Sustainable stabilisation of expansive road pavement foundations, pavement thickness optimisation and defects analysis

Amakye, Samuel

Abstract

Expansive road subgrade has been in existence for decades resulting in major road pavement defects, high maintenance/construction costs and detrimental environmental effects associated with using traditional cement and lime in subgrade stabilisation. Taking a sustainable approach, this research aims to address these issues using waste and industrial by-products (i.e. brick dust waste (BDW), ground granulated blast-furnace slag (GGBS), recycled plastic (RP) and recycled glass (RG) as partial replacements for cement and lime in subgrade stabilisation. The study investigates the sample characteristics, mineral structure, Atterberg limit, compaction, California Bearing Ratio (CBR), swell and microstructural properties of treated and untreated expansive subgrade materials.
Sustainable waste materials and industrial by-products at proportions of 23.5%GGBS, 23.5%RP, 23.5%RG, 23.5%BDW and 11.75%GGBS, 11.75%RP, 11.75%RG, 11.75%BDW were used to achieve the optimum results. This reduced 20%Cement and 8%Lime (control mix design) to 2.5%Cement and 2%Lime. The 2.5%Cement was later eliminated and GGBS increased to 26% to see the effect on subgrade. Untreated high plasticity index (PI) (103) subgrade recorded Optimum Moisture Content (OMC) of 34.46% with a standard deviation (SD) of 23.41% and Maximum Dry Density (MDD) of 1.25Mg/m³ (SD=0.31%). A Liquid Limit (LL) of 131.26% (SD=18.18%) and Plastic Limit (PL) of 28.74% (SD=1.85%) were also recorded for untreated high PI subgrade. Untreated extremely high PI subgrade (249) recorded higher OMC of 40.97% (SD=9.42%) and MDD of 1.17 Mg/m³ (SD=0.28%) with much higher LL of 294.07% (SD=48.48%) and PL of 45.38% (SD=1.13%). CBR values for untreated subgrade increased from 0.6% (SD=4.38%) to 109% (SD=34.10%) and 200% (SD=53%) and up to 220% (SD=54%) after 28 and 90 days of curing when 20%Cement+8% Lime were partially replaced with 23.5%GGBS, 11.75%GGBS+11.75%BDW and 26%GGBS. Swell values reduced from 56.76% (SD=7.72%) to 0.04% (SD=0.01%) after 20%Cement+8%Lime were partially replaced with 23.5%GGBS and 11.75%GGBS+11.75%BDW translating into reduced pavement thickness and depth of construction when pavement design was conducted in the study.
Road pavement thickness of 700mm and depth of construction of 800mm recorded for untreated subgrade with CBR values less than 2% reduced to 40mm and 50mm with CBR values between 80-100% when 20%Cement+8%Lime were partially replaced with 23.5%GGBS and 11.75%GGBS +11.75%BDW. Pavement design conducted using CBR values between 80-100% achieved for waste-treated subgrade in accordance with Design Manual for Roads and Bridges (DMRB) recorded a slight reduction in pavement thickness with reduced stresses responsible for pavement defects. A gradual reduction in CBR values from 230% (SD=54,61%) to 16% (SD=29.81%) for high PI subgrade and from 200% (SD=47.79%) to 15% (SD.=20.44%) for extremely high PI subgrade was observed after ten (10) wetting-drying cycles when 20%Cement+8%Lime was partially replaced with 23.5%GGBS and 11.75%GGBS+11.75%BDW. These acceptable CBR values achieved for wetting-drying cycle were due to the formation of high Calcium Silicate Hydrate (CSH) gel in the mix where up to 44.87% (SD=11.98%) of calcium (Ca) was recorded after 28 days of curing. Mix design 2%Lime+2.5%Cement+23.5%GGBS was selected as the optimised and most viable mix design in this study followed by mix design 2%Lime+2.5%Cement+11.75%GGBS+11.75%BDW due to their ability to achieve acceptable results for the set objectives including reduced Life Cycle Cost (LCC).
Furthermore, a 55% reduction in LCC (£268,433,336) was observed for a kilometre (km) of road subgrade treated using 23.5%GGBS; whilst a high LCC of £488,754,774 was recorded for a km of road subgrade removed and replaced with foreign materials. Sustainably treated subgrade using 23.5%GGBS recorded 21% lower embodied carbon (0.0018 Co₂e/kg); whilst subgrade treated using 20%Cement+8%Lime recorded high embodied carbon of 0.0084 Co₂e/kg. Based on these findings, the study concluded that the engineering properties of expansive subgrade can be enhanced with reduced pavement thickness/construction depth, defects, carbon emission and overall LCC using sustainable waste as additives in subgrade stabilisation. However, the findings are based entirely on laboratory generated data and not field data. Therefore, as a next step, and before widespread uptake is considered, it is important that the findings are tested and verified in real-life field setting .

Citation

Amakye, S. Sustainable stabilisation of expansive road pavement foundations, pavement thickness optimisation and defects analysis. (Thesis). University of the West of England. Retrieved from https://uwe-repository.worktribe.com/output/10267642

• Thesis Type: Thesis
• Deposit Date: Dec 21, 2022
• Publicly Available Date: May 30, 2023
• Public URL: https://uwe-repository.worktribe.com/output/10267642
• Award Date: May 30, 2023