Groundwater degradation
Dec 29, 2022
By: Omri Nissani, Civil Engineer, Licensed Insurance AgentFirst Article in the Series
"You will draw water with joy from the springs of salvation" (Isaiah 12:3)
Groundwater lowering is a common practice in Israel that refers to the artificial regulation of the groundwater table at construction sites. There are several techniques for implementing groundwater lowering, the main ones being:
Pumping groundwater and discharging it through municipal drainage or sewage systems.
Permeation drilling, which diverts groundwater to another area of the site (either a deeper aquifer or a sub-aquifer).
Groundwater lowering is required based on a preliminary geotechnical survey and is common in areas known to be prone to high water tables, especially when the project involves underground work and there is a risk that the water table will rise above the work area. As the name implies, groundwater lowering is intended to lower the water table beneath the construction line, ensuring a safe and dry working environment.
From: Omri Nissany, Civil Engineer, Licensed Insurance Agent
First article in a series
"And you shall draw water in a pitcher from the springs of salvation" (Isaiah 12:3).
Groundwater depletion is a common practice in Israel that refers to the artificial regulation of groundwater levels at construction sites. There are several techniques for performing groundwater depletion, the main ones being:
· Pumping groundwater and disposing of it based on municipal drainage or sewage systems.
· Penetration drilling aimed at removing groundwater to another area of the plot (to a deeper aquifer or to a subaquifer).
Groundwater lowering is required in accordance with a preliminary soil survey and is common in areas known to be "prone to disaster" due to being saturated with groundwater. Especially when the project includes underground works and there is a concern that the groundwater level will rise above the work area. Groundwater lowering, as the name suggests, is intended to lower the water and lower its level below the line of the works.
The preliminary survey
Any real estate project that involves excavations in the ground (for example, for basements, underground parking, etc.) requires the developer to perform trial drilling, obtain an opinion from a land consultant, and, if necessary, an opinion from a hydrologist. This method is intended, among other things, to determine what the water level is on the site and whether there is no escape from carrying out groundwater depletion.
As a result, the relevant execution methods are examined. It is generally recommended to perform tests close to the excavation date in order to prevent changes that may occur in the groundwater level.
The preliminary survey for planning the implementation method for groundwater dewatering on the site is derived from an in-depth (literally) analysis of data such as the type of soil, the level of soil conductivity, the desired depth for dewatering, and the radius of the dewatering.
During the preliminary tests, as mentioned, trial drilling and laboratory tests are carried out to monitor the quality of the water. It should be noted that the groundwater level varies according to the seasons of the year and in some cases there can even be gaps of up to 2 meters in the level between summer and winter. Since construction is not done in one day, seasonal changes in the level must be addressed with a forward-looking perspective .
The drilling also includes the removal of impurities in the water and testing for salt concentration. These are parameters that can have an impact on the cost of groundwater degradation, as the lower the quality of the water, the lower the payment rate to the authority (up to a complete exemption).
Any real estate project that involves excavation (for example, for basements, underground parking, or similar) requires the developer to conduct trial drilling, obtain a geotechnical engineer’s report, and, if necessary, a hydrologist’s assessment. This process is intended, among other things, to determine the water table on the site and whether groundwater lowering is unavoidable.
As a result, the relevant execution methods are evaluated. It is generally recommended to perform these tests close to the excavation date to prevent changes that may occur in the groundwater table over time.
The preliminary survey for planning the groundwater lowering method at the site is based on a thorough analysis of data such as soil type, soil conductivity, desired lowering depth, and the radius of lowering.
During these preliminary tests, trial drilling and laboratory analyses are conducted to assess water quality. It should be noted that the groundwater table fluctuates seasonally, and in some cases, differences of up to 2 meters can occur between summer and winter. Since construction does not occur in a single day, seasonal changes in the water table must be taken into account with a forward-looking perspective.
The drilling also includes testing for contamination and measuring salt concentrations. These parameters can affect the cost of groundwater lowering, as lower water quality may reduce the fee payable to the relevant authority—even to the point of full exemption in some cases.
