Methods of Demolition of Old Structures

 Methods of Demolition of Old Structures

The techniques of demolition can be studied under two heads: -

1)      Manual Demolition

2)      Engineered Demolition.

In manual demolition ,the demolition of a structure is brought down by  use  of traditional tools such as sledge hammers and it is carried out by manual labour. In case of engineered demolition, the structure is demolished by means of sophisticated equipments such as water jets, Diamond cutting tools, wrecking balls, explosives and many more.

Engineered Demolition is further sub-divided into two categories:-

1)      Explosive Demolition

2)      Non-Explosive Demolition.

Explosive demolition or implosion is the process of bringing down the structure by means of explosives such as TNT or RDX, whereas non-explosive demolition is the breaking of structure by wrecking balls, Large hydraulic excavators, high pressure water jets, Precision diamond cutting tools etc.

Factors to be consider for Selection of Appropriate Techniques:

·              Safety of personnel and public,

·              Working methods,

·              Legislation applicable,

·              Insurance cover.

 

 

                             Flow chart of the activities involved in a demolition project

METHODOLOGY

Manual Demolition-

 

This technique is normally carried out by contractors using manual tools which are portable. Sledgehammer, pneumatic hammer and drills are used, supplemented by lifting tools such as cranes and tackle. This technique is considered non-engineered. The demolition contractor’s survey of available documents and field condition are the primary basis for equipment selection and removal sequence. This method can be chosen if the structure under consideration is a small one like a Bungalow or a two to three storied structures.

ENGINEERED METHODS

Engineered demolition involves the analytical evaluation of a concrete structure during all stages of demolition to verify the adequacy of its strength and stability. The deconstruction analysis is essentially a reverse process of the original construction of structure, with revised partial framing and alternate load patterns. The process of engineered demolition should include a thorough review of the existing condition of the structure, its proximity to other structures, utilities above and below grade, the preparation of demolition sequence, the procedure, the assignment of equipment to be used, an analysis of actual and altered structural models and the stability analysis of the structure.

 

1-A wrecking ball is a heavy steel ball, usually hung from a crane, that is used for demolishing large buildings.

 It was most common during the 1950s and 1960s.

With the invention of hydraulic excavators and other machinery, the wrecking ball has become less common at demolition sites because its working efficiency is smaller compared to that of long reach excavators. Although the wrecking ball is still the most efficient way to raze a concrete frame structure, it is decreasing in use.

Wrecking ball technique

In olden days the ball used for this work used to be a spherical one, but now-a-days the ball which is being used is of a pear shaped as shown in the picture above that is the lower portion is bulged while the upper portion is flat. Due to this the lifting and the swinging can be done easily. Also, this ball is much stronger as compared to its predecessor, since it is made by forging, due to which it gets compressed and stores a large amount of energy.

But now a days this method is becoming obsolete as it is quite slow, hence it is giving way to the new advanced methods.

2-Pusher arm technique

In this technique the structure is demolished by using Hydraulic excavators. Hydraulic excavators may be used to topple one- or two-story buildings by an undermining process. The strategy is to undermine the building while controlling the manner and direction in which it falls. The demolition project manager/supervisor will determine where undermining is necessary so that a building is pulled in the desired manner and direction.

 

For large, tall, storied buildings large hydraulic excavators of a height of 1000 feet are being used now a days.

Use of Large hydraulic excavators for the demolition of Terminal 2-A of Mumbai Domestic Airport.

 

In order to control the dust water hoses can be made use of  . Hoses may be held by workers, secured in fixed location, or attached to lifts to gain elevation.

3-Thermic lance technique

Netherlands  is specialized in the use of thermal lances.

Used when corroded or damaged parts cannot be dismantled with hydraulic equipment.

In this method the structure is demolished by making use of oxy-acetylene flame. Holes are drilled in the member which is to be demolished and a temperature of 5500 degree Celsius is created. This high temperature melts the steel and concrete.  The entire operation is done by using a welding torch.

In the last 5 years the use of the thermic lance for deconstruction and maintenance purposes has increased significantly.

The job can be done in a fraction of the time taken by conventional methods.

Thermal lancing can also be used for demolition of steel structures.

Advantages of thermal Lancing

1) Thermal lancing allows for demolition in areas that require minimal dust and noise.

 2) After the thermal lance cuts the steel, a crane can lift the steel from the building.

