Builiding Technology 2008 Question 2A

Explain the purpose of groundwater drainage at construction site

Answer:

Water is always present int he ground the amount and level of it depends upon the particular climatic season and the nature and composition of the ground sheta. Water in the ground can be classified by its position and is identified as surface water, 90% of which percolates into the ground to form the ground water. Further down is the subsoil water the height of which is known as the water table level and its position relative to the earths surface will vary with seasonal change, as detected in diagram below:

In almost all construction sites, excavation work will be carried out and the ingress of water can give use to the following problems:

1. Flooding of the excavation. This is obviously undesirable as this will in most cases impede work progress. Engineering work in most instances progress best under dry condition.

2. Collapse of the ground. The volume and strength of the soil, in particular the clay depends on the water content. The more water in the clay the greater the volume and lower the strength. There is interest in the strength of the clay as an engineering material, for example, to construct an embankment or cutting or stand a building on it, as with less water content the clay contracts and the strength increases and for the engineer, this is a fundamental property of the material.

3. Undermining of supports to excavations. Support are generally necessary in order to maintain safe working conditions for the operations and the public; allow the work to be carried out unhindered and without delay; provided protection to adjacent properties and protect services contained in the ground.

4. Water logged working conditions, besides making at cumbersome for operations, often slows work progress and in extreme situations responsible for equipment failenes and losses.

From the above, we can see that the control of ground water is very important in terms of maintaining the materials in both the permanent and temporary conditions. excavations might easily encounter both ground and subsoil water entering the trenches and excavations, particularly where they might be considerable depth. The water must then be chained or controlled for the following purposes.

1. To reduce flow of water into the excavation or trenches.

2. To assist in supporting the excavation safely so that all operators can work in minimal damages.

3. To increase the bearing capacity of the soil. Thus will strengthen the soil to take on the loading of the building that is to be constructed on it.

4. To control ??tesian preserve.

5. To offset quicksand conditions due to hydrostatic pressure.