Technical factors influencing decisions to recommend underpinning of low-rise buildings on shrinkable clay

Foundations to buildings which fail, are frequently underpinned and since 1971 whenhouse insurers began to offer cover for damage due to "subsidence", the underpinningindustry has grown to enjoy a multi-million pound turnover. Much of England southeastof a line from the Humber to Exeter, has a sub-soil of clay. Although it has beenknown for more than a century that this material shrinks when dry, failures on clay todwelling houses have lately become a subject of interest to householders, buildingprofessionals and insurance companies in particular. This study concentrates on thetechnical factors taken into account when professionals investigate a failure to a lowrisebuilding on shrinkable clay; from the preliminary hypothesis that underpinning isdone too often. A body of technical literature exists and is brought under survey so asto isolate those main factors which in the amalgamated view of the leading authors arecentral to a well-based decision to carry out,or to refrain from, underpinning.Consulting engineers in Essex were surveyed hi order to discover volume and qualitydata about their subsidence work on a firm-by-firm basis: also to conduct case studiesof actual-practice examples in relation to the regime of main factors identified by theleading authors. The regime was applied to 282 cases of failed buildings in the light ofa response by 63% of the sample from the exploratory questionnaire, to the effect thatin their opinion underpinning was indeed carried out too often. The preliminaryhypothesis appeared to be correct. From analysis of the case-data it appears that inactual practice the fields of investigation of causes identified by the authorities wererespected by the respondents who were able to supply a full body of data. Thestrongest correlation in respondents' data as an indicator of the actual decision tounderpin or not however was the initial level of damage. Damage is an effect and atendency was found in decisions towards the exclusion of analysis of data emergingfrom the investigation of cause. In short, the quantity and not the quality of damageappears to bear much the stronger relation to a decision to underpin