A methodology for the design of basement retaining walls and foundations
Background to the structural design guidance given in TBIC - Guidance Document – Basements for dwellings - TBIC/007.
It will be apparent that the more rigorous the analysis and the better the consideration to the provision and distribution of reinforcement the better will be the ability of the structure to resist water ingress. Clearly, this will also depend on workmanship and attention to detail (e.g. joints) but the design is paramount to obtaining a robust water-resisting structure.
Unfortunately, the structural design of small basements is often rather limited due to the value of the structure and the lack of readily available robust design guidance.
The new Basement Guidance publication (the replacement of the Approved Document – Basements for dwellings) has been extended to include assessment of the whole basement structure based on soil/structure interaction. Allowance has also been made for the effects of two-way action resulting in some adjustment factors for the wall’s aspect ratio. These have been determined from common elastic bending moment coefficients found in a number of publications as they conveniently include both triangular and rectangular loadings and two-way spanning for both propped and un-propped upper horizontal supports.
In order to achieve good water-resistance it is desirable to assess the structure as a loaded box so that the design allows for the bending moments and stresses induce. However such design can be a complex and a time consuming process and to simplify design a methodology to take account of a loaded box (including walls with openings) has been developed in order to produce simple design tables in the new Basement Guidance Document.
The methodology used for the structural section in the Guidance Document is outlined within this publication, which includes the formula used for the assessment of soil structure interaction and the bending moment coefficients used for two way spanning walls and raft, and that used for internal loads on a raft foundation.