3) Under this method, demolition of a building occurs from the building's top to its bottom.

4) Due to its burning process, very little noise is created from the dismantling of steelwork, in comparison to other more explosive and machine orientated techniques.

5) Thermal lancing also provides vibration-free demolition.

6) Cost of demolition is moderate

4-Pressure Jetting technique

In this technique the destructive energy of water is made to use. A high-pressure jet is made to impinge on the surface of damaged concrete, which causes the removal of concrete from that location. This technique is very suitable for repair and maintenance purposes also.

The removal of concrete takes place due to high pressure of water and due to cracks on the concrete surface, in which water enters and due to it exerts pressure thus tearing the surface apart.

Since the upper surface of concrete has been eroded, a new layer can be easily applied as there will be a strong bond due to the indentations on the parent surface.

5-Saw Cut Method

In this technique a flywheel made up of high strength steel is made to cut the structure apart.

This method is ideal for cutting of bridges. In fact, the JJ flyover had been brought down by this method. Care should be taken that the Upper deck of the bridge is collected after the bridge is cut.

The cutting wheel used for the work is available in various sizes depending upon the type of work to be completed. The  cutting face of the wheel is impregnated with small bits of Diamond. Coolants should be continuously be sprinkled in order to control the temperature of the equipment.

 

6-Green Break Technology

¨  Used for under water demolition.

¨  With the Green Break Technology Non-Detonating Safety Power Cartridge, one can precision fracture specific underwater targets, without any damage to underwater sea-life, or structural foundations, with no shockwaves caused.

¨  we can achieve sequential blasting underwater which radically reduces blasting time and construction / demolition costs.

¨  GBT can be used underwater in highly sensitive areas, as well as for large scale demolition projects.

 

Microwaves Technique

When microwave energy is passed into the mass, its transfer causes heat, resulting in vaporisation of the absorbed liquid. Concrete disintegrates in this process.

 

 

Demolition of structure using Implosion Technology:
a CASE STUDY

Introduction

In western railway, Mumbai division, there was a G+3 storied building numbered as  25/T. It was built in 1924, constructed by BB & CI railway as a first cement concrete residential structure in Mumbai. It was having 80 Nos. (4×20) type I Quarter with total land area about 430 Sq. m. The total height was 13.20 m. above rail level.

 Demolition by Implosion

Implode’ means make a building collapse down on its footprint. The basic idea of implosion is quite simple, If we remove the support of structure of a building at a certain point, the structure of a building above that point will collapse. The explosives are used in these methods are such that the whole structure should fall towards its center of its gravity .

Drilling of holes for placement of explosive

    4 holes of 25mm diameter, 20cm deep are provided at each column.

 Type and Quantity of Explosive Used

There are two types of explosives commonly used for implosion RDX & gelatin. The electronic detonators were used to ignite the explosive. 250 gm of explosive in central 24 columns. & 125 gm in outer 16 columns.

Test Blasting had to be carried out in-order to check the impact and efficiency of the blast.

Wrapping of Holes

The wrapping of holes is done to ensure that due to explosion of charges the debris does not fly in air. For this purpose, the holes were covered with gunny bags and iron net after placement of explosive and detonators.

 

Ballasting

Ballasting of explosives work  are carried out in a controlled manner, such that there will be a time gap of 1/100 second between two successive blasts. The trigger of charges is done in such a control manner so that the noise pollution and air pollution should be minimum.The central column is first triggered and then blast proceed towards outside to produce three way action and hence results the fall of existing structure on its footprint.

Falling of structure

Once central support/column will be destroyed and adjoining columns will be weakened, due to its gravity the entire mass will come down on its footprints without damaging any adjoining asset.

 

Applications

 

• Secondary breaking in mines & quarries.
• Civil construction, demolition
• Tunneling, box cutting
• Site clearing
• Swimming pool excavations
• Residential, commercial & industrial site use
• Trench digging
• Farming operations
• Dimensional stone dressing and sizing
• Mining, open case and underground operations
• Delicate rescue work under ground or in collapsed concrete structures
• Rock and concrete cutting on delicate structures like dam walls
• Rock splitting on sensitive road cuttings. Removing over hang and dangerous rocks.
• Rock cutting close to sensitive services like water supply pipes, gas and fuel lines, electricity supply lines Concrete cutting in reconstruction of buildings.