We created a multi-purpose development in the Capital

Proposed Site Layout

The site utilised the following features

• The new construction comprised of a ring-shaped set of commercial units at its base with a car parking facility placed in the central area.
• The first floor podium slabs were placed on top of the commercial buildings, supporting 3 towers, up to a maximum height of 15 storeys.
• The centre of each podium slab needed to function as a garden space, bringing an ambient green area to the residential development.

We were commissioned to construct 2 new buildings featuring a ring of commercial units and 5 residential towers

Initial Parameters

Some of the first requirements of the structural engineering work were

• Due to the design fully maximising the available site area, we were limited in the types of foundation we could use for the project – particularly with the columns in the perimeter of the building.
• Drainage also presented a number of problems as we had to reach a fixed drain on each site, and the design required all drainage to sit inside the structure.
• A large degree of onsite attenuation was required due to restricted discharge into the existing main sewer network.

The site was designed to maximise the available area leaving only small areas for foundation piles

Foundation Challenges

The following issues occurred with the concrete piles

• We had to create concrete piles in the peat rich soil. After calculating the loads we recommended 750mm piles carrying 220 tonnes per pile submerged to a depth of at least 20m.
• All of our designs were based on this pile diameter, which then governed the spacing of each individual pile and thus the supporting columns throughout the structure.
• The appointed specialist piling contractor informed HA Consulting Engineers that they had encountered a thick level of gravel stone on a previous project at depth in the soil and the size of the pile would need to be changed to 450mm at very short notice.

Smaller piles than originally specified had to be used due to problems with the soil substructure

Testing and Revising

To ensure the newly specified piles would support the proposed structure

• We requested that testing was conducted to test the load-bearing capacity of the smaller diameter piles.
• The piles failed during the first load test. An investigation was launched into the reasons behind this failure.
• Poor grade concrete and an inefficient curing process were found to be the root cause of the failure.
• With a third of the working piles already installed at the time of the first load test, we then had to trace the source of this concrete and conduct an additional test to ensure no working piles were affected.

After extensive testing we approved the piles as a suitable structural support for the new construction

Further Challenges

Before the work commenced we had to contend with the following difficulties

• The revisions and testing to the pile structure created a 2-month delay in the project whilst we waited for the concrete to cure. Using our latest computer design software and integrated reinforced concrete detailing we were able to reduce this delay considerably.
• Due to the cost of certain cosmetic bricks, we had to revert to a solid brickwork wall on part of the structure rather than a lightweight brick slip façade, more than doubling the load at each floor level.
• To accommodate this extra load we completed a total redesign of each building at extremely short notice, utilising the foundations that had already been installed.

Despite an increase in the overall load of the buildings, we adapted the design to utilise the pre-built foundations