Research: A look inside the Soil

Apr 05, 2024

A new research facility at the University of Bristol will examine the way buildings and infrastructure interact with the ground under dynamic loading. It is preparing for its first projects.

The Soil-Foundation-Structure Interaction (SoFSI) facility at the University of Bristol is part of an initiative to drive down the cost of infrastructure while extracting increased value from it.

The facility is being developed by a research joint venture called the UK Collaboratorium for Research on Infrastructure and Cities (UKCRIC).

The UKCRIC is a network of 13 universities which is delivering 10 national laboratories which seek ways to underpin the renewability, sustainability and improvement of infrastructure and cities in the UK and abroad.

The University of Bristol received £12M from the Engineering & Physical Sciences Research Council (EPSRC) to build the SoFSI laboratory. The government is investing £140M in all such facilities across the UK.

The research facility sits at the edge of the Mendip Hills in Somerset

The SoFSI laboratory is in a large outdoor compound at Bristol University’s Langford campus, 22km south of Bristol at the edge of the Mendip Hills.

Its state of the art equipment will enable researchers from academia and industry to explore the many complex ways in which structures interact with their foundations and the surrounding ground.

“The critical infrastructure that we can test is really diverse,” explains University of Bristol professor of earthquake engineering Anastasios Sextos, who is also the laboratory’s academic director.

“It relates to a wide civil engineering field, including bridges; buried pipelines such as water, wastewater and natural gas; and offshore or onshore wind turbines,” he adds.

The facility currently houses two major pieces of equipment. The first is a 4m deep, 6m long, 5m wide soil testing pit. The pit is surrounded by a strong floor of 5m deep solid concrete to support the mounting of heavy equipment. The second is a 6m long, 4m wide 30t biaxial shaking table.

A 1m by 1m multi-axis hexapod shaking table – the final piece of equipment – will soon be delivered to the facility.

All equipment is hydraulically operated and supplied from a hydraulic power unit in a separate room in the compound.

The compound itself has been purpose-built for the SoFSI facility’s research aims. Its 50m high ceilings can accommodate 8m tall cranes while its backyard is spacious enough for trucks to offload large deliveries of sand and concrete needed for testing.

There are smaller structures testing facilities at the University of Bristol faculty of engineering. But the SoFSI laboratory will enable researchers to conduct experiments on a much larger and even full scale and to help answer questions that cannot be resolved by conventional, smaller scale testing.

The soil pit will house pile foundation tests at scale, enabling researchers to see how they interact with the ground.

The biaxial shaking table sits on four hydraulic actuators and can simulate ground motions that take place in an earthquake. It can also be used in conjunction with a soil box which can be mounted on the table to test the effect of seismic movements on different types of ground.

The soil testing pit

The facility’s first project is called Plexus Plus. It will look at abutments in concrete integral bridges, and how the backfill behind these structures responds to the different static and dynamic loads imposed by traffic and other environmental factors.

Sextos says this project is important, because integral abutment bridges – mostly overpasses of motorways or railway bridges – are increasingly popular in the UK. Abutment bridges have no joints or bearings between deck and abutment and as a result do not deteriorate as quickly over time and have fewer maintenance issues.

The problem with integral abutment bridges is that thermal expansion and contraction of concrete caused by changes in temperature, and dynamic loads from vehicles, mean that the superstructure is continuously moving. This can cause the structure to gradually deteriorate and puts the soil-backfill-foundation system under long term cyclic motion and fatigue.

Other geotechnical issues can arise from a structure’s interaction with the ground. These include soil ratcheting, when the backfill behind an abutment stiffens in response to repeated expansion and contraction.

To study these geotechnical phenomena the SoFSI team will carry out tests to accurately monitor the stiffness behind abutment backfill and the distribution of the pressures behind the abutment wall.

The soil pit will be filled with 80t of sand 2.5m deep, and two towers of 40 concrete blocks weighing 1.5t each will be positioned on either side of it. Between the blocks the team will place a vertical precast concrete slab, hinged at the base and fitted with sensors.

The slab will be pushed back and forth by an actuator to mimic the loading experienced by the abutment of an integral bridge.

The actuator simulates the sideways load imposed on the abutment by the bridge deck.

The experiment will test the deformation of the sand backfill under stress. It will also measure the pressure profile behind the wall when this happens and how the soil stiffness changes with deformation. The test will additionally look at how a failure surface in the soil wedge behind the wall develops.

The highly controlled environment of the soil pit provides an ideal environment for this testing.

