HOLT CASTLE

aka: death of a fieldwork site, a tale of burial by shotcrete

JLC summer 2006.

WJEC Geology GL3

Key Idea 4Engineering activities

Problems of ground instabilityCuttings – 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

Noticeboard

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

June 2005.

Beautiful cross bedding

palaeocurrent

Cross bedding

Cross bedding, here damaged by rock bolts

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

Nettles are 1.45m high!

June 2006, no nettles, but loads of machinery!!!

Cross bedded area

Faulted block

Ancient castle, walls overhanging, very overgrown with vegetation

Fenced off

Viewpoint under shady tree!

Faulted block

castle

Work already underwayFaces bolted to secureBefore shotcrete applied

June 2006: castle wall being shored up

Castle, large overhang!

Use of metal pillars to support Before metal cage and shotcreteapplied

Helpful, onsite workers!

High nettles, not mowed by council due to fencing

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 weakenedthe 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 cageused during drillingfor rock bolts

ROCK BOLTINGGalvanised bolts

Average 2.7m depthTakes 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

Fingerpointing 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 createdby using wire ties

Metal pad to spread force over widerarea

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

Sheeting to minimise damage to grass Tester samples, to test set rate

Control panel for speed and mix variations

• 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

Hose 1: compressed airFrom an on site compressor

Hose 2: larger, concrete feedconcrete mixed with SA160 to shorten the setting time to approx15 minutes

Sprayed over the site,Much reliance on “eye and experience” technique of operator

Blue sheetingCovers up areas not to be sprayed

Hose only operating at ¼ speedotherwise too much force and the guy would be thrown out of the cherry picker!

After 3 sprayings of shotcreteMetal framework“thickening up”and being covered

Blue sheetingTo 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!

June 2006, during

August 2006, after

Note concrete colour to blend but not an exact matchSoil has been back filled

June 2006: castle wall being shored up

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

August 2006 after shotcrete completed

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