One of the great challenges of civil engineering is the proximity of water to engineering structures. To solve this problem, we must remove water from the construction site so that water does not penetrate them after the construction of the structure. Removing water from the construction site is called drainage. Drainage literally means drying water that seeps from the ground or from a rock. Drainage is usually said to prevent water from approaching the bottom of the excavation and the walls of the excavation, and draining prevents water from entering the bottom of the excavation and the walls of the excavation.
To solve the problem of providing parking in tall structures, earth-moving and deep excavation is inevitable. Often the following issues and problems can be encountered during excavation:
- Groundwater aquifers
- Adjacent to semi-saturated and saturated soils
- Aggressive chemical conditions
And these problems cause serious damages such as:
- Water seeps into the structure
- Settlement and demolition of foundations and structural skeletons
- Corrosion of steel and concrete due to the presence of sulfate or chlorine and even the phenomenon of carbonation
- Increasing project time and cost
- Demolition of the earthen forehead of the excavation
- Reduction of bearing capacity of soil under the foundation
- Drilling of boreholes
- Side problems during shotcrete execution or execution problems such as trapping machinery
Drainage history
From the past until now, various methods such as bituminous waterproofing or asphalt felt have been used to drain and dewater the excavations. But the following:
- Investigation of the vulnerability of these products to environmental conditions
- Uncertainty about how they will work in the long-term (the life of this waterproofing is short and it cracks after about 5 years)
cause that these methods are less used today and alternative and better methods are used for drainage and dewatering of the construction excavation.
Different drainage system methods
1) Sump Pumping
The most convenient method of draining the bottom of the excavation is the sump pumping method, which is done by collecting water in a sump, which is usually installed in a corner of the ground) and pumping it out of the excavation.
2) Deep well
Deep well method consists of several wells with immersed pumps. Water is pumped upwards from each of the wells and directed to the out-of-project point by streams or tubes.
3) Well Point System
A set of small diameter wells that are connected to a tube at the top and this tube is connected to the pump at the end, by which the water level in the wells is lowered
4) Educators
In low permeability soils, water is kept away from the bottom of the excavation by creating a vacuum in each well.
5) Vacuum wells
Vacuum wells method is adapted from the deep well method (method 2). Water is pumped by submersible pumps and the power of the pumps is generated by vacuum pumps above the wells.
6) Horizontal Wells
In Horizontal Wells method, flexible perforated horizontal tubes are used, which are installed by a special device.
There are other less commonly used methods of drainage:
7) Relief wells
8) Artificial recharge
9) Siphon drains
10) Chemical Consolidation of Soil
11) Dewatering by Constructing Sand Drains
12) Dewatering by Freezing Process
Sand drainage method for drainage of the bottom of the excavation
13) Dewatering by Electro-Osmosis
Different methods of dewatering the excavation wall
1) Membrane dewatering method with geosynthetic products
In this method, a dewatering curtain is created between the structure and the surrounding environment using geosynthetic membrane dewatering layers such as geomembrane, GCL, etc. The thickness of the layer is calculated based on the hydrostatic pressure of water. Some of the suitable geosynthetic products for membrane dewatering can be named as follows:
- Geomembrane sheets with dewatering properties with different thicknesses such as PVC / VLDPE / HDPE / LLDPE
- Bentonite GCL sheet with dewatering property
- Dimpled sheet with drainage and water conductivity
- Geotextiles with drainage and water conduction properties and protection of geomembranes
In each project, based on the efficiency and characteristics of geosynthetic products, a product or a combination of different types of geosynthetic products is used to dewater and drain the walls of the excavation and underground structures.
2) Dewatering method using structural members (impermeable concrete)
Different excavation stabilization methods have two different approaches to dewatering:
- Stabilization with dewatering capability: dewatering diaphragm wall, sheet pile and so on
- Stabilization without dewatering capability: soil nailing, piling and so on
In non-dewatering stabilization methods, assuming the water level is higher than the excavation bottom level, it is often necessary to dewater the foundation and retaining wall.
In order to dewater the concrete used in the foundation and retaining wall, two points are important:
1) Concrete quality:
Factors affecting the quality of concrete include the following:
- Control and reduction of water to cement (W/C) ratio
- Use of materials with continuous granulation and the desired filter percentage
- Suitable slump control for concrete
- Use of quality cement
2) Additives:
Additives lead to dewatering of concrete by reducing capillary pores and cracks in concrete. Here are some suitable additives for dewatering concrete:
- Types of water reducers and concrete lubricants
- Volumetric crystallizers of concrete
It should be noted that in order to completely dewater the structure, the following items are also important:
- Proper and principled installation of water stops
- Injection tube in executive joints
- Proper design of concrete cutting
- Necessary controls in reinforcement and molding
- Selecting the appropriate concreting method to prevent cold joints
Links:
Federal Highway Administration (FHWA)
pureeffect , Construction Dewatering Treatment Systems
aquatechdewatering , Dewatering
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