Engineers are using geosynthetic reinforced soils to ensure that bridges can retain consistent surface levels at the points where they connect to embankments.

A perennial problem for road engineering is the frequent disparity in surface levels which arises at the juncture between a bridge and the embankment to which it connects, caused by the bridge structure rising slightly higher as it exerts pressure upon the soil below its surface.

Dante Fratta

Dante Fratta

The “bump in the road” that this creates is more than just a minor annoyance for engineers and drivers. The surface level discrepancy between the bridge deck and the embankment can damage vehicles traversing the bridge and can create hazardous road conditions, which increases the likelihood of accidents.

The height disparity also makes the bridge junction far more difficult to lay asphalt upon or repair, and in places which experience severe cold weather can leave the bridge deck susceptible to damage from snow ploughs.

Engineers from the University of Wisconsin-Madison believe they have now found a solution to this problem through the use of geosynthetic materials to put the bridge and embankment on an even playing field.

UWM Civil and Environmental Engineering associate professor Dante Fratta proposes dispensing with the traditional method of supporting the bridge and road via separate structures, with the bridge deck propped up using rigid piles and embankments borne by compactable soil.

He advocates placing both the bridge and the embankment on a single support which is firmed up using geosynthetic reinforced soils (GRS), in order to ensure their surface levels are always consistent.

According to Fratta, the use of a single foundational support means that the bridge deck and the embankment will always remain at the same level, even in the case of deformation of the pavement.

Fratta and a team of engineering colleagues have already trialled the technique in the real world, testing it on a small bridge which traverses a creek on State Highway 40 to the south of Bloomer in Wisconsin’s Chippewa County.

mississippi diagram

The Wisconsin Department of  Transportation (WisDOT) adopted Fratta’s approach during the construction of the bridge in the summer of 2012, creating a 36-metre long foundation to bear both the bridge deck and the roadway embankment simultaneously.

The location of the bridge makes it an outstanding test subject given that the frac sand mining boom in the west of the state has resulted in an increased amount of heavy vehicle traffic along the highway route.

WisDOT has monitored the bridge to assess its performance when subjected to the rigours of heavy cargo traffic, as well as whether the geosynthetic soil is capable of withstanding the erosion and scour which occurs on waterways and river banks.

Initial surveys have found that the Bloomer bridge is performing well, with the only glitch being the embankment rising above the bridge deck by around three centimetres in winter as a result of water penetrating the soil and freezing solid.

Fratta believes the problem can be easily remedied, however, by altering the composition of soils in the embankment, and using gravel instead of fine particles to reduce the incidence of swelling.