FIG 3.11 Settlement or subsidence. The crack widens with height.
FIG 3.12 Subsidence damage inside house.
insuring against such matters, minor cracks reported by his surveyor need not concern him since the very fact that the surveyor has reported the damage may have under-mined the basis of any later claim.
FIG 3.13 Subsidence damage outside house.
Remedies
Damage caused by the action of tree roots and proved to be due solely to the presence of trees, and no other cause should not normally require underground repair. It should be suffi cient to recommend that the off ending trees be removed and that the site be given a number of wet winters to stabilise before the damage is made good by stitch-bonding cracked brickwork, repairing pointing and making good rendered surfaces in the normal way. If the off ending trees cannot be removed, because they are either protected by tree preservation orders or in other ownership, then deeper foundation bases may be required to accommodate present and future root activity. An interrupted foundation should be used to ensure that expansion and shrinkage rates in the subsoil are the same on all sides of the foundation at diff erent times of the year. If trees are a contributory cause of damage but the foundations are also inadequate for other reasons then an underground repair may be necessary in addition to the removal, pollarding or crowning of off ending trees.
Traditional underpinning, which is an expensive and labour-intensive process, com-prises the provision of an additional foundation below the existing foundation. This may be achieved by alternately excavating bases, usually 1 m 3 in extent, around the
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perimeter of the structure and under the internal load-bearing walls. Each base is cast incorporating steel reinforcement married into adjoining alternate bases so that a com-plete continuous foundation is provided under the existing foundation to whatever dimensions are specifi ed. When the bases have hardened, the space between new and old foundations is fi lled by dry packing with a hard cement mortar. During the course of such underpinning, allowances have to be made for diverting and repairing any services encountered. After the work is fi nished, a suitable period is allowed for consoli-dation of the structure on the new founconsoli-dation, and then any cosmetic repairs, masonry repairs and levelling up are completed.
Cavities should de filled with concrete to ground level on all types of non-basement footings. Some provision for water to weep from base of cavities is advisable, especially in conditions of high exposure and to elevations in normal conditions facing the prevailing weather.
Trench fill permits the use of narrow, machine-excavated trenches and is ideal on sites where the bearing capacity of the subsoil is high but there is need for stiffness and depth.
This foundation may be unsuitable for subsoils subject to seasonal change where different lateral forces may arise on each side and cause lateral displacement.
50 mm cavity (maximum permitted – 75 mm)
FIG 3.14 Modern deep strip or trench-fi ll foundation.
As a general rule such underpinning will be applied equally to all bearing walls, other-wise a danger of structural movement between those parts of the building which have been underpinned, and those parts which have not, arises. This may not apply on a sloping site where the underpinning depth may step up with the site slope and remain in the same stratum of subsoil.
Special diffi culty arises with semi-detached or terraced structures in multi-ownership since if the general rule mentioned is to apply, the whole structure or terrace would need underpinning. In such situations a degree of engineering judgement is required and it may be necessary to undertake partial underpinning only. If so, it is recom-mended that the depth of the underpinning bases be graduated down from the point where non-underpinned walls end to avoid a sudden marked change in overall founda-tion depth at any specifi c point. If party walls have to be underpinned, hopefully this will be by agreement since although a party wall can be physically underpinned from one side only, the bases themselves will be trespassing beneath and adjoining owner-ship and permission is then needed. There is also a risk that partial underpinning of, say, a semi-detached house and the party wall will result in any future stresses being trans-ferred to the adjoining house. This could cause movement relative to the party wall with consequential damage. Before advising on such works legal advice should be taken as to the implications and means by which the adjoining owner might be warned about the proposed works. A party wall may be underpinned by Notice under the London Building Acts (Amendment) Act 1929, part of the special code for party structures which applies in London now extended to England and Wales in The Party Wall etc. Act.
As an alternative to the traditional underpinning described, a number of specialist con-tractors now employ special systems to provide greater depths using a system of pads and beams. Pads are cast at corners and within the structure at the main point-loads to considerable depths if necessary. The pads are then linked by narrow reinforced ground beams cast directly under the original foundations. Such systems reduce the overall amount of excavation necessary and provide interrupted foundations which could be recommended on certain types of shrinkable clay, especially, if live tree roots are present in close proximity.
If a surveyor is asked to deal with underpinning he or she should be satisfi ed that it is necessary in the circumstances that the shoring and other temporary support works will be adequate, and that the procedure for casting a new foundation will not subject the structure or any adjoining structure, to additional distress.
Generally, with traditional underpinning, bases should be cast leaving at least two equivalent areas of support between so that bases 1, 4 and 7 would be cast, allowed to harden, dry-packed completely and the packing set. Then bases 2, 5 and 8 might be excavated similarly, followed by bases 3, 6 and 9. In soft ground even greater spacing of excavations may be required to avoid further structural damage. In all cases excava-tions should be open for the least possible time since some subsoils deteriorate when exposed. All excavations should be adequately planked and strutted at all stages of the work to avoid lateral displacement of the adjacent subsoil and the bottom of any underpinning excavation should be carefully checked to ensure that it is sound, level and dry before concrete placement begins.
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If it is suspected that the structure under review has been underpinned in the past, the reasons for this should be ascertained and details obtained if possible. Nowadays, underpinning works require local authority consent under the Building Regulations and most recent works should appear in their records. Before 1976, such work was regarded as outside the scope of the Building Regulations or bye laws and normally went unre-ported to the local authority.