“Here we know everything,” says Sextos. “We know the properties of the soil, we measure the forces and displacements, and we measure the pressures that we can have even behind the wall and on the reaction wall around the pit. Therefore, we can very accurately understand the entire performance [of the abutment-soil system] in 3D.”

The team will also measure the active and passive pressures in the abutment using “novel instrumentation techniques”, says Sextos. These include ground penetration radar, standard LVDTs [linear variable differential trans-formers], displacement transducers and possibly high resolution pressure cells buried within the ground material.

Up until now reliable large scale experimental testing of the backfill in integral bridge abutments under laboratory conditions has not been possible. Designs of these structures have previously been based on analytical solutions. By supplying missing field data, Sextos hopes the project will improve those analytical models, and thereby improve the design and longevity of integral abutment bridges.

The biaxial shaking table

Another upcoming project in the soil pit will test the foundations of offshore wind turbines. The SoFSI team will work with the University of Oxford on the project, which is currently in its proposal stage.

The project will study two issues: the interaction of different natural loads like wind, waves and earthquakes on the performance and dynamic properties of a turbine, and the forces exerted by waves on the seabed.

Offshore wind turbines are exposed to dynamic wind and wave loading, so testing the interaction between their foundations and the seabed over time is crucial.

Sextos explains: “Under continuous environmental loads, the foundations of offshore wind turbines get softer with time, thus influencing the dynamic properties of the system.”

To study the interaction between turbine foundations and the seabed the team will place a 6m high model of a wind turbine in the soil pit. A reaction frame will be mounted around this on the strong floor and a set of electric actuators will then be attached to this to simulate wind forces on the turbine. A second actuator will apply and simulate the wave loads at the turbine’s base in the soil pit.

Data for wave loading and dynamic water pressure applied to wind turbines already exist, so the team will programme the actuators with these values.

Recreating wet sand to mimic ground conditions for offshore wind turbine foundations will be more challenging.

“Practically the presence of water makes things more complicated in the soil pit,” notes Sextos. Although the pit can be drained via sumps in its floor, pumping out the water from the saturated sand will take some time. The pit will also have to be lined. Because of this the team will start by using dry sand.

A key aim of the facility is to inform designs that are safer and more cost efficient. A lack of understanding about how structures interact with their foundations and the surrounding ground has led to conservative design, explains Sextos.

This has resulted in structures with higher safety factors than necessary and this has an “unbelievably high impact” on construction costs, he says. The result is structures which can be more resource intensive, take longer to construct and which are more expensive to maintain.

“High safety factors arise from the uncertainties from the soil [ground] which, given the nature of the clays and sands, are much higher than the uncertainties of the structures,” he adds.

“If, by rigorous testing, we are able to reduce these unnecessarily high safety factors, wherever they exist, then these funds can be used to improve safety somewhere else in the system.”

Research at the laboratory also aims to find ways to reduce the carbon footprint of structures by helping to develop designs which use fewer materials. Another aim is to make existing structures last longer and be more resilient to climate change.

As Sextos notes, the laboratory has an environmental dimension “both explicitly and implicitly” because it is also testing renewable energy structures.

As well as this, it is promoting research into novel materials – such as metamaterials. Another future project will involve testing nano composite slabs, which are much thinner than traditional 150mm thick concrete slabs, under dynamic loads on the strong floor.

The laboratory can also support a wide range of other academic and commercial research. This includes testing aerospace and aerodynamic problems. The SoFSI team is currently exploring the use of the shaking table to mimic the launch of a satellite.

The facility will also work with the nuclear industry, which requires “equipment qualification” – an approach which ensures that all safety critical components in nuclear facilities meet certain quality standards.

It has used the large shaking table to certify equipment such as cabinets containing sensitive electrical equipment to test how they might behave in an earthquake. And it can place a model of a small modular reactor in the soil pit to see how this reacts with the soil.

Perhaps because of its wide range of research applications, the SoFSI facility is off to an exciting start.

It has already been successful in its first application to be part of a European consortium of 12 other major research facilities. This £11.6M project of which £1M comes to SoFSI will give international partners access to the facility to test their ideas with the Bristol-based team.

The SoFSI team also includes: University of Bristol senior lecturer in civil engineering Flavia De Luca, University of Bristol lecturer in civil engineering Raffaele De Risi, University of Bristol chair in soil-structure interaction George Mylonakis, University of Bristol technical manager David Williams, University of Bristol civil engineering research fellow Tony Horseman, UKCRIC research project manager Patrick Tully, University of Bristol professor of earthquake engineering Adam Crewe and University of Bristol research fellow at the department of civil engineering Matt Dietz.

Want to read more? Subscribe to GE’s enewsletters and follow us on Twitter and LinkedIn