Introduction and Structure of High-performance Basements and Foundations

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We will now discuss high-performance basements, foundations cellars and related underground buildings.

Introduction

We will now deal with parts of buildings other than foundations lying below ground level. For the purposes of this book, a basement is taken to be that part of a building which is below ground level and which is used as habitable accommodation; whereas a cellar, also below ground level, is not used as habitable accommodation.

Basements are generally reasonably well finished and heated; cellars are usually unoccupied areas probably originally built to store coal, and for other long term storage requirements. Cellars are typically poorly finished, and may be damp and lack adequate ventilation.

Existing cellars about to be converted into habitable accommodation, or basements about to be upgraded, may be inadequate in terms of thermal insulation, ventilation or provision of heating.

Cellars which have been used for fuel storage are likely to be contaminated by salts. Original finishes may have deteriorated and subsequent use of lightweight plasters in a relatively damp environment may have accentuated dampness and condensation problems.

--- A basement, probably built for storage but now used for recreational purposes.

--- A coal cellar (with accumulated leaves) from the nineteenth century which has remained derelict for many years.

Basements tend to be difficult to ventilate properly and may often require special measures to avoid condensation. Bathrooms and other utility rooms, which require only limited natural lighting, are often located in basements and invariably generate high moisture vapor levels; the vapor will need to be extracted or diluted by ventilation. There can be risks of surface and interstitial condensation with balconies and access ways above basements; these problems are analogous to those sometimes encountered with flat roofs. Using basements as habitable accommodation will usually require particular attention to be paid to the likely performance of the building fabric and the servicing systems if satisfactory living conditions are to be provided.

Many cellars and basements, because they are at low levels and often near to water tables or tree root systems, are at risk of structural damage. This topic is dealt with separately.

Furthermore, most old cellars have no dampproofing and penetrating dampness can be a significant problem. Other matters which need consideration include ventilation, thermal performance, condensation, lighting, fire protection, and means of access and escape in case of fire, all of which may be more difficult to achieve in basements than in rooms above ground. With underground buildings, also known as earth sheltered buildings, many of the performance requirements are similar to those of cellars and basements, but, because the whole building is underground rather than just a part underground, there may be additional considerations relating to means of access, lighting and other services.

Basements in the housing stock.

--- Traditional basements and cellars in urban areas were frequently constructed below the level of adjoining streets. Pavements were carried on brick arches over the coal cellars

Of these, over three quarters date from before 1907.

Dampness is the most common defect encountered, though it’s not quite as common as one might expect, with just over 1 in 20 of all basements showing some signs of penetrating or rising damp. The occasional structural problem was observed in basements, but instances were too few to draw any statistically valid conclusions.

In urban areas, cellars were commonly used for the storage of solid fuel. The circular cast iron manhole covers set into pavements in front of Victorian and Edwardian dwellings are still a familiar sight although the use of gas for heating now means that little coal is now stored. The other main use of cellars was for general storage, or, in a few dwellings, and taking advantage of more stable temperatures at low level, for the storage of wine.

A current estimate of the number of houses built with some form of basement construction, mainly to accommodate sloping ground, is roughly 100,000/year.

Changing attitudes to cellars and basements in housing:

As part of a study of utilization of space in housing, an examination was made in the late 1970s by Govt. Agency investigators of the feasibility of providing habitable basements in newly constructed housing. In the study, alternative methods of constructing basements were examined, basement house plans were developed and a comparison was made, on a cost and amenity basis, of basement houses and comparable non basement houses.

It was shown that, in general, basement houses were more expensive on both flat and sloping sites than the nearest comparable non-basement houses. When measured as cost per unit of useful shell area (i.e. the floor area excluding circulation areas and garage), the basement house was 22-28% more expensive on flat sites and 8-11% more on sloping sites.

However, the notional plot area required for basement houses is less, giving scope for higher densities of development and reduced land costs.

A further move to encourage the incorporation of basements into new housing came with the formation by the U.S. Cement Association of the Basement Engineering Group in 1990. There was a realization that there was scant information on structural design and that the information on waterproofing needed reviewing. Cost estimates for various site layouts were prepared, and builders and owners encouraged considering incorporating a basement.

