First, we will have a brief introduction of foundation system for tall buildings, then we will talk about the key design issues and design processes. After that, we will look at 2 cases – Incheon Tower in South Korea and Nakheel tall tower in Dubai. For the Incheon Tower, Coffey was appointed as the geotechnical designer and for Nakheel tower, we were appointed as geotechnical peer reviewer.
In the past 2 decades, the number of buildings with a height in excess of 200m is increasing. Predictions from council on tall buildings and urban habitat show that by next year, the total number of tall buildings will reach 680 and 83 of them are supertalls which are over 300m high. The design of these supertalls are challenging as the traditional design approach cannot be applied with confidence. It is necessary to come up with some innovative design approach for the design of a cost effective foundation to satisfy the long term performance requirements.
Now, we look at the structure of our tall building design team, at the top, we have a project principal who is responsible for the supervision of the design team, review design process and provide technical advice to the team and clients. The next level is the project manager. The project manager will be responsible for the preparation of proposal and cost estimation of the design services, interact with clients and supervise the design process. Then the next level is the design engineers who will be responsible for the interpretation of test data, development of geotechnical models and undertaking foundation analyses.
Piled rafts are a cost-effective form of foundation for tall buildings as the raft and piles can combine to transfer the applied load from the structure to the supporting soil. By placing the piles strategically, the overall and differential settlement can be controlled, therefore, the piles are often used as settlement reducers. In this case the load carried by the piles is considered as a secondary issue in the design.
The design issues that need to be addressed in the design of tall building foundation. The ultimate capacity of the foundation subjected to vertical, horizontal and moment loading combinations. The influence of cyclic nature of wind and earthquake on foundation capacity and movements. The overall settlement and the differential settlement within the footprint of the building and the differential settlement between the high rise and low rise areas. Sharing of loads between the raft and piles and the distribution of load along the piles. If excavation is required for the construction of pile cap, it may impose external ground movement which could induce additional load and movement to the pile. Earthquake can cause serious damage to buildings, so it is necessary to assess the response of the structure foundation system to earthquake excitation and the possibility of liquefaction of the surrounding soil supporting the foundation. Lastly, is the dynamic response of the structure foundation to wind induced forces.
Before carrying out the foundation design, we need to have a well planned design process. The design process has to include several phases: The first phase is subsurface exploration to carry out a desktop study of all available geotechnical data including previous site investigations and geotechnical engineering recommendations in the vicinity of the site. Then to perform site specific geotechnical investigation to explore the soil strata profile, groundwater conditions and perform in-situ and laboratory testing to get the soil properties. The second phase is foundation design. In this phase, first we have to develop geotechnical models based on the available geotechnical information from Phase 1. Then we can carry out preliminary foundation design using simplified geotechnical models and simple analysis method. Upon receiving the geotechnical information from the site specific geotechnical investigation and structural loads from structural engineers, we can then revise the geotechnical models and perform detailed design to assess the performance of the foundation under different loading conditions. If excavation is required during construction, we have to design the retention system to control groundwater inflow. Then we need to carry out and assessment on the impacts of construction on adjacent properties and other facilities within the site. Lastly, is to carry out an assessment of seismicity of the site including changes in soil and rock conditions during earthquakes and possible effects on the foundation system.
Limit state design is used for the foundation design. The ultimate limit state in which the design strength has to be greater than or equal to the design action. The design strength is calculated by applying a reduction factor f to the ultimate strength. For the serviceability limit state (SLS), the settlement and augular distortion or rotation of the foundation under working load have to be less than the allowable values
Incheon Tower is a super high rise twin tower proposed to be constructed in Songdo in South Korea. Each tower consists of 151 storeys with a height of 600m and basements of approximately 8.5m deep. The 2 towers is connected by three skybridges. The geotechnical designer is Coffey in associated with Jin-Young ENC in South Korea, the structural designer is Thornton Tomasetti and the architect is John Portman and associates. For this project Frances was the project manager and Helen was the design engineer.
The tower is to be constructed on reclaimed land comprised of approximately 8m of loose sand and sandy silt underlain by approximately 20m of soft to firm marine silty clay, followed by 2m of medium dense to dense silty sand and then the residual soil and weathered rock. The rock material within about 50m from the surface have been affected by weathering and the strength has reduced to a very weak rock or a soil like material.
This figure shows the inferred contours of the soft rock surface. Based on the borehole data, the top of the soft rock surface within the foundation footprint has a variation of up to 40m. To capture the variation of ground conditions, the footprint of the tower was divided into 8 zones with the appropriate geotechnical models and parameters.
The performance of the foundation was assessed using the different computer programs. Program CLAP was used to assess the overall stability under ultimate load combinations. Program GARP was used to assess the foundation settlement under vertical and moment loadings. Finite Element Program PLAXIS 3D was used to carry out an independent check of the foundation settlement under vertical and horizontal loads.
