From 44 gallon drums and cavers to concrete, 'caissons' and engineers - creating an entrance shaft into Ruakuri Cave, Waitomo, New Zealand

John Ash

Setting: Ruakuri Cave is situated at Waitomo Caves, New Zealand. It is a 5 km plus network of large vadose river canyons and smaller phreatic passages cut through a fault-bounded block of Oligocene Otorohanga Limestone. The system has three main levels of passages, two dry entrances, three walk- climb in stream entrances, two abseil shafts, one siphon, one excavated entrance, two possible digs and three reasonable question marks.

History: In 1904 tours accessed the cave via a sloping rock debris pile leading down into a wide passageway at the base of a 60m high cliff face. Over the last 4-500 years New Zealand native Maori had used this entrance, and higher openings in the cliff face, as a waahi tapu or burial site. In 1906, after paying compensation to the tour pioneer, James Holden, the government took over the operation. On Waitangi Day, February 7, 1988, the cave was closed to dry tours as a result of a land dispute between descendants of James Holden and the government operator at the time, the Tourist Hotel Corporation (at that stage THC also operated the Glowworm Cave and Aranui Cave). Subsequent cultural issues ensured that the waahi tapu entrance would not re-open. The dry tours ceased to operate. For the next 17 and a half years the Holden family investigated alternative means of accessing the cave. These included: 1989-1992: Consulting with the Department of Conservation and Maori elders (kaumatua) for a culturally acceptable means of re-opening the traditional entrance. This negotiation was finally abandoned. 1993-4: Tunneling into the upstream section of the cave and creating a boat ride to link up with cantilevered walkways that would connect with the old tour route. This initiative stimulated competing activity from a neighbouring landowner. Eventually it resulted in the Holden's purchase of 5 acres of adjoining land, thereby ensuring the protection of this section of the cave. 1994: Enlarging the stream entrance used by Black Water Rafting adventure tours. A few quick cash-flow calculations ensured that the wetsuit-clad, cave rafters were left in peace. 1995-2000: Engaging a developer to obtain relevant Resource Consents and to investigate excavating and enlarging the Rimrock extension - a passage that was extended, explored and mapped, with the assistance Black Water Rafting guides, in 1996-7. The extremity of this passage was connected to the surface by drilling from a ground survey point. Further investigations involved a baseline report on the cave climate, trials with a 40,000 psi rock-cutting water blaster and consulting with underground pipe-thrusting experts and theme park ride designers. In the final analysis the term "open cheque book engineering" was coined. The developer went into voluntary receivership. 2000-2003: Forming a new partnership with APR, one of the consulting companies involved in obtaining the Resource Consents, and The Legendary Black Water Rafting Co. (BWR). This partnership acquired the assets of the former developer, which included the consents and ten 2.4m by 2m concrete sewer pipes. Focus now shifted from the Rimrock passage to the Drum entrance, which had been excavated by recreational cavers and lined with 44 gallon drums in 1968. It was situated about 80m north of the previous site, just within the Ruakuri Bush Scenic Reserve administered by the Department of Conservation (DOC).

Engineering: Because this section of cave lay under crown land administered by DOC, a concession needed to be applied for and a new set of conditions added to those in the original Resource Consent. The developers now had to monitor: the cave climate for: • air flow • temperature • humidity • carbon dioxide and • volatile organic compounds • vibration and speleothem damage • stream sediment loads • air and water pollution • cave sediment excavations • cave biota and • energy inputs. Limits of acceptable change were established and standards were set for all earth and engineering works. Engineering designs evolved - from simply enlarging and shotcreting the existing Drum entrance, to sheet-piling a cylinder directly at the entrance, to sinking a caisson offset from the entrance and tunneling into the cave. The approach depended on how much geotechnical information one was prepared to collect, engineering expertise and creativity, tenacity and the size of one's cheque book. In 2003, Tourism Holdings Ltd (THL), who operate the Glowworm Cave and Aranui Cave, made an offer to purchase the Ruakuri development as well as the Legendary Black Water Rafting Co. At the time THL were New Zealand's largest tourism operator; they had the capital backing and the move made strategic sense. Back to a three cave operation. Extra geotechnical drilling was carried out and the nature of the fault line on which the Drum entrance was situated was evaluated. It was then decided to proceed with an excavation off-set from the present cave access and connect this to the entrance chamber by a tunnel. Contracts were let to construct a top-down caisson that would be shotcreted to a wall thickness of 300mm. Geotechnical cloth and plastic drainage grid would be installed around the perimeter of the structure to maintain the hydrological integrity of the host ground. Detailed surveying was carried out to calculate the final baseline of the excavation and the orientation of the inter-connecting tunnel. This information was critical if speleothem damage within the first chamber was to be avoided or minimised. The inter-connecting tunnel was to be formed by pipethrusting the ten surplus 2.4m diameter sewer pipes from the floor of the caisson directly into the cave. It would pass beneath the original 44 gallon drum entrance. Consent parameters were continually monitored and temporary 'airlock' doors were installed in the pipe tunnel prior to breakthrough into the cave. Once access to the cave proper had been achieved, in-fill sediments in the Drum passage were hand excavated and removed by wheelbarrow, skip and crane. Sediment floors and banks were channel sampled for later analysis. Buried, broken speleothems were also marked and collected for later study. Steel formwork was then attached to the walls of the entrance cylinder in preparation for the pouring of a 150 metre, 1 in 10, spiral concrete access ramp. Power supply cables were readied for feeding into the cave before the entire structure was roofed over. 'Airlock' doors were installed in the entrance and exit openings to the caisson as well as in both ends of the inter-connecting tunnel and a water 'shower' established - dripping from the roof onto a large piece of karren in the centre of the floor. The latter feature gave some degree of control over the humidity and temperature of the air in the entrance cylinder. The finishing touches involved theming the outside of the structure that protruded above ground level with gunite, to make it look like, and blend in with, surrounding karst outcrops - and burying, landscaping and planting the roof. Finally, on July 28, 2005, after addressing all of the stakeholders' needs and having staff swim 200 metres to work for eighteen months, dig out 300 tonnes of silt by hand, mix 1000 cubic metres of concrete and lose 10 kilograms in weight each, a 17 and a half year project reached a successful conclusion. Ruakuri Cave re-entered the tourist arena as an innovative 1.2 km long, dry, wheelchair accessible journey.