry Research and Information Association, London (CIRIA) Report 97 on prescriptive design and Utilities Technical Liaison Committee (UTLC) (2003).

 All excavations must be properly designed, and Competent Person are required to take adequate precautions to ensure public safety. Specific designs with detailed drawings shall be prepared for excavations meeting the following criteria:

i. Deeper than 4.5m and greater than 5m in length, and

ii. Liable to affect any road, building, structure, slope steeper than 30˚, water main 75mm in diameter or greater, the affected area being defined as within 45˚ line up from the base of the excavation to the ground surface.

 It is also not desirable to have the whole length of a long trench opened up at any one time, even with support. Excavations should be in sections of shortest practical length, preferably not more than 100 meters long.

H.1.2 Technical Consideration

 The design and construction of trenching and shoring works shall consider the following failure mechanisms:

i. The collapse of vertical sides of poorly supported trenches which may cause subsidence of the neighbouring ground or the death of workers in the trench.

ii. Heaving or softening at base of excavation due to high ground water pressures which may adversely affect the foundation for pipe laying.

iii. Heaving at base of excavation due to shear failure of the soil causing local subsidence inside the trench.

iv. The erosive action of ground water, washing sand and silt into the trench and causing local subsidence.

v. Consolidation of neighbouring compressible soils due to local reduction of ground water table which may cause settlement of adjacent foundations or the opening of sewer joints, etc.

 Consideration shall be given to loads imposed on the open trench such as vehicular loads and loads due to excavated spoil or stockpiling of materials near the excavated trench.

 Trenches located at the toe of a slope or earth-retaining structure can reduce its stability and should be avoided wherever possible. Where it is unavoidable, trench supports must be designed to ensure that the stability of the nearby slope or earth-retaining structure is not compromised by the trenching work. Stability analyses of the affected slope/structure may be required.

 Pipe trenches shall be backfilled with suitable materials and shall be properly compacted in accordance with the specifications.

H.1.3 Drainage Consideration

 Drainage measures shall be provided regardless of the excavation depth.

This is especially important when trenches are located uphill or in close proximity to slopes and opened during the rainy season. Effective measures shall be implemented to:

i. Minimise the ingress of surface run-off.

ii. Control infiltration of rainwater and water runoff from an open trench into nearby slopes.

 Up-stands used together with trench covers have been found to be effective in dealing with surface runoff. Up-stands are provided on either side of the trench and can be made of compacted earth fill bunds cemented together.

The height of the up-stand shall be determined by the designer but shall not

 Pumping from small sumps shall be provided for all trenches located at slope crest, on slopes or in flood prone areas and opened up in the rainy season.

 High ground water table levels can be controlled by means a dewatering system. However, dewatering can cause the consolidation of neighbouring compressible soils and thus, the settlement of adjacent foundations and utilities, opening of sewer joints, etc. Where this risk arises, assessments should be carried out by a qualified Professional Engineer with appropriate experience to ensure the safety of adjacent structures/infrastructures.

H.1.4 Shoring

 Construction works shall be supervised by the Competent Person who is responsible for the Temporary Works design and who shall also ensure that trenches are constructed strictly in accordance to the design and contract specification, and safety requirements.

 Where the ground conditions are poor and have little “free-standing” time, the installation of sheeting, excavation and insertion of waling and struts shall proceed in stages until the full excavation depth is achieved.

 Under no circumstances should workers be permitted to work in an unsupported trench that is deeper than 1.2m.

 When removal of struts are required in order to give access during lowering of pipes, equipment or compaction of backfill, only the minimum practicable number of struts should be removed. The ground shall be adequately supported at all times while workers are in the trench.

Figure H.1: Typical sheet pile shoring detail with steel struts and water beams Notes:

1. All dimensions are in millimeters (mm).

2. The sizes of the structural members (e.g. sheet piles, struts and water beams) and spacing between struts shall be adequately prescribed based on actual excavation depth, ground conditions and other factors affecting loading on the shoring system.