Rainwater that falls on impervious surfaces has to go somewhere. In densely populated urban areas, where sewer systems are reaching their capacity limits and natural infiltration areas are lacking, decentralized stormwater management is becoming increasingly important. In this context, shaft infiltration is one of the most technically mature methods: It directs stormwater through a specially designed shaft directly into permeable soil layers, relieves the sewer system, and closes the natural water cycle in a very small space. Those who understand how this technology works, its requirements, and its limitations can implement it effectively and in compliance with regulations.
- What a shaft infiltration system is and how it differs from other infiltration systems
- What hydrogeological requirements must be met at the site
- How a shaft infiltration system is technically constructed and what components it comprises
- Which standards, regulations, and official requirements apply
- When a shaft infiltration system is appropriate and when other forms of infiltration are preferable
- How the design process works and which parameters are critical
- What errors frequently occur during planning, construction, and operation
- How maintenance and inspection ensure long-term functionality
What Is a Percolation Well: Definition and Classification
A soakaway, also referred to as a drainage shaft in technical literature, is a system for the decentralized infiltration of stormwater, in which runoff is directed through a vertical, tubular shaft into deeper, permeable soil layers. Unlike surface infiltration, which distributes water over a wide area across the ground surface, or basin infiltration, which temporarily stores water in a shallow depression in the terrain and allows it to slowly infiltrate, the infiltration shaft works specifically at depth. It thus bypasses near-surface, low-permeability layers and taps into deeper soil horizons with higher hydraulic conductivity.
Systematically, shaft infiltration is classified as a decentralized stormwater management system, which is grouped under the term “stormwater infiltration” in German planning law and water management. The overarching goal is to approximate the natural water cycle: precipitation should infiltrate as close as possible to where it falls, rather than being discharged via pipes into receiving waters or wastewater treatment plants. Thus, shaft infiltration is a tool of the “sponge city” strategy—that is, the approach of making cities more resilient to heavy rainfall events and dry periods through decentralized water retention and infiltration.
Distinguishing it from related systems is essential for planning. The infiltration trench—a ditch filled with gravel or plastic blocks—distributes water horizontally into the ground and is suitable for larger areas with uniformly permeable subsoil. The infiltration shaft, on the other hand, is designed for localized discharge into deeper layers and is particularly suitable when the upper soil layers are not permeable enough to absorb sufficient water, but deeper layers exhibit suitable infiltration properties. This distinction is not merely theoretical but is crucial for selecting the right system for a specific site.
Hydrogeological Requirements: What the Soil Must Provide
No infiltration system works without a suitable subsoil. For infiltration shafts, the hydraulic permeability of the soil—expressed by the permeability coefficient kf—is the key parameter. The kf value describes how quickly water flows through the soil and is expressed in meters per second. According to worksheet DWA-A 138, the authoritative German standard for the planning and design of stormwater infiltration systems, certain limit values apply to infiltration systems: The kf value should be in the range of approximately 1 × 10⁻⁶ to 1 × 10⁻³ m/s. Soils with lower values allow water to infiltrate too slowly to enable economically viable systems; soils with very high values, such as pure gravel or coarse sand, allow water to infiltrate quickly but offer little filtration and can pose a risk to groundwater if the water is not adequately pretreated.
Therefore, before planning any infiltration well system, a site investigation must be conducted to determine the stratigraphic structure of the subsoil and the kf values of the relevant horizons. Methods for this include sieve analyses, field infiltration tests, and the evaluation of borehole profiles. It is particularly important to know the highest expected groundwater level: A sufficient distance must be maintained between the bottom edge of the infiltration shaft and the maximum groundwater table; otherwise, the infiltrating water will reach the aquifer directly and unfiltered. The DWA-A 138 standard specifies minimum distances for this purpose, which vary depending on soil conditions and the intended use of the facility.
Soils with a high clay content, compacted backfill, anthropogenic deposits, or sites with known contaminated sites are generally unsuitable for infiltration shafts or require extensive special investigations. The geological conditions also play a role: Karst areas, where underground cavities exist, are problematic for infiltration systems because water can enter deep aquifers uncontrolled. In such cases, consultation with the responsible water authority is essential before a shaft infiltration system is even considered.
Technical Design: Components and Construction Principles
At its core, an infiltration shaft consists of a vertical pipe or shaft element made of concrete, plastic, or reinforced concrete that is installed in the ground and whose walls are perforated or slotted. Through these openings, the water introduced into the shaft seeps laterally and downward into the surrounding soil. The shaft is typically surrounded by a gravel filter pack, which protects the slits from silting up and improves the hydraulic connection to the soil. This filter gravel bed is encased in a geotextile—a water-permeable nonwoven fabric—that retains fine particles from the surrounding soil and prevents fine material from entering the gravel.
