REMOVAL OF 50 TONNE TREE OVER RIVER BANK
History
The large 44 metre tall Eucalyptus teriticornis failed via a total root ball severance and slid root ball first into the water, then the main part of the tree collapsed onto the two main water Pipe lines. This type of collapse was estimated using the QTRA assessment as being 1/500,000 probability of occurring. Had the tree failed in a general lean over collapse it would have had the mass, volume and velocity to sever through both these pipes. This would have catastrophic results as each 1.9 metre diameter pipe carries 1.5 million litres of water per hour
We were unsure of the extent of damage caused to the pipe as we could not gain access to see exactly what, if any structural problems may have occurred during the tree failure. It was suggested that this could be a finger in the Dyke type scenario. In other words once pressure was relieved off the pipe from the tree, it could possibly burst.
We went through the various options for the removal of the tree off the pipelines and these were:
Helicopter: Declined due to: We couldn’t get the helicopter in close enough to the fallen tree due to the height of the trees either side of the two banks. Rotor wash could possibly have dislodged the tree off the top pipe or compromised the climbers ability to work safely over the pipeline.
Crane: Due to the proximity of the fallen tree from the road we would have had to close down the road for approx 3 days. This was unacceptable as this road is a main thoroughfare with heavy traffic flow. Option two was to place the crane onto the eastern bank. However the amount of soil removal, compaction and stabilisation was impractical to accommodate a 350 ton crane.
Large tow truck / Bull dozers: Declined due to: Couldn’t get heavy plant onto site due to the proximity of water mains under the surface of the soil. Also certain parts of the entrance track had a PSI rating of 310 PSI. This meant that no vehicle over 5 ton could safely access the site let alone work any where near the gulley.
Barge with Crane. Declined due to: Unstable in shallow water, with the creek being a tidal flow catchment, when the tide went out the barge would have become unstable as it rested on the bottom of the creek bed. If we had used this type of operation it would have had major problems with the collective failures in the river bank areas.
All the above options still required the climber to have independent access to the tree and the most suitable option that we came up with was to design and manufacture, a large A frame tripod to elevate the Tyrolean traverse system. These A frames had to be designed and manufactured to be able to withstand approx 7 ton loading and have a optimum working height of 5 metres high. This had to be manufactured in such a way as the whole system could be dismantled and manhandled into position. Each portion of A Frame could not exceed 110 kilo gram in weight.
Due to the general logistics and massive safety variables it was decided that the most effective, fiscally viable and safest way of completing the tree removal was to have 2 A frames for the critical Tyrolean (climbers skyline) and then another 2 A frames for the main haul Tyrolean system.The skyline span across the river from bank to bank was approximately 142 metres across and the mid span vertical height was 32 metres to the river below.
Stage 1.SET UP
The setup for this operation was done in many stages. The preliminary part of the operation was to have the areas both side of the river bank cleared of lantana and medium scrub so site access was free and unencumbered.
Then the Main Skyline A Frames were manually installed on the opposing banks. Each one of these A-frames weighed in at 585 kilos each. They came in 7 main pieces, which were the head, and three primary legs with three secondary leg boots. The A frames were partially erected laying on the ground and then winched into an upright position for the final positioning. The final placement of the legs was done using crowbars and various winch and rigging systems. Due to the weight issues of the A frames placement was critical.
Each A frame was held to the ground using 4 2.5 ton ratchet straps anchored to the ground.
Stage 2. REMOVAL OF BRANCHES
This portion of the operation consisted of the sectional removal of all the branches under 300 mm in diameter being removed off the fallen tree. On three of the main branches that were in proximity to the pipe we had to place a plumb Bob (string line with weight at the end). this was put into place to check if the tree moved at all, during the dismantling operation.
Each of the branches that were removed had to be done like playing a big game of pick up sticks. With much higher stakes of course. There was a high threat of total collapse of the tree if any branches were not removed systematically and with extreme caution. Each branch was secured to the small haul line ratchet block via a rigging system called a spider leg. This was two lengths of ropes attached to the hook at the end of the block.
Preload was taken up on the branch and the Arborist cut the branch free. The free branch was then lifted via the haul line ratchet block and pulled back to the bank using a Toyota Land cruiser Capstan winch attached to the haul back line. This branch was processed at the landing site and the haul system returned to the arborist.
Stage 3. REMOVAL OF MAIN TRUNK
Due to the fact that no machinery over 5 ton could get to the site, and excavation was out of the question due to proximity of the underground pipeline. Once all the branches were removed off the pipelines and brought back to the bank to be processed we had to then start lifting the twenty five ton stem with approximately 20 ton of soil still on the root ball off the pipe lines.
This was done using three 10 ton pulley blocks with five sections of 60 metre half inch chain. Each block was manually tightened using 6 men on the ratchet chain. Once the block came to its end of its travel we had to shift to the next block set. When all the systems were at full return capacity the arborist had to reset the chains by climbing onto the chains and shortening them ready for the next cycle.
The remaining stem had to be manually/mechanically removed from the pipe proximity.
All the 10 ton ratchet blocks were anchored to other trees around the area and anchor points had to be redirected to get the most effective lifting angle. The stem was first lifted to a near vertical position and then it was pulled at a forty five degree angle away from the pipe line.
Some of the technical logistics and equipment specs.
Main Skyline Haul Line. 13 mm steel cored double braid wire rope. Swl 10,000 kilo, length 190 metres.
Critical Tyrolean line. Ansteel blue 10 mm diameter, swl 18,000 kilo length 185 metres. Purchased via USA, special delivery
A-Frames X 4 each frame weighed 585 kilograms. And consisted of 7 parts that were dismantable for carrying into site and erected using manpower only. These were primarily designed by Daniel Oaten had to be certified via a structural engineer andmanufactured in Brisbane. The A frames had to take torsion loading of up to 7 ton and have an optimum working height of 5 metres, with legs that could be self levelling to work on variousterrain angles. The main head of the A frame had to be horizontal.
Chains. 290 metres of chain were used for this job. Approximate weight 1.7 ton.
Ratchet Blocks. Three ten ton chain ratchet blocks. These had a working pulley chain length of 6 metres. Each block weighed 75 kilo and were used as the primary leverage for pulling up the main log off the pipe. These were anchored to trees on the site. Each block had x 2 8 metre 6 ton capacity slings to secure the blocks to the trees.
A 500 kilo capacity ratchet block for use on the Main haul line for lifting off the branch sections. This was secured to the haul back line so that the sections could be brought back to either bank.