Gerry Manley (Letters, GE February and March 05), tells me that he heard of alleged failed grouting sites in the Great Western Territory ('Railways - appropriate technology', GE January 05) through former colleagues.
I fully accept he reported this in good faith. I am aware of problems with some sites treated using 50mm tubes between Swindon and Shrivenham; my section neither designed nor supervised those works.
The British Railways embankment grouting system was based on forming a strong, thick (50-150mm), continuous seam of cementitious grout along the slip surfaces, tension cracks and imminent tension cracks by means of hydrofracture displacement grouting followed by penetration grouting into the overlying ballast pocket to expel water and stabilise the ash/ballast fill.
The information in Ciria report C550 Infrastructure embankments is restated in the 2nd Edition (2003) C592 where section 5.6.1 includes 'Pressure and fracture grouting'. The fi gure 5.14 'Principle of injection grouting of embankment' shows neither the continuous grout seam nor the embankment pockets.
The report says: 'Care should be taken that the injected grout does not cause heave of the embankment, propagation of the fracture to the surface of the embankment, or block embankment drains.' This excludes all the 600 sites I worked on because we followed this guidance. Extra grout and points were specifi ed for old counterforts, transition to underbridges, wing walls and for certain foundation soils to deal not just with the slip but with the railway.
The sites included a few cuttings plus design for embankment slips on French railways where drainage had failed and a new retaining wall had been pushed over. It does not include many sites treated on the Eastern Region, London Midland Region and Southern Region.
The contractor involved on the sites reported: 'Pressure grouting with a sand-cement mix was used to fill small voids in the bank material and so minimise future settlement and reduce maintenance costs.' (sic).
This was similar to the approach of the 1947 Great Western Railway embankment trials, which also failed.
The BR (Western Region) engineers then developed equipment, techniques and rules resulting in the present successful method. Supervision in their compliance is essential, especially to ensure that the grout is injected as recorded.
Doug Ayres, soil mechanics engineer, British Railways Board (retired)