## 4 Micro-Scale Analysis  
### Nuclear Magnetic Resonance (NMR)  
The nuclear magnetic resonance (NMR) test was carried out using a MicroM12-025 VR nuclear magnetic resonance analyser manufactured by Suzhou Newmax Analytical Instruments Co., Ltd. (Suzhou, China) with an instrumental strength of (0.28 \(\pm\) 0.05) T, a resonance frequency of 12 MHz and a sampling coil size of 25.4 mm. Figure 15 shows the steps of the nuclear magnetic resonance (NMR) test. NMR is a technique often used in the field to analyse porosity, bound water saturation, pore size distribution, permeability and reservoir parameters or to observe the state of fluid distribution. When the sample is placed in a magnetic field, hydrogen plasmons absorb the energy of the signal and resonate when a certain pulse frequency is emitted. The relationship between the signal amplitude and the relaxation time \(T_{2}\) of the pore water in the soil sample is obtained during the test. Since the sample is in a saturated state, \(T_{2}\) can be regarded as the vector size of the pore, which is given by the following:  
\[\frac{1}{T_{2}}\approx\rho_{2}\frac{S_{pore}}{V}=\frac{\lambda\rho_{2}}{d} \tag{9}\]  
where \(T_{2}\) is the lateral relaxation time (ms), \(\rho_{2}\) is the relaxation coefficient, \(S_{pore}\) and \(V\) are the pore surface area and volume, respectively, \(d\) is the pore diameter, and \(\lambda\) is a factor related to the pore shape. According to the equation, the relaxation time \(T_{2}\) is proportional to the pore volume \(V\), i.e., the larger the relaxation time \(T_{2}\) is, the larger the pore volume \(V\). Figure 16 shows the distribution curves of the transverse relaxation time \(T_{2}\) for ordinary clay and for the EPS-modified lightweight soils with different sisal dosages at 5% EPS volume fraction. According to the above figure, the control and 1.0%, 1.2% and 1.4% sisal fibre admixtures all had bimodal distributions, which was due to the nonuniform diffusion coupling between the water molecules that remained in the saturated and unsaturated pores, where the trend of the signal intensity of each major peak was between 0.1 ms and 1 ms and that of the minor peaks was between 10 ms and 50 ms. However, the change in the peak value of the soil samples in each test group was relatively more obvious in the main peak, and the peak value of the secondary peak was too small and almost unchanged. Some scholars have shown that the relaxation time and soil pore size are directly proportional to the corresponding peak signal intensity, the number of pores is directly proportional to the pore size, and the peak area can indicate the soil pore size, that is, the pore volume and the water content of the specimen are positively correlated [17; 27; 28; 29]. The comparison between controls 1 and 2 showed that the peak signal intensity and peak area of control 2 were larger, which indicated that the EPS-modified lightweight soil had more continuous small pores than the normal clay soil, the internal pore space was larger, the water content was higher, the EPS incorporation made the internal structure looser, and the number and size of the pores were larger than those of the normal clay soil. Comparisons between control 2 and the other test groups showed that the peak signal intensity of the sisal fibre significantly weakened after the incorporation of the sisal fibre. With increasing amounts of sisal fibre, the peak signal intensity decreased, and the peak area gradually decreased. This indicated that a series of physical reactions occurred after the sisal fibres were incorporated into the soil, and the soil grid structure that formed was  
Figure 15: Test procedure for nuclear magnetic resonance. conducive to improving its strength and dynamic properties. In addition, the sisal fibres made the soil particles more compact, which was equivalent to filling the soil pores and made the number and volume of the pores decrease continuously.