HOLT CASTLE

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
(No Transcript)
6
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 1600s and many local
  buildings are also made of the sandstone

7
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

8
June 2005.
9
Beautiful cross bedding
palaeocurrent
10
Cross bedding
11
Cross bedding, here damaged by rock bolts
12
June 2005 overgrown with nettles, cross bedding
just visible
13
Holt-Coddington Fault, Tear fault, 2.5 mile
displacement
14
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

15
Nettles are 1.45m high!
16
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!!!
17
castle
Faulted block
Work already underway Faces bolted to
secure Before shotcrete applied
18
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
19
Close up of overhanging castle wall
20
Overhang has been in this state for more than 20
years. Ivy coverage has not been maintained and
this has weakened the structure.
21
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, its from the Welsh word
  to keep

22
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

23
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

24
ROCK BOLTING
Cherry picker cage used during drilling for rock
bolts
25
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
26
ROCK BOLTING
Rock bolt right through the cross bedding!
27
ROCK BOLTING
Rock bolts glued in place Finger pointing to
where the glue has oozed out
28
Rock bolts

• To provide support for a metal framework to be
  attached before the shotcrete is sprayed on

Metal frame/cage
29
Metal cage is created by using wire ties
Metal pad to spread force over wider area
30
Shotcrete

• Sprayed on concrete
• Ordinary concrete delivered by wagon

Local concrete delivery person!
31
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
32

• 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

33
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
34
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
35
After 3 sprayings of shotcrete Metal
framework thickening up and being covered
Blue sheeting To stop overspray
36
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!

37
June 2006, during
38
Note concrete colour to blend but not an exact
match Soil has been back filled
August 2006, after
39
June 2006 castle wall being shored up
40
July 2006, rock bolted, and framework applied,
first layers of shotcrete
41
August 2006 after shotcrete completed
42
I am very grateful to the guys on site from
SprayCon and Colin Jones (Rock Engineering) Ltd
for their patience in answering my questions!
43
(No Transcript)
44
(No Transcript)