‘Architects and town planners are designing multi-storied commercial and residential buildings, which significantly increase the demand for water supply. Is the additional water supply available there? The Central Ground Water Board has identified about 800 regions in India in which ground water level is plunging at an alarming rate. These regions are located in rajasthan, Madhya Pradesh, Punjab, Haryana, Gujarat, Bihar, Delhi and TamilNadu’, says Prof. Prithvi Singh Kandhal, Associate Director Emeritus, National Centre for Asphalt Technology, Auburn University, Alabama USA
According to the 2019 data of CGWB, for every 125 units of ground water being taken out in Jaipur, only 100 units are replenished by rain. It is estimated that the ground water level in Jaipur is falling at the rate of about one meter every year. According to the CGWB of India, all underground water will be depicted in Jaipur in about 10 years. There is some water in rocks below 100-m but that water contains harmful elements in it and may not be safe for drinking.
We must act now to recharge the ground water in over-exploited/critical areas of India. The Ground Water Advisory Council on Artificial Recharge of the Ministry of Water Resources has suggested that there is a need to develop separate technologies for recharge specifically for urban areas. This paper addresses that very need.
The ground water problem was also felt in the US in urban areas, where rainwater simply runs off without charging ground water. The Franklin Institute of Philadelphia, Pennsylvania was tasked in early 1970s to develop technologies to address the problem of plunging water table in urban areas. The author had the privilege of brainstorming with the Franklin Institute researchers in developing the concept of porous asphalt parking lot for urban areas. This concept was tried in some pilot projects and was successful. The concept was later fully developed in the 1980s. At present, it is being used in many states of the US primarily for storm water management. The State of California has built over 150 projects since 1980. About 95 per cent of rainwater falling on a porous asphalt parking lot goes to recharge ground water. Even in case of open ground with vegetation in rural areas, only about 33 per cent of rainwater goes to recharge ground water, primarily due to evapotransportation losses. This proven concept of building porous asphalt pavements was declared Outstanding Engineering project by the American Society of Civil Engineers.
Concept of porous asphalt pavement technology
This technology is based on building porous asphalt pavements, which can be used for parking lots, recreational areas, or low volume streets and roads. The porous asphalt pavement works like:
The top 50-100mm thick asphalt layer is designed specifically to make it porous. Rainwater goes through it rapidly without any ponding at the surface. The water is then stored in an underlying open-graded stone bed. From there, water percolates slowly into the underlying natural soil. There is hardly evaporation loss. Porous parking lots or streets can be integrated with roof rainwater harvesting systems in the buildings adjacent to it. There is no need to bore deep wells or construct deep pits.
A typical cross-section of the porous asphalt pavement system consists of the following components (Fig. 2)
- Open-graded, porous asphalt course 50-100mm thick
- 12.5mm size aggregate choking layer. 25-50mm thick (this is placed over the stone bed so as to stabilise it and facilitate asphalt paving over it)
- Clean, uniformly graded 40-5mm size crushed aggregate compacted layer to act as a water reservoir (typically it is 225-mm thick and contains more than 40 per cent voids to accommodate rainwater)
- Non-woven geotextile to separate the soil sub grade and water reservoir course, so that soil particles do not migrate from the sub grade into the stone water reservoir course thus choking it.
- Uncompacted natural soil subgrade
As mentioned earlier, rooftop rainwater harvesting systems of the buildings, adjacent to porous parking lots or streets can be integrated into the porous asphalt pavement. A typical surface rainwater harvesting system for buildings consists of the following elements:
- Vertical down pipes for carrying the water from the roof to ground level and a horizontal pipe system for connecting all down pipes
- A silting pit filled with a steel screen
- A soaking well with cement ring and shaft filled with their media consisting of large stone, medium size stone and coarse sand.
If the rooftop rainwater harvesting is integrated with the porous asphalt pavement, the above highlighted 2 items are not required. The water from the rooftop is carried directly to the stone water reservoir and dispersed there with a perforated water pipe. This means no silting pits and soaking well or bore hole which involves a lot of costs. In case of streets, water from the roof top of the buildings on the street can all be diverted to the stone water reservoir course. Another major advantage of this technology is that the water recharging the underground water is pure and free of contaminants.
