MEASURES TO COLLECT WATER

MEASURES TO COLLECT WATER

24/09/2020

The water collection pit is the most basic measure of the method of lowering groundwater - draining the foundation pit. In essence, it involves allowing groundwater to enter the trenches and collect it in a sump, and then pump it out for disposal.

1.Introduction

Scott (1980) defines a  sump (sump ) as a pit that collects water before it is pumped out by dropping a pump into the pit.

By definition, a sump must be lower than the surrounding ground so that all the water can flow into the pit because of gravity. For construction projects, water is removed from the sump using a suction pump or submersible pump.

The combination of sumps and drainage channels increases the efficiency of surface and groundwater control. Pump type selection should also be considered

2.Scope of application of pits

The water collection pit is the most basic measure of the method of lowering groundwater - draining the foundation pit. In essence, it involves allowing groundwater to enter the trenches and collect it in a sump, and then pump it out for disposal. A sump is used for two distinct purposes, although the form of a sump may be similar for either requirement:

  1. Collect surface water runoff entering it using a collection ditch or channel to discharge to a treatment point or area ( see section Surface water pumping ).
  2. Collect and pump discharge water to lower the groundwater level to dig shallow soil; Pumps may also be required for gravity drainage at the base of retaining walls of sloped roofs.

The most common method of treating water from a sump is pumping. Usually, each sump is equipped with a submersible pump (slurry pump type). Slurry pumping can be a very efficient and economical method to achieve low water level lowering in coarse-grained soils (eg gravel, coarse sand) or in fractured rock.

Under some conditions, the use of sumps can lead to major problems. These problems arise mainly, because water flow into the excavation can have an unstable effect on the fine soil. This can lead to soil particles being washed away and causing subsidence. Powers (1985) lists soils in which the use of slurry pumps poses a risk of fine particle washout including:

  1. Fine sand
  2. Non-stick dust and soft clay
  3. Weathered rock formations in which fissures can corrode and expand due to high water velocities
  4. Rocks in which cracks are filled with sand, dust or soft clay, which can corrode
  5. Sandstone has an unconsolidated layer, which can be washed away

In these soils, even the best sump pumping systems can experience problems. Potential problems can be avoided by using a ground water lowering system using wells (point wells, deep wells, or ejectors) with reasonable design and installed filtration. Provided the wells are located outside of the main excavation area, these methods have the advantage of drawing water out of the excavation, improving stability, and avoiding problems associated with sump pumping.

3.Water collection pit

Pumping water in the sump is easy to carry fine-grained sedimentary materials, when operating the pump should be suspended approximately 300 mm from the bottom of the sump to allow the sediment to accumulate. This does not apply to small and shallow pits where only the suction pipe is located in the sump.

A common arrangement for a sump is to drop the pump in a sump or similar container with multiple holes punched through the wall. A layer of gravel is poured around the outside, which acts as a filter.

The sumps should be dug deeper than the foundation excavation depth at the outset and must be maintained throughout the construction period, although further digging can be made as needed, but it is more beneficial to dig deeper at the outset:

  1. The filter layer is rearranged and works more efficiently to minimize the loss of fine-grained soil
  2. Keep groundwater always below excavation level at all construction stages
  3. Allows to speed up excavation when necessary

Most sump pits are made by digging, the walls should be stabilized temporarily with slabs, before placing the bucket.

surface water sump surface water collection system

4. Problems when lowering groundwater level by sump

If the water entering the sump will carry fine particles from the soil, the pumped water will have a load of sand, silt, and clay particles. Sump pumps typically pick up some sediment in the water and are likely to continue operating, unless the sediment load is particularly high, at which point they can choke with sand and mud. However, this dirty water discharge can cause environmental problems.

There are an increasing number of situations when sump pumping may be technically feasible, but where environmental concerns are associated with the discharge of "dirty" water it will be difficult to take a sump measure. Discharge of sediment carrying water can cause many potential problems. If water is discharged into the drain, sediment can build up in the drain, reducing capacity and causing the drain to clog. If water is discharged into a water source, the sediment will have an adverse effect on aquatic plants, fish and insects.

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