Structures

Designing Concrete Basements

 

Charles Goodchild outlines the main considerations for the structural design of subterranean spaces

There are many ways of constructing basements, with many complexities and specialities. This article will outline some of the key considerations for their structural design. A successful basement is a collaborative effort, so the whole project team should be engaged in their delivery.

Construction types

BS 8102:2009 defines three main methods or basement construction types for providing protection against ground water:

  • Type A Barrier or membrane protection with a waterproofing layer located either externally, internally or sandwiched within a structure of reinforced concrete or masonry.
  • Type B Structural integral protection: reinforced in-situ concrete is designed to be water-resistant by controlling any cracking and using tried-and-trusted details such as water bars.
  • Type C Drained protection: an internal cavity system, which allows any water seeping through external walls and floor to drain to a sump and be pumped away.
    BS 8102 also divides basements into three grades of internal environment and gives associated illustrative building uses:
  • Grade 1 is for where some seepage and damp patches are tolerated, such as car parks.
  • Grade 2 allows no water penetration, but some damp is tolerable and some ventilation may be required in permanent workshops or garages.
  • Grade 3 is where no water penetration is tolerated, and ventilation, dehumidification or air conditioning is required. This grade is appropriate for residential or commercial accommodation.

The choice of protection type is heavily influenced by the client’s needs, in terms of the grade of basement, and an assessment of site conditions and the risks from groundwater. It may determine whether a combination of construction types is sensible – this is relatively common and is an NHBC requirement. Based on the water-table level, BS 8102 contains advice, summarised in table 1.

Type A will require specialist advice in all but the most benign conditions. Type B is water-resisting reinforced concrete construction and will most likely form part of the structural engineer’s brief. The structural engineer is also likely to be involved with larger Type C basements because one of the prerequisites is to use the outer structure to keep water out of the cavity as much as possible.

Surveys and initial feasibility studies

Generally, layouts should be as simple as possible. Aligning the basement with the superstructure (or vice versa) simplifies construction. An appropriate site investigation and thorough evaluation of its findings is an essential part of any basement design. A major part of that is to determine the groundwater level, which affects both the choice of construction type and risks during construction. Besides its influence on the type of protection, a high water table means buoyancy, lower soil strengths, high lateral pressures, potential flooding and, probably, a difficult and muddy site. During construction, water levels can be managed with pumping and cut-off walls but these require space and permissions.

On rural and residential sites it may be sensible to batter the sides of the excavation. On many urban sites any basement usually extends to the site boundaries and care will be necessary when working close to adjacent structures – party-wall agreements may be required. Care should also be taken over any adjacent services or tunnels.

There are many ways of temporarily supporting basement excavations. Piled walls are popular as they can be incorporated into the permanent works. Sheet piling is not as popular as it once was and concrete piles are now used extensively. Contiguous (ie, spaced) piles can be used to retain soil while additional secant (overlapping) piles help to seal off groundwater and/or unstable ground. On anything other than a dry site, these walls will need a second line of defence – such as a membrane, a lining wall, a facing wall or a drained cavity.

Cantilevering piles will make construction simpler, but at depth, propping, with associated capping beams, wallings etc. gives greater overall economy. At even greater depths, diaphragm walls may be necessary – but the costs become huge.

Continuity of the defence system(s) is essential. One leading contractor does all work in BIM, making sure that at least two completely continuous waterproofing layers can be demonstrated and that problem areas such as service entries and corners are thought through before going on site. Designers also have to consider water run-off from surfaces and cavities above.

The correct specification and installation of the waterproofing system or systems is fundamental to success. It will be useful to talk to specialists. The websites of the Basement Information Centre and the Property Care Association contain useful information and details of specialists who can help with specification and details. BS 8102 emphasises the inclusion of a specialist waterproofing adviser on the design team.

By considering all these issues, feasible construction methods should emerge and scheme designs can then be developed and discussed with the client and design team and, hopefully, the constructor. Generally, costs increase with depth: costs increase massively with high groundwater.

Structural design

The structural design of concrete basements considers both the ultimate (strength, equilibrium, geotechnical) and serviceability (deformation and cracking) limit states. For reinforced concrete basements, it’s not the concrete that leaks, but cracks, construction joints and the results of bad workmanship. Provided that the appropriate concrete and other materials are specified and workmanship can be assured, structural design is mainly about controlling crack widths.

Having said that, the ultimate limit case must be considered for the various loads and load combinations according to the rules in Eurocodes 2 and 7. Normally, slabs have permanent and imposed actions and may be subject to heave; walls have pressures due to imposed load, retained soil and/or compaction on the walls. And, of course, water pressure affects both.

Serviceability design is mainly about crack control. When concrete sets it is usually warm due to the heat of hydration. When it then cools, the concrete will try to contract. If that contraction is restrained by, say, a wall sitting on a base or slab, then the wall will experience tensile stresses in areas of restraint. Those tensile stresses may be sufficient to cause through-cracking and the width of those through-cracks must be controlled to accepted limits using reinforcement. These early-age thermal effects are often critical. Longer term, seasonal temperature effects and, in dry basements at least, drying shrinkage set up tensile stresses in the same way and may be more critical. The same phenomena occur with any adjacent slab or wall pour, but they become critical in basements and water-retaining structures.

In calculations, designers usually deal in strains, rather than stresses. Strains due to the additional short (early-age), medium (seasonal) and long-term (drying) effects are added. The total strain is then compared to the concrete’s time-dependant strain capacity to determine whether the section is likely to crack or not. With reasonably thick sections, it is usually found (or, as a reasonable worst case, assumed) that the section will crack. The calculated strain is used to determine how much reinforcement is required to keep any cracks to an acceptable theoretical width.

Another check ensures that the reinforcement will not yield at the first crack – it is desirable to have lots of small cracks rather than a large, uncontrolled one. This minimum reinforcement can be about 0.58% to Eurocode 2. If there are truly stiff restraints either end of a section then this “end restraint” condition results in even greater reinforcement requirements. 

Design crack width limits depend on the water table. Eurocode 2 is not that helpful but guidance is given in The Concrete Centre publication, Concrete Basements. For Type B protection, a through-crack width limit of 0.30mm may be appropriate where the water table is low, 0.2mm where it is variable and 0.05-0.20mm where it is high. High water tables can mean potentially punitive amounts of reinforcement.

For Type A protection, the structure itself is not intended to be watertight: masonry is often used. Where concrete is used, then a maximum 0.3mm through-crack width should be acceptable to the membrane. In Type C protection, 0.3mm may be appropriate although 0.05-0.2mm will improve overall water resistance.

Traditionally, it is usual to deal with flexural crack widths separately. These cracks are limited to the usual 0.3mm at the surface. However, as they only go part-way through the section, it is assumed that they do not impair water resistance.

Further reading

Concrete Basements, published by The Concrete Centre, available from www.concretecentre.com