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ULTIMATE TEST

TESTING

Engineering on a grand scale in an active earthquake zone needs big foundations. That's why the world's biggest pile tests are currently taking place in Taipei.

When Testconsult's Dick Stain got a call from Taipei-based Great Asia Engineering Consultants (GAEC) a few months ago saying it was planning a very large dynamic test 'and would he come over and help them out' he had little expectation of what lay in store.

But in Taiwan civil engineers think big. Mainly this is to do with the elements. Typhoon Aere dumped a staggering 1.5m of rain on the island in just three days in August; and sitting in a subduction zone, where the Philippine sea plate is being pushed under the Eurasian plate, an earthquake in 1999 caused 9,909 buildings to collapse completely.

Despite these deterrents, Taipei also boasts the world's tallest building which will be opened before the end of the year. Standing 509m, Taipei 101, so called because of the number of storeys, is the world's only super-tall building in an active earthquake zone.

'The scale of engineering and the pace of the place is staggering, ' Stain says. GAEC's general manager Chien-Shun Huang, for example, mentioned he had recently carried out a 5,800t static load test, which Stain says 'to our knowledge, is the largest static load test ever carried out on a single pile' This test was carried out on a 2.5m diameter pile on the Uni-President International Tower (UPIT) site, adjacent to the 101 building.

The UPIT building has 32 floors above ground (132.5m height) and seven basement levels (32.4m depth). Very high superstructure column loads are carried by 115 piles, varying in diameter between 2m and 3m.

GAEC carried out four static pile load tests at the site, making use of a patented test beam arrangement.This included 11 beams in three layers, with the reaction force provided by 12 sets of 600t hydraulic jacks and 12 sets of load cells.

But while static load testing gives the greatest certainty over pile performance, such tests are expensive and time consuming. It was against this background that GAEC was interested in developing a high capacity dynamic pile testing capability for testing piles for the planned 15 storey Champion Site residential building.

Dynamic tests typically involve dropping a weight on to the pile and measuring the dynamic behaviour. The trick is converting this response into a static equivalent.

'One of the limitations of dynamic testing, ' explains Stain, 'is that dynamic testing cannot 'predict' pile behaviour, it can only measure it. So to determine ultimate behaviour, an impact large enough to mobilise all the resistances is required.

In effect this means achieving rupture at all levels, including the toe.'

Despite such fundamental limitations, proponents of dynamic testing claim useful predictions can be achieved, and the method is 'massively quicker and vastly less expensive' which as Stain says is an enormous advantage. The key thing is that engineers understand the limitations and treat the results accordingly.

Chien-Shun says: 'With the SIMBAT method, we particularly liked the direct method of measuring pile displacement using a noncontact digital theodolite. This gives a very accurate time history of the pile movement under load and allows determination of both elastic and permanent settlement. After the 2,500t test on the Champion site, we are confident we made the right choice.'

The purpose of the testing at the Champion site was to verify the ultimate capacity of a 24m long, 1.2m diameter pile, socketed 4m into sandstone. Anticipated capacity was about 3,000t.

GAEC designed and constructed a special 35t drop mass, made up of layers of 50mm steel plates fixed on a 150mm thick base plate.

The guide frame and release mechanism were set up to give a drop height of up to 2m. This is the largest falling mass system ever used for SIMBAT dynamic testing and made use of a Seacatch release mechanism, which can release up to 50t under load.

'The testing was not without difficulty, ' says Stain. 'The largest drop mass we have worked with before was 20t - jumping up to 35t puts the whole thing in a different league and we had some initial problems with ground waves upsetting our theodolite.'

Other adversities were less technical: 'When you are used to UK conditions, the summer heat and humidity of Taipei was draining.'

Nevertheless, the drop system itself performed flawlessly. Teething problems with the Seacatch release mechanism were resolved in time for the full test procedure.

The largest impact was with a drop height of 1.6m, which produced a dynamic reaction in excess of 30,000kN. Static behaviour was computed up to 25,000kN and was almost entirely elastic and recoverable.

Furthermore, says Stain, the signal matching confirmed the expected resistances from borehole data and indicated a sound pile toe and socket within the sandstone.

The SIMBAT dynamic load test was developed in France by Jean Paquet at the CEBTP, in the late 1980s. Since then Testconsult has developed the method and applied it to virtually all pile types, from 150mm diameter micro piles, tubular steel piles and large diameter bored shafts.

The method consists of instrumenting the pile top with two strain gauges, two accelerometers and a theodolite target. A mass is then dropped on to the pile top and the strain, acceleration and theodolite signals are captured and digitised in a special high speed data logger.

The use of the optical, non-contact theodolite is crucial as it allows the whole deflection cycle to be accurately determined and this in turn is used in the data processing.

When a pile is impacted in this way a compressive wave travels down the shaft. As it reaches 'soil restraints', part of the wave is reflected back up the shaft. The same happens at the pile toe. So at any moment there are both upward travelling waves as well as downward travelling waves. It is the upward waves that quantify the soil restraints on the pile and determine the pile capacity.

Signal processing involves conversion of strain to force, integration of acceleration to velocity, verification of the integration constants using the theodolite data as reference, separation of forces into upward and downward components and measurement of dynamic reaction, Rdy.

The dynamic reaction is then converted to static reaction and a Static Load versus Settlement plot is obtained. This is then verified using computer modelling and wave matching techniques.

One of the most important aspects of dynamic testing is confirmation by signal matching. A computer model pile is created with 20 or more discrete layers. The model consists of masses, springs, dashpots and sliders, the latter three corresponding soil elasticity, soil viscous behaviour and soil rupture value.

This model is then subjected to the same downwards force as the real pile and parameters are adjusted until the model behaves in the same way as the real pile - a process known as wave matching.

Once a good fit has been obtained, and this can be done manually or automatically 'with a little human intervention from The dynamic response is converted into a static equivalent;

wave matching reveals the distribution of lateral resistances down the shaft and at the toe, which can be correlated to a static force/ settlement plot.

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