Grouted anchors and soil nails: all you need to know about inspection, condition assessment and remediation
John O’Donovan, GDG Principal Engineer and co-author of CIRIA guide C794 outlines the importance of grouted anchors and soil nails to our infrastructure.
In our industry today, there is a plethora of information concerning the design and construction of grouted anchors and soil nails, but very little, if any, guidance on their inspection, condition assessment or remediation. A new CIRIA guide C794 has therefore been produced to address this omission.
As my co-author, Devon Mothersille, has quite rightly said “an anchor or nail that is designed and constructed correctly, to modern standards, needs minimal maintenance”. When we started drafting C794 we discovered that many of our codes and standards had also adopted this position too. Asset owners, such as Highways England and Network Rail, own many anchored and soil nailed structures which form critical infrastructure. Many of these structures were constructed before BIM and Digital Twins, and paper records (if available) have obviously degraded over time. Given this situation, asset owners must decide whether the supported structure is performing satisfactorily in accordance with acceptance criteria.
Even with accurate records, this would not be an easy issue to resolve, as access to anchors and nails is usually difficult. The assessment on the performance of the structure is principally the responsibility of the asset owner. The guide presents clear and robust information on the steps that can be taken to assist such an evaluation. Guidance is provided addressing intrusive inspection beneath the anchor or nail head, typically the critical section in respect of corrosion, and outlines specialist techniques for safe inspections. Regular inspections carried out in relation to the risk associated with the structure can help identify the changes over time and have been shown to extend service life.
When inspections need to be supplemented with more quantitative results, testing may be required. Lift-off testing for grouted anchors is covered in detail, providing guidance on the procedure, as well as specialist techniques such as the pressure gauge observation technique. Additionally, methods for connecting to anchor heads, with insufficient tendon projection above the head block are outlined. Safety procedures for anchor inspection and lift-off testing are also included, together with practical options for safe working, such as anchor head restraint and safety cages.
Information on monitoring of grouted anchors and soil nails is provided, in particular, the frequency of inspections, type and level of monitoring in relation to defined risks. Monitoring can seem onerous, but it need not be so, as it can be relatively inexpensive, may be combined with inspections in some cases, and can be highly effective in achieving extended service lives.
Asset owners will be aware of the structures that are anchored or supported by soil nails within their network or facilities and should understand the consequences of failure. The CIRIA C794 guidance document provides information on the assessment of both grouted anchors and soil nails, as well as options for remediation and repair, to assist the asset owner in making informed decisions.