Ground Improvement with Conventional and Novel Binders

Cambridge University
Geotechnical Engineering Group



Deep soil mixing (DSM) is a ground improvement technique used in geotechnical industry and is becoming increasingly popular due to its versatility to difficult ground conditions. In DSM, basically a cementitious material is added to the soil and mixed with it mechanically to enhance engineering characteristics of problematic soils. DSM is categorised in to two types based on the nature of binder addition, namely wet and dry mixing method when grout and dry binder is used. Portland Cement (PC) is the most common binder used in DSM applications. PC, while being an excellent material in terms of fulfilling technical requirement in many modern construction techniques, in recent years attracted increasing scrutiny in respect of sustainability due to its high CO2 emission and vast use of raw material during its energy intensive manufacturing process. Studies have shown the cement industry constitutes to 2% of global primary energy consumption and responsible for 5% global carbon dioxide emission. Hence, the use of alternative materials in soil mixing has attracted attention in the context of increasing sustainability of the technique and also to overcome technical deficiencies associated with the use of PC.

The objective of the research reported here was to perform comparative study on performance of a wide range of binder blends in ground improvement relative to the performance of PC. Novel binder blends were prepared from industrial by-products with cementitious properties such as ground granulated blastfurnace slag (GGBS), pulverised fuel ash (PFA), furnace bottom ash (FBA) and cement kiln dust (CKD) along with conventional binders. Also, novel binders such as magnesia cement, zeolite cement and slag-magnesia blend were also studied to test their applicability in ground improvement. Various binder blends were produced from basic binder materials with different proportions and the resulting binder blends were used to stabilise different model soils which represent basic soil types namely clay, silt, clayey silt and sand. Both wet and dry mixing methods were used in stabilisation and the mechanical properties of stabilised soils such as bulk density, strength, stiffness and failure strain were measured at various curing times. Effects of various process variables such as soil type, binder content, grout dosage, water to binder ratio, grout rheology and curing temperature were studied on soils stabilised by novel binders and compared with the corresponding results from PC.

Novel binder blend of GGBS and reactive magnesia at 9:1 ratio was formulated and this binder resulted in very high strength in all soil types and almost always outperformed all other binders including PC. Better performance of PC+GGBS binders than PC in all soil types was witnessed and the strength of the stabilised soil increased with GGBS content in the binder up to 75%. Strength provision of magnesia cement with/without PFA depends significantly on PC content of the binder. PC+PFA and PC+FBA binders showed their strength provision depends largely on PC content of the binder in sand stabilisation whereas inclusion of PFA and FBA contributed to strength in stabilisation of cohesive soils composed of kaolin and silica flour. Also, the influence of particle size difference between PFA and FBA was insignificant on strength of stabilised clay. Zeolite-cement blend and PC+CKD binder resulted in higher strength than PC in stabilisation clayey silt after 90 days of curing and their effect on sand was minimal on early curing time. GGBS+CKD binder produced strength results comparable to PC after 90 days of curing in sand and clayey silt.

Comparisons between wet and dry mixing methods, reflecting the role of water added at different stage of mixing, showed wet mixing is more effective in terms of strength provision in cohesive soils. On the other hand, dry mixing was more effective in sand stabilisation compared to wet mixing. Sustainability of the binders in soil stabilisation was compared relatively using indices which were developed in current study by incorporating technical performance, environmental impact and economical aspect of binders. Strength of stabilised soil, embedded carbon and cost of the binders were used in the analysis. Outcome of the analysis showed the PC+GGBS binder with 75% GGBS content is relatively more sustainable binders than others. This binder showed sustainability indices about 7 and 9 times higher than PC in stabilisation of clayey silt and sand respectively.