The estimates showed that a house wholly below ground might cost 11.5% more and a house with a basement partly below ground might cost only 4.1% more than a house without a basement. Where ground conditions would require deeper (2m) foundations then the percentage increase reduces to 9.7% and 2.0% respectively. However for the same size house (normally, 130 m^2 ) the site can be 3m narrower, with a potential saving of 21% of land costs. Energy costs will also show a possible saving of between 4.4-5.6% and 6.1-9.5% for semi-detached and detached properties respectively.

A number of potential problems need to be faced when considering the conversion of a cellar into habitable accommodation, and these are considered in turn.

Basements in the non-housing building stock

There is no generally available information of the incidence of basements in the non-housing sector.

However, it’s a matter of common observation that many buildings built in areas where land values are very high do exploit the additional potential for accommodation and car parking provided by one or more basements.

As will be seen: very deep basements of several storeys below ground level do have considerable implications for fire precautions and means of escape.

Structure

Cellars and basements by their nature form the lowest parts of buildings, and may be required to provide whole or partial support to the remainder of the carcass. The assessment of their structural condition, of whatever materials they are made, is therefore of paramount importance in assessing the structural condition of the building as a whole. The older the building, the more crucial it will be to determine the structural condition of the cellars and basements.

Characteristic details

Crypts

As was noted in Floors and flooring, large arched or barrel vaulted basements of major public buildings such as castles date from Roman times. Later medieval church crypts had more complex intersecting vaults with piers at relatively close centers.

In parallel with the later barrels came the rather more slender ribs of the intersecting voussoir arches, infilled with stone slabs to carry the ground floor above. In Renaissance times, vault spans tended to become larger.

Domestic cellars and basements

Floor construction

Cellar floors are often poorly constructed and in a poor state of repair. Some comprise compacted earth or rock. Concrete floors in older properties may be only a thin layer of weak concrete. Many cellars have stone or slate flag floors, in which the flags have either been laid directly onto the soil below or have been bedded in a thin layer of weak concrete. Cellars with suspended timber floors are rare.

Occasionally the cellar floor will have been replaced or upgraded with a modern concrete floor which includes a damp-proof membrane. In many cases, where the cellar lies beneath only part of the building footprint, the ground floor above and alongside is of mixed construction (i.e. some rooms may have solid concrete floors and others may have suspended timber).

Wall construction

Where a cellar is located in soil rather than rock, certain walls within the cellar will act as retaining walls; they will vary considerably from building to building. In a few cases the cellar may simply have been dug into the bedrock and the rock then forms the walls. In soil, more usually, the walls will be of stone or brick construction, typically poorly constructed without any vertical barrier to dampness.

Sometimes they are finished internally with a coat of sand and cement render or, occasionally, plaster, but this is often poorly applied.

Ceiling construction

In most cases the ceiling will comprise nothing more than the suspended timber floor of the room above. At best this may be covered with some kind of wooden or plaster boarding to provide a ceiling. It’s rare for the cellar ceiling to be in good condition.

As a consequence the ceiling is likely to be very drafty. A more enlightened owner will have taken the opportunity to improve the thermal performance by placing sheet insulation between the joists or by means of quilts suspended on netting.

Sometimes the cellar ceiling is formed in vaulted stone or brick, or occasionally in suspended concrete. In these cases the ceiling is likely to provide reasonable resistance to airflow from the cellar into the rooms above.

Non-domestic cellars and basements

Most city center commercial properties and many industrial buildings include the provision of basements. These basements may be of considerable depth where they are used to provide underground parking facilities. In commercial developments, in congested city areas, basements usually occupy the whole site area to maximize the use of valuable land space below ground. As such they will provide the walls that are used to support the site boundary and which should at the same time avoid settlement or damage to adjacent roads or property.

The conventional methods for constructing these retaining walls are time consuming and expensive.

That this is so is largely due to the extensive nature of the temporary works needed for their construction.

However, new developments in techniques have made it possible to reconsider the whole concept of retaining wall construction so that construction time and money can be significantly reduced. The methods now available make it possible to install the main wall structure (permanent works) before any excavation of the basement is started.

