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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 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: • 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
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
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
Beautiful cross bedding palaeocurrent
June 2005: overgrown with nettles, cross bedding just visible
Holt-Coddington Fault, Tear fault, 2.5 mile displacement
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
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!!!
castle Faulted block Work already underway Faces bolted to secure Before shotcrete applied
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
Close up of overhanging castle wall
Overhang has been in this state for more than 20 years. Ivy coverage has not been maintained and this has weakened the structure.
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”
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
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”
ROCK BOLTING Cherry picker cage used during drilling for rock bolts
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
ROCK BOLTING Rock bolt right through the cross bedding!
ROCK BOLTING Rock bolts glued in place Finger pointing to where the glue has oozed out
Rock bolts • To provide support for a metal framework to be attached before the shotcrete is sprayed on Metal frame/cage
Metal cage is created by using wire ties Metal pad to spread force over wider area
Shotcrete • Sprayed on concrete • Ordinary concrete delivered by wagon Local concrete delivery person!
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
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
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
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
After 3 sprayings of shotcrete Metal framework “thickening up” and being covered Blue sheeting To stop overspray
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!
Note concrete colour to blend but not an exact match Soil has been back filled August 2006, after
July 2006, rock bolted, and framework applied, first layers of shotcrete
I am very grateful to the guys on site from SprayCon and Colin Jones (Rock Engineering) Ltd for their patience in answering my questions!