Any pumping of groundwater is subject to Israeli water law to obtaining approval from the Water Authority. The approval process involves many other entities beyond the operating contractor and the company specializing in groundwater depletion. These include the local authority, the Ministry of Environmental Protection, water corporations, and more.
Often, the local authority will not allow a building permit to be obtained without approval from the Water Authority regarding the relevant solution to groundwater degradation on the site.
We must understand that the current trend among local authorities in Israel is to minimize the impact on groundwater, and in particular, pumping rates for the benefit of groundwater depletion.
The guiding principle is an environmental concept that strives to maintain the aquifer in a natural water balance. Similarly, the authorities seek to prevent a burden on water infrastructure and reduce the damage that may be caused to nearby structures. It is known that groundwater depletion in dense urban areas may lead to high fluctuations in the level to the point of soil erosion, foundation undermining, and more.
On the other hand, given the fact that the amount of land in Israel is limited and that much real estate activity is carried out in the central region, which is located above the coastal aquifer, groundwater depletion is not uncommon. Quite the contrary.
After the developer has made the decision regarding the method of execution of the groundwater depletion, he must submit a professional opinion to the Water Authority that includes a lot of data, such as an estimate of pumping quantities, a current measurement of the groundwater level, an estimate of the change in the level following the pumping, recommendations regarding the method of water disposal, results of analytical tests for the water, and more.
Regulation and the environmental approach of local authorities
· Deep wells – groundwater depletion by pumping from deep wells based on deep drilling. Submersible pumps are lowered and inserted into the base of the well, while the well is lined with pipes. This method involves drilling using a dedicated machine while preventing collapse by polymer solutions. The wells are located on both sides of the excavation, with precise planning of the distance between them to ensure a controlled lowering of the groundwater level.
· Shallow wells – a groundwater depletion technique in which the groundwater level is drained for localized depletion according to a planned radius of influence around a suction body based on the strength of the suction. The method involves a sequence of "pins" (suction bodies) and overlapping radii of influence.
Main methods for performing groundwater degradation
Key considerations in groundwater depletion planning – between geology and budget
Like any engineering task, groundwater depletion depends on a wide range of planning considerations. Some stem from geo-hydrological conditions and some are related to commercial-business aspects such as the needs of the project being executed, budget considerations, regulatory conditions, execution methods, and more.
With regard to geo-hydrological aspects, it is important to note that the groundwater level is a variable and dynamic level that is influenced by seawater, rainfall, adjacent and previous groundwater depressions, etc. Similarly, great emphasis is placed on the type of soil, as clay soil has low hydraulic conductivity relative to sandy soil.
The needs of the project impact the groundwater level at various points, such as ground stabilization, the extent of underground construction, shaft excavation, etc. The drilling depth is derived from the planned structure and, as a result, defines the lining requirements, the extent of pumping, and sealing.
Entrepreneurs often decide on the execution method based on economic costs, and in some cases, excessively high costs may make economic viability unnecessary in the first place.
To read the second article in the series, click the link
To read the third article in the series, click the link
To read the fourth article in the series Link
Similar to any engineering task, groundwater lowering depends on a wide range of planning considerations. Some arise from geo-hydrological conditions, while others relate to commercial and business aspects, such as the project’s requirements, budget considerations, regulatory conditions, execution methods, and more.
Regarding geo-hydrological aspects, it is important to note that the groundwater table is dynamic and variable, influenced by seawater levels, rainfall, previous or nearby groundwater lowering, and similar factors. Soil type is also a key consideration: clay soils, for example, have lower hydraulic conductivity than sandy soils.
Project requirements affect groundwater lowering in various ways, including soil stabilization, the scope of underground construction, shaft excavation, and more. The drilling depth is determined by the planned structure and, consequently, defines the shoring requirements, pumping volume, and waterproofing measures.
Often, developers decide on the method of execution based on economic costs, and in some cases, excessively high costs can make the project economically unfeasible from the outset.
Read the rest of the series:
[Second article in the series – link]
[Third article in the series – link]
[Fourth article in the series – link]
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