Client name: Songdo Landmark Corporation

Duration: 2007 - 2010

Location: Incheon, South Korea

Developing innovative, cost effective and uniquely designed piled raft foundation system for this super-tall building.    

Building a supertall twin tower on sand

102 Incheon Tower is a twin-tower super high rise being constructed in Songdo, South Korea.

Built on reclaimed land the Songdo development covers over 6km2. Ground conditions largely comprise of approximately  8m of loose sand and sandy silt, underlain by 20m of soft to firm marine silty clay, 2m of medium dense to dense silty sand, and then residual soil and weathered rock.

Establishing accurate knowledge of the ground conditions was essential in the development of an economical foundation system which would perform to expectations.

Borehole data showed that the top of the soft rock surface within the foundation footprint had a variation of up to 40m. To capture the variation of ground conditions, the footprint of the tower was divided into eight zones with the appropriate geotechnical models and parameters.

There were complex interactions between the site’s geology and the large loads of the super structure including the very high wind (lateral) loads created by the building’s height.

This was further compounded by the complex underlying geology of the reclaimed land (geological boundaries sheared and crushed seams), and the need to control differential settlement of the two towers that were horizontally adjoined by three sky bridges. The sensitivities of the building were significant and settlement predictions had to account for not only how much the structure was going to settle, but also the differential settlement between any two points of the building.

Given all these factors, traditional empirical design methods could not be applied as they provided no precedence on which to validate results.

In the design of foundations for tall buildings, both lateral and vertical loadings are of great importance. A small rotation at the foundation will be magnified to a very large degree at the top of the structure due to the height of the building, which will then affect the serviceability and functionality of the building.

Designing an economical pile raft foundation system for a superstructure

Coffey was engaged to undertake the foundation design in conjunction with local Korean consultant Jin-Young ENC.

The design consists of two 151 storey towers connected by three sky bridges and basements to depths of 8.5m. The 102 Incheon Tower will stand 600m in height and be regarded as one of the world’s top five tallest towers.

Designing foundations for very tall buildings is a specialist field, as conventional design approaches cannot be applied. Mega buildings, such as 102 Incheon Tower, are supported on relatively short foundations and Coffey’s main role was to predict how the foundations would behave over time under very large building loads.

To facilitate a robust assessment process, the team utilised a suite of commercially available and innovative in-house developed computer programs that enabled the detailed analysis of large groups of piles to be undertaken.

It also allowed the team to incorporate factors that included pile-soil-pile interaction effects, multiple pile lengths and varying ground conditions in the foundation design.

The geotechnical and structural design teams worked collaboratively through the design process. This collaborative approach is particularly important for the foundation design as the overall pile group behaviour needed to be adequately captured in the structural design, and in turn the wide range of loading conditions needed to be adequately assessed in the geotechnical design.

In the initial stages, the pile head stiffness at each pile location was assessed by Coffey and passed onto the structural designers to put into the super structure design. A separate analysis was then undertaken by the structural designers to assess the foundation response and load distribution across the raft in relation to the super structure. The foundation response to these revised loads was then re-assessed by Coffey and updated pile stiffness values computed. This process was repeated until compatibility between the applied loads and foundation response was achieved.

To support the large vertical loads and restrain any horizontal displacement due to wind and seismic loading, a pile raft foundation system was designed to support the super structure.

The raft provided additional support that enabled Coffey to optimise the number and vary the length of piles required across the site.

The post-design process was extended in order to obtain the actual response of the ground and the piles due to various loadings. From the results of pile load tests carried out during the detailed design phase, the prediction of pile behaviour was refined and the pile capacities updated.

Geotechnical and structural collaboration led to innovative and cost-effective design

The final design was innovative, cost effective, and driven by an intensely interactive design phase between the geotechnical and structural engineering design teams.

Post design, additional modifications to the design were then possible to potentially deliver more cost savings.