The foundation has a bow tie shape. It consists of a 5.5m thick raft supported by 172 piles with a diameter of 2.5m. The raft is embedded into the ground with the base located at about 14m below the ground surface. The length of piles varies between 36m to 66m with a minimum socket length of 2 times pile diameters into soft rock.
GARP was used as the main design tool for the settlement assessment under vertical loading. In the GARP model, the piled raft is in contact with the underlying soil but not with the surrounding soil. The overall settlement under the unfactored dead and live loads was 67mm with a differential settlement of 34mm. In PLAXIS 3D, 2 cases were considered. Case 1 is similar to the GARP analysis in which the piled raft is in contact with the underlying soil only. For case 2, the piled raft is embedded in which the raft is in contact with the underlying soil and the soil surrounding the raft. The basement walls were modelled in the analysis.
This is the mesh used for the PLAXIS analysis, the dark blue elements represent the basement walls supporting the sides of excavation. The soil layers are modelled as Mohr-Coulomb material to allow for non-linear behaviour. Point loads were applied at the column locations and uniform loads were applied at the corewall locations. This plot shows the load displacement curve for both cases. The solid line is the displacement for case 1 and the dotted line is the displacement for case 2. We can see that the displacement is reduced when the resistance of the surrounding soil is considered.
The performance of the foundation under horizontal loading is a critical issue in the design. Program CLAP was used to assess the lateral stiffness of the pile group in which the raft is not in contact with the underlying soil. The lateral stiffness of the raft and basement wall were calculated separately. From CLAP analysis, the lateral deflection of the pile group was 22mm. Again, PLAXIS 3D was used as an independent check. Three cases were considered. Cases 1 and 2 were the same as for the vertical load, case 3 was the for a pile group only.
The performance of the foundation under horizontal loading is a critical issue in the design as the building is subjected to very high wind load. This load will then transfer to the foundation. Program CLAP was used to assess the lateral stiffness of the pile group in which the raft is not in contact with the underlying soil. The lateral stiffness of the raft and basement wall were calculated separately. From CLAP analysis, the lateral deflection of the pile group was 22mm. Again, PLAXIS 3D was used as an independent check. Three cases were considered. Cases 1 and 2 were the same as for the vertical load, case 3 was the for a pile group only.
For this project, Coffey was appointed as geotechnical peer reviewer. Frances was responsible for setting up the peer review process and review the geotechnical designers report and foundation design method provided by the foundation designer. I was responsible for undertaking independent analyses for the foundation system proposed by the foundation designers.
Nakheel Tower is one of the multi-billion dollar projects in Dubai. The tower is proposed to have more than 200 storeys high with a height exceeding 1km. This tower will be one of the centre pieces of Nakheel Harbour. Now we look at the ground conditions. The geotechnical profile consists of 20m thick sand layer underlain by cemented carbonate siltstone with gypsum layers up to 2.5m thick occurring at depths greater than 75m below ground level.
One of the main features observed from the material was the existence of bond yield strength that controls the compressibility of the material. If the imposed stress on the ground is less than the bond yield strength, the soil will behave as a very stiff material otherwise, the compressibility would increase which results in excessive settlement. Therefore, in the foundation design, the stress on the ground has to be limit to be below the bond yield strength.
The foundation consists of a raft with a thickness varies up to a maximum of 8m supported by a total of 392 barrettes. The sizes of the barrettes are 2.8m x 1.2m and 2.8m x 1.5m and the length of the barrettes are 37m 42m and 72m.
As the geotechnical peer reviewer, we are involved in reviewing the geotechnical designer’s report and the foundation design method provided by the geotechnical designer. Then we have to develop independent geotechnical models based on the available geotechnical data and undertaking independent analyses for the foundation under different loading conditions. In the independent analyses, Program CLAP and PIGS were used to assess the overall stability of foundation under ultimate limit state load combinations. Program GARP was used to assess the settlement under serviceability loads and PLAXIS 2D was used as a check for the GARP results. In PLAXIS 2D analysis, an asymmetric model was used and the barrettes were modelled as equivalent circular rings. From GARP analysis, the maximum settlement was 95mm which agreed well with the settlement from the designer.
In conclusion, we have discuss about the key design issues that are required to be addressed and the design process that needs to be considered in the foundation design for super tall buildings. It is necessary to have a good understanding of the ground conditions in the development of geotechnical models and parameters. In the design, the performance of the foundation under lateral loads is a critical issue, therefore special consideration has to be given in adopting the parameters for assessing the lateral response. In the past few decades, female engineers have been involved in different aspect of engineering and they are often given the opportunity to undertake different roles in major projects.
Challenging Design: Foundations for Tall Buildings Helen Chow & Frances Badelow