# Stabilisation of Soft Clay, Quick Clay and Peat by Industrial By-Products and Biochars
## 5 Conclusions  
Four biochars (BCs) and five industrial by-products (IBPs) were characterised and tested for stabilisation effects on three natural Norwegian soils: the Onsey and Tiller-Flotten clays and peat. The following conclusions were made:  
* Two of the biochars (BC1 and BC4) had beneficial stabilisation effects (i.e., strength and stiffness) on the Onsey clay with a relatively high water content (\(\sim\)73%). All four biochars had negative stabilisation effects on the Tiller-Flotten clay with a relatively low water content (\(\sim\)42%); however, BC1 and BC4 had the least negative effect.  
Figure 8: Strength of mixtures of biochars and IBPS relative to mixtures with 100% CEM as a binder for the peat.  
* Almost all of the IBPs had positive stabilisation effects on the two clays. The greatest effect was observed with the paper sludge ash (PSA) and bottom ash (BA). The ladle slags (LS1 and LS2) had a negligible effect on strength and stiffness development.
* Three biochars (BC1, BC3 and BC4) had a positive stabilisation effect on the peat. Three IBPs had negative stabilisation effects, whilst the ladle slags LS1 and LS2 again had a negligible effect.
* The stiffness-to-strength ratios (\(E_{50}/\tau_{max}\)) of most of the mixtures ranged between 200 and 400, except for all peat samples, which had \(E_{50}/\tau_{max}\) values of approximately 50. The low \(E_{50}/\tau_{max}\) values also applied to some mixtures with biochar and all PSA-stabilised specimens.  
The varying results in strength for the biochars are believed to depend on the biochar characteristics such as grain size and water absorption capacity in relation to the porosity and water content of the soil. The results indicate that the stabilisation effect of the biochars increases with increasing water content of the soil; however, this has not been investigated in detail. Further research on biochar in soft soil stabilisation is needed as the climate mitigation potential is considerable.  
No apparent correlation between stabilisation effect and the chemical composition of the biochars or IBPs was observed. This highlights the difficulty in predicting strength development based on chemical and mineralogical compositions alone and stresses the importance of performing laboratory testing.  
The need for further studies is noted. In particular, further research should investigate the observed large strength and stiffness differences when stabilising different clays. In addition, further studies are needed to explain the effects of, for example, the grain size distribution of IBPs and biochars and the effects of the binder uniformity of biochar on the stabilisation effect.  
Conceptualization, S.R. and P.P.; methodology, S.R., C.S. and P.P.; software, S.H.; validation, S.H., P.P. and M.L.; formal analysis, S.H. and C.S.; investigation, S.H.; resources, S.R., P.P., G.C. and M.L.; data curation, S.H.; writing--original draft preparation, S.H.; writing--review and editing, S.H., P.P., S.R., G.C., C.S. and M.L.; visualization, S.H.; supervision, P.P.; project administration, S.R.; funding acquisition, S.R. and G.C. All authors have read and agreed to the published version of the manuscript.  
Not applicable.  
All data are available from the corresponding author upon reasonable request.  
The authors are grateful to [PERSON] and [PERSON] at the NGI geotechnical laboratory in Oslo for performing many of the laboratory tests presented herein. The authors are also grateful to the staff at Nemko Norlab, SINTEF and X-ray Mineral Services for performing several of the characterisation investigations. The authors are especially grateful to [PERSON] at X-ray Mineral Services for contributing to the interpretation of the XRF and XRD analyses. The contributions from the industrial partners Norske Skog Skog, Bergene Holm, Lindum and Celsa Steel Services are appreciated. The authors appreciate the valuable comments from the anonymous reviewers, which improved the quality of the manuscript.  
The authors declare no conflict of interest.