The Pinnacle – Foundation Design
Overview of the Project
The Pinnacle (formerly known as the DIFA Bishopsgate development) will be a 63 storey landmark building in London. It will join The Gherkin, Tower 42, Heron Tower, and 122 Leadenhall St to form a distinctive cluster of tall buildings in the City.
The proposed redevelopment will comprise the demolition of the existing 5 to 8 storey office buildings and the construction of a 290m high tower. Existing basements and ground levels across the site will be deepened and a new site wide basement constructed to a level common with the existing basement slab level of 38 Bishopsgate. The building has a gross floor area of 138,000m2 including 3 basement levels.
The Pinnacle (formerly known as the DIFA Bishopsgate development) will be a 63 storey landmark building in London. It will join The Gherkin, Tower 42, Heron Tower, and 122 Leadenhall St to form a distinctive cluster of tall buildings in the City. The proposed redevelopment will comprise the demolition of the existing 5 to 8 storey office buildings and the construction of a 290m high tower. Existing basements and ground levels across the site will be deepened and a new site wide basement constructed to a level common with the existing basement slab level of 38 Bishopsgate. The building has a gross floor area of 138,000m2 including 3 basement levels.
Overview of the Geotechnical Solution
The proposed building will be founded on a combination of piles founded in the London Clay and the Thanet Sand strata. Where required, the new basement walls are to be constructed within a secant pile wall.
The foundations consisted of the deepest and largest built to date in the City of London at 2.4m diameter, 63m long bored piles base grouted in the Thanet Sand. Reuse of the existing underreamed piled foundations in London Clay supplemented by new large diameter bored piles and minipiles was adopted over part of the site.
Pile loads from this structure were up to 45MN, and this meant that the piles had to extend into the Thanet Sand stratum, 63m below ground level. The decision to have single deep base grouted bored piles per column also meant that not only were the piles the deepest but also the largest ever built in the City, at 2.4m diameter.
Pile Lateral Load and Moment Capacity
The bearing piles are also designed to resist applied lateral loads due to out of balance earth pressures and wind load. In addition, the bearing piles must be designed for moments applied due to eccentricity of the connection between the pile and columns due to positional tolerances.
An assessment has been made of the effect of lateral loading and moments applied to the piles using Alp. For each pile diameter, plots have been produced for increments of applied lateral load and applied bending moment against induced bending moment and shear force. These have been used to calculate the forces and therefore reinforcement required in each individual pile.
The bending moment profiles in the piles due to applied lateral loads and moments are illustrated by the Alp plots.
Tensile Pile Capacity
The main bearing piles at The Pinnacle will be subject to tensile forces as a result of ground heave caused by demolition of the existing buildings and excavation of the new basement.
The calculation of tension forces in the piles due to the heave movement is required to enable appropriate design of the tensile pile reinforcement.
The pile in heaving ground will be subject to shear strain. The pile’s neutral axis lies along the horizontal where the cumulative shaft resistance curves meet. The point at which they meet is known as the neutral point or point of zero shear strain. At this point the Maximum Tension Force (MTF) will occur. However the neutral point (and so the MTF) will be lower in a pile that is subject to a compressive force. When there is a swelling force the MTF could be increased if the swelling force is larger than the applied compression load. This is illustrated below.
A linear elastic method is used to model the behaviour of the reinforced pile and to calculate the tensile forces generated in the pile, the maximum tensile force and required tension reinforcement. This requires an understanding of the likely heave movements in the ground and the variation of these with depth, which are calculated using Vdisp V18.2 (currently Oasys Pdisp).
The same methodology is used for the both the short term and long term loading condition.
Pile Reinforcement Design
The pile reinforcement design is specified to be the worst case of:
Reinforcement required to resist forces and moments induced by lateral loads and eccentric column connections;
Reinforcement required to resist ground heave induced tension;
Minimum pile reinforcement in accordance with BS EN 1536:2000.
Consequently, pile reinforcement design has been considered in both the lateral load and bending moment design and the tension design described.
Furthermore, the section capacity of each pile has been checked using Force-Moment Interaction charts created using AdSec .
These charts calculate the capacity of each reinforced section for various combinations of moment and axial force. The following load combinations were used:
Maximum Tension and Bending Moment
Maximum Compression and Bending Moment
Maximum Compression (No Heave forces) and Bending Moment
The shear reinforcement in the piles is provided in the form of helical link bars. The quantity of helical links in each pile was determined using a calculation sheet which checks shear in combination with bending moment and axial force.
The shear force is the sum of the out-of-balance earth force and the shear due to eccentric column loading. The shear due to eccentric column loading has been calculated using the Oasys Alp analysis described.
Embedded Retaining Wall Design
The design of the hard firm secant pile retaining wall to be constructed for The Pinnacle redevelopment consists of two different pile diameters: 1200mm diameter piles spaced at 960mm diameter centres (male to female) and 840mm diameter piles spaced at 540mm centres (male to female). Only the male piles will be reinforced.
For all sections except the vehicle lift, the wall is propped in the short and long term by the permanent ground floor and basement slabs. It is noted for section 3-3 the ground floor slab does not act as a permanent prop, in this case the B1 slab acts as permanent prop.
For the section at the vehicle lift, the wall is propped by the lowest basement floor slab. A continuous ‘waling wall’ will prop the wall continuously from ground floor slab level to the base of the basement.
Five different design sections are considered:
Section 1-1: section constructed using low headroom Martello rig;
Section 2-2: general section through the wall with capping beam at +16.0mOD;
Section 3-3: general section through the wall with capping beam at +13.4mOD;
Section 4-4: general section through the wall with capping beam at +14.0mOD;
Section 5-5: section at vehicle lift.
A typical construction sequence for one of these sections is shown below:
0. Initial condition
Excavate to +11.4 mOD
Install temporary prop and excavate to +7.0 mOD
Install temporary prop and excavate to +3.0 mOD
Construct B1, B2 and B3 slabs & remove temporary props
Long term case
The secant pile wall is designed using both unfactored soil parameters for Serviceability (SLS) and factored soil parameters for the Ultimate Limit State (ULS) in accordance with BS 8002:1994 and CIRIA C580. Oasys Frew was used to analyse the wall and calculate the bending moments, shear forces and deflections of the wall, as well as the prop forces.
The design methodology for vertical load considers both total and effective stress design.
Shaft capacity is only assumed below the excavation level. For the effective stress design, the input horizontal effective stresses are those calculated from the Frew retaining wall analyses. Because the wall is carrying vertical loading, wall friction may not be generated on the back of the wall. Therefore, no benefit of wall friction (δ=0) is taken on the active side in the Frew analyses.
In the summer of 2009, piling had been completed and workers began excavating deep down, ready to begin constructing the basements. The first blue crane base was put into place in October 2009.