Ground Water Control Water can be classified by its relative position to or within the ground.
Ground Water Control Problems of water in the subsoil: – A high water table could cause flooding during wet period. – Subsoil water can cause problems during excavation works by its natural tendency to flow into the voids created by the excavation activities. – It can cause an unacceptable humidity level around finished building and structures.
Ground Water Control Control of ground water always referred to temporary and permanent exclusion. – Temporary exclusion: lowering of the water table and within the economic depth range of 1500mm using subsoil drainage methods, for deeper treatment a pump or pumps are usually used. – Permanent exclusion: the insertion of an impermeable barrier to stop the flow of water within the ground.
Jetted Sumps – Almost the same with simple sump methods of dewatering. – A borehole is formed in the subsoil by jetting a metal tube into the ground by means of pressurised water to a depth within the maximum suction lift of the extract pump. – The metal tube is withdrawn to leave a void for placing a disposable wellpoint and plastic suction pipe. – The area surrounding the pipe is filled with coarse sand as filtering media.
Wellpoint systems – This is a method of lowering the water table to a position below the formation level to give a dry working area. – Jetting into the subsoil a series of wellpoints which are connected to a common header pipe which then connected to a vacuum pump. – Commonly use in trench excavation. – If the proposed formation level is below the suction lift capacity of the pump a multi-stage system can be employed.
Ground Water Control Thin Grouted Membranes – Work as permanent curtain or cut-off non structural walls or barriers inserted in the ground to enclose the proposed excavation area. – Suitable for silts and sands and can be installed rapidly but they must be adequately supported by earth on both sides. – The only limitation is the depth to which the formers can be driven and extracted.
Contiguous Piling – Forms a permanent structural wall of interlocking bored piles. – Alternate piles are bored and cast by traditional methods after which the interlocking piles are bored using a special auger or cutter. – Suitable for most types of subsoil and has the main advantages of being economical on small and confined sites; capable of being formed close to existing foundations and can be installed with the minimum of vibration and noise.
Ground Water Control Contiguous Piling – To ensure a complete interlock of all piles over the entire length may be difficult therefore the exposed face of the piles is usually covered with a mesh or similar fabric and face with rendering or sprayed concrete. – Suitable for structures such as basements, road underpasses and underground car parks.
Diaphragm Wall – Are structural concrete walls which can be cast in- situ or using pre-cast concrete methods. – Suitable for most subsoil and their installation generates only a small amount of vibration and noise. – The high cost of these walls makes them uneconomic unless they can be incorporated into the finished structure. – Normally use for basements, underground carparks and similar structures.
Pre-cast Concrete Diaphragm Wall – Have some applications with in-situ concrete diaphragm walls. – Lack in design flexibility. – The panel or post panel units are installed in a trench filled with a special mixture of bentonite and cement with a retarder to control the setting time. – This mixtures ensures that the joints between the wall components are effectively sealed. – To provide stability, the panels of posts are tied to the retained earth with ground anchors.
Grouting Methods – Are used to form a curtain or cut-off wall in high permeability soils where pumping methods could be uneconomic. – The curtain walls formed by grouting methods are non-structural therefore adequate earth support will be required and in some cases this will be a distance of at least 4m from the face of proposed excavation.
Ground Water Control Grouting Methods – Grout mixtures are injected into the soil by pumping the grout at high pressure through special injection pipes inserted in the ground. – The pattern and spacing of the injection pipes will depend on the grout type and soil conditions. – Grout types: Cement grouts Chemical grouts Resin grouts
Ground Freezing Techniques – Suitable for all types of saturated soils and rock and for soils with a moisture content in excess of 8% of the voids. – The basic principle is to insert into the ground a series of freezing tubes to form an ice wall thus creating an impermeable barrier. – Takes time to develop and the initial costs are high.
Ground Water Control Ground Freezing Techniques – The freezing tubes can be installed vertically for conventional excavations and horizontally for tunneling works. – Normally using magnesium chloride and calcium chloride with a temperature of -15 to -25 degree Celsius which takes 10 to 17 days to form an ice wall 1m thick. – Liquid nitrogen could be used as the freezing medium to reduce the initial freezing period if the extra cost can be justified.