Availability of suitable anchors
Rescue operators are very experienced at identifying and evaluating anchors for normal rescue loads . Finding anchors for guying large trees may be more challenging .
A 3000kg rescue vehicle without chocked wheels will be dragged on wet grass at forces of around 10kN – a surprisingly low figure , but the coefficient of friction between car tyres and grass is only 0.35 and the impact of even light rain will make this worse . The use of a vehicle as an anchor may be suitable for trees with little back lean , or trees which are generally stable where the guy is more for peace of mind than primary support .
A 30T excavator may have a maximum drawbar pull of around 290kN . Although this is significantly less than the ‘ worst case scenario ’ of our completely dislodged , heavily tilted pine tree example from earlier , excavators can be used to pull or push over all but the most heavily leaning trees .
As has been previously discussed , a critical consideration is the angle between the guy line and the arm of the leaning tree .
The further away from the base of the fallen tree that the anchor point can be , the better the angle of attachment and consequently the higher the moment that an equivalent force exerted on the guy will apply to the tree . This function follows a sine curve and can be plotted on a graph as an easy visualisation ( see image below ).
The image compares three guying options using the two trees behind the partially failed tree . Whilst none are ideal , a significant improvement in the security provided by the guy can be achieved with even a small investment of time to improve the angle of attachment . The three different rope angles in the image are shown on the graph by dotted lines of the same colour as each rope .
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