Once the wall has been established, excavation can proceed. As most retaining walls of this type require support from the permanent structure built within them, temporary support is needed as an interim measure as the excavation takes place. The most favored method of support, ground anchors, enables the working area of the site to be free of any obstruction to the continuity of the works. Internal raking shoring, by contrast, leads to a great deal of obstruction which inevitably causes inefficient sequencing of construction operations and to increases in cost.

Three basic methods for providing perimeter construction are in use today:

  • - diaphragm walling
  • - contiguous bored piles
  • - secant piling

All these methods involve operations that require the services of a specialist subcontractor.

----

--- Stages in the construction of a diaphragm basement wall using a bentonite slurry to maintain the walls of the excavation until the reinforcement and concrete are placed:

Stage 1 Excavation of panel. Excavation kept filled with bentonite suspension:

Stage 2 Panel, on completion, full of bentonite.

Reinforcement about to be lowered.

Stage 3 Reinforcement inserted and concrete poured in panel. Concrete placed so that bentonite is displaced.

Continuous guide walls

Excavation by grab

Concrete into tremie

Bentonite displaced

Stop end tubes

----

Diaphragm walls:

Diaphragm walls are commonly used in clay and sand with gravel areas. The resulting wall is substantially watertight. The method of construction works as follows.

Guide walls, spaced to be the final wall thickness apart are first constructed by the main contractor. Between these walls a trench is excavated to the required depth by the specialist contractor. As excavation proceeds, the sides are prevented from falling in by keeping the excavated area filled with bentonite slurry. This is a 3-6% mixture of a form of diatomaceous earth in water. The reasons why this mixture is so effective in preventing a trench of considerable depth in sand, sandy clay or gravel from collapsing involves a complex chemical process and won’t be considered in detail here. Suffice to say, it works extremely well.

The methods of excavation vary with the specialist contractor in question but usually involve hydraulic grabs, rotating wheel cutters or other means. As excavation proceeds, so it’s kept full with the bentonite liquid and the excavation carried out in bays in the manner illustrated. Alternate configuration for the bays is usually adopted unless the engineer requires a wider spacing to maintain the stability of the adjoining boundaries.

--- Cast-in-situ piles conform to the final shape of the excavation, leaving a rough surface when exposed. Light construction Male piles reinforced with mild steel bars; Heavy construction Both piles reinforced with rolled steel sect ions;

--- Light / heavy forms of Libore secant piling

Once a bay has been excavated, prefabricated reinforcement is lowered into the excavated area, displacing some of the bentonite as it does so (stage 2). The surplus liquid is pumped back to storage tanks, where it’s cleaned ready for further use. A special quality of the bentonite is that it does not adhere to the reinforcement.

Concrete is now poured into the trench by means of a tremie (stage 3).

Bentonite is displaced as the pour proceeds and is fed back to the storage tanks. On completion, all the bentonite is displaced. As the reinforcement does not cling to the bentonite, the normal bond between the concrete and the reinforcement is obtained. The bentonite may become contaminated with sand or soil fines during the process, but it’s easily cleaned before reuse.

The particular advantages of this method are that:

- installation is free from vibration and excessive noise

- walls are constructed with minimum disruption to adjacent areas

- these walls form a dual purpose:

they avoid the need for any temporary sheeting to the excavation and they become the final structural wall of the permanent works. (Some form of internal facing is usually needed for cosmetic purposes)

- diaphragm walls are substantially watertight

- diaphragm walls need support and this may be provided by the permanent structure within the basement or by ground anchors acting outside the walls

With the last point above, either temporary support will be needed until the permanent structure within the basement is complete, or ground anchors can be installed at an appropriate moment in the excavation sequence and no temporary support will be required.

Contiguous bored piles

As the name suggests, bored piles are installed as close together as possible to form the perimeter wall before any excavation takes place. The accuracy of placing the piles depends upon the type of pile and the method of placing, but piles cast-in-situ will conform to the final shape of the excavation.

