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

Wilkin, D

Abstract

Foundations to buildings which fail, are frequently underpinned and since 1971 when
house insurers began to offer cover for damage due to "subsidence", the underpinning
industry has grown to enjoy a multi-million pound turnover. Much of England southeast
of a line from the Humber to Exeter, has a sub-soil of clay. Although it has been
known for more than a century that this material shrinks when dry, failures on clay to
dwelling houses have lately become a subject of interest to householders, building
professionals and insurance companies in particular. This study concentrates on the
technical factors taken into account when professionals investigate a failure to a lowrise
building on shrinkable clay; from the preliminary hypothesis that underpinning is
done too often. A body of technical literature exists and is brought under survey so as
to isolate those main factors which in the amalgamated view of the leading authors are
central 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 quality
data about their subsidence work on a firm-by-firm basis: also to conduct case studies
of actual-practice examples in relation to the regime of main factors identified by the
leading authors. The regime was applied to 282 cases of failed buildings in the light of
a response by 63% of the sample from the exploratory questionnaire, to the effect that
in their opinion underpinning was indeed carried out too often. The preliminary
hypothesis appeared to be correct. From analysis of the case-data it appears that in
actual practice the fields of investigation of causes identified by the authorities were
respected by the respondents who were able to supply a full body of data. The
strongest correlation in respondents' data as an indicator of the actual decision to
underpin or not however was the initial level of damage. Damage is an effect and a
tendency was found in decisions towards the exclusion of analysis of data emerging
from the investigation of cause. In short, the quantity and not the quality of damage
appears to bear much the stronger relation to a decision to underpin.

Citation

Wilkin, D. Technical factors influencing decisions to recommend underpinning of low-rise buildings on shrinkable clay. (Dissertation). University of Salford

• Thesis Type: Dissertation
• Deposit Date: Jul 2, 2021
• Award Date: Mar 1, 1993