Design and Construction of Porous Asphalt Pavements
It is recommended that the in-situ soil permeability infiltration rate is 12.5-mm per hour. However, 2.5-mm per hour is acceptable by suitably increasing the thickness of the stone reservoir course. In Jaipur, the infiltration rate of the local soil is significantly higher than12.5-mm per hour. Soil investigations should be carried out by noting and/or test pit to test for permeability, determining the depth of high water table, and determining depth to bedrock.
Compacted stone layer should be placed directly on natural soil subgrade (bed) because fill is not recommended. Although a flat soil bed is preferred, slope of natural soil bed should be limited to 5 per cent. The thickness of compacted stone course (containing about 40 per cent voids) should be designed to accommodate intensity and amount of rainfall prevailing in the region. Typical designs are made for 6months/24 hour rain storms. Conservative designs are based on 20 year/24 hour rain storms, which can range from 35-mm to 400-mm in 24 hours.
The structural design of the pavement including the compacted stone course and porous asphalt wearing course should be based on traffic using the facility. Normally, porous asphalt pavements are recommended for parking lots, recreational areas, and low traffic roads (with limited truck use). The construction of porous parking lot does not require any special material on equipment and this concept can be used on light traffic roads and streets.
Work site should be protected from heavy equipment so that the natural soil subgrade (bed) is not compacted otherwise its permeability may be reduced. Before placing the stone reservoir layer, place a filter fabric over the soil bed so that soil particles do not migrate upwards and clog the stone reservoir layer. Place the porous asphalt course last on the entire project so that it is protected from construction debris. It should also be protected from soil laden runoff.
Before placing the 50-100mm thick porous asphalt course, place 25-50mm thick layer of 12.5-mm size stone to stabilize the surface of the stone reservoir course and facilitate the paving operation. The porous asphalt course should be designed as per established guidelines for open-graded asphalt friction courses. Normally, the asphalt mix would have 6 to 6.5 per cent bitumen by weight of mix. Traffic should be restricted for 24hours after construction of the porous asphalt wearing course.
Performance of Porous Asphalt Pavement and its implementations
The dramatic performance of porous asphalt pavements in the US is clearly visible in Fig. 4, 5 and 6.
Fig. 4 shows a parking lot which is porous where the cars are parked whereas the driveway between the parked cars is dense asphalt. During rain, water is standing on the driveway but has percolated into the porous parking area. Fig 5 shows two parking lots just after rain. The one in the background is conventional dense asphalt parking lot whereas the one in the foreground is a porous asphalt parking lot. Their relative appearance after rain is so very clear.
Fig. 6 shows views of a highway in Chandler, Arizona during rain. The left lanes were constructed with porous asphalt and the right lanes were constructed with conventional dense asphalt. After 20 years in service, the porous asphalt on this highway is still functional. This highway is in semi arid region of Arizona with very low rainfall similar to Rajasthan.
It is absolutely clear that the porous asphalt technology works. 95 per cent of the rainwater falling on porous asphalt pavement goes to recharge the ground water. Therefore, its effectiveness in capturing rainwater is very close to paved catchment area around a “kund” (underground rainwater storage tank) found in semi arid regions of Rajasthan. Fig 7 shows a “kund” in the native village in Churu District of Rajasthan. Such a “Kund” provides water for about 10 families for the whole year, despite scant rainfall. It shows the power of a very small catchment area in supplying water. Therefore, the potential of recharging ground water from porous asphalt parking lots and low-traffic streets in urban areas such as Jaipur in huge and unimaginable.
Fig 8 shows a glimpse of current water crisis in Jaipur. According to James Clarke, “A politician thinks of the next election. A statesman, of the generation”. A politician will just order more water tankers whereas a statesman will implement rainwater harvesting technologies for the next generation. Unfortunately, India does not have statesmen at the state or national level. Therefore, it is all the more necessary for the public, architects, and town planners to be proactive in the national interest and try to incorporate rainwater harvesting techniques in their designs and planning.
Porous asphalt pavement is one of the responses to plunging ground water table in Jaipur and elsewhere in India. It can be integrated with the roof rainwater harvesting system effectively and economically. Properly designed and constructed porous asphalt pavement can last more than 20 years. Such a pavement can be used for parking lots, recreational areas, and low volume roads and streets. Government should encourage construction of porous asphalt pavements in urban areas. Architects and town planners should be proactive by incorporating this unique rainwater harvesting system while designing residential buildings, commercial buildings, and new townships.