Deep basements: tackling land constraints through smart design and construction

Deep basements are increasingly common in central business districts, downtown areas and prime high-rise developments where land is both scarce and costly. They are also prevalent in cities with limited parking spaces due to inadequate urban planning, where stricter parking requirements are often imposed on new developments.

As such, it is essential for engineers to understand and highlight the key design and construction considerations at an early stage. Doing so enables them to provide informed advice to clients, including developers, architects, project managers, and contractors. While these considerations are particularly relevant to projects in the Middle East, many are equally applicable to developments worldwide.

Design Considerations

Ground water pressure

In developed urban areas located near the sea, the water table is often encountered close to the surface. As basement depth increases, uplift water pressure rises proportionally, creating both global uplift forces and localised high-pressure effects. Several engineering solutions can be employed to counter these pressures, including increasing the structural mass, installing tension piles, constructing robust retaining walls, and using ground anchors. An alternative approach, more common internationally but rarely adopted in the Middle East, is the installation of a drainage system that activates when rising groundwater triggers it. While effective, this solution is typically considered costly and therefore less favoured in the region.

Concrete durability

Groundwater in the region is often highly saline and typically contains significant concentrations of chlorides and sulphates. This creates two main risks: sulphate attack on the concrete itself and reinforcement corrosion due to chloride penetration. To address these challenges, several protective measures can be employed, including optimising the concrete mix design, ensuring adequate concrete cover, limiting crack widths, applying waterproofing membranes, and, where necessary, incorporating cathodic protection.

Permanent retaining wall design

The permanent retaining wall must be designed to resist a range of loads, including lateral soil pressure, hydrostatic water pressure, surcharge pressure and lateral seismic earth pressures. Furthermore, crack widths in the retaining wall must be limited as per code requirements and local standards to ensure durability and serviceability.

Basement slab diaphragm action

The basement slab plays a critical structural role by acting as a horizontal diaphragm. It restrains the perimeter retaining walls and transfers lateral earth pressures to the building’s lateral stability elements, such as core walls. Key considerations for engineers include the presence of large openings near the retaining walls or stability elements, as well as any slab offsets between these elements, which can compromise load transfer and diaphragm effectiveness.

Foundation design

The foundation solution is typically chosen from three main options: raft foundations, piled foundations, or pile-assisted raft foundations. The latter is often the most effective choice for tall buildings, as it provides additional stability but also requires detailed analysis carried out in close coordination with geotechnical engineers. The decision on which foundation solution to adopt is influenced by several key factors, including the overall height of the building, the depth of the basement, the level of the groundwater table, the allowable bearing pressure of the soil, site boundary conditions, as well as the time and cost implications of the construction process.

Temporary (enabling) works considerations 

Temporary or enabling works are typically assigned to a specialist contractor and cover a defined scope of activities. These generally include site setup and hoarding, site survey, verification of the geotechnical investigation, and dewatering. Dewatering is one of the first considerations once the site has been prepared and surveyed, and it must be carried out before subsurface excavation for shoring and foundations can begin. For deep basements, dewatering often represents a significant running cost, as it is usually required until the middle or even the end of the structural works. To mitigate these costs, some engineers explore alternative foundation solutions, such as piled foundations, which allow the dewatering system to be switched off once the basement structure has been completed.

Temporary shoring wall (diaphragm wall / secant wall)
In the Middle East construction practice, the shoring walls are treated as temporary structures, used only during construction. A second concrete wall, the permanent retaining wall, is then built to resist long-term lateral earth pressures. By contrast, in North American and European practice, the shoring wall is often designed to serve as the permanent wall as well, which reduces material use, time, and cost.

At the concept stage, it is essential to allow for the combined thickness of the temporary shoring wall, construction tolerances, and the permanent retaining wall. This ensures that the permanent wall does not extend beyond the site boundary, which could otherwise affect the design and regulatory compliance. The required thickness of the shoring wall is influenced by several factors: the depth of the basement, the magnitude of earth pressures, site-specific constraints, and the degree of internal or external bracing provided.

Excavation

Site excavation is often a time-consuming element of the enabling works package, with its duration influenced by the area, depth, and accessibility of the basement. Excavation activities must be carefully coordinated with the overall construction methodology for the basement to ensure efficiency and safety.

Temporary bracing

Ground anchoring
The use of ground anchors depends on the soil conditions and the diaphragm wall design. Either a single line or multiple lines of anchors may be required. Since ground anchors typically extend beyond the building footprint, it is essential to consider the implications of temporary works encroaching outside the site boundary. 

To implement a ground anchoring system, anchors must avoid conflicts with existing services and utilities along access roads and waterways. Additionally, approvals from local authorities and No Objection Certificates (NOCs) must be obtained. For adjacent empty plots owned by private developers, temporary anchoring would also require the consent of the plot owners. Where ground anchors cannot be used on one or more sides of the site, internal bracing provides an alternative solution. The layout and location of internal braces depend on the geometry of the basement footprint, construction logistics, and site constraints. Steel members are recommended for internal bracing, rather than concrete, due to the high demobilisation costs associated with concrete systems.

Piling
Pile design is typically a specialist task carried out by the piling contractor. During the design process, structural engineers must make informed assumptions regarding pile layout, diameter, and length to guide foundation design. Close coordination with geotechnical engineers and piling contractors is essential to ensure the piles are appropriately sized for the project conditions.

Instrumentation and monitoring
Instrumentation of the enabling works, along with monitoring of groundwater levels and shoring wall movement, is critical to ensure all parameters remain within allowable limits. It is important to note that basement design solutions are not always conventional and may vary depending on site-specific conditions and boundary constraints.

Procurement route

For deep basement construction, clients generally adopt one of two procurement approaches. They can either appoint the temporary or enabling works contractor first, and then hand over the works to the main contractor once piling is complete, or they can appoint the main contractor from the outset, engaging the temporary/enabling works contractor as a subcontractor under the main contract.

This article was originally published on Construction Week online.