The SILVERON Recharge Shaft

The ever-increasing population, urbanization, industrialization and rise in agricultural activity are major reasons for growing water demand. Rapid decline in the water table can be attributed to over exploitation of ground water resources for meeting these demands.

The basic purpose of artificial recharge of groundwater is to replenish water into aquifers that have depleted due to excessive groundwater extraction. Artificial groundwater replenishment systems involve techniques that modify the natural movement of surface water and use civil construction methods to enhance the sustainable yield of groundwater in areas where over extraction has depleted underground aquifers.

SILVERON’s dedicated team led by Sunil Sharma started experimentation to determine the various factors affecting groundwater conditions such as thickness of alluvium, depth to rock, the extent and depth of aquifer and quality of ground water. The team conducted several Geophysical Vertical Electrical Soundings  in the targeted areas.                                          

In our earlier blog Geophysical Survey: An essential tool for Rainwater Harvesting, we discussed the importance of variations in soil formation and presented sample data from our records showing major variations in ground strata.

Explains Ground Resistivity Tests for Rainwater HArvesting
Clockwise from top left: (a) Soil-Layers cross-sec view (b) Team SILVERON performs Ground Resistivity Test (c) snapshot of Soil Resistivity Testing and (d) Imaginary Ground Profile Illustration

Geophysical measurements are based on the fact that subsurface consists of a sequence of distinct layers of finite thickness. Each layer is assumed to be electrically homogeneous and isotropic and the boundary planes between layers are assumed to be horizontal.

Lessons from Field Experiments

Based on the Geophysical Survey and analysis of soil samples obtained from various depths while drilling trial bores using a Rotary drilling machine, it was observed that the changing compositions of soil components are very diverse. The soil strata keeps changing as we drill deeper into the earth and we are likely to encounter some layers that absorb more water, some which absorb less and some which absorb no water.

Since it is impossible to modify this naturally occurring soil strata, hence the research team came to the conclusion that for rain harvesting, Diameter of the recharge bore has less meaning while its depth is more important.

Team SILVERON also studied existing rainwater harvesting designs and observed that most structures had worked well in the initial months but gradually their efficiency reduced, and water started stagnating in the structures. This happened as most existing structure designs were not in line with natural laws but rather attempted forcing water absorption against natural processes.

The conclusion drawn was that for rain harvesting, supporting natural process is a sure path to success while going against nature is a recipe for failure.  

In view of the above valuable lessons, Sunil Sharma and his team started experimenting with multiple design ideas to be able to develop an Eco-friendly shaft design which would recharge the ground water in a sustainable way closest to the natural recharge process.

The Vision

SILVERON’s dream was to create a Rainwater Harvesting System that recharges ground water aquifers and simultaneously enriches the root zone with moisture supporting trees, shrubs and foliage. The vegetation generated by this enriched root zone forms a network of roots, binding the soil together to check soil erosion and provide fodder for animals.

Increased moisture content in the top soil and vegetation raises the humidity in the air that in turn helps in the survival of diverse interdependent friendly organisms like bacteria and earthworm etc living both over and below the soil surface.                                   

The conceived design would allow water to flow down an unlined borehole in direct contact with soil starting from the root zone. This matches the natural process where the rainwater is not flowing through artificial ducts such as a failure-prone slotted pipe system .

An Ingenious Design

We had determined that the soil profile is not uniform and there are layers of varying composition, porosity & permeability. It is conceivable that in the natural process, rain water on the surface would runoff since it was the easiest path and droplets which percolated into the ground due to gravity encounter other obstacles in the flow that may delay movement and point of contact with groundwater.

Slow Natural Ground Water Recharge

To facilitate both vertical and horizontal flow of water across the varying permeability of soil strata, SILVERON shaft depth was planned in a way that pierces through all layers and provides easy passage for the flow of water. The shaft’s bore hole is filled with material of high permeability providing an automatic connection between all layers in the ground.

Natural Ground Water Recharge supported by Rainwater Harvesting Shaft

It was observed that the recharge shaft had its own absorption (or intake) speed and any excess quantity of water reaching it would tend to runoff on the ground surface.

