Many civil engineering or smaller projects involve excavations below groundwater level. Dewatering is a term to describe the removal of groundwater from within a soil material and is carried out to ensure excavations are undertaken in dry and stable conditions. The risk of not controlling groundwater can have catastrophic effects by groundwater ingress and instability, or excessive groundwater pore pressures resulting in heave or quick conditions.
Construction Dewatering is a common term used to describe the temporary lowering of groundwater level within an aquifer or depressurizing a sub-artesian or artesian head.
Permanent Dewatering covers the installation of continuing groundwater control system. Ground conditions and objectives dictate dewatering requirements and appropriate method can be determined by pumping testing’s. The dewatering mechanism can encompass gravity drainage such as deep wells using submersible borehole pumps or applying a vacuum to a soil material using ejectors or vacuum well point systems.
A deep well typically consists of a borehole fitted with a slotted liner and an electric submersible pump. As water is pumped from a deep well, a hydraulic gradient is formed and water flows into the well forming a cone of depression around the well in which there is little or no water remaining in the pore spaces of the surrounding soil.
Deep wells work best in soils with specific permeability profiles with the amount of drawdown that a well can achieve is limited only by the size of the pump.
Deep Wells are typically drilled using cable percussive or rotary drilling methods, terminating typically in a 300mm or 250mm bore.
Well pointing or vacuum well points are a very versatile and economic method of groundwater control and have an advantage of being installed relatively quickly and at close centers are very effective in fine grained or stratified soils.
Well points consist of small diameter tubes with a filter intake at the bottom, they are typically installed around the perimeter of an excavation or parallel to a pipeline by jetting or auguring techniques. Vacuum and flow is generated by a dewatering pump, which in turn limits drawdown to ~5-6m depth. For greater depths a second stage well pointing system can be installed at a lower level.
Ejectors or Educators are very effective in soils of low to medium permeability where the pumping water level is too deep for well points but yields too small to allow the use of a electric submersible borehole pump.
Ejectors are commonly used up to depths of 50m, can run at times of zero water entry into the well and if sealed can generate a vacuum in the well to increase efficiency.
Dewatering, Groundwater Control, Groundwater Remediation, Pumping Tests & Site Investigations, Landfill Leachate Control, Geothermal
Well points are essentially shallow wells, which offer an economic and versatile method of groundwater control. Their close spacing and use of vacuum is particularly effective in stratified and fine graded soils.
Well points consist of small-diameter tubes with slots near the bottom that are inserted into the ground from which water is drawn by a vacuum generated by a dewatering pump.
Well points are typically installed at close centers in a line along or around the edge of an excavation. As a vacuum is limited to 0 bar, the height to which water can be drawn is limited to approximately 6 meters.
To enable functionality at greater depths, well points can be installed in stages, with the first stage reducing the water level by up to five meters, and a second stage, installed at a lower level, lowering the water level further.
Basement dewatering and waterproofing refer to techniques used to remove and prevent water from entering the basement of a house or other commercial building.
Effective below -ground waterproofing will include both drainage and sealers.
Waterproofing is needed anytime a structure is built at ground level or below ground. Waterproofing and drainage considerations are especially needed in cases where ground water is likely to build up in the soil and raise the water table. This higher water table causes hydrostatic pressure to be exerted underneath basement floors and against basement walls.
Hydrostatic pressure forces water in through cracks in foundation walls, through openings caused by expansion and contraction of the footing-foundation wall joint and up through floor cracks. Hydrostatic pressure can cause major structural damage to foundation walls and is likely to contribute to mould, decay and other moisture-related problems.
Methodologies
The three measures developed to prevent this problem differ greatly in ideology and design. The first is interior wall and floor sealing, with the other two methods being interior water drainage and exterior drainage combined with waterproofing coatings.
Interior Sealants
Interior sealers will not provide permanent protection from water infiltration where hydrostatic pressure is present. Interior sealers are good for preventing high atmospheric humidity inside the basement, from absorbing into the porous masonry and to prevent spelling.
Spelling is a condition where constant high humidity or moisture break down masonry surfaces causing deterioration and shedding of the concrete surfaces.
Interior water drainage
Although interior water drainage is not technically waterproofing, it is a widely accepted technique in mitigating basement water and is generally referred to as a basement dewatering solution. They function by draining underground water from alongside the foundation footers and underneath the basement floor.
They then channel it with a French drain, PVC pipe, or through a patented product to a sump pump system, which will then pump the water from the basement. Foundation sump pumps are best installed by a professional dewatering contractor to ensure maximum effectiveness and project reliability. In a simplified model, the following would occur
- Water enters the home via the basement wall/floor joint, through cracks in the foundation walls and/or holes created by faulty or decaying masonry/brick.
- A perimeter trench drain such as a French drain collects the water before it enters into the basement.
- Wall vapor barriers/retarders and drip moldings are used and incorporated into the sub-slab perimeter drain to collect water coming from wall cracks and other foundation wall defects, such as pipe protrusions.
- The drain directs the water to a sump pump.
- The sump pump directs the water out of the house.
Interior basement waterproofing systems should be prepared to work in the case of a power outage, the failure of a sump pump, and in the face of overwhelming torrential rain.
