Demolition by implosion technology of serene towers at Kochi, India

02060


Introduction
Urbanisation has accelerated the growth of the urban environment, resulting in the construction of high rises.Urban planning and space restrictions have led to the birth of new high-rises and the razing of existing buildings.High-rise building destruction is a challenging task, especially in cities. Demolition of buildings is typically done when a structure had reached the end of its useful life, when it has suffered substantial damage from corrosion, fire, etc., when local utility needs have changed, when a building violated the law, or in other situations.Demolition of any structure is a ground-to-earth technique which means destroying or falling down a building with the help of equipment, machinery, explosives or manual techniques without affecting the surroundings.Demolition is a simple process for small buildings or houses.The building is brought down either manually or mechanically using large hydraulic equipment: elevated work platforms, cranes, excavators or bulldozers [5].After pre-demolition tasks, including surveying, removing hazardous materials, and a stability analysis report, the demolition method is chosen.Implosion is defined as the designed collapse of a building using explosives within its footprint [1].
High-rise building destruction can be completed more quickly and at a lower cost using controlled explosions and delayed detonation techniques.Implosion is the deliberate placement of explosives and the timing of their detonation to collapse a structure while causing the most minor physical injury to its immediate surroundings.The damage potential of buildings/structures, however, varies widely depending on the type and quality of construction, the nature of excitation, the surrounding soil strata, structural resonance, etc. [7].During the demolition by blasting techniques, it is to be ensured that the impact of vibration induced by the touchdown of the collapsing structure does not adversely affect the safety of the buildings nearby and delayed detonation techniques are adopted for the same [6].Controlled explosion using the delayed detonation technique was successfully used for the demolition of five residential towers situated in Cochin, India, in January 2020.[7].This paper deals with the reason behind the demolition of Serene Towers, the engineering challenges behind the demolition of the towers, various preventive measures adopted to control the impact of blasts and the field monitoring carried out to measure vibrations.

About the Project -Serene Towers
Maradu is located just 7kms away from Kochi.Maradu Grama Panchayat was formed in the year 1953 and was upgraded to a municipality in the year 2011.In 2006, the Panchayat, without obtaining the mandatory permission from the Kerala Coastal Zone Management Authority (KSCZMA), issued building permits to the builder and the construction of Serene towers started in the year 2007.People started occupying the flats in the year 2012.The Serene flats were located on the shores of the Vembanad backwaters at Kundannoor.It comprises two towers of 16 floors with 60 apartments each with a total built-up area of 2 lakhs square ft. Figure 1 shows the location and image of Serene Towers before demolition.The area's soil profile comprises very loose topsoil, followed by very soft marine clay extending to a depth of 15 metres.Below this, medium stiff laterite strata were noted extending up to 32m.From 32 to 36m, very dense sand with decayed wood was noted.This was followed by very dense coarse sand extending up to 48m.Below this hard clay was noted extending up to a depth of termination of the borehole at 50m.As the topsoil in this area comprised of fill followed by very soft Cochin marine clays up to a depth of 15 metres, bored cast in-situ piles terminating in the very dense sand at 40 to 48m were adopted for high-rise constructions.

Background and Timeline
In 1996, the Coastal Zone Management Plan was introduced, and Maradu, a Grama panchayath, was marked in the CRZ III Category.In 2006, the Panchayat, without obtaining the mandatory permission from the Kerala Coastal Zone Management Authority (KCZMA), issued building permits to the builders in the consideration that since massive development is happening in the area, the high density of population and the proximity to Cochin corporation, the area can be treated as in CRZ II category.During construction works of the towers, a vigilance probe pointed out that the sanction violated CRZ rules.The apartment builders approached the court and got a stay order for the same, and the work of the apartment was completed in 2010 and was handed over to residents.Kerala Coastal Zone Management Authority (KCZMA) became a party to the case and moved to the Supreme Court.After a lengthy legal battle at various courts, the Honourable Supreme Court finally issued an order in September 2019.In its order, the Supreme Court division bench said permission granted by the panchayat was illegal and void and directed the state government to raze four buildings, namely, H20, Serene (Tower 1&2), Coral Cove Apartments and Golden Kayaloram.The Supreme Court order came as a rude shock to the residents.Several political parties and organisations rallied behind the residents and staged protest marches, as depicted in Figure 2.

Major Challenges for Implosion
The significant challenges for demolition were that 40 odd houses surrounded the Serene Towers in a densely populated area, a ten-storied hotel complex in the adjacent plot and the proximity to backwaters.Utmost care has to be taken to carry out an explosion without affecting nearby houses and damaging the surrounding built environment.The plan was to make the apartments collapse into the space between two towers.Figure 3 indicates the significant challenges in the demolition process.Apart from the above, a few additional challenges were needed to be considered during the implosion of Serene Towers.The structures are located only about 15 metres from the tidally-influenced Vembanadu Lake, Kerala's most prominent and conceivably the longest lake in the nation, which was a severe cause for concern.Since the Serene Towers were constructed on soft Cochin marine clays, the stability of the soil beneath them was also considered.Soil examination was conducted to ascertain if the land could withstand the impact of tons of debris crashing down during the implosion.
Dust from implosion is likely to engulf an area of about 100-meter radius, Debris from the demolition may fall onto neighbouring buildings or into nearby waterbody it may obstruct the natural flow of water and removal of debris waste from the lake will be complex.There may be chances of severe damage to nearby structures from tremors caused in the ground by implosion, etc. Adequate precautions were taken, and careful planning was made to reduce implosion-related damage.

