Supporting a deep excavation adjacent to existing buildings and with limited working space is always a challenge. In Cavan, Ireland an innovative method of using raked buttress piles to support a 3-storey basement excavation has been successfully used for a large multi-storey retail development, designed with the help of Oasys software.
A deep basement ranging in height from one to three storeys was required for the development’s underground parking and storage. The site is located adjacent to a hotel, offices, industrial buildings and a national road. Options for supporting the walls of the excavation during construction were limited as owners of the neighbouring buildings opposed the use of tie-backs, lack of working space prevented the use of temporary props and the set-back between the wall and site boundary was minimal.
How Oasys proved invaluable
The team of designers and engineers from Parsons Brinkerhoff (Ireland) Limited and Alan Traynor Consulting Engineers designed an alternative solution to overcome the space shortage. A contiguous wall of 600mm diameter piles varying in lengths from 9-17m was constructed around the site by the contractor Quinn Piling. In the one-storey section of basement the piles were left as cantilevers. In the two and three-storey sections 600mm diameter buttressing piles were installed at an angle of 10° every 2m behind the pile wall. Excavation was carried out in two stages – firstly to 1m to allow construction of a reinforced concrete capping beam and then to formation level followed by construction of permanent the floor slab.
Oasys programs were used throughout the wall design. The overall stability of the contiguous pile wall was analysed with Oasys Frew. The program calculated that the pile length in the two-storey section was required to be approximately 12m with a retained height of 7.5m.
Oasys Frew was used for the serviceability analysis of the contiguous wall. Data was entered into the model in six stages:
- Initial conditions
- Installation of pile wall
- Installation of buttress piles
- Excavation to 1m and constructing pile cap
- Complete excavation
- Construction of floor slabs and permanent walls.
Conservative values were used for earth pressures and the drainage behaviour of Irish glacial till. Frew predicted a maximum lateral deflection of 17mm in the cantilever single basement section, 13mm in the buttressed double level basement and a service load on the buttress piles of 505kN/m.
Oasys AdSec was used in the steel reinforcement design for the contiguous pile wall. The reinforcement was designed to resist the bending moments and shear forces estimated by the Oasys Frew analysis.
Monitoring of the wall for movement was carried out over a period of five months. The maximum deflection of the cantilever wall was measured as 12mm and the maximum deflection of the buttressed wall was 7mm, both lower than the values predicted by Frew.
The use of raked buttress piles has provided an economical and readily available solution for maintaining the overall wall stability of an excavation within tight space constraints. It has proven an effective method for limiting the wall deflection in a deep excavation.
This case study is based on the paper ‘An innovative use of bored tension piles in embedded retaining wall design and construction’ by Abid O. Adekunte, Van Elle Limited, Pinxton, Nottinghamshire, U.K., an Oasys customer. Abid would like to pass on his gratitude to the management of P.Elliot (Ireland) Limited, who were the main contractors for the project, for unrestricted access to site and monitoring data. The contributions of Quinn Piling Limited, Alan Traynor Consulting Engineers and Parsons Brinckerhoff (Ireland) Limited to the successful completion of the project is also acknowledged.
"I've used Oasys software over the years. I find it very reliable, powerful, consistent and user-friendly. That's why I use the programs on all my Pile design projects."