Basements and Special FoundationsWhispers article and sneak preview for 24th June London Club talkSimon Pole BSc C Eng FIStructE MICE MRICS, is a practising structural engineer,designing all manner of buildings in London and the Home Counties and alsoadvises many Adjoining Owners’ Surveyors as ‘advising’ engineer (P and T guidancenote 10 September 2010). As a Building Surveyor he surveys residential propertiesat the point of sale.SummaryFirst, a shock tactic in the hope you will read on!Is today’s Building Owner’s Surveyor now acting as Advocate to the Building Ownerand design team?Engineers’ drawings have changed. I now see structural phrases on drawingsdictated by Building Owners’ Surveyors such as notes referring to mass concretestructural blinding, reinforced concrete underpinning, specifications for buildersdescribing the formation of a ‘wall’ in 1m lengths, superfluous blocks of massconcrete under reinforced retaining walls, forming no engineering function, delayingconstruction work and compromising health and safety in excavations.It would seem that the spirit of the Act is dying in many instances and the credibilityof the surveying and engineering profession is under threat if we cannot sort this outand provide more consistent advice to our appointing owners and clients. Can weplease review the Section 20 interpretation of the various structural components sothat the true spirit of the Act can be restored and Surveyors can be given the tools todo their job more effectively and without so much disagreement and loss of time andmoney to clients.Two questions keep arising during retrofit basement designs:a) What makes a special foundation ‘special’ and why does this structure receiveparticular attention in the legislation?b) Why is there so much disagreement between surveyors over theinterpretation of simple foundation structures within the Act if neither party hasa promotional role?I have recently written an article for the Structural Engineer journal suggesting thatthe engineering profession needs to do more to train engineers in Party Wallprocedures; when the Act applies, how to design responsibly with a duty of care tothe Adjoining Owner and how to act in an advisory capacity to the Adjoining Owner’sSurveyor. See separate paper if interested.
Health warnings, advance apologies and olive branches are all offered early in thispaper, intended to provoke healthy discussion, drawn from a number of exceptionalcases in recent times to highlight a potential problem for the future. It concerns methat my observations and experiences are drawn from persons who are experts intheir field.I worry how the non expert can be expected to apply the Act given the lack ofagreement between experts. Our building stock is at serious risk of futureredundancy if clever engineering solutions are increasingly allowed to prejudiceneighbouring property.In thirty years of practice, I have noticed a marked change in two aspects of PartyWall procedures which concern me, one technical and one procedural.The first change is a technical point and concerns the complexity of basementconstruction resulting from commercial pressure on space. This leads to increasinglycomplex foundation solutions where the Building Owner’s substructure is rigidlyconnected to, or is part of, the Party Wall foundation. See figures 3 and 4 below.Almost all are ‘Special Foundations’.The second change I have noticed is the increasing trend for some Building Owners’Surveyors to act as ‘Advocate’ to the Building Owner with regard to avoiding theSpecial Foundation aspects of the Party Wall Act, whilst still prejudicing the AdjoiningOwners’ rights. I suspect this is, in the main, simply to avoid complicated discussionswith Appointing Owners and other surveyors resulting from a lack of clarity in theSection 20 interpretation of the structural terms ‘Foundation’ and ‘SpecialFoundation’. These ‘interpretations’ are no longer sufficient to deal with modernbasement construction techniques in my opinion. A proportion of this advocacy ishowever clearly one of commercial expectation to satisfy an Appointing Owner anddesign team.The rights of Adjoining Owners are now regularly being prejudiced by a more‘commercial’ application of the Party Wall Act and the Section 20 Interpretationsneed urgent review and must include ‘engineering’ definitions.From all of the above, there is a tremendous opportunity for the Pyramus and ThisbeClub to lead the way. Cross party discussions between surveyors and engineers,RICS and I Struct E could see further technical guidance notes following thoseexcellent articles issued to all members dated September 2010. A series of guidancenotes or a further revision of the Green book in due course could update the‘interpretation’ of Section 20 of the Act which would provide invaluable help to PartyWall Surveyors and design engineers alike.IntroductionToday’s basement extensions are becoming as common as loft conversions. Theyare becoming larger, with more innovative engineering design and constructiontechniques creating significant underground development. Some are builder-led withlittle ‘engineering’ input. Others are highly engineered.
