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HOLT CASTLE

HOLT CASTLE. aka: death of a fieldwork site, a tale of burial by shotcrete JLC summer 2006. WJEC Geology GL3. Key Idea 4 Engineering activities Problems of ground instability Cuttings – rock strength, stable and unstable patterns of geological structures. The site:.

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HOLT CASTLE

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  1. HOLT CASTLE aka: death of a fieldwork site, a tale of burial by shotcrete JLC summer 2006.

  2. WJEC Geology GL3 Key Idea 4 Engineering activities Problems of ground instability Cuttings – rock strength, stable and unstable patterns of geological structures

  3. The site: • Only 15 minutes from my College, a public access site (so no permission needed), no specialist footwear required • A RIGS site with a notice board about the geology and history of the area • Red sandstone, showing cross bedding, and pebbles carried by flash floods (Bunter Pebble Beds) • Holt Coddington Fault (only surface outcrop), and some small scale faulting • Shows 3D outcrop due to extraction of rock

  4. Noticeboard

  5. The site: • Also the site of a SAM - Scheduled Ancient Monument “Holt Castle”, 10 years ago fenced off with metal fence to deny access, although all the local families have their scars from “falling off the castle” • Romans originally extracted red sandstone from here, castle built in 1600’s and many local buildings are also made of the sandstone

  6. June 2005 • A couple of years previously the site was given an explanation board (Local Authority) • The site had also been given some fences, although these were not to prevent access to the rock faces

  7. June 2005.

  8. Beautiful cross bedding palaeocurrent

  9. Cross bedding

  10. Cross bedding, here damaged by rock bolts

  11. June 2005: overgrown with nettles, cross bedding just visible

  12. Holt-Coddington Fault, Tear fault, 2.5 mile displacement

  13. June 2005 • Site overgrown with nettles • This is due to the fences not allowing the Local Authority lawn mower access close up to the rock faces, and also less human and animal trampling to keep them down

  14. Nettles are 1.45m high!

  15. Ancient castle, walls overhanging, very overgrown with vegetation Faulted block Cross bedded area Fenced off Viewpoint under shady tree! June 2006, no nettles, but loads of machinery!!!

  16. castle Faulted block Work already underway Faces bolted to secure Before shotcrete applied

  17. Castle, large overhang! Use of metal pillars to support Before metal cage and shotcrete applied High nettles, not mowed by council due to fencing Helpful, onsite workers! June 2006: castle wall being shored up

  18. Close up of overhanging castle wall

  19. Overhang has been in this state for more than 20 years. Ivy coverage has not been maintained and this has weakened the structure.

  20. June 2006 • the site workers say … • CADW have found some money and decided the castle was unsafe and are going to carry out remedial work • CADW: are the Welsh Assembly Government Historic Environment division, it’s from the Welsh word “to keep”

  21. Remedial work • stability of overhanging corner (and another wall of the castle), to be given metal reinforced concrete cage to support it • Rock bolting of weaker faces, and shotcrete to be applied

  22. Consultation? • NEWRIGS had prepared a report on the geology of the site for CADW, 2 years previous to this, but had not been consulted since • In discussion with the on-site CADW archaeologist “CADW will protect the SAM at all costs, including covering the rock faces with concrete”

  23. ROCK BOLTING Cherry picker cage used during drilling for rock bolts

  24. ROCK BOLTING Galvanised bolts Average 2.7m depth Takes about 5 minutes to drill Screw pattern to attach nuts These are not STRESSED anchors They are not under tension

  25. ROCK BOLTING Rock bolt right through the cross bedding!

  26. ROCK BOLTING Rock bolts glued in place Finger pointing to where the glue has oozed out

  27. Rock bolts • To provide support for a metal framework to be attached before the shotcrete is sprayed on Metal frame/cage

  28. Metal cage is created by using wire ties Metal pad to spread force over wider area

  29. Shotcrete • Sprayed on concrete • Ordinary concrete delivered by wagon Local concrete delivery person!

  30. Concrete fed through hopper, SA160 added to control set time, and mix sent to sprayer Control panel for speed and mix variations Sheeting to minimise damage to grass Tester samples, to test set rate

  31. SA160 added to make concrete set • Aim for setting time of 10-15 minutes • But once a few layers have been sprayed it is left for 24 hours to cure • Operator works from a cherry picker and has full control of both cherry picker and shotcrete hose

  32. Sprayed over the site, Much reliance on “eye and experience” technique of operator Hose 1: compressed air From an on site compressor Hose 2: larger, concrete feed concrete mixed with SA160 to shorten the setting time to approx 15 minutes

  33. Hose only operating at ¼ speed otherwise too much force and the guy would be thrown out of the cherry picker! Blue sheeting Covers up areas not to be sprayed

  34. After 3 sprayings of shotcrete Metal framework “thickening up” and being covered Blue sheeting To stop overspray

  35. Shot crete final coat • Final coat will have a colour added • But CADW are adamant that this should NOT exactly match the rock faces • So it can be seen that this has been added and is not natural!

  36. June 2006, during

  37. Note concrete colour to blend but not an exact match Soil has been back filled August 2006, after

  38. June 2006: castle wall being shored up

  39. July 2006, rock bolted, and framework applied, first layers of shotcrete

  40. August 2006 after shotcrete completed

  41. I am very grateful to the guys on site from SprayCon and Colin Jones (Rock Engineering) Ltd for their patience in answering my questions!

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