At the top of the shaft is an inlet through which stormwater is fed from the drainage pipe. A prefilter or sediment trap is typically installed upstream of the inlet to trap coarse solids, leaves, and sediments before the water enters the shaft. This prefilter is crucial for the long-term operation of the system: Without it, fine particles would clog the shaft and the surrounding filter gravel within a few years, drastically reducing the infiltration capacity. The shaft itself is accessible via an inspection cover, which allows for inspection and maintenance.
The depth of an infiltration shaft depends on the stratigraphic structure of the subsoil and the required infiltration volume. Typical depths range from two to six meters, and in some cases may be deeper if the permeable layer begins at a greater depth. The diameter varies depending on the manufacturer and design, but is often between one and two meters. For larger catchment areas, multiple infiltration wells are connected in series or in parallel to achieve the required infiltration capacity. The hydraulic connection between multiple wells is established via connecting pipes that allow water to overflow from one well to the next.
A key design feature is the emergency overflow line. It activates when the infiltration capacity of the shaft is exceeded during a heavy rain event and the water level in the shaft rises to a defined level. The emergency overflow discharges the excess water in a controlled manner into the sewer system or another drainage outlet, preventing uncontrolled water discharge onto the ground surface. This safety feature is not optional in terms of design and construction; rather, it is an integral part of a system that complies with regulations.
Standards, Regulations, and Official Requirements
The planning and construction of infiltration systems in Germany are governed by a multi-tiered regulatory framework. The central technical regulation is Worksheet DWA-A 138 of the German Association for Water, Wastewater, and Waste (DWA), which contains planning and design principles for stormwater infiltration systems. It defines requirements for site suitability, pretreatment of the infiltrating water, design, construction, and operation. In addition, there are fact sheets and guidelines issued by the federal states that specify state-specific regulations and requirements, as water law in Germany is a matter for the federal states.
Under water law, the infiltration of stormwater is generally not subject to a permit in most federal states, provided certain conditions are met: The areas into which the water is discharged must be considered to be only slightly contaminated; this typically includes roof surfaces without problematic coatings or street drains without elevated levels of pollutants. For areas with a higher potential for contamination, such as heavily trafficked parking lots, gas station forecourts, or industrial sites where chemicals are used, further pretreatment of the water is required before it can be allowed to infiltrate. In such cases, a permit under water law must be obtained, and the responsible local water authority must be involved in the planning process at an early stage.
In addition to water law, building code requirements must be observed. In some zoning plans, decentralized stormwater management is explicitly mandated or encouraged; in other cases, development agreements or municipal drainage ordinances specify what proportion of stormwater must be retained on the property and allowed to infiltrate. Planners must be familiar with these requirements and integrate them into the design phase. Coordination with the relevant authorities—the civil engineering office, the water authority, and, if applicable, the public health department—is part of the regular planning process and is not a bureaucratic obstacle, but rather an expression of the principle of protecting soil and groundwater.
Design: How to Properly Size a Soakaway
The design of a infiltration well follows a hydraulic calculation approach that links the relevant input parameters: the connected catchment area, the design rainfall runoff, the hydraulic permeability of the subsoil, and the required storage volume of the well. The starting point is the precipitation intensity, which is derived from statistical analyses for the design rainfall at the site. In Germany, the Kostra data from the German Weather Service (DWD) serve as the authoritative basis for these precipitation statistics; they provide rainfall amounts for each location for various return periods and duration classes.
The infiltration capacity of the shaft is determined by the wetted surface area of the shaft wall and the kf value of the surrounding soil. The larger the perforated area and the higher the permeability, the more water the shaft can absorb per unit of time. The storage volume of the shaft buffers peak runoff that temporarily exceeds the infiltration capacity and releases the water into the ground at a controlled rate. Design in accordance with DWA-A 138 ensures that the shaft does not overflow even under the selected design rainfall without activating the emergency overflow. For residential properties, a return period of five years is often selected; for public areas or facilities with increased operational safety requirements, longer intervals may be mandated.
A common mistake in practice is undersizing the pre-filter or neglecting the sedimentation chamber within the manhole. If the manhole is hydraulically correctly sized but insufficient volume has been provided for the deposition of fine particles, it will silt up faster than planned and lose its function. Proper design therefore takes into account not only the hydraulic performance verification but also the maintenance intervals and the sedimentation behavior of the connected catchment area.