Encouraged by the success of recharge due to the vertical inter-connectivity that the shaft provided through natural formations inside the ground, it was decided to interconnect the adjoining recharge shafts by PVC pipe at the top so that the excess water entering the shaft could reach an adjoining recharge shaft without evaporation losses. This would help in avoiding flood hazard during storm showers by capturing the rainfall run-off which would otherwise overwhelm sewer or storm drains and also results in soil erosion.

Shaft Inter-connectivity boosts Rainwater Harvesting
Shaft Inter-connectivity boosts Rainwater Harvesting

The team while examining choked recharge structures observed that most rain water harvesters focused entirely on silt removal before the water was allowed to enter the recharge structure.  SILVERON recognizes that silt is the finest particles of soil, suspension of silt in flowing rain water is natural and since the turbulence is high during monsoon season, silt does not get time to settle making the water appear murky. 

The challenge is not only to ensure unhindered recharge performance in condition of normal silt naturally flowing into the structure but also to incorporate design features that would make silt removal and cleaning an easy & cost effective process.

Natural performance of the SILVERON Recharge Shaft in a village pond

SILVERON designed recharge shafts such that the silt which enters the shaft in the previous monsoon is removed easily by back wash arrangement before onset of next monsoon, at almost negligible cost and as simple process.

Removing silt through backwash

SILVERON shaft was designed conceived that rain water transported from roof tops, paved surfaces or low-lying areas to the recharge shaft would percolate and make the soil wet all around the entire depth of the borehole.

This in turn would attract naturally percolating rainwater coming from any direction, any distance and at any depth – since the shaft column is available to provide easy downward flow passage. The main design principle was to let nature develop an inter-connected network of streams below ground howsoever minute, since once created these channels would remain available as a path for water to flow when it rains.

Naturally Percolating Streams Find Recharge Shafts
Naturally Percolating Streams Find Recharge Shafts

The SILVERON Edge

We at SILVERON, understand the importance of considering all natural factors when designing a rainwater harvesting structure. These variations make the scope of work differ from location to location. Site specific work of such magnitude calls for years of experience and understanding on the part of the harvester.

Ingenuity, untiring efforts, trials, determination to succeed and decades of experience stand behind the performance of SILVERON Rainwater Harvesting Systems.

The wide acceptance and appreciation of the performance of SILVERON recharge shafts makes SILVERON the first choice in the field of water conservation.

Understanding Earth & Soil

Experiencing nature gives us immense joy & pleasure. Be it in walking along a park trail lined by lush green trees, seeing vast green spaces or perhaps even seeing puddles and little streams after a rain shower. We love the sound of the humming birds, enjoy the weather and the fresh cool breeze. We love the sight of bright flowers all around and the squirrels crossing our path.

Does anyone spare a moment to think about the earth under their feet?

The world continues to move from the stone age to the space age because the principles of physics remain the same. It is only as we understand them better and experiment do we make advancements that benefit our lives.

We know more about the Sun, Moon and other planets in space but hardly know our Earth. Ever wondered how we can accurately predict solar and lunar eclipses but are caught off guard by disastrous flash floods, earth quakes, tsunamis and landslides?

The earth underneath us is so diverse and alive that the more one thinks about it, the more magical and mysterious it appears. It is the place where at each moment every law of physics is in action and which is holding within it the entire periodic table, innumerable organic and inorganic compounds and minerals and air and water and the living and the decomposing.

The core of the earth is about 6300 Km. from where we stand but humans have not been able to drill below 12 km (see ref). Most people do not even know that the ground well below our feet is may be hotter than the surface of the sun.

The layers of earth based on chemical variations from shallowest to deepest are:-

Layers of the Earth
  1. Crust: Earth’s crust is the outermost layer of earth and ranges from 5–70 kilometers in depth.
  2. Mantle: Mantle lies between Earth’s crust and dense super-heated inner core and is about 2,900 km. thick.
  3. Outer Core: Earth’s Outer Core is largely liquid iron layer of the earth that lies below the mantle. The outer core is about 2,300 km. thick.
  4. Inner Core: Earth’s inner core is the innermost geologic layer of the Earth. It is primarily a solid ball with a radius of about 1,220 km. The inner core is believed to be composed of an iron–nickel alloy with some other elements.

