How Expansive Soils Subgrades Affect Pavement Durability
The performance of pavement is significantly affected by the swelling and shrinking of expansive subgrade soil in response to changes in moisture.
According to an article by Geosciences, expansive soil poses a significant annual cost for the maintenance and repair of structures built upon it. The cost of this annual expense ranges from 9 to 15 billion USD, with 50% of the expenses being associated with highways and streets. Integrating the behaviour of expansive subgrade into pavement design can substantially reduce future maintenance costs.
What happens to road pavements constructed on expansive soil subgrades?
Expansive soil covers vast areas of land across the globe, causing damage to road pavements as they shrink and swell. Uplifting and longitudinal cracking of pavement are common examples of the problems caused by such soils.
A soil is considered expansive when it has a high percentage of clay minerals that have very small, plate-like particles that can absorb large amounts of water which cause the soil to become plastic and sticky as it expands. On the other hand, in dry conditions, these same soils become hard and begin to shrink, causing cracks to appear in the ground. This is known as shrinkage cracking, which may also be known as block cracking, map cracking, and chicken wire cracking. As the pavement subgrade soil undergoes repetitive cycles of wet and dry periods each year, severe cracking develops in the pavement due to such high concentrations of stress. Some examples of this can be seen in Figure 1.
Figure 1: Damaging effects of expansive soil subgrades
Solutions for Dealing with Expansive Soils
Excavation and Replacement: Remove the expansive soils and replace them with suitable fill materials.
In-situ Stabilisation: Use a lime treatment to chemically stabilise the in-situ soil to mitigate its expansive properties.
Cover thickness: Increase the pavement's thickness to mitigate the reactive pressures transmitted through the various pavement layers.
Geosynthetic Support: Use geocomposites and geogrids as these are highly effective in reinforcing and stabilising the base of roads.
Drainage: Install impermeable barriers or impermeable shoulders and table drains to control subgrade moisture conditions.
Polymer stabilisation: Polymer subgrade stabilisation involves the addition of polymer to expansive or unstable subgrade material to make it suitable for construction.
Mechanical subgrade stabilisation: This refers to subgrade compaction or densification using mechanical force. Mechanical subgrade stabilisation employs techniques such as vibration, rammers, rollers, and blasting to eliminate air voids within a soil mass. It is seen as a more intensive method of subgrade stabilisation when compared to other approaches.
Case Study Example
During an investigation for pavement design, non-destructive structural evaluations were carried out using the Falling Weight Deflectometer (FWD) and laboratory geotechnical tests to assess the subgrade’s stiffness and density. These tests revealed a subgrade that was expansive and of low strength, so lime stabilisation was ultimately considered as the solution. Click here to read the full case study.
Figure 2: FWD testing of subgrade
Reference
Ahmed A, Hossain MS, Pandey P, Sapkota A, Thian B. Deformation Modeling of Flexible Pavement in Expansive Subgrade in Texas. Geosciences. 2019; 9(10):446. https://doi.org/10.3390/geosciences9100446
Amakye, S.Y. and Abbey, S.J. (2021). Understanding the performance of expansive subgrade materials treated with non-traditional stabilisers: A review. Cleaner Engineering and Technology, 4, p.100159. doi:https://doi.org/10.1016/j.clet.2021.100159
Pavement Interactive (n.d.). Shrinking and Swelling Soils – Pavement Interactive. [online] Available at: https://pavementinteractive.org/reference-desk/design/design-parameters/shrinking-and-swelling-soils/#google_vignette