INADEQUATE SPECIFICATIONS ON CRB WALLING LEADS TO WALL FAILURES:
Article Date: 31 January 2007
 South African concrete retaining block (CRB) wall contractors are being handed job specifications which lack essential information, and in many instances, are brazenly inadequate, particularly as they appear in tender bill of quantity briefs.
Silvio Ferraris, vice president of the Concrete Manufacturers Association’s (CMA) CRB Wall Division, says that a typical specification reads as follows: “Construct a 116m² wall with retaining blocks on a straight and curved section at an angle of 60º and fill in retaining blocks with selected material”.
Ferraris notes that it is impossible to price accurately on such a brief especially if it is not accompanied by a set of drawings, which often is the case.
“Does the brief indicate that the wall is 116m long and one metre high, or is it 38 metres long and an average height of three metres? Is it against ‘cut’ or ‘fill’? Are there loads above the wall and how close are these loads to the top of the wall?”
Ferraris says it is virtually impossible to make valid assumptions with no drawings, no site to view and a poorly described bill item.
“Contractors generally react in one of two ways to this type of specification. They can either make certain assumptions in their favour, such as a low wall, which will enable them to submit the lowest possible quote, or, if the site is ready for viewing, they can visit it, make more accurate assumptions, and quote more appropriately. Although the higher quote reflects the contract requirements far more accurately, the client is more likely to opt for the lowest quote.
“Then, when the winning contractor arrives on site he generally finds that the project involves a higher wall as well as other elements not dealt with in his quote, such as poor access, more backfill, fabric reinforcement, drains and stronger foundations. He then either cuts corners, building a wall in which the risk of failure is high, or he revises his quotation, and the project goes hopelessly over budget. Either way it eventually ends up costing considerably more.
“There is no excuse for the submission of inadequate CRB wall specifications, nor for the omissions of drawings. All CRB wall specifications should be accompanied by a set of drawings showing heights, etc. And for those not sure what a comprehensive specification entails, the CMA’s CRB wall installation manual includes a typical bill of quantities and covers all the requirements for constructing a CRB wall. The Association has also produced a film (available on CD) which provides step-by-step construction guidelines.
“CRB wall specifiers should also refer to two SABS standards, SANS 207 for geofabric material used to stabilize crb walls, and SANS 508 : 2007, a specification for retaining blocks. The latter is in its final draft form and will be available for comment during 2007,” concludes Ferraris.
CAPTION
This wall, which retains a four metre high bank and forms an integral part of a beautifully landscaped garden, is an excellent example of how a concrete retaining block (CRB) wall should be constructed. The wall was built with Corobrik’s Geolok 400 blocks on a concrete foundation.
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