Our aim here is to give some insight to our readers that our earth is magical and that the earth’s resources are major components which impact the environmental factors which make planet earth habitable.

We do not wish to wade into a theoretical discussion about permeability or porosity of soil or about range of validity of Darcy’s law or velocity of flow of water. We would restrict our discussion to shallow depth of the earth’s crust since no man has ever explored the other layers in person – in fact at a maximum depth of 12kms, our civilization has barely scratched the surface.

The most common components with varying percent found in composition of the soil.

CapacitySandClaySilt
AerationGoodPoorMedium
Water-HoldingLowHighMedium
Compact-abilityLowHighMedium
DrainageHighSlowMedium
Leakage-preventionPoorGoodPoor
Result of tilling after rainfallGoodPoorMedium
Erosion by waterMediumLowHigh
Erosion by airMediumLowHigh
Capacity to hold plat nutrientsPoorHighMedium
Capacity to shrink or swellLowHighLow
Decomposition of organic matterFastSmallMedium
Soil components and capacity

Sand has pockets which hold air hence aeration is good and the voids are interconnected hence water holding capacity is low and drainage rate is high. This causes the compact-ability to be low and its use as a sealer to prevent leakage is not advisable.

This is the reason why it is beneficial to till the sandy land after rainfall. Planting of trees and bushes and construction of check dams are recommended to prevent soil erosion by air and water.

Some form of clay when added to sandy soil, enhances the cultivation output because it reduces soil erosion both by air and water and increases the compact-ability, water retention capacity & plant nutrient holding capacity of the soil.

If we explore the properties of soil we come across a list which is unending.

  • Soil not only serves as an anchor but also provides the required minerals and water to the plants. In fact this resource provides 99% of the food consumed by human beings.
  • Soil is a major raw material required for manufacture of many types of building materials. Soil is the foundation of construction projects.
  • Soil absorbs and cleans rainwater as it percolates through it and also holds it in the aquifers. This quality of soil not only serves as a source of ground water when needed but also protects us from floods and in time of droughts.

Interestingly, it has been observed that many animals deliberately ingest soil as it absorbs toxins, and facilitates digestion and checks diarrhea and it is also a source of rare minerals. The practice of eating soil is referred as Geophagy

Soil protection and conservation is very important since soil is the home for many organisms living in such interconnected harmonious diversity both inside and outside the soil like earthworms, snails, slugs, millipedes, centipedes, potworms, nematodes, bacteria, fungi and algae, to name a few. Aerobic processes of soil have a major role to play in waste management while handling effluent from septic tanks and elsewhere.

It is estimated that soil restoration will offset the effect of increases in greenhouse gas emissions and slow global warming. About 60% of the biotic content is carbon and this makes the Biological component of soil is very important. Even in desert crust, cyanobacteria which are microorganisms related to the bacteria but capable of photosynthesis, lichens and mosses capture and hold a good quantity of carbon by photosynthesis.

When we talk about water conservation, it is intricately tied to soil conservation as well. Preventing the erosion of soil through water conservation, rain water harvesting, green-belt development, responsible agricultural and industrial practices therefore has many inter-linked benefits – from improved soil quality to reducing the effects of global warming.

Geophysical Survey : An essential tool for Rain Water Harvesting

The main reason for a rapid decline in the water table can be attributed to our ever-increasing exploitation of ground water resources for meeting the growing water demands of agriculture, domestic and industrial purposes. Increasing urbanization and industrialization have also resulted in ground water pollution causing adverse effects on the health, environment and imbalance in the ecosystem.

The basic purpose of artificial recharge of groundwater is to replenish water into aquifers that have been depleted due to excessive ground water extraction.

