# Stabilisation of Soft Clay, Quick Clay and Peat by Industrial By-Products and Biochars
## 3 Results
### Stabilisation Effect  
Figure 4 shows results from the UC tests, i.e., strength and stiffness for the stabilised specimens cured at 28 days. The stabilised Onsey clay (see Figure 4a) obtained \(\tau_{max}\) values ranging between 50 and 100 kPa, except for CEM as a single binder with \(\alpha\) of 60 kg/m\({}^{3}\), i.e., CEM-60, which reached almost 200 kPa. Adding 100 kg/m\({}^{3}\) of biochar had varying stabilisation effects. Biochars BC2 and BC3 had negative stabilisation effects on the strength development, whilst BC1 and BC4 had positive stabilisation effects. The same differences were also observed for \(E_{50}\) (see Figure 4b). The stabilisation effect of adding the calcium-based IBPs also varied. However, all IBPs had positive stabilisation effects with increased strength. The PSA and BA in particular seemed to give a relatively high strength and stiffness contribution. The Tiller-Flotten clay (see Figure 4c,d) showed a considerably greater strength development, where \(\tau_{max}\) ranged from just below 200 kPa up to approximately 550 kPa. However, for the Tiller-Flotten clay, all biochars had a negative stabilisation effect, although to a lesser degree for BC1 and BC4. For the IBPs, the same pattern as that for the Onsey clay was observed, i.e., the PSA and BA had the highest stabilisation effect. The results for the stabilised peat are shown in Figure 4e,f. All biochars, especially BC1 and BC4, had a positive stabilisation effect with an increased \(\tau_{max}\) compared to CEM as a single binder. The effect of adding IBPs, however, reduced the strength considerably, except for LS1 and LS2. The same pattern was obtained for stiffness \(E_{50}\). The average relationships between stiffness \(E_{50}\) and strength \(\tau_{max}\), often referred to as the rigidity index, are plotted in Figure 5. For the clays, the ratio \(E_{50}/\tau_{max}\) was generally between 200 and 400, equivalent to \(E_{50}/UCS\) between \(\sim\)100 and \(\sim\)200, which is in line with other studies [29; 82; 83]. The CEM and PSA stabilised specimens deviated from this (CEM-30/PSA-30), where the ratio \(E_{50}/\tau_{max}\) was \(\sim\)90, i.e., considerably lower  
Figure 3: Results from thermogravimetric analyses of binders: **(a,b)** CEM and IBPs, **(c,d)** biochars. than that for the other IBPs. It was further observed that specimens stabilised with biochar in general had a lower ratio \(E_{50}/\tau_{max}\) compared to CEM as a single binder and CEM combined with IBPs. This implies that stabilisation with biochar improves stiffness to a lesser degree than strength. For the peat specimens, the ratio \(E_{50}/\tau_{max}\) varied between \(\sim\)40 and \(\sim\)100, independent of IBP or biochar, in line with previous studies on biochar in peat [27]; however, there was also an exceptionally low ratio for the CEM-PSA-specimen, where \(E_{50}/\tau_{max}\) was \(\sim\)13. Figure 4: Results of shear strength (\(\tau_{max}\)) and stiffness Youngs’ modulus (\(E_{50}\)) from UC tests for the (**a**,**b**) Onsoy clay, (**c**,**d**) Tiller-Flotten clay and (**e**,**f**) peat. Error bars show the standard deviation. The results from the strength activity index (SAI) are shown in Figure 6. All biochars and IBPs reduced the strength but to various degrees, ranging from -70% reduced strength for BC4, BA and FA to -90% reduced strength for BC3. On average, the strength reduction was 81% for the biochars compared to 73% for the IBPs. This difference was very much affected by the low reduction for BC3 (93%). It should, however, be noted that the variation for each mixture was relatively large, as shown by the error bars. Figure 5: Relationship between shear strength (\(\tau_{max}\)) and stiffness Youngs’ modulus (\(E_{50}\) ). Average values for the three replicate specimens for each mixture. Points denoted ‘w/CEM and IBP’ show mixtures with both CEM as a single binder and CEM and IBP in combination. Figure 6: Results of the strength activity index (SAI) for 100% CEM and mixtures where 20% of the CEM was replaced with biochar or IBPs.