Underground Excavation problems

Greetings!
This post contains the brief lecture notes on the topic "Underground Excavation problems".

There are three types of underground excavation problems, which we come across in general:

1. Stability of excavation
2. Dewatering
3. Effect on the adjoining structures

In my previous post, I discussed the first part of this topic, i.e. Stability of Excavation. In this post, I will take up the second part, i.e. Dewatering.

Dewatering:

Dewatering is required for deep construction excavations, such as power houses, pumping stations, bridge foundations and so on. Practically any foundation below water table will require dewatering, however, if the soil has got low permeability i.e. less the 10^(-6) cm/sec, much problem due to seepage of water is not posed, because the discharge is small. 

However, if the discharge  is more, that is case of permeable soils having permeability more than 10^(-5) cm/sec, dewatering is essentially required. Extent of dewatering will depend upon characteristics of soil, presence of water bearing stratas, aquifer parameters and the field permeability.

 The dewatering job is to estimate the rate seepage of water, for this a central circular well is constructed, and the rate of discharge is estimated for a fully penetrating well. 

The rate of discharge will depend upon the geometry and extent of excavation and the desired level of lowered ground water table. 

A no. of wells are placed around the periphery of excavation and the water is pumped out simultaneously out of the wells.

There are three methods of dewatering:

1. Sump Pumping
2. Deep well pumping
3. well point dewatering.

Sump Pumping: If the soil is cohesive, having low permeability, shallow trenches are dug along outer edges of the excavation to collect water and to pump it out through the pumps. It is particularly suitable in clayey and silty clay, where the rate of seepage is low.

This method is inexpensive and easy to operate. The greatest depth upto which this method can be adopted is 5 to 6 m below the pump level.

Deep Well Pumping: Deeps wells are installed along the sides of excavation and water is continuously pumped out. This method is suitable for highly permeable soils such as sand and silty sand. A group of wells is installed, then the effect of pumping is observed, if sufficient control is not achieved, additional wells or pumps may be added. 

Deep wells must be constructed in such a way that they remove the large quantity of water, without allowing the silt to enter into the well casing.

Well Point Pumping: It consist of installation of nos. of well points around usually excavation. The vertical raiser pipes are connected to the header main on ground. The water is drawn into the riser pipe and discharged by the header pipe. The distance between the well points is 1 to 1.5 m. 

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