(A) General
Mechanical demolition generally involves the use of large machinery with attachment to dismantle the building from outside. The common mechanical methods include the use of a pusher arm, wire rope and clam shell. The operations of these applications are illustrated in Figure 4.18. These methods shall only be applied to isolated buildings on relatively flat ground. The concerns and good practices of the mechanical demolition generally included the following:
(1) The machine shall be operated on smooth and firm ground. It shall also have adequate counter-weight to prevent overturning during the operation;
(2) The equipment and accessories such as attachments and rope shall be inspected frequently and shall be repaired or replaced whenever necessary;
(3) The impact of the collapsed structural sections on the floor or ground shall be checked to prevent the potential overloading of the suspended floor, vibration and disturbance to adjacent properties and damage to underground utilities;
(4) The site shall have full time security to prevent unauthorized personnel entering the site. No person shall stay within the working area of the machine and the building while the machine is operating;
(5) Sufficient water spray or other anti-dust precautions shall be provided to minimise air pollution by dust;
(6) The cab of the machine shall be equipped with impact proofed glass and its construction shall be robust enough to protect the operator from flying debris; and
(7) A spot person shall be on site full time to provide guidance and assistance to the operator in the demolition process. In addition to the above, specific criteria for each mechanical method are discussed in the following sections:
(B) Mechanical Method by Pusher Arm
Mechanical pusher arm involves the use of machines equipped with a pusher arm attachment for applying horizontal thrust to demolish the structural element. Special conditions for pusher arm demolition are listed below:
(1) The pusher arm shall be constructed of steel or equivalent material and shall have adequate strength to operate on the building; a crane boom shall not be used;
(2) Minimum safety distance of 0.5 times the height of the building element being demolished shall be maintained between the machine and the building for pushing into the building;
(3) Minimum safety distance of 1.5 times the height of the building element being demolished shall be maintained if structural elements are pulling out of the building;
(4) The point of application of pushing shall not be less than 2/3 of the height and not more than 600 mm below the top of the wall; and
(5) The pusher arm method shall be limited to buildings less than 15 m high.
(C) Mechanical Method by Deliberate Collapse
Mechanical demolition by deliberate collapse generally consists of systematic removal, or weakening of the key structural elements to induce the collapse of the structure. Special conditions for deliberate collapse are as follows:
(1) Minimum safety distance of 1.5 times the height of building element being demolished shall be maintained throughout the operation;
(2) The procedures shall be carefully designed and executed in the removal of key structural elements so that there will be no pre-mature collapse and the structure will collapse onto the anticipated area; and
(3) Application of the deliberate collapse method shall be limited to buildings less than 15 m high.
(D) Mechanical Method by Wire Rope Pulling
Mechanical demolition by wire rope pulling generally involves the use of an earth mover machine or mechanical winch device equipped with heavy steel wire for pulling down structural members. Special conditions for wire rope pulling are listed in the following:
(1) A safety distance of 1.5 times the height of element to be demolished shall be maintained between the machine and the building during the pulling;
(2) The machine shall always travel parallel to the line of pull during the pulling operation;
(3) In the case when pulling is done by a pulley, such a pulley device shall be securely anchored;
(4) The wire rope or chain shall be composed of steel with tensile strength not less than 4 times the theoretical force required to perform the pulling;
(5) The wire rope used for the operation shall be inspected for wear and damage at least twice daily and replaced as necessary;
(6) Any sharp edge that is to be wound by the rope shall be protected to minimise the possibility of cutting or wearing of the rope during pulling;
(7) The bottom of the wall may be pre-weakened with care and protection to ensure controlled collapse;
15 m high; and
(9) All workers shall stay away from the area within reach of the rope or wire in case it breaks.
(E) Mechanical Method by clam shell
Demolition by clam shell typically involves the use of a crane equipped with a clam shell attachment which progressively bites away the structure. Special conditions for clam shell are listed in the following:
(1) A minimum safety distance of 0.5 times the height of the building element being demolished shall be maintained between the machine and the building during the operation; (2) The process of biting off the structural elements shall begin
from the top and progress downwards; and
(3) The clam shell shall be operated not less than 1 m above the structure being demolished.