## 6 A New Method to Determine the T\({}_{2}\) Cut-Off Value  
Larger pore sizes occupy unrelaxed spin states and more time to reach the pore wall is required. Low-field NMR could add confidence to estimates of bound fluid, free fluid, and permeability [38]. Based on an expectation that producible fluids most reside in the larger pore spaces, and bound fluids are mostly held in the smaller pores, a \(T_{2}\) cutoff may be applied to the \(T_{2}\) distribution that divides the total NMR porosity into bound and free-fluid porosity [22]. The \(T_{2}\) cutoff separates free fluid from bound fluid, so the water saturation can be predicted. [PERSON] et al. and [PERSON] et al. also showed that the influence of pore size distribution and the difference in the response depending depend on the fluid type [52; 53]. More than separating free fluid from bound fluid, NMR can also discriminate porosity as clay-bound water, capillary-bound water, and movable fluid.  
The proportion of different water types in soft clay becomes a critical component to evaluate soft clay strength. There are two methods to determine the \(T_{2}\) cutoff value. The first one compares the \(T_{2}\) relaxation time spectrum before and after high-speed centrifugation. The other method is experience-based judgment called the \"half-amplitude method\". According to the summary of cutoff values of various soft clays, for predominantly single peak \(T_{2}\) relaxation time spectra less than 10 ms, the amplitude midpoint on the right limb of the relaxation time curve was set as the cutoff value. The normal two methods can determine the cutoff value between absorbed water and free water. This study provides a new method to determine the cutoff values between clay-bound water, capillary-bound water, and free water. It is well known that water freezes strongly with the binding force between hydrogen, the absorbed water could not be frozen by -20 \({}^{\circ}\)C. However, the smaller the pore size is, the more proportion absorbed water occupied. The pore water in different pore size has different freezing speed. So, the cutoff values of water types could be found by comparing the relaxation time spectra during different freezing durations.  
From Figure 18, the evaluated cutoff value of bound water and free water from the three freezing methods cluster around 2-3 ms. The cutoff value between bound water and free water from half-amplitude, high-speed centrifugation, and freezing methods are 2.25 ms, 2.8 ms, and 3.0 ms, respectively [43]. The clay-bound water and capillary-bound water had an approximate demarcation point between 0.2 and 0.3 ms, which was the most accurate available range currently. The freezing method result was similar to the cutoff values from the other two methods.  
Comparing the three test methods: the half-amplitude method is an empirical method, which is simple to determine the \(T_{2}\) cut-off value of the movable fluid. Meanwhile, the accuracy of the evaluation cutoff from empirical methods is difficult to guarantee. The high-speed centrifugation method has strict laboratory apparatus requirements and experimental operation to obtain a precise result. However, the operation of the high-speed centrifugal method is cumbersome; the freezing method proposed by this paper can simultaneously measure the cut-off value among the strong bound water, weak bound water,and the movable fluid. Moreover, the operation of the freezing method is relatively simple, and the test results have a high accuracy.  
Therefore, the freezing method to test the \(T_{2}\) cut-off value is a suitable method to distinguish the three water components, which makes up for the blank test of the cut-off value of strong and weak bound water. The freezing method provides a pioneering new idea for determining the \(T_{2}\) cut-off value of NMR. Since the proportion of different water types determines the mechanical properties of saturated soft clay. Thus, the new way to determine the \(T_{2}\) cutoff value is meaningful to predict long-term subway settlement.