(1)P Any scheme for removing water from the ground or for lowering the water pressure shall be based on the results of a geotechnical or hydrogeological investigation.
(2) Water may be removed from the ground by gravity drainage, by pumping from sumps, well points or bored wells, or by electro-osmosis. The scheme adopted will depend on:
- the existing ground and ground-water conditions;
- the characteristics of the project: e.g. excavation depth and extent of dewatering.
(3) Part of the dewatering scheme may be a system of recharge wells at some distance from the excavation.
(4) In the dewatering scheme the following conditions should be considered, as appropriate:
- in the case of excavations, the sides of the excavation remain stable at all times under the effect of ground-water lowering; also, excessive heaving or rupture of the base, for example due to excessive water pressure beneath a less permeable layer, does not occur;
- the scheme does not lead to excessive settlements or damage to nearby structures;
- the scheme avoids excessive loss of ground by seepage from the sides or base of the excavation;
- except in the case of fairly uniformly graded material, which can establish itself as a filter material, adequate filters are provided around the sumps to ensure that there is no significant transportation of soil with the pumped water;
- water removed from an excavation is normally discharged far enough from the excavated area;
- the dewatering scheme is so designed, arranged and installed as to maintain the water levels and pore-water pressures anticipated in the design without significant fluctuations;
- there is adequate margin of pumping capacity and back-up capacity is available in the case of breakdown;
- when allowing the ground-water to return to its original level, care is taken to prevent problems such as collapse of soils having a sensitive structure, e.g. loose sand;
- the scheme does not lead to excessive transport of contaminated water to the excavation;
- the scheme does not lead to excessive extraction in a drinking water catchment area.
(5)P The effectiveness of dewatering shall be checked by monitoring the ground-water level, the pore-water pressures and the ground movements, as necessary. Data shall be reviewed and interpreted frequently to determine the effects of dewatering on the ground conditions and on the behaviour of nearby structures.
(6)P If a pumping operation is to continue over a long period of time, the ground-water shall be checked for the presence of dissolved salts and gases, which could either result in corrosion of the well screens or cause clogging of the screens by the precipitation of salts.
(7)P Systems for long term dewatering shall be designed to prevent clogging by bacterial action or other causes.
5.5 Ground improvement and reinforcement
(1)P A geotechnical investigation of the initial ground conditions shall be carried out before any ground improvement or reinforcement method is chosen or used.
(2)P The ground improvement method for a particular situation shall be designed taking into account the following factors where appropriate:
- thickness and properties of the ground or fill material;
- magnitude of water pressure in the various strata;
- nature, size and position of the structure to be supported by the ground;
- prevention of damage to adjacent structures or services;
- if the ground improvement is temporary or permanent;
- in terms of anticipated deformations, the relationship between the ground improvement method and the construction sequence;
- the effects on the environment including pollution by toxic substances or changes in ground-water level;
- the long-term deterioration of materials.
(3)P The effectiveness of the ground improvement shall be checked against the acceptance criteria by determining the induced changes in the appropriate ground properties.