Artificial groundwater replenishment systems involve techniques that modify the natural movement of surface water and utilize suitable civil construction techniques in order to address issues such as:

  • Enhancement of the sustainable yield in areas where over-development has depleted the underground aquifers. Storage and conservation of excess surface water for evolving future requirements
  • Improvement in the quality of existing ground water through dilution.
  • Avoiding flooding of roads during storm showers by capturing the rainfall run-off which would otherwise overwhelm sewer or storm drains.
  • Help reduce soil erosion and flood hazard.
  • Provide an eco-friendly method of water resource conservation.

In this post we discuss the importance of performing a Geophysical Survey of an area targeted for ground water recharge or extraction. We have presented sample data from our records showing variations in soil formation with changes in location and how a Geophysical Survey can help in planning & designing structures for groundwater extraction and artificial ground water recharge.

GEOPHYSICAL SURVEY 

In order to know the ground water condition , thickness of alluvium, depth to rock, the extent and depth of aquifer and quality of ground water, Geophysical Vertical Electrical Soundings are conducted in the specific areas. The results of these investigations and their interpretation forms the basis of identification of sites for construction of rain water harvesting structures.

Objectives of a Geophysical Survey:

  • Determine the depth and thickness of saturated aquifer zones.
  • Quantify the expected alluvium thickness and depth to rock at the sites.
  • Determine the variation of quality of water with depth.
A Geophysical Survey In Progress
A Geophysical Survey In Progress

Investigation Methodology

The Geoelectrical resistivity technique uses an artificial current source whereby a low frequency (4HZ) current is used. Controlled amount of current is introduced into the ground through current electrodes and measurements are carried out with the help of potential electrodes.

The current and potential electrodes are placed in various configurations, but the most extensively used electrode configuration for subsurface investigation is the Schlumberger Configuration. In this configuration the four electrodes are placed symmetrically along a straight line, the current electrodes are on the outside and the potential electrodes on the inside along the array.

With this configuration, to change the depth range of the measurements, the current electrodes are displaced outward. When the ratio of the distance between the current electrodes to that between the potential electrodes becomes too large i.e. more than 5 times, the potential electrodes must also be displaced outward, otherwise the potential difference becomes too small to be measured with sufficient accuracy.

In the Schlumberger Configuration, the apparent resistivity (Pa) is calculated by the formula.

Where, Pa is apparent resistivity.
L is half of the distance between current electrodes.
l is the half of the distance between potential electrodes.
π is constant,
∆v is potential difference and
I is amount of current.
K is a constant known as Geo-electric factor and based on the type of electrodes configuration.

Geophysical measurements are based on the assumption that the subsurface consist of a sequence of distinct layers of finite thickness, each of these layers is assumed to be electrically homogeneous and isotropic and the boundary planes between subsequent layers are assumed to be horizontal.

The resistivity data are interpreted using the Schlumberger Sounding Data Processing and Interpretation Programme.

For ascertaining ground water levels, the resistivity response depends primarily on the amount of impregnating water, the conductivity and quality of water and manner in which water is distributed. The first two factors have a nearly linear relation with the resistivity while the influence of the third factor is more complicated and depends on the nature of aquifer material.

In summary, it can be stated that a dry soil formations, whether porous or non porous are practically poor conductors and hence the resistivity will vary with amount of pores and quality of water. The chemical quality of ground water corresponds with aquifer resistivity.

 LIMITATIONS:

There are certain inherent limitations in estimating water withdrawal levels and lithology, which are being mentioned as under:-

1.  Scientific and human error possibilities tothe extent of 15% cannot be ruled out both in quantitative and qualitativeassessment of ground water.

 2.Increased ground water development activity in and around the area investigatedmay also effect rate of subsurface flow of water in the area in the years tocome.

Thus, any recommendations for construction of structures (Tube- well/ Dug cum bore well) for ground water development are based on indirect science of Geophysical investigations.

Presented below are data from 3 different sites to show different soil formations calling for appropriate modifications in rain water harvesting designs.

Why do some Rain Water Harvesting systems fail?

When water is not absorbed by a Rain Water Harvesting (RWH) system and it stagnates around the structures or if the structure itself collapses, we consider it as a failure of the RWH system.

There are innumerable designs and ideas floated on the internet for Rain Water Harvesting. We evaluated several designs and have found that many of these have inherent shortcomings which can cause RWH systems to fail. We observed that these designs have not been properly tested in real-world scenarios over a period of several monsoons or their designers simply do not report failures.

