Builiding Technology 2005 Question 2B

By using diagrams explain the types of ground water drainage system.

Answer:

Groundwater drainage can be achieved with one of more of the following methods, depending upon the ground conditions:

1. Pumping from open sumps within the excavation

2. Installing well points or ?? wells outside the excavation

3. Sheet piling around the excavation.

1. The first one is the popular method of dewatering the subsoil. This method can be used in most soil types and is economical in operation. Normally sumps are sited within the excavation area, but maybe outside it when large quantities of water have to be pumped. A minimum of two pumps are used on all but the smallest sites.

Pumping From Sumps

The sump is excavated to the fall depth required to drain the excavation before the excavation reaches the level of the groundwater. To prevent loss of soul fines when pumping fillers should be used. Drainage channels should bearing forwards the sumps to prevent water remaining on the surface of the excavation.

2. When the water table is too high or the subsoil is too porous to contain and dispose of the ground water, a system known as well pointing should be considered.

WELL POINTING

Well pointing is a method that uses mechanical wells 75mm in diameter closely spaced in the ground around the excavation water is withdrawn from the well by a suction pump connected to a ring main well points are effective up to 5 meters below the suction pump in lef and therefore at greater depths they well need to be set in stages. Suitable for most Greu??lar  materials and affordable, the well point system is a very popular system used to day. Some 15 to 20 well points can be operated with the same pump.

In excess of 5 meters, deep board wells may be employed ?ere a pump is lowered to the bottom of the deep well to dewater the ground. The bottom of the deep well to dewater the ground. The hole is bo?ed with conventional drilling or bouing machines and steel living is left in place of 300 to 600 mm in diameter. Another steel tube is placed inside the well, in which a submersible pump is placed.

3. Sheet piling is used to seal off the flow of water when there is a layer of impermeable strata below the reduced level. In cases such as this, the water can be virtually sealed off. This form of piling is the most commonly used for permanent and temporary work to control ground water and support excavations steal sheet piles that interlock to form a steel wall, are hammered into the ground are held stable by either candider action or are propped across excavation.


SHEET PILING CUT OFF WALL

It should be noted that this may not work in all circumstances due to the presence of an i?ntesian head of pressure.