The public are concerned about the stability of neighbouring property and the impacton ground water and underground streams. Local Authority Town Planning policy ischanging as a result, requiring early engagement of engineers and Party Wallsurveyors. This is essential when engineers specify ‘Special Foundations’ as theymay not be permitted by the Adjoining Owner even if the surveyors agree and thiscan lead to months of abortive time and costs.In my formative years, I had not heard of the Party Wall etc Act or The LondonBuilding Acts as a Graduate Engineer, until the first project requiring Party Wallprocedures tripped me up! 30 years later I meet many engineers and some havevery little knowledge of the Act and do not know when to advise their clients that theAct applies to their designs.In recent years, as a building surveyor I have surveyed all manner of properties withbasements extending from the front of the front garden to the very end of the reargarden. Some of these have created restrictive development potential for theneighbouring property and new owners, surveyors, designers and estate agents arecursing the previous generation of development. This is a relatively newphenomenon but is increasingly common.A brief recapThe derivation of the London Building Acts stemmed from the experiences learntfollowing the Great Fire of London and the rebuilding of many terraced propertieswith fireproof walls built from stone and brick.The principle of the Act was for Adjoining Owners to have equal repairing liabilitiesso that Party Walls did not fall into disrepair. Furthermore the Act developedperiodically and the spirit of the Act was to allow a Building Owner reasonabledevelopment of the Party Wall, whilst protecting the interests of the Adjoining Owner.Of considerable importance however, is the fact that the London Building Acts weredrafted at a time when complex basements were rare. Unfortunately the Section 20interpretation of simple structural components can no longer keep up with thetechnical complexity or commercial pressures of today.Party Walls were built on their own simple foundations immediately below groundlevel. Party Wall considerations were based on ‘above ground level’ separatingwalls, not subterranean living adjacent to someone’s foundations. New terraces ofhouses are normally built this way. It is the relatively new phenomenon of retro fitbasement and subterranean living that has created our current ‘Special Foundation’dilemmas in the main.I doubt that those who crafted the simple Section 20 ‘interpretations (not definitions)for the structural elements (walls, foundations and special foundations) evenanticipated subsequent generations would spend so much time deliberating overtheir meaning.Party Wall foundations no longer simply support vertical loads from plain masonrywalls above ground. They resist significant horizontal and overturning forces in many
instances now. I would make specific reference to a specific Party structure; thereinforced retaining wall here, since, to an engineer, a retaining wall is an intrinsicpart of geotechnical/ foundation engineering, resisting vertical horizontal andoverturning forces and normally providing ‘support’ to a wall and the AdjoiningOwner’s soil and foundations. It is too simplistic to call part of this single structure a‘wall’ and part of it a ‘foundation’. To ‘want’ to do so is to be an advocate, in myopinion, as it ignores the ‘Special Foundation aspects. See later.The majority of points made in this article are based on reinforced concreteunderpinning (‘Special Foundation’) of terraced property but other areas where PartyWall engineering problems arise also include:a) Raising a substantial Party Wall in less durable materials.b) Raising a Party Wall partly on the Party Wall itself and partly on adjacent newframed structure.c) Raising the Party Wall on a cantilever (framed structure).d) Significant vertical chases/ slots in Party Walls (eg for columns, services etc).e) Method/ site related matters associated with deep excavations and temporaryworks.f) Work to Party Floors (separating floors between flats).Special FoundationsIt is important to remind ourselves why there is a ‘special foundation provision withinthe Party Wall Act and what this means for Adjoining Owners and their advisors.Special Foundations are defined specifically in section 20 of the Party Wall etc Act1996 (and previously in the 1939 London Building Acts) as “an assemblage ofbeams or rods employed for the purpose of distributing load”. In basementconstruction this normally means a reinforced concrete foundation, oftenasymmetrical about the Party Wall and often integral with the Building Owner’s raft/ground beams/ pile caps. In the 1920’s and 30’s this was more commonly associatedwith steel beams forming foundation grillage bases for framed structures, whichsometimes imposed on the Adjoining Owners land, local to column bases but did notprovide wholesale support to the Party Wall.Why are Special Foundations singled out and why do they require specific consentfrom a neighbour? The Green book says “Special means that reinforcement is usedwhich often creates complex foundations relying on integrity for strength .” Thisdoes perhaps not go far enough to explain the significance as ‘integrity’ only infers‘quality’.It engineering terms this complexity could be further explained by the ‘distribution ofloads or forces to adjacent structures’. In other words, that portion of the foundation
immediately beneath the Party Wall is not strong or sufficiently stable enough on itsown and the applied forces need to be resisted by adjacent structures remote fromthe Party Wall foundation. These might be so remote that the Adjoining Ownercannot subsequently use this method of support. Furthermore the distributed loadsare transferred into the current Building Owner’s structure, therefore any future useof the wall may damage/ cause settlement/cracking of the current Building Owner’spremises. The most extreme example of this is a cantilevered piled raft supporting aParty Wall. The neighbour cannot realistically underpin the piles or add additionalload to those piles in the future. If he excavates, he removes the skin friction from thepiles and this could have disastrous consequences to the current Building Owner.Three decades of changeThe 3 diagrams below are indicative of the change in underpinning technique duringthe 30 or so years of my working career to date. The diagrams are simplified andexclude non structural detail such as waterproofing and drainage.The changing trend is achieving faster construction and achieving more space forthe Building Owner, but at what cost to subsequent neighbouring properties?Figure 1. 1980’s: A typical mass concrete underpinning scheme. A separatebasement raft slab and wall or tanking/ dry lining/ drained cavity were provided.
Figure 2. 1990’s: To save space, the mass concrete is replaced by reinforcedconcrete (‘Special Foundation’) but it is separate from the Building Owner’sbasement raft and other foundations.Fig 3. 2000’s: To save space and improve watertighness, reinforced underpinning isintegral with the Building Owner’s raft foundation, sometimes piles, swimming pool,lift pit etc.
Fig 4. 2010’s: Substantial superstructures often include piles and more substantialraft foundations.Over three to four decades we have seen the demand for additional space virtuallyeliminate the mass concrete underpinning in Figure 1. It is used in domesticsubsidence repairs and the most utilitarian basements where ground water is less ofan issue and dry lining with a drained cavity is sufficient (Ciria report 139 and BS8102, grade 3 environment)Today’s trend for deeper and higher specification basements (High quality residentialhabitation (grade 4), paper archive storage etc) and faster construction methods(top down construction etc) has seen increasingly complex construction. Architectshave ‘sculpted’ the space available for clients and have asked engineers toincreasingly merge the Building Owner’s foundation structure with the Party Wallfoundations.The trend is for the Party Wall foundation to become part of the Building Owner’sfoundation structure (Figures 3), to save space for the Building Owner. In recentmonths I have viewed proposals for deep piled underpinning of Party Walls, complexraft structures, some piled and all manner of part mass and part reinforced concreteunderpinning, with endless arguments regarding what constitutes a wall, afoundation and a special foundation. All have in common, a highly engineeredbasement for the Building Owner that is rigidly connected to the Party Wallfoundation, making any subsequent use of the Party Wall difficult without impartingloads and settlements in