Through our decades of experience designing & developing Rain Water Harvesting solutions, we recognize the multitude of factors which can lead to failures of Rain Water Harvesting structures.

The idea of this post is not to comment on the design of any individual person or entity but to select a few basic reasons which can defeat the entire RWH effort.

To illustrate these scenarios, we have randomly picked up two designs for Rain Water Harvesting structures commonly circulated on the internet, which in our experience are not successful in practical applications.

a. RWH systems using hollow cement rings with slots

This design proposes using hollow cement rings with holes to construct a Rain Water Harvesting bore up to a depth of 10 feet to 30 feet, with a diameter of about 3 feet approximately. The hollow cavity of the bore stacks cement rings of diameter 2.5 to 3 feet over each other from the base till the top. The top is covered with a slab as you see in the pictures below

Hollow cement rings with slots
Hollow cement rings with slots

Expectation:  The designers of such a system imagine that the rain water will fill the hollow space in the pit and the water will pass through the slots in the cement rings and when it comes in contact with the soil surrounding the cement rings, the water will be absorbed by the soil.

Reality:  In practice, while the rain water fills the empty space in the pitit thereafter moves out of the holes in the cement rings and forms a watercolumn outside the rings. Now as the soil which has come in contact with thewater dissolves in this water column, this mixture of mud and clay re-entersthe hollow space and tries to fill the hollow space with soil.

This processcontinues as and when the remaining empty space is filled with water. Thisprocess stops once the empty space in the rings is completely filled andcompacted by the adjoining soil.

Collapsed Hollow Cement Ring Based RWH Structure
Collapsed Hollow Cement Ring Based RWH Structure

At this stage,any chance of ground water recharge stops due to compaction of soil. The soilwhich has moved in through the holes in cement rings leaves a hollow space adjoiningthe structure causing it to eventually collapse around the structure as isvisible in the photo above.

This fundamentally flawed design increasesthe risk of soil shifting and structural damage to nearby constructions.

b. RWH Systems using percolation Bore-Pits

This design proposes creation of percolation bore-pits for of rain water absorption through creation of 15 to 30 feet deep bores where the top end is enclosed in a 2 x 3 feet deep bore-pit covered with a perforated RCC slab as shown in the image below.

Percolation Bore Pit Design
Standard Percolation Bore-Pit Design

Expectation: The designers here imagine that the flowingrain water over paved surfaces (such as streets or parking lots) will effortlesslyfall into the chamber through the perforated RCC cover slab and thereaftersettle around the bore-pit to eventually be absorbed by the bore.

Reality: Having observed a practical implementation of this design on one of our sites, we recognized that when water flows over a perforated slab it forms a film of water and thereafter most of the water flows and passes over. Some rain water laden with silt and clay enters the perforation and deposits in the pores in the shape of a cone further reducing the opening size of the hole on the inside of the slab and finally the hole gets completely blocked.

Clogged Percolation Covers With Silt
Perforated cover slab choked by silt

Further, below the cover in the bore-pit in the silt-removal chamber:

Expectation: It is imaginedthat the flowing rain water that reaches the silt removing chamber will gentlyfall on the coarse sand and all the silt and clay suspended in the rain waterwill be restricted by the coarse sand from moving further down. After this onlyclean water will pass through the pebbles and enter the recharge pit pushingthe air out through the air vent and letting the soil absorb the water.

Reality: During rainfallthere is massive turbulence in water leading to the water appearing muddy andalso there is no time for decantation – silt mixed with clay and small pebbles flowswith rain water and fill the chamber to the brim as intake speed of anyrecharge structure is relatively slow.

The turbulence in the chamber also disturbs and suspend the coarse sand inthe water. This water with suspended silt, clay, coarse sand and small pebbles formsa paste which tries to find a passage into the air vent chokes the air ventpipeshown below the perforated cover within the chamber.

Once the air-vent pipe gets choked, the recharge process completely stopssince choking of the air vent pipe is like somebody closing a pipette with athumb to stop the water column from falling down.

