The Picnic Chamber Viewing Platform - Royal Arch Cave, Chillagoe
General
Queensland Parks and Wildlife Service (QPWS) manages three guided tour caves at Chillagoe, one of which is the Royal Arch Cave. A daylight chamber near the entrance is known as the Picnic Chamber.
The Picnic Chamber is considered by many to be one of the most scenic chambers in the Royal Arch Cave. This daylight chamber acquired its name because it was a very popular picnic spot in the first three decades of this Century.
The guided tour takes a fairly lengthy pause of around ten minutes in this chamber. In past times it was considered appropriate to install a picnic table in the centre of this chamber to give people a place to sit. The table served its purpose well enough in the periods of lower visitation but was inadequate in high visitation periods when group size could be up to forty. This resulted in people having to stand lined up on the concrete path around the table and on the approaches to the table. It was not appropriate to allow people to stand off the path because they would be standing on mud which supports the cave flora (algae and mosses) or they would be standing on limestone formation.
Often elderly or infirm people would opt to stay at the table while the rest of the group continued through the cave. These people would rejoin the tour on the way out of the cave.
QPWS decided in 1997 that the table should be replaced by a minimum environmental impact viewing platform offering more seating area and a much larger area for people to stand. A design was developed, plans drawn and certified, with construction commencing in August 1998.
Design
Our first and most obvious consideration was to design something to fit into the existing space available without having to modify the cave features. We ended up with an "L" shaped platform with the leg of the L measuring five metres by three metres and the foot of the L being two metres square.
Consideration also had to be given to how to connect the platform up with the existing pathways. We decided to alter the pathways to suit but keep them similar in style to the existing pathways.
There were a number of constraints to consider in our design options. These were:
- Processes and materials used in construction had to have as low an impact as possible on the cave environment.
- The visual impact had to be kept to a minimum.
- All materials had to be as durable as possible.
- The flooring of the platform had to be non-resonant and not capable of becoming slippery.
- The platform had to be built from materials which were able to be carried into the chamber by hand.
- The cost of the structure had to be kept within budget allocations.
- The construction had to be simple enough to allow for the building skills of the available staff.
The final design consisted of a "Corrotech" deck supported by large hardwood bearers on top of stumps made of 150mm PVC stormwater pipe filled with reinforced concrete and set into a good sized concrete footing.
"Corrotech" is a product developed by Corrosion Technologies in Doveton Victoria. It is basically pultruded fibreglass produced in a number of configurations. It can be obtained as round handrail sections, square post sections and a grate type decking.
Hardwood seats were provided down two sides of the leg of the L and across the bottom the L making a total of eleven metres of seating.
Materials
A lot of thought was given to materials and the advantages and disadvantages of the different options available. It was decided early on in the design process that galvanised materials were not to be considered due to their tendency to leach zinc which is toxic to many of the lower plant forms. It was decided that 316 grade stainless steel would be the most suitable option for bolts and brackets.
Stainless steel bolts were very expensive to buy so we used stainless threaded rods cut to the required length with a nut each end. Other advantages of this system are that it gives infinite adjustability should the timber shrink with age and it makes it much easier to work out your bolt requirements. The only disadvantage of using stainless steel for the brackets was that it is very difficult to cut and drill and as there were over forty brackets each with four 9/16 holes a lot of time was used in their preparation.
Hardwood was chosen for the bearers because it is easier to work than any of the metals, is durable (we have some thirty year old timber boardwalks which continue to meet safety requirements) and it tends not to resonate as readily as metals.
Other reasons we avoided metals was their cost, weight and specialist skills and equipment required to work alloys such as aluminium. The thought of electrolysis occurring in aluminium joints also made us uneasy.
Bearers made of "Corrotech" were also considered but the cost and size required to have the requisite load bearing properties were both enormous.
"Corrotech" grated decking was chosen as being the most suitable decking material for a number of reasons. It is quiet, inert, grippy and available in any colour required. Although it is more expensive than other options we thought it was worth a try.
The top of the decking is coated in very grippy silica chips. It is virtually custom built as special nosings are incorporated into the decking sheets where ever there is an entry point onto the platform. The price for the decking worked out to about two hundred and twenty five dollars per square metre.
Construction
Work was begun on the platform on the seventeenth of August with setting up of the profiles. The holes were then dug and the footings and stumps poured, the path was altered to match the level of the platform on the top end and the bearers were bolted in place before the corrotech was screwed down. Although it only takes one sentence to describe it actually took two months to get this far. Due to budgetary timelines we had to commence the project at the time of year when we are busiest with school group tours. Therefore work on the platform could only be undertaken when guiding schedules allowed.
During construction an effort was made to minimise the amount of sawdust left in the cave environment. A tarpaulin was spread under the work area when the saw was in use and often a vacuum cleaner was used to collect the dust as it came off the saw.
We attempted to finish the timber with a product called "Woolube" which is basically lanolin extracted from wool. In a situation where the timber is not subjected to humid conditions it probably would be an excellent timber conditioner but in our situation it was not suitable and actually appeared to promote the growth of mildew.
Eventually the platform was completed after further delay caused by the wrong size stair treads having been sent. This would not have caused such a hold up if I had realised earlier that they were incorrect. Although I measured the decking material on arrival, I overlooked the stair treads and didn't realise the error until it was almost time to install the stairs.
Things we will do differently next time
1 - Measure the stair treads. This exercise was a valuable learning experience.
We now know that next time we should try to allow for drip zones during the design phase. This will mean having to wait until the wet season to do part of our planning. Alternatively we will need to think well ahead and do a site plan and drip zone survey as soon as a project is envisaged. The consequences of not doing this for our platform are that we have two drip points directly above the seats with the result that the ends of our seats are now more prone to rot than they should be.
We also now realise that if we wish to use timber inside a cave it should be durability class one hardwood with a low moisture content and it should be sealed to resist fungal attack. Oil finishes do not appear to be effective.
Platforms decked with Corrotech need to incorporate a catcher tray below the Corrotech where appropriate. It quickly became evident once we got the floor down that any small items dropped will go straight through and shall be difficult to retrieve. With our platform it was not appropriate to include a catcher tray because it is in a daylight Chamber and we hope to have regeneration of plant life below the platform.
Conclusion
This platform was our first attempt at producing a low impact, high capacity development within any of the Chillagoe Caves. Previous developments had been undertaken in both the Trezkinn and Donna Caves but both of these were done before the effects of zinc on the cave environment were considered. Both of the developments were done with extensive use of galvanised materials and although we have no concrete proof that zinc leaching will have an adverse effect in a dark cave, the evidence suggests that it is not a good alternative in daylight areas.
The finished platform looks and appears to perform as well as we had hoped and with the lessons learned from its construction we hope to be able to do an even better job next time.