Foundation Remediation using Expansive Polyurethane Resin


Figure 1: Injection of Expansive Polyurethane Resin to remediate cracks in a structure.


Figure 2: Expansive Polyurethane Resin structure.

Figure 3: Interaction of two shots of resin in a 20 mm width crack. First shot is cracked by the second one. Macrovoids are visible.

Introduction

Injection of expansive polyurethane resin is a common underpinning solution for individual houses, buildings and paving slabs. The pressure exerted during the chemical reaction producing the resin lifts the structure. When injected in expansive soils, which are very sensitive to water content changes, it is of particular importance to understand how soil-resin composite material behaves and how the resin may affect the soil’s hydraulic properties and its swelling behaviour.

Indeed, desiccation cracks created during the soil shrinkage are filled with resin, meaning that any future wetting of the soil would lead to enhanced swelling that may over-lift the remediate foundation.

The Polyurethane Resin

Exothermic reaction between a polyol and isocyanate
Reaction time in the ground ? 30 s
Expansion pressure : up to 10 MPa
Increase of volume in case of free expansion: 40 times
A closed cell structure is formed (Figure 2)

Resin formation in the ground

  • The resin permeates into the soil following the weakest path,
  • The resin is able to crack the soil and to fill and/or enlarge the existing cracks.
  • The resin is also observed to enter cracks as small as 0.1 mm - The resin injection is made in multiple shots, allowing the resin to expand after each one => Interaction of shots (Figure 3) - The resin injected into the ground is a heterogeneous material having several sub layers and some possible macro voids

Permeability of the resin

  • Tests performed in Rowe Cells with GDS Pressure Controllers,
  • Injection pressure of 25 kPa
  • Steady state flow only for lightest resin (density of 30 kg/m³) => permeability of about 10-8 m/s to 10-9 m/s,
  • Denser resin is waterproof due to the closed cell structure except in case of defect.
  • Resin resists a pressure up to 200 kPa.

Permeability of the soil mass

  • Soil mass is a dual permeability system (bulk clay + cracks)
  • Resin fills the cracks preventing water to flow => decrease of permeability
  • In situ air permeability tests have been performed to quantify the impact of the resin (Figure 5)
  • Results show a decrease of permeability up to a factor 50 (Figure 6)

Perspectives

Future work will focus on swelling behavior of composite soil and on the interaction between swelling soil and remediate structure.


Figure 4: Permeability of the resin as a function of its density
Figure 5: Experimental set up for in situ air permeability tests.
Figure 6: Evolution "flow rate-pressure" for both injected and non injected zones at different depths.

Investigators