---- Contiguous piles are normally restricted to ground conditions where naturally dry soil conditions exist. However, a method of sealing the gaps between piles to provide a watertight structure is under development.

As with diaphragm walling, temporary support is usually provided by ground anchors or raking shores within the basement area. The advantages are similar to those for diaphragm walls, with the exception of watertightness. Against, is the need for much more effort to provide an acceptable internal face finish than needed with the diaphragm wall.

In cost terms, the differences between diaphragm walls and contiguous piles are likely to be small.

Choice is a matter of comparing costs, together with any special requirements that the supervising engineer may feel favors one or the other method for structural reasons.

Secant piling

Secant piling is a development of the bored pile principle. The name secant is used since adjacent piles cut into each other, forming a cut out in the shape of a secant. In this configuration, a watertight wall can be achieved.

Until relatively recently, secant piling was mainly used in heavy civil engineering for dock walls, major retaining wall structures, and in cut and-cover railway construction and similar situations. There is now a lighter weight system on the market which claims to be competitive with the standard contiguous bored pile approach. As the methods used are different, they are described separately.

Libore secant piling

This system is named after the boring rig used. This is of very heavy construction and capable of boring through almost any type of strata, whether dry or water-bearing, and with ancillary equipment which can break up boulders. In spite of its robustness, the equipment is capable of forming retaining walls in restricted areas, with minimum disturbance to adjoining property, services and people.

The first stage of installation involves boring alternate piles (female) at centers less than twice the diameter of the selected pile size. As a second stage, the gap is closed by cutting the male piles into the female piles. In this way the surfaces are bonded together and a watertight joint results. As will be seen, the female piles are not reinforced unless a very heavy wall is needed. For heavy situations, the female piles are reinforced with steel 'I' sections or square box arrangements of reinforcement (allowing room for the secant cut-out).

Alternatively, all piles can be reinforced with steel 'I' sections.

As might be expected, with the heavy equipment involved the result is more expensive than the methods previously described. Its value is in situations requiring watertightness and strength characteristics which coincide with difficult boring conditions with which the other systems cannot cope.

Stent wall secant piling

This approach operates in quite a different way from the Libore method.

Here the retaining wall is formed of alternate piles of bentonite/cement and reinforced concrete piles with secant interlocks. The installation sequence is shown. The piles are constructed first, along a line 100mm outside the line of the following concrete piles. Once they have set to the right strength, they are followed by the concrete piles which, on installation, cut secants out of the piles. The result is a watertight wall which is claimed to be no more expensive than a diaphragm wall or contiguous pile wall.

Again the construction is silent, with the advantages similar to the diaphragm wall, but claimed to be cheaper.

Main performance requirements and defects

Walls forming a cellar or basement usually act as retaining walls; and since they are completely or almost completely located below ground level, they often need to resist quite large lateral loads.

Where a building is piled and also has a basement, probably in an urban area, the pile heads must be finished below basement floor level. Driven piles may be installed from the ground surface and cut down as excavation proceeds; alternatively, piles may be installed from the base of the excavation. However, the ground conditions are likely to be inferior to those at the surface, especially in wet weather, leading to problems with heavy piling rigs.

Bored piles can be formed from the ground surface prior to excavation and concreted only up to the required level, the remainder of the bore being temporarily filled with sand. These piles help to reduce the heave when the basement is excavated in clay soils liable to swelling as the load is removed.

--- Stages in the formation of Stent secant piling.

EXAMPLE: Secant piles

The interaction of a secant pile retaining wall with the over-consolidated London Clay in which it was constructed has been monitored by Govt. Agency investigators. The wall was 20 m deep and formed by 1.2 m diameter bored piles at 1.1 m centers. The depth of embedment was 11 m. The slab bridging the wall with the adjacent wall acted as a prop, but there was some compressible packing between the slab and the wall.

Lateral earth pressures and pore water behind and in front of the wall were monitored during wall construction and subsequent excavation. Vibrating-wire strain gauges were used to estimate the bending moments in the wall and the prop load.

Observations gave some insights into the complex interaction between the retaining wall and the stiff clay, particularly where it relates to the use of retaining structures in over-consolidated clay soil where the initial in situ horizontal stresses may be much larger than the vertical stresses.

