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Unconventional design lays foundation for expansion of Bangkok’s largest mall

Megabangna Shopping Centre expansion

Megabangna Shopping Centre expansion

08 June 2017 - An innovative foundation design from Aurecon helped expansion work on one of Asia’s largest malls stay on schedule while also ensuring no damage was done to the existing structure. Aurecon pushed the boundaries of traditional foundation design in Thailand, with a unique hybrid piling and structural solution.

Pile driving is not for the faint-hearted. Using heavy equipment to punch pre-cast concrete piles into the ground is noisy work - and requires plenty of brute force. It’s an activity that can be seen frequently on project sites across the rapidly evolving Thai capital, Bangkok, where the local soft soil is something of a double-edged sword. On the one hand, having softer soil means piles can be driven more easily; on the other hand, the softer soil means bigger and longer piles are needed to deliver the necessary load-bearing capacity.

So when leading Thai developer S.F. Development Co., Ltd., approached Aurecon to help with the expansion of the famous Megabangna Shopping Centre, the company knew immediately that one of the biggest challenges would be managing the groundworks. Having heavyweight piling rigs driving the pre-cast concrete piles just three metres from one of South East Asia’s most sophisticated retail facilities would not be easy for two reasons.

Firstly, all the nearby shop units featured large expanses of glass in their facades making them highly sensitive to even low levels of vibration. And secondly, one of the stipulations of the project was that disruption to the existing shopping mall – a lively and vibrant retail centre – must be minimised.

The expansion included a 7-storey car park and 3-storey retail complex with a total gross floor area of 57,000 m2. For the groundworks, Aurecon initially looked at a traditional grid of standard-sized bored piles of 600 mm diameter. But the design team quickly realised that this could not be installed in the required timeframe. Aurecon needed a solution where heavy piling work (and its associated vibration) near to the mall was minimised. Also, the design should be fast and easy to implement on site, helping construction to be completed quicker, and allowing the new extension to be up and running as quickly as possible.

With these issues guiding the company’s design thinking, Aurecon’s engineers created a multifaceted solution which offered both the construction efficiency and peace of mind required. The design incorporated larger driven piles with greater spacing across the majority of the expansion to minimise the number of foundations. In addition, different piles were used in the area immediately next to the existing mall and the ground was partially pre-bored, in order to lower the impact forces from pile driving.

Disrupting foundation design

Having a design with fewer piles spread over a wider area means bigger structures are needed. In this case, Aurecon specified 525 mm square piles for the majority of the groundworks, and 600 mm spun piles in sections closest to the existing mall. In early discussions for this design, there were questions from both the client and piling contractors, as both pile types are designed for larger capacities than those typically found on Bangkok job sites. Heavier equipment is needed to drive the piles, and, in addition, there were concerns that the proposed grid would not reach the required strength to support the expansion.

Worakarn Aimdee, Aurecon’s Geotechnical Engineer on the project, explained the theory behind its design.

“Almost everyone we showed the design to was doubtful to begin with, but once we worked through our geotechnical modelling and calculations, everyone understood our thinking,” he said. “On this project, there were specific requirements for the timing and the impact on surroundings that simply couldn’t be accommodated in more standard designs. We knew we had to engineer an unconventional solution to help our client realise their dream of developing its retail expansion.”

Aurecon was able to call on its experience from designing the original Megabangna Shopping Centre to support its work on the expansion, where 450 mm square piles had chosen to support the development. The load capacity of any pile depends on its size and the soil type around it. Because Aurecon had detailed information on the local soil parameters and a record of the capacity of the piles in the existing mall, it was able to apply this information to the design of piles for the expansion.

For the car park and the retail buildings, most groundworks would use bored piles. However, as construction work was being fast tracked, Aurecon specified driven piles which are faster to place. For the 600 mm spun piles, a hybrid installation solution was devised, with the top 10 m to 12 m of soil pre-bored before the remaining 20 m of the pile was driven. This meant lower impact to surrounding structures from the pile driving process, while also limiting the cost and time needed for boring. In the end, vibration levels in the existing mall were lower than anticipated, with zero damage recorded in the original mall, including the glasswork.

Megabangna is owned by SF Development Co., Ltd. The existing mall opened in May 2012 with a gross building area of 400,000 m2. The new expansion adds19,000 m2 of retail space plus 38,000 m2 of parking space. On both the original build and the new expansion, Aurecon provided all civil and structural design work, as well as mechanical, electrical and plumbing design services. Groundworks on the expansion, including earthworks on site, started in November 2016 and finished in early February 2017. All construction work is scheduled to end by November 2017, with the expansion due to open in December 2017.

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