Typical Applications and Limitations of Shaft Infiltration
Pit infiltration is particularly suitable for sites where the upper soil layers are poorly permeable, but deeper layers exhibit good infiltration properties. Typical applications include single-family home lots with roof areas serving as catchment areas, commercial areas with large roof areas and limited land area, as well as infill development projects in existing neighborhoods where there is insufficient space for trough or surface infiltration. Particularly in urban infill development—one of the central themes of current urban development—well infiltration is often the only practical option for retaining stormwater on the property.
Well infiltration has its limitations where the subsoil is fundamentally unsuitable, where there is suspicion of contaminated sites, or where the groundwater table is so high that a sufficient distance from the bottom of the well cannot be maintained. Furthermore, in areas with heavily contaminated catchment zones—such as streets with high levels of heavy metals or hydrocarbons—soakaway systems are not permitted without extensive pretreatment. In such cases, alternative systems such as soil retention filters, activated soil zones, or cisterns with restricted discharge into the sewer system should be considered. Pit infiltration is a powerful tool, but not a universal solution: its suitability must be assessed on a site-specific basis and cannot be assumed across the board.
Compared to trough infiltration, shaft infiltration has the advantage of requiring little surface area. A infiltration shaft with a diameter of one meter takes up hardly any space at ground level and can be integrated beneath paved areas, lawns, or plantings. This advantage is significant in practice because land is scarce and expensive in urban areas. At the same time, compared to a trough, a soakaway is less visible and therefore less useful as a design element: it is a piece of technical infrastructure, not a green space with added benefits for biodiversity or quality of life.
Maintenance, Inspection, and Common Operational Issues
A shaft infiltration system is not a maintenance-free system. Its long-term functionality depends crucially on the regular removal of sediment, leaves, and fine particles from the pre-filter and the shaft. Experts recommend an inspection at least once a year, and on an as-needed basis following heavy rainfall events. During the inspection, the fill level of the sludge trap, the condition of the geotextile, the permeability of the filter gravel bed, and the function of the emergency overflow are checked. Pits that have not been maintained for years often exhibit significantly reduced infiltration capacity because the gravel bed has become clogged with fine material.
A common design flaw is the lack of or insufficient pretreatment of the inflowing water. When rainwater from green roofs, gravel surfaces, or paths is directed into the manhole directly without a prefilter, fine particles, organic material, and sediments enter the filter gravel bed unfiltered. The result is accelerated silting, which, in the worst case, can lead to the system becoming completely inoperable within a few years. The investment in a high-quality pre-filter with sufficient sedimentation space pays for itself through extended maintenance intervals and a longer service life for the system.
Installation errors often involve the geotextile: If it is laid incorrectly, damaged, or installed with insufficient overlap, fine material from the surrounding soil migrates into the filter gravel and causes silting from the outside. Choosing an unsuitable geotextile—such as a nonwoven fabric that is too dense and impedes water permeability—can also reduce infiltration performance from the outset. Proper installation therefore requires careful material selection and construction supervision that documents the correct installation conditions.
Pit Infiltration in the Context of Stormwater Management and Urban Development
The infiltration shaft is not an isolated technical component but rather part of a larger system. In integrated stormwater management, it is often combined with other elements: Green roofs reduce runoff and improve the water quality of the runoff, swales distribute water over a wide area, retention basins buffer peak flows, and cisterns enable the use of rainwater for irrigation or toilet flushing. Within this system, shaft infiltration serves the function of deep infiltration when near-surface methods are insufficient or not feasible.
In the context of climate adaptation, decentralized stormwater management is gaining strategic importance. More frequent and intense heavy rainfall events overload combined sewer systems and lead to flooding in urban areas. At the same time, longer dry spells cause groundwater levels to drop, which puts strain on urban trees, groundwater-dependent ecosystems, and the drinking water supply. Infiltration wells contribute to groundwater recharge and are thus not only a tool for reducing runoff but also for water conservation. This dual function makes them a key component of the “Sponge City” strategy, which is increasingly being established as a guiding principle in German urban planning.
For landscape architects, urban planners, and open-space planners, this means that infiltration pits are not a purely technical issue that can be delegated to civil engineers. It is part of open-space planning because its location, its integration into pavement structures, and its connection to vegetation and surface drainage require planning decisions that shape the overall appearance of the outdoor space. Those who incorporate infiltration wells into open-space planning from the outset, rather than retrofitting them into finished designs, achieve better technical results and more design-coherent solutions. The quality of stormwater management is not determined by the infiltration well itself, but by the interaction of all surfaces, pipes, and infiltration elements that drain a property or neighborhood.