A proper sump pump, backup sump and/or battery backup sump pump should be installed in a large sump pit with an airtight lid for safety and to keep humidity from seeping through to the basement environment, where it can promote mold growth. This airtight practice will also reduce the possibility of dangerous radon gases for entering the living space.
Despite widespread sump pump failures, top-of-the-line sump pump systems are more reliable than ever.
Exterior Waterproofing
Exterior waterproofing prevents water from entering foundation walls therefore preventing the wicking and molding of building materials. Waterproofing a structure from the exterior is the only method the IBC (International Building Code) recognizes as adequate to prevent structural damage caused by water intrusion.
Prior to the 1980′s much of the original exterior waterproofing was actually damp-proofing using a degradable asphalt-based covering. Today, however, Polymer products will completely waterproof an exterior foundation wall. This material has a half life in the thousands of years which make it ideal for a long term exterior waterproofing solution.
Asphalt and tar based compounds are affected by soil pH.And break down after 10-20 years, thus making that type of waterproofing ineffective over time.
Polymer-based compounds
Over the past ten years, polymer-based waterproofing products have been developed. Polymer-based products last for the lifetime of the building, and are not affected by soil pH. Polymer-based waterproofing also has the advantage of a low enough viscosity that it can be sprayed directly onto a wall.
Here groundwater must be lowered further than six meters below ground and especially where the shallow aquifers include low permeability materials such as silts, muds or sands, ejectors are often the best dewatering solution.
For a more detailed explanation of ejector systems, please view the tabbed section Ejector Overview and Glossary.
Project Dewatering has one of the UK’s best drilling capabilities of any groundwater company, through its close association with Drilcorp, the UK’s most successful specialist drilling company, and are able to use a variety of drilling methods to deal with all types of ground conditions.
Pumping test (or aquifer test) is conducted to evaluate an aquifer by stimulating the aquifer through constant pumping, and observing the aquifer’s response (drawdown) in observation wells.
Pump testing is a common tool that hydro geologists use to characterize a system of aquifers, aquitards and flow system boundaries.
A slug test is a variation on the typical pumping test where an instantaneous change (increase or decrease) is made, and the effects are observed in the same well. This is often used in geotechnical or engineering settings to get a quick estimate (minutes instead of days) of the aquifer properties immediately around the well.
Pumping tests are typically interpreted by using an analytical model of aquifer to match the data observed in the real world.
When applied to the design of dewatering systems, a pumping test gives the best information on the drawdown level, flow rates and unforeseen factors generated upon pumping
Groundwater remediation is the process by which water pollution is extracted and filtered.
While groundwater is a readily available source of drinking water, it is also used by farms to irrigate crops and by industries to produce everyday goods.
Most groundwater is clean, but groundwater can become polluted, or contaminated as a result of human activities.
The many and diverse activities of man produce innumerable waste materials and by-products; these are often deposited or stored on land surfaces where by percolation they eventually get carried downward contaminating the underlying groundwater and therefore jeopardizing the natural quality of it.
As a result, contaminated groundwater becomes unsuitable for use.
Using contaminated ground water causes hazards to public health through poisoning or the spread of disease. Contaminants found in ground water cover the entire range of physical, inorganic chemical, organic chemical, bacteriological, and radioactive parameters.
Pollutants and contaminants can be removed from ground water by applying various techniques thereby making it safe for use.
Increasing the cleansing or removal of polluted groundwater is required to enable Civil Engineering and Housing projects to be carried out. The type of groundwater remediation method is dependent on the type of pollution and ground conditions.
Groundwater remediation techniques span biological, chemical, and physical treatment technologies.
Most groundwater treatment techniques utilize a combination of technologies.
Some of the biological treatment techniques include bioaugmentation, bioventing, biosparging, bioslurping, and phytoremediation.
Some chemical treatment techniques include ozone and oxygen gas injection, chemical precipitation, membrane separation, ion exchange, carbon absorption, aqueous chemical oxidation, and surfactant enhanced recovery.
Physical treatment techniques include, but not limited to, pump and treat, air sparing, and dual phase extraction.
Arab dewatering is a leading provider of groundwater control services.
We specialize in the design, installation and maintenance of dewatering systems.
Our technical expertise is underpinned by in-house design capability, comprehensive drilling experience and ongoing research into predictive modeling and groundwater management.
Pressure Relief Wells
Pressure relief wells are a method of controlling excess pore water pressures in confined aquifers.
The artesian or sub-artesian head within a confined aquifer can be dangerous if the overburden pressure is equal or less than that of the confined pore water pressure, and if not dealt with it will result in piping or heaving of the base of the excavation.
There are two basic means of relieving this pressure
1. The use of Passive Pressure Relief Wells ; and
2. The use of active pumping pressure relief wells, which can consist of either deep wells Orwell points.
The recommended solution is to adopt an active pumping pressure relief system.
The reason is that this system gives you greater control over the problem area.
While passive relief systems can be a more economical solution, they can lead to difficult working conditions with groundwater seepage onto the excavation floor, necessitating a progressive sumping arrangement to remedy.
Passive relief systems can also prove difficult to seal effectively upon completion of the project.
A variation of a passive well is sand drains, which is provides a drainage path between two aquifers. Typically, they are used to drain a perched aquifer, which cannot dewater by gravity drainage into a dewatered lower aquifer.
They are particularly effective where the perched water would cause stability problems to an open-cut excavation, or to speed up the consolidation of silts and soft clays.