Field Monitoring and Vibration measurement
The building blasting and subsequent collapse result in a combination of ground vibrations: blasting vibration, backlash vibration, and touch-down vibration [2], [3].It could damage nearby structures if its amplitude is high.[4].The vibration measurement was carried out during the controlled demolition operation of the Serene Towers at ten locations located at radial distances varying from 28 m to 200 m from the buildings.Vibration measurement stations for Serene Towers are shown in Figure 4. Vibrations generated due to the demolition of buildings were measured with the help of piezoelectric-type accelerometers fixed directly on the concrete blocks embedded in the ground.The time history of vertical acceleration during the controlled demolition operations was continuously recorded and stored.Ground-borne vibrations were measured at different distances from the target buildings at locations of importance, primarily in the vertical direction and also in the horizontal direction at a few locations.The analysis of the processed data comprises the identification of peak particle acceleration (PPA) and determining peak particle velocity (PPV) and frequency content.The frequency content of the ground vibrations was determined from the Fourier Spectra obtained from the measured time history of acceleration using Seismosignal 2020 software.The time history of acceleration during the controlled demolition operations was continuously recorded.From the recorded time history of acceleration, the peak vertical acceleration was identified, and the Serene Towers had two peaks each, indicating the blasting of Towers 1 & 2, shown in Figure 5.For Tower 1 demolition, peak acceleration near the Serene apartment location varied from 4.92 m/s 2 to 14.18 m/s 2, while for Tower 2, the values lie in the range of 1.38 m/s 2 to 12.75 m/s 2 .The duration of the ground vibrations recorded was 17 secs for Serene Tower 1 and 18 secs for Serene Tower 2. The peak particle velocity (PPV) measures the damage potential of vibration.The time history of velocity is computed from the measured time history of acceleration using the Seismosignal 2020 software.The peak particle velocity near Serene Tower 1 varied from 33.3 mm/s to 218.1 mm/s, while for Serene Tower 2, it varied from 20.7 mm/s to 84.8 mm/s.Typical velocity-time history plots recorded stations of the Serene Towers are presented in Figure 6.It is found that the peak particle velocity of the ground-borne vibrations tends to attenuate as the distance from the impact increases.The prediction of PPV at different locations is possible if an attenuation relationship can be established using the measured vibration data.The attenuation relationship is developed from the collected data, and it is standardised to the form where PPV is Peak Particle Velocity at any distance from the impact source in mm/s, K and C are site constants, and  is the distance from the impact.The attenuation relationship from the data collected for Serene Towers 1 and 2 is shown in Figures 7 and 8.

Precautionary Measures Adopted
As safety measures, once the charging of explosives commences in the building, only the core members of the demolition contractors and authorities were allowed to enter the premises.A safety zone of 50m and an evacuation zone of 200m from the implosion sites were adopted to protect them from flying debris and to ensure safety if something goes wrong from the plan.On the day of the blast, the people, vehicles and pets were evacuated to safe areas, and about 2000 police personnel were present to control the traffic and an assembly of about 10,000 spectators who had come over to witness the demolition.The traffic along the highway was blocked 30 minutes before the scheduled time of the blast.As a preventive measure against flying debris, vulnerable areas were covered with geosynthetics and tarpaulins.To protect the flying debris from falling into the backwaters, protection using netting and geosynthetic barriers were made.To prevent the debris from blast being thrown away, the charge columns were covered with chain links and geotextiles.One of the significant challenges is the dust factor and the dust cloud formed due to the expulsion of air entrapped between the floors during the collapse.The effect of dust will depend upon the weather conditions and the direction of the wind.For the Maradu blast, it was predicted that dust clouds were likely to engulf at least an area of a 100m radius and would take about 30 minutes to subside.Water from fire engines and the nearby river using water boats was sprayed immediately after the clearance from the demolition core team to control the dust impact.To control the vibration transmitting through the ground, soil impact berm cushions are formed.Due to the vibrations/ tremors caused by the implosion, cracks or damages are likely to develop in other neighbouring structures in the area.The Association of Structural and Geotechnical Consultants was tasked to survey the structural integrity and assess the damages that can occur to the nearby buildings due to the demolition of the Apartment Complexes.Detailed site inspection of all surrounding buildings in the safety zone was conducted to assess the condition and structural integrity of the building.Visual inspection and non-destructive testing of the neighbouring residence were carried out in December 2019.Existing cracks and damages in the buildings were noted during Rapid Visual Assessment, and photos and video recordings were made.After the successful demolition, the impact on neighbouring buildings due to implosion was monitored by comparing the data collected before.From the study, it was noted that there were only minor damages, such as upheaval of soil leading to the undulation of the ground causing dislocation, breaking of tiles and a few glass windows that occurred during the demolition process to the neighbouring structures, and it was subsequently rectified.

7
Controlled Demolition of Serene Towers  were removed, thereby reducing the building weight by approximately 50%.An estimated weight of 19000 tonnes was expected to hit the ground in phases during the implosion of Serene Towers.1200 kg of explosives were put for the blasting inside the structural columns and shear walls in charge holes and protected using steel wire mesh and geosynthetics to control the flying debris.The primary supports of the structure are taken out in sequential manner with delayed detonation technique in both horizontal and vertical direction so that the structure will collapse on its weight to the direction and location it was pre planned.

Fig. 1 .
Fig.1.Location and Image of Serene Towers

Fig. 2 .
Fig.2.Protest of Inhabitants and political parties

Fig. 4 .
Fig.4.Vibration measurement stations for the demolition of Serene Towers

Fig. 9 .
Fig.9.Image of Serene Towers during Controlled Demolition Figure.9.shows Serene Towers during the demolition.The buildings demolished were RCC-framed structures with shear walls and supported by pile foundations.To reduce the impact of blasting, infilled brick walls, part of the shear walls and all hazardous materials