St Mary’s Church Stabilisation

Grade 2 listed St Mary’s Church is located in Doverdale in Worcestershire (figure 1). Its original construction was dated as far back as the 12th century with subsequent restorations in the centuries that followed. In the 2000’s the church had shown evidence of subsidence and in 2012 Geobear, under its former name Uretek, completed stabilisation works to the west wing of the building.

Figure 1: St Mary’s Church, Doverdale UK

In 2021 the East Wing of the building began to show signs of significant structural movement. Large cracks were forming in the walls of the building (figure 2), therefore stabilisation work was necessary to protect the integrity of the building.

The cause of the movement was driven by clay shrinkage which was exacerbated by a large tree near the treatment area. Clay shrinkage occurs when the moisture content of the clay soil is reduced, this causes the structure above to sink slightly causing cracks.

Figure 2: Cracks to the exterior wall of the east wing

Site Investigation data showed that the ground beneath the structure was made up of a loose/soft made ground/ sandy clay of up to 1m below ground and a stiff-firm clay layer below that with a modified PI of between 12 and 25 reaching a depth of 2m. 

Geobear solution

Geobear designed a solution based on the site investigation data. The solution requires a geopolymer resin to be injected into the ground beneath the structure, at a number of locations and depths around the east wing.

The geopolymer resin is injected as a two-component liquid. A reaction occurs when the two components mix in the ground causing a material expansion. The expansion of the geopolymer material results in compaction of the clay soils, preventing the shrink-swell capability and stabilising the ground beneath the structure. 

The east wing required multiple injections of geopolymer to a depth of 2m below the existing foundations. Shallow trenches were dug out around the treatment locations to expose the foundations (figure 4).

Figure 4: Injection tubes in locations

During the injection process, laser monitoring is used to both verify the stabilisation of the area and to ensure no lifting of the building occurs. When  0.5mm of movement is registered on the laser receiver, this indicates the ground beneath is stabilised (figure 5).

Figure 5: Laser monitorin24% improvement was achieved at level 1.

Standard penetrometer testing was also used to confirm the ground improvement in terms of its bearing capacity. Figure 6 indicates the improvements at each of the three levels where a 24% improvement was achieved at level 1.

Figure 6: SPT results

Geobear could deliver the stabilisation work of the project within two days. Using self-contained units, minimal plant and equipment were needed on-site (figure 7). The alternative option for the structure would have been underpinning; this would require large-scale excavation and many weeks of disturbance to the site. Additionally being a listed structure, underpinning poses greater risks of damage to the church than a geopolymer injection solution.

Figure 7: Geobear units on location