With the fascination caves heightened since the Wild Boars soccer team became trapped for a harrowing 13 days in a cave formation in Thailand last month, we thought we’d talk about some caves a little closer to home and how vibration from development might affect them.
The South Australian Naracoorte caves first came to global scientific attention after 1969, when cave explorers entered relatively inaccessible limestone chambers and found fossils of marsupial lions, enormous kangaroos and giant monitor lizards.
The find created a buzz around the world with the Naracoorte Caves now known as one of the world’s best fossil sites.
Fast forward to today and the caves and their extensive fossil record are now recognised as the Naracoorte Caves National Park and are a World Heritage Listed site.
The park is a visitor destination, with a camping ground and caravan park, dormitory accommodation for groups, picnic grounds and a licensed cafe. The caves are also of important ecological significance as thousands of southern bent-wing bats breed there each year.
As is often the case in National Parks, construction work does sometimes need to occur. Whether its footpath or road creation for accessibility to amenities, Resonate are often called in to assess the noise and vibration impact of certain activities and provide recommendations to help with management of these activities.
The vibration investigation
The intent of our investigation was to understand the vibration transfer from surface level construction activities to the cave decorations. Of particular importance was the impact of vibrations on speleothems, or straws, which are structures formed in caves by the deposition of minerals from water over thousands of years. In this case, the longest straw in the cave is approximately 200 cm long.
We have many tools at our disposal to measure and test potential impacts of noise and vibration. We also refer to published papers, where similar studies may have been completed in other parts of the world. This was the case for this study.
Measurements were carried out with two vibration instruments with intent to simultaneously measure both surface and cave vibrations. The surface measurements were carried out directly above the cave measurements at two locations.
The vibration source used was a Wacker Neuson compactor (name plate mass of 450kg) with a measured forcing frequency of 73Hz. The compactor was operated by traversing the machine continuously over the cave near the two measurement locations.
The results indicated that surface vibration measurements were likely to be generally representative of that measured in the cave in the straw locality. During this time, vibration was easily felt within the cave including on the hand rails which were connected to the cave floor.
Since the late nineties, several investigations have been made to check the influence of earthquakes on speleothems in caves. Of relevance is paper “Prevention of speleothem rupture during nearby construction” (2002) published by Lacave et. al. The paper documented the results of a study which evaluated the risk of speleothem rupture caused by explosive shots that were planned for the excavation of a new highway tunnel.
Relatively long and slender speleothems are the most vulnerable for two reasons:
- The longer the speleothem, the greater the lever of the inertia forces and bending moment, and the smaller the diameter, the higher the maximum tensile bending stress for a given bending moment, and the smaller the diameter, the higher the maximum tensile bending stress for a given bending moment.
- The longer the speleothems, the higher the probability that they contain a weak section, i.e. an internal structural irregularity with a low rupture stress.
From the vulnerability curves presented by Lacave et. al., it was also possible to estimate the percent probability of rupture vibration level for the speleothems located in Naracoorte Caves.
The results showed that the cave area near the straw decoration was sensitive to surface vibration. We understood that a relatively thin rock layer exists between the surface and the straws (i.e. approximately 3 m) and also noted that the rock protruded above the surface soil layer in places. The cave vibration level also doubled in magnitude when the small compaction unit traversed over the cave rock layer protruding through the surface soil layer.
Given this, we recommended that care be taken with general construction activities, with vibratory compaction plant, rock breaking, jack hammering or ground ripping. The use of vibratory compaction plant above 3 tonnes would not be recommended while operating above or very near to the cave footprint.
We also suggested that when appointing contractors for future works, a vibration management plan be requested as part of the contract. The plan should document how the works will be managed to ideally not generate more than 6 mm/s PPV at the surface through construction works. We adopted 6mm/s PPV with intent to reduce the probability of rupture to 0%.