GROUNDWATER CONTROL. Ground Water Control Water can be classified by its relative position to or within the ground

GROUNDWATER CONTROL

Ground Water Control

Water can be classified by its relative position to or within the ground.


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.


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.



Temporary exclusion– Simple Sump Pumping– Jetted Sumps– Wellpoint Systems


Simple Sump Pumping– Suitable for trench work and/or where small

volume of water are involved.



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– The metal tube is withdrawn to leave a void for placing a disposable wellpoint and plastic suction pipe.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.





Permanent exclusion– Thin Grouted Membranes– Contiguous Piling– Diaphragm Wall– Precast Concrete Diaphragm Walls– Grouting Methods– Ground Freezing Techniques


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.


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.


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 groutsChemical groutsResin 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 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.


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GROUNDWATER CONTROL. Ground Water Control Water can be classified by its relative position to or within the ground