When building on soft subsoils, a combined pile raft foundation (CPRF) is often significantly more cost-effective than a traditional pile foundation. As part of the “Im Glattgarten” project in Wallisellen, we were able to cut the costs for the foundation in half. Rather than a great deal of concrete, a streamlined solution of this nature instead requires a great deal of analytical work on the part of geotechnical engineers. The project in Wallisellen also involved an in situ field test with two bored piles fitted with testing instruments.
When experienced geotechnical engineers say that it is possible to save money on a building’s foundation, clients are all ears. This is exactly what happened during the course of Plazza AG’s “Im Glattgarten” project in Zürich-Wallisellen – a building project that will contain more than 200 rental flat units and 1,700 m2 space for light industry. The subsoil under the former industrial complex, on which six seven-story buildings are to be built, consists of clay that is essentially as soft as butter. For this reason, the original plans involved a solid bored pile foundation consisting of piles up to 40 metres in length, which would transfer the load from the buildings down to the moraine layer below. But the geotechnical engineers at Basler & Hoffmann were able to find a more efficient solution, and create a new plan on behalf of the client. Instead of using a vertical pile foundation, they recommended a combined pile raft foundation which distributes the load onto a base plate without requiring a reinforced foundation, and which involves markedly shorter piles of just 15 to 20 metres. The savings: approximately 4,000 m3 less concrete would be required in the base plate, and 7,000 fewer metres of piles, which corresponds to total savings of between CHF 3 and 4 million. The shorter piles also cut down the construction period considerably.
Field test with high-tech piles
In order to optimise a foundation, one first needs to understand the subsoil. Rather than investing in concrete, building engineers must invest in analytics. That is why Carlo Rabaiotti and Cornelia Malecki are on location at the construction site in Wallisellen today. The two geotechnical engineers are monitoring the test instruments fitted to two test bored piles. The construction site does not really look like much right now. A lone pile drilling machine stretches toward the sky; alongside it, the reinforcement cages for the two piles are standing by. A small troupe of people wearing bright orange have gathered nearby. At first glance it is clear that these are not your average reinforcement cages: a glowing blue device is sending pulses of light between the reinforced bars; the longer cage is equipped with two of these devices. “Those are presses,” Cornelia explains. They drill the test piles into the ground with a force of up to 10,000 kN – more than the planned building load. “This allows us to determine the upper limits of the load-bearing capacity. At the same time, the presses are also pushing upwards, which allows us to measure the maximum surface friction.” In order to perform these kinds of tests, a great deal of measuring technology must be installed in the piles: blue wires criss-cross through the piles from top to bottom – fibre optic sensors that measure the compression and expansion. Furthermore, around 20 strain gauges are mounted onto each pile. The high-tech reinforcement cages have now been fully fitted with test equipment and can be lifted into the borehole.
Testing and evaluation
The test piles are extremely important for the geotechnical engineers: “The in situ field test with these two piles will tell us how well the results of our soil investigation and our FE model correspond to the actual behaviour of buildings and the soil,” Carlo says as, next to where he is standing, workers begin to pour concrete into the reinforcement cage. The testing method used here, known as the Osterberg method, was named after the American professor Jorj O. Osterberg. “Depending on the results of the tests, we could potentially streamline the actual foundation even further,” says the engineer, who has a doctorate in Geotechnology. The actual construction phase, when the remaining 300 piles will be drilled, is scheduled to begin in two months.
Precision that exceeds expectations
Two weeks later: the test results are in. What is the verdict? The subsoil behaves almost exactly as shown in the FE model. The foundation fits perfectly. Shouldn’t this be a reason to celebrate? Carlo and Cornelia agree, and yet they are a bit disappointed: “We can cut around 300 metres of total pile length. We thought that it would be even more than that.” Now building can begin as planned and the geotechnical engineers will continue their measurements. Eight piles and eight of the building’s pillars will be fitted with fibre-optic expansion sensors so that the engineers can observe the building’s behaviour all the way through to the final construction phase.
Less concrete – more expertise
Doesn’t that involve a lot of work? “Not at all,” says Carlo, shaking his head energetically. “An optimised solution like the one in Wallisellen involves four mandatory steps: in-depth soil investigation, an FE model, in situ field testing and fitting the final structure with testing equipment. An FE model is not worth much unless you can validate it with actual measurements. And, the more test results we have, the better the models become. The amount of work we have to do is quite low compared to the overall reduction in cost.” A point that has been proven time and again in a number of projects.