First results from the site investigation for the new CN Gallery building in Kuala Lumpur gave the design and build team a good idea of the type of material which lay below the site. But what it did not reveal was quite how varied the limestone stratum was or the impact this would have on foundation construction.
Local contractor Invescor and consultant Robert Benaim & Associates won the design and build contract for the 30 storey development. The structure has three towers, a podium and six storey basement car park. Preliminary site investigation showed that the ground below the building was overlain by alluvium, with karstic limestone between 4m and 48m depth.
Based on the investigation, a top-down construction solution was proposed for the basement. This was to be supported on piles of between 1500mm and 2200mm diameter and 18m-long plunge columns installed in empty bores, each carrying nominal loads of 20MN.
At the time of tender Benaim had thought the site investigation and borehole logs were inadequate, as the bores had not fully penetrated the limestone. Neither had the anticipated limestone voids been mapped with accuracy.
'We had worked in Kuala Lumpur before,' says Benaim technical director Jim Paterson 'and encountered karstic limestone when piling for foundations of the STAR light rail system. We knew it was a variable, irregular strata.'
Benaim warned the contractor about the variable nature of the ground and advised it to allow for further investigation.
These additional boreholes, drilled after the contract was let, revealed that the founding limestone layer is characterised by cliffs plunging some 40m in depth, steep sided valleys, large voids, and wedges of limestone pinnacles penetrating the alluvium deposits above it. The situation is, according to Paterson 'exceptional, even by Kuala Lumpur standards.'
The surface of the karstic limestone is scarred with deep channels. The stratum is also riddled with both active and redundant water channels and cavities. The problem was how to design and form deep pile foundations, probing for a substantial rock socket into the voided and jagged rock profile down to 40m depth.
An overriding factor was that variations in foundation design could not compromise the development nor jeopardise the tight budget and construction programme. 'There was no way the 16m deep basement could be removed from the scheme because of the ground conditions. Having opted to go top down, we had to drill blind bores from the surface rather than excavate to lowest basement and pile from there. We had no choice but to find a practical solution and minimise risks,' says Paterson.
Design and construction of each pile required individual consideration. Information from the site had to be assessed and accommodated in the pile design as cavity treatment and pile construction progressed, each pile being treated as a unique site investigation.
The pile-specific site investigation boreholes were sunk in parallel with the construction of the 24m deep secant piled wall for the basement, to depths ranging from 15m to 50m below ground, penetrating up to 12m into the limestone. This was necessary as piles were designed with a rock socket of 6m and it was vital that the socket was not bridging any voids below. Changes in drilling rate and resistance were used to assess cavity depth.
The bedrock profile of the karstic limestone was then mapped using borehole logs and showed solid bedrock and the top of cavernous voids. But before German piling contractor Bauer could start work, local cavities had to be grouted up.
Bauer pumped over 1000m3 of grout into the south west corner of the site to grout the pile positions. It was also evident from the geological records that the sloping bedrock had voids of less than 0.5m in width, which would not be picked up when drilling for the boreholes, which meant there was a risk that further grouting would be needed at pile locations.
It was decided that cavity treatment was required to the depth of the rock socket and should continue for a further 6m below the design toe level, to ensure the rock socket integrity.
Pile design was checked and adjusted to maintain adequate shaft friction resistance and end bearing capacity with grouting results. Where significant cavities in the limestone were identified during pile construction, the effect this would have on the adjacent pile was also assessed and toe level adjusted if necessary.
In preliminary pile design the rock socket depth was sized such that the service load was carried by shaft friction, with the ultimate load and overall factor of safety shared between shaft friction and end bearing. The design assumed that the full shaft friction was mobilised from a point 500mm below the underside of the lowest basement slab. However, service load is resisted by a combination of shaft friction and end bearing, so pressure grouting of the pile base was used to improve stiffness under working loads.
After a pile installation under bentonite, 18m long plunge columns were lowered down the bentonite filled bore. The base of the each plunge column was then embedded 5m into the pile concrete, with the remainder forming the support for the basement floors above.
A weak layer of concrete was tremied into the bore displacing the bentonite, this being broken out around the steel column during excavation for the basement slabs.