## 2 Methodology
### Materials  
A soil collected in central Portugal, near the city of Coimbra, was used in the experimental study. The soil is mainly composed of silt (\(\sim\)66%) with some clay (8-12%) and sand (17-22%) particles, having in its composition a high organic matter content (9.3%), which is mainly responsible for the plasticity characteristics of the soil (liquid limit \(\mathrm{w_{L}}\approx 71\%\) and plastic limit \(\mathrm{w_{P}}\approx 43\%\)). The natural soil exhibits a high water content (80.9%), high void ratio (2.1) and low unit weight (14.6 kN/m3), and is classified by the Unified Soil Classification System [35] as OH, organic silt with high plasticity [36; 37; 38]. These characteristics give the soil a poor geomechanical behavior (low strength and high deformability); thus, a ground improvement technique should be adopted to allow for any construction on it [39]. In the present work, chemical stabilization was selected to improve the soil properties by mixing the soil with a binder and a suspension of \"properly\" dispersed carbon nanotubes. Although the soil is slightly acid (pH \(\approx\) 4.5-5.3), which may restrain some binder reactions, it exhibits high silica (\(\sim\)62%) and alumina (\(\sim\)16%) content, allowing a long-term strength improvement [7; 8]. The soil was chemically stabilized with Portland cement type I 42.5 R, applied in a quantity of 175 kg/m3 (kilos per cubic meter of soil). Table 2 presents the main characteristic of the cement particles. The high specific surface of the cement particles and the fact that they are slightly negatively charged should be highlighted. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Surfactant** & **CNTs** & **Time** \\ \hline
**Name** & **Conc. (\%)** & **Conc. (\%)** & **(Days)** \\ \hline \multirow{3}{*}{-} & \multirow{3}{*}{-} & \multirow{3}{*}{-} & \multirow{3}{*}{0.001} & \multirow{3}{*}{7/28} \\ \cline{3-3}  & & & 0.001 & \\ \cline{3-3}  & & & 0.01 & \\ \hline \multirow{3}{*}{Visocrete} & \multirow{3}{*}{3} & \multirow{3}{*}{0.001} & \multirow{3}{*}{7/28} \\ \cline{3-3}  & & & 0.001 & \\ \cline{3-3}  & & & - \\ \hline \multirow{3}{*}{Glycerox} & \multirow{3}{*}{0.5/1/2} & \multirow{3}{*}{0.001} & \multirow{3}{*}{7} \\ \cline{3-3}  & & & 0.01 \\ \hline \multirow{3}{*}{Amber 2001} & \multirow{3}{*}{0.1 */0.5/1 */2} & \multirow{3}{*}{0.001} & \multirow{3}{*}{7 *} \\ \cline{3-3}  & & & 0.01 \\ \cline{3-3}  & & & 0.01 \\ \hline \multirow{3}{*}{Amber 4001} & \multirow{3}{*}{0.5/1/2/3} & \multirow{3}{*}{0.001} & \multirow{3}{*}{7} \\ \cline{3-3}  & & & 0.01 \\ \cline{3-3} \cline{1-1} \cline{3-3}  & & & 0.01 \\ \hline \end{tabular}
\end{table} 
Table 1: Experimental testing plan. Carbon nanotubes were selected as an additive for the chemical stabilization of the soil, aiming to improve the geomechanical behavior of the composite material while promoting a reduction of the quantity of Portland cement added. Multiwall carbon nanotubes (MWCNTs) were chosen for the present work mainly due to economic factors; MWCNTs are significantly less expensive than singlewall carbon nanotubes and, so far, only MWCNTs are produced at an industrial level. According to data provided by the manufacturer, MWCNTs CN7000 have a mean diameter of 9.5 nm, a mean length of 1500 nm, a mean specific surface of 275,000 m\({}^{2}\)/kg (1000 times greater than cement particles) and are composed of 90% pure carbon with some metal oxides (10%). MWCNT characterization was complemented with the evaluation of its density (1.7 g/cm\({}^{3}\)) and charge (\(-\)25.2 mV) [8; 21]. MWCNTs were applied in two small concentrations (0.001 and 0.01% \(w/w\) referred to the dry binder mass) to keep costs under control.