During rainy seasons, the danger of water seeping into a structure increases with a rise in the water table. This water enters through seepage or leakage and damages the internal structure due to corrosion. Generally, the corroded reinforcement steel bars are responsible for the collapse of a structure as they cannot be restored once rusted.
Sometimes, the serviceable part of a structure has to be located in an area that is in close vicinity to water. Then in such cases, it becomes necessary to design a facility that is completely waterproof and effectively prevents the ingress of water. In the construction of a subaqueous facility, provisions should be made to restrict the complete entry of water into the structure.
In this article, our main focus would be on the provision of a complete drain system. Subsequently, the focus will be on the process of construction of the drainage facility.
Generally, the drainage facility consists of a filter system, a conduit system, a collector chamber, and a disposal unit.
1. Construction Techniques for Drainage Prevention
The construction techniques to avoid the entry of seepage water into a structure are described below :
1.1 Foundation Drains
When the water table rises up to the foundation level or above the foundation level, provision should be made to remove the water from the proximity of the structure.
When the depth of the water table is not much above the foundation level, a provision for foundation drains should be made. The foundation drains are very useful in removing the water from the neighboring area of the footing level.
A foundation drain consists of a filter system that is attached to perforated pipes on one side. The function of the filter system in the drain facility is to avoid seepage erosion due to the heavy flow of water from the underground during heavy rainfall.
The underground water will then come in contact with perforated pipes and will be led towards the collection chamber from where it would enter the disposal unit.
The disposal unit can be pipes with a diameter bigger than that of the perforated pipes. This disposal unit may work on the gravity flow principle where it connects them to the storm drain system or dry well, etc. If the gravity flow is not feasible, then an external pumping system has to be installed.
1.2 Blanket Drains
When the seepage of water is intruding the basement floor slab, a blanket drain should be provided. Blanket drains remove the water while maintaining the continuity below the basement floor slab. Therefore, it reduces the uplift force and increases the stability of the structure.
The mechanism of blanket drain consists of two filter layers. The outermost filter layer consists of finely graded material, and the innermost filter consists of coarse-graded material.
These filter layers will act against the variable hydraulic pressure during the heavy inflow of water into the basement floor slab. Further, the innermost layer will lead the water towards the collection chamber.
Generally, the collection chamber is located at the underside of the basement floor slab. From the collection chamber, the water is pumped out and disposed to the storm drain facility.
Blanket drains are very useful in case of masonry dam and also beneath the pavement surface. In the case of masonry dams, they are provided under the dam floor to reduce the effect of seepage pressure head.
In pavements, they are installed below the base layer and are very effective in reducing the upward capillary flow, which otherwise can lead to the pothole formation.
1.3 Interceptor Drains
Interceptor drains are provided in highways and runways. Construction of interceptor drains should be parallel to the shoulder. The main purpose of such drains is to reduce the water movement below a certain level from the pavement base course.
Interceptor drains are very effective in keeping the subgrade and base soil dry. Therefore, it achieves the satisfactory requirement of strength and stability criteria. They are also effective in holding the surface or subsurface water flow, trying to enter into the base surface. This is how the drains prevent the development of excess pore water pressure and reduce the chances of seepage erosion.
1.4 Drain Away Panels
In case the moisture penetrates the walls of the foundation due to subsurface water flow movement, then the situation will deteriorate during heavy rainfall as more and more moisture will begin to enter the foundation wall and consequently reduce the strength of the foundation.
The drain away panels are instrumental in waterproofing the walls and ultimately reducing the built-up pore water pressure. They are available in two forms: the most popular one is the two-sided drain away panel, and the other is the single-sided drain away panel.
1.4.1 Two Sided Drain Away Panel
This panel consists of the innermost layer of concrete covered by a plastic membrane, acting as the core of the panel. Further, the plastic core includes a non-woven geotextile filler on both sides. In addition to that, it includes a sleeve for the perforated pipe through which the drained water can be taken out. Generally, it is used for the protection from seepage in foundations, retaining walls, tunnels, bridge abutments, etc.
1.4.2 One Sided Drain Away Panel
It is useful for protection against the drainage water in the horizontal direction, such as in rooftop decks, multilevel story gardens, floor slabs, landfill caps, etc.
Its working is similar to the two-sided panel. However, the only difference is that the geotextile layer is attached only to one side of the panel, leading the flow of water through one side. The collection and disposal of water can be achieved with the help of a perforated pipe.
If the drainage system is installed in an area of compressible soils, then a reduction in the pore water pressure will increase the effective stress on the soil grains. Furthermore, the increase in effective stresses can lead to the consolidation of the soil strata. Thus consolidation can cause drawdown in that particular region.
Filter is the most essential component of the drain system, which prevent the drain system from piping failure. If the proper filter system is not installed, then erosion of surrounding soil can lead to the formation of cavity. Thus the cavity formation initiates the process of piping failure, which can lead to the collapse of the structure. The soil erosion may also lead to the failure of pavements and basement floors. Such a phenomenon is mostly observed in sandy soils.
1. For perforated clayey pipe : ASTM C 498
2. For Concrete drain pipe : ASTM C 118
3. For reinforced concrete pipe: ASTM C 76
4. For perforated concrete drain pipe: ASTM C 444
5. Polyvinal chloride drain pipe: ASTM D 2729
6. Aluminum alloy perforated pipe: ASTM B 745
7. Steel metallic coated perforated pipe: ASTM A 760
8. Polyethylene tubes and fitting material: ASTM F 405
9. Perforated bituminised fiber pipe: ASTM D 3356