GROUND IMPROVEMENT TECHNIQUES BY A. DEEPTI

INTRODUCTION Ground water is usually considered as one of the most difficult problem that has to handled in any civil engineering construction. Impact of ground water in construction project is enormous. It effects the design of structure, construction procedures and over all project cost. Current trends of population growth have increased the land values, creating demand for development of sites which were previously considered unsuitable : often ground water is a problem that must be solved.

INTRODUCTION The purpose of dewatering is to control the surface and subsurface hydrologic environment in such a way as to permit the structure to be constructed “in dry.” Dewatering means “the separation of water from the soil,” or perhaps “taking the water out of particular construction problem completely.” This leads to concepts like pre-drainage of soil, control of ground water, and even the improvement of physical properties of soil.

FUNCTIONS OF DEWATERING Dewatering is carried out for one or more of the following reasons: To provide a dry working area for construction. To reduce lateral pressures on foundations or on retaining structures, To reduce compressibility of granular soils. To increase bearing capacity of foundations. To prevent liquefaction due to upward gradients. To prevent surface erosion. To prevent or reduce damage due to frost heave.

METHODS OF DEWATERING Shallow Excavations Sump Pits Interceptor Ditches Intermediate Excavations (Depth > 9 m) Well Points Vacuum Dewatering Electro-osmosis Deep Excavations Jet or venturi pumps

Shallow Excavations Sumps Sump : A pit in which water collects. By definition, a sump is at low level in relation to the surrounding ground surfaces so that any water will flow to it due to gravity. It involves allowing ground water to seep into the excavation, collecting it in sumps and then pumping it away for disposal. Sump pumping can be very effective and economic method to achieve modest draw downs in well graded coarse soils (such as gravelly-sand, sandy-gravels and coarse gravel) or in hard fissured rock

Sumps & interceptor ditches The essential feature of this method is a sump below the ground level of excavation at one or more corners or sides. In order to prevent standing water on the floor of excavation, a small ditch is cut around the bottom of the excavation. In case of large excavation which have to remain open over long periods, it is advisable to design these drainage ditches with much more care. The greatest depth to which water table can be lowered through this method is about 8 m below the ground surface.

Sumps & interceptor ditches The sump must be substantially larger than that needed to accommodate the pump, because the surface water flowing to the sump is likely to carry the fines. These are likely to be abrasive and capable of causing wear and tear to the pumping equipments Adequate provision should be made for periodic servicing of the equipment and removal of accumulated sediment.

Sumps & interceptor ditches Advantages: This is the most economical method of ground water lowering. This method is very easy and rapid. Disadvantages: Flow of water into excavation can have destabilizing effect on fine grained soils. There is risk of collapse of sides of an embankment. Risk of instability of base due to upward seepage towards sump

Sumps & interceptor ditches Soil types where use of sump pumping has a significant risk of causing loss of fines include: Uniform fine sand. Soft non-cohesive sitls and soft clays. Sand stone with uncemented layers that may be washed out

Well point systems Well point system essentially consists of series of closely spaced small diameter water absorption points (screens) connected to suitable pumping systems. A perforated well points is typically about 0.7 to 1.0 m length and 40 to 50 mm in diameter. Well points connected to a common header(150 mm diameter) which carry away the water by gravity with assist from a pump The header pipe is normally made of high impact plastic, although steel pipe is some times used especially when there is risk of damage due to construction activities.

Well point systems Header pipe is typically supplied in 6 m length and is joined onsite by simple couplings Not effective when D10 < 0.05 mm Generally produce a drawdown of 15'-18' below the centre of the header Placed at spacing of 1 – 3 m apart around the perimeter, but generally depends on Permeability of soil Time available to effect the draw down

Well point systems Advantages: This is the most economical method of ground water lowering. This method is very easy and rapid. Level of expertise needed is not sophisticated and are readily available Disadvantages: Serious limitation is suction lift. Excessive air can be drawn into the system through joints in pipes, valves etc

Vacuum dewatering When silts and other fine grained soils are encountered, gravity is insufficient to draw the water to the well; thus, a vacuum is applied to the well point system to assist in the water removal System can operate in several stages depending upon the conditions of the site

Electro Osmosis Reuss (1809) was the first to observe water flow in soils as a result of a direct electric current passing through it. This uses an external electromotive force across the solid-liquid interface. Reuss experiment is shown in the figure:

Electro Osmosis Useful for low permeability soils (2 x 10-4 < k < 2 x 10-6 ft/min) When direct current is applied to a saturated soil, water migrates to the cathode (negative electrode) Well point installed at cathode to collect water Has the additional benefit of consolidating soils and allowing them to gain strength Method usually expensive

Foundation drains Drains necessary to keep water away from existing structures Although possible to seal a foundation (especially a small one), a better approach is to drain the water away from it Foundation drains run parallel to foundation, generally run perforated pipe with coarse filter material (such as gravel) surrounding the pipe Gravity preferred drainage; sump if necessary

Blanket drains Similar in concept to foundation drains except more generally applied A “blanket” or layer of permeable material is placed under a structure or area requiring drainage which allows the water to exit either by pumping or gravity flow Normally consists of a layer of coarse material with a sand layer on top of it