Other factors like the shallow depth of pit, chances of choking of recharge bore pit by infiltrating coarse sand mixed with silt and clay entering with rain water into the bore hole as well through the pebbles, the unmentioned size of pebbles and unclear relevance of the diameter of the pit are likely to jeopardize the performance of this structure design.

The success of SILVERON rain water harvesting designs is a result of decades of untiring experimentation and experience in field work across diverse topographies.

At SILVERON, Learning & Innovation are the heart of what we do.

Using roads as catchment for ground water recharge

Every monsoon we see familiar scenes of flooding in our towns and cities overwhelming our urban infrastructure, inundating our roads and highways with huge quantities of water for days. On the other hand, for remaining months of the year our cities have to fight a never-ending battle against water scarcity and depleting water tables.

This imbalance is a man-made crisis caused by concretization of surfaces, filling up of ponds and lakes for urban land-use, increasing density of building infrastructure and reduction of green spaces around our cities.

How can we address this imbalance?

A good starting point is to re-phrase the problem of flooding in our cities. Rather than consider it only as a storm-water drainage issue we should consider using our roads and highways as water catchment areas for ground water recharge.

The massiveness of the roads & highway infrastructure as rain water catchment area would be astonishing once we take into account the length and breadth of this catchment. While water scarcity is looming large in most parts of India we have comfortably preferred to ignore the hundreds of thousands of square kilometers of ready in-hand catchment.

Using our urban infrastructure to also help recharge our ground water reserves is a sustainable approach with many benefits however, it requires careful study to be designed and implemented correctly.

Some people are of the opinion that rain water falling on the roads should not be recharged since there are contaminants like rubber remnants from friction of tires and oil spillage on roads due to vehicular traffic.

Surface contaminants from vehicular traffic
Surface contaminants from vehicular traffic

Some engineers avoid rain water harvesting beside the road due to fear of road collapse due to shifting of soil.

A road cave-in during the monsoons
A road cave-in during the monsoons

Some people wonder if it is even possible to hold back, guide and recharge the rain water falling on roads and highways.

Typical water-logging during monsoon rains

To begin with, we at SILVERON firmly believe that every drop of rain water must be prevented from running off long distances on the road, must be prevented from evaporation and must be recharged into the ground close to where it falls.

There are surely some contaminants on the roads but most of them are not water soluble and also during the rains the dilution levels are extremely high hence we should not lose out on this opportunity.

Recharge of rain water along these highways also support the idea of recharging rain water where ever it falls thus benefiting the entire area at large.

It must be underscored that recharging rain water close to a road is a highly specialized work since there is a risk of shifting of soil from under the road into the rain water harvesting structure leading to development of hollow space below the road which may not be visible at the first instance but may cause caving over time creating risk for commuters.

We at SILVERON have years of experience in building rain water harvesting and ground water recharge structures that are designed to perform alongside roads. Through are experience, we have following suggestions to offer:

  • Highways should have a proper slope on both sides from the center for water to immediately flow towards the edge of the road. This will not only prevent the road from damage but will put the water into shallow storm water drain running along road’s edges.
  • Storm water drain should have baffle walls a regular intervals. and these drains may not be covered and instead filled with 40 mm gravel to prevent any paper trash, poly bag, cloth etc. from chocking the drain while allowing the water to easily enter it.
  • A SILVERON design recharge shaft should be constructed on the outside of the drain preferably between two baffle walls and connected to the drain .
  • The recharge shaft design has to be modified such that the water from the drain is released into the recharge shaft bore sufficiently below the ground level so that it can percolate deeper into the ground. This will not disturb the compaction of the road.
  • Restaurants, shops or petrol pumps abutting the highways should ensure that they put slabs to protect these drains from getting clogged with sand or trash.

SILVERON designed catchment systems collect rain water run off from the road into the storm water drains. The gravel in these drains filters the water and prevents trash like polythene bags, paper etc from chocking the drain. Rain water percolates into the drain and moves through the connecting pipes to be recharged by the recharge shaft.

When conserving rain water, we just need to have the will that creates the way.