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Tigers regain their strength

SOUTH EAST ASIA

South East Asia's key economies have bounced back from the 1997 financial market collapse and are expected to report strong growth this year. According to a report in the New Straits Times last month, major South East Asian nations should end 1999 with gross domestic product growth rates ranging from 2.9% to 6.3%. Singapore leads the way, followed in turn by Malaysia, Thailand and the Philippines. Only Indonesia has failed to turn its economy round. It is a rapid climb-back for the region which last year plunged into its worst recession in decades.

Spectacular growth should underpin confidence in construction and see a return to the phenomenal activity of the mid 1990s. Over the following pages Ground Engineering highlights some of the region's geotechnical projects.

How Bangkok keeps it up

The world's longest elevated expressway, a six-lane viaduct running for 55km south from Bangkok, will soon be fully open to traffic. Ground Engineering looks at how difficult foundation challenges were overcome.

Thailand's Bang Na-Bang Pli- Bang Pakong Expressway (or BBBE) elevated highway is in effect a 27m wide, 55km long bridge. Add another 40km of ramps and toll stations, all constructed on top of a 10 lane highway that had to be kept open throughout, and the foundation work was going to be as much a logistical challenge as a technical one.

German contractor Bilfinger + Berger Bauaktiengesellschaft, leader of the joint venture contractor BBCD, founded the 15m high bridge on more than 2,000 single piers with an average load of 40MN on each standard pile cap. The soft soil conditions of the Bangkok alluvial plain required loads to be carried by piles to 30m to 60m below ground level.

Ground conditions consist of 15m to 25m of soft marine clay overlaying a relatively persistent layer of stiff silty clay with a thickness varying from nothing to more than 15m. These clays are underlain by a medium-dense to dense silty fine sand at 20m to 35m, which over much of the route includes layers of stiffer clay, typically 3m to 5m thick. Below this is a stiff to hard clay layer and finally a second sand layer which comes in below about 45m depth.

Combined, these conditions made a precise prediction of the foundation depth very difficult. An initial borehole investigation, with SPTs to 50m depth, was undertaken at 150m to 200m intervals. Given the viaduct span-length of 40m to 45m, detailed design data for each pier was interpolated between borehole locations.

However soil strata proved to vary considerably, particularly as distance from Bangkok increased, which meant the assumptions could not be relied upon.

After nearly a year of piling and, as a senior Bilfinger + Berger engineer puts it, 'many unnecessary non-conformances due to the changing soil conditions and restrictive QA-procedures (ISO 9000)', additional soil investigation was undertaken with one CPT at every pier-location.

These detailed results, combined with a common (in Europe) design approach using Koppejan/van Veen's 4D/8D method to evaluate the necessary driving depth, reduced the non-conformances immediately. Tests to confirm these assumptions were performed and a detailed design for every driven pile was introduced. Savings in pile lengths are reported to have completely offset the additional investigation costs.

Limited space and sensitive surroundings required the use of 1,300 bored piles (1.2m diameter) for 218 piers, while the majority of the Y-shaped columns (1,899) were placed on 23,000 precast driven spun concrete piles of 0.8m diameter, manufactured by the joint venture partners in a factory built near the site.

The standard pilecap design comprised either six bored piles of 7,000kN capacity or 16 driven piles, each of 2,500kN capacity. Most driven piles were installed by conventional driving, others were formed using the 'auger press system', which is common in Asia.

This Japanese system uses a continuous flight auger travelling at the base of the hollow concrete spun pile to pass through the upper soft clay layer of up to 20m thickness. Penetration, in the 'press' phase, results from the dead load of the pile and the load of the 'excavated' soil transported to a bucket on top of the slowly rotating auger.

After the first one or two pile sections are installed, the auger is extracted from the pile, the motor is turned sideways (by turning the boom) and a piling hammer takes over.

Bored piles were designed using test results from six full-scale in- situ static load tests carried out by a local test company. In comparison with the earlier investigated soil profile, an estimated foundation level for each pier was established from the original borehole investigation, but actual foundation depth was modified according to the profile revealed during pile boring, assuming unit values determined from the static load tests for each soil layer. All bored piles were toe-grouted to reduce initial settlement.

During excavation, the first 15m was temporarily fully cased because of the very soft clay. The remainder was excavated using a bucket with bentonite suspension for stabilisation.

Strict tolerance requirements - nearly twice the normal Bangkok standard - for minimised deviation, settlements and the high working loads, meant that all bored piles had to be tested for verticality and integrity.

Integrity tests were done by incorporating four full-depth steel tubes within the reinforcement cage and measuring the ultrasonic durability in six profiles after concreting. This was only feasible on every pile because of the low manpower costs in Asia.

Foundation design had to ensure that no differential settlement would result where a pier founded on bored piles was adjacent to a pier founded on driven piles.

For equipment, the joint venture BBCD used six Liebherr HS 843 rigs with Menck MHF 5-12 hydraulic freefall hammers, with seven subcontractor piling rigs (most of them old Japanese models), and six Soilmec and Casagrande rigs from two other local subcontractors.

Average progress for driven piles was about 5 to 6 piles/day/rig (dayshift only) for conventional driving and 3 piles/day/rig for auger press driving. All driven piles were coupled by welding up to three sections of single piece precast concrete spun-piles between 11m and 18m long each, with a maximum driven length of 47m.

Bilfinger + Berger's mechanical department in Mannheim and Wiesbaden developed a new mechanical coupling which reduced the welding time by more than 50%, and B + B believes this will prove a useful tool on major jobs in the future.

Bored piles were installed up to 62m deep with an average progress of 1.5 piles/day/rig (working 24hrs) by using kelly bars up to 67m length.

Piling works for this $1billion design and construct project started in March 1996 and were completed in July this year. In these 41 months about 10% of the contract sum was spent on the foundations alone. Apart from the technical difficulties, a significant challenge was keeping the 10-lane highway open to traffic on both sides of the working area throughout the entire construction period.

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