--- In spite of the mixture of materials in this domestic cellar, no structural distress is apparent.

Vertical and horizontal loads Basements have to resist horizontal loads from the retained ground. Also, hydrostatic pressure can contribute uplift forces on the structure. For new construction, simple design rules for structure such as those used for walls in the Building Regulations are not appropriate and specific calculations are required.

So far as an existing building is concerned, some reliance can be placed on the fact that the building has already stood the test of time, but any changes in use and circumstances that might disturb a structural condition that has existed for many years still must be considered. In particular, restraint provided by internal walls and floor structures to the outer basement walls may be significant and should not be underestimated. They should not be removed without adequate assessment of the consequences. Also, it’s important to take into account the effects of changes which might influence the moisture level in the ground around the building.

Horizontal loads can be imposed on walls by nearby foundations (surcharging). The likelihood and magnitude of surcharging should be evaluated when considering the placement of additional foundations.

To assess theoretical load capabilities, actual wall, floor and foundation constructions will need to be identified. Site investigations are advisable to assess loadings from retained ground and groundwater.

Specific structural guidance has been produced by the Basement Design and Engineering Group. U.S. Standard Codes of practice on foundations), masonry and loads on walls in water retaining structures all have some relevance.

The effect of basements on foundation design

The additional load on the ground caused by a building may be reduced by providing a basement or a light hollow box foundation, the weight of which is less than the weight of ground excavated. No additional load is applied to the ground until the total weight of the building and the subsurface structure is greater than the total weight of ground excavated; until these conditions are achieved the building actually tends to float (see the EXAMPLE below).

Excavations for the construction of large basements may lead to heave in certain circumstances. E.g., when the new Center was being built on the south bank of a river, it was estimated that excavations in the clay for the large basements would create upward displacements of the underlying tunnels of 20-30mm.

Distress in cellar and basement structures

Settlement of foundations or horizontal displacement of walls may result from ground movements, clay heave or excessive loading. Bulging or cracking of enclosing walls would suggest that the walls have inadequate strength.

A visual survey should identify the more obvious signs of distress such as bulging walls, uneven floors, cracking and misalignment. Paneled finishes on walls may need to be removed to allow inspection of the condition of the structure behind.

Work on site

A basement constructed well before the superstructure of a building can present problems and, in waterlogged soil, may require continuous dewatering until much of the building has been completed. If tanked, upward displacement caused by flotation is a real possibility.

Workmanship

Work on construction and waterproofing of basements is normally carried out by specialist contractors with experience of the various methods and practices.

Supervision of critical features Before any work is specified, it’s important to check which walls are earth retaining and which are freestanding. If part of a partition, floor or built-in structural timber is to be removed during rehabilitation work, it will also be important to check the structural implications.

Inspection

The problems to look for are:

  • cracking of masonry walls
  • cracking or movement of vaults
  • leaking surface water drains causing washout of foundations
  • sulfate attack in walls
  • rot in built-in structural timber

EXAMPLE:

Measuring distress in basements at the house ...

Automatic instrumentation systems were designed and installed to monitor the movements of the House during the construction. Ground movements below foundation level were measured by 'strings' of Govt. Agency electrolevels installed in tubes grouted into boreholes.

Electrolevels on beams and wall plates were used to determine vertical and horizontal movements of the structural elements of the basements. An independent check on basement movements was obtained by a water leveling system. Attention was focused on the ballroom area of the building which has had a long history of structural damage from tunneling operations in the vicinity of Bank underground station. Here, as well as the wall plate electrolevels, crack monitoring was carried out by displacement transducers; wall movements were monitored at ceiling level by invar wire extensometers. Govt. Agency load cells were also mounted on pre-tensioned bars installed to tie this part of the building together prior to construction of the DLR overrun tunnel at Bank. The instrumentation provided comprehensive data on the pattern, rate and extent of building and ground movement, enabling informed decisions to be made on the timing of construction phases of the DLR overrun tunnel and subsequent House refurbishment.

 

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Updated: Sunday, May 12, 2013 8:52