# Numerical Simulation on Backfilling of Buried Pipes Using Controlled Low Strength Materials
## 2 Pipe and Backfill Material Parameters
### Performance Test of Controlled Low Strength Material  
The CLSM mixture ratio was tested in Table 2. Engineering excavation soil and sand were used as fine aggregate, and the engineering excavation soil was replaced by 10%, 20% and 30% of sand, cement and fly ash as cementitious materials.  
Natural river sand as fine aggregate is medium sand. The fineness modulus is 2.4. Excavation soil comes from the Wuhan project. In this study, a 5 mm sieve was used for screening, and large particles were screened out. The liquid limit of excavated soil is \(\omega\)l = 15.3, the plasticity index is lp = 17.3, and the natural moisture content is 20.18%. In this study, Portland cement with compressive strength of 42.5 MPa and Class F primary fly ash were used.  
The unconfined compressive strength test used cube specimens with dimension of 70.7 mm \(\times\) 70.7 mm \(\times\) 70.7 mm. After 28 days' curing of CLSM, the strength test was carried out in the YAW-4605 pressure testing machine. CLSM's stress-strain test used size of rectangular specimen, which was 70.7 mm \(\times\) 70.7 mm \(\times\) 210 mm. After 28 days' curing in the curing box, the stress-strain test was carried out using a servo pressure testing machine. Unconfined compressive strength and stress-strain tests are implemented in accordance with \"GB/T 50081\(-\)2019. Standard Test Method for Physical and Mechanical Properties of Concrete\" [20].  
The stress-strain curve of CLSM is shown in Figure 3. When the amount of excavation soil is 10%, CLSM reaches the peak stress, then the specimen is destroyed, and the stress decreases rapidly. With the increase of excavation soil content in CLSM, the plastic strain of CLSM gradually increases. CLSM reaches peak stress drop more slowly, has a large plastic deformation, and will not be destroyed immediately. CLSM reaches peak stress drop more slowly. The material has a large plastic deformation and will not be destroyed immediately. It can be concluded that increasing the clay content in the aggregate can increase the plastic deformation of CLSM.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Excavation Soil**} & \multicolumn{5}{c}{**Material (kg/m\({}^{3}\))**} \\ \cline{2-6}  & **Cement** & **Fly Ash** & **Water** & **Sand** & **Excavation Soil** \\ \hline
10\% & 75 & 200 & 340 & 1192.5 & 132.5 \\
20\% & 75 & 200 & 370 & 1060 & 265 \\
30\% & 75 & 200 & 425 & 927.5 & 397.5 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Mixture ratio of test.  
Figure 3: The stress–strain curve of CLSM.  
For the comparison of CLSM's unconfined compressive strength and elastic modulus, as shown in Figure 4, the modulus of elasticity and compressive strength change trend is the same. When the excavation soil content is 10%, CLSM has the largest modulus of elasticity at 290 MPa. When the excavation soil content in CLSM increased to 30%, the elastic modulus of CLSM is only 130 MPa. In the numerical simulation that follows, CLSM with an excavation soil amount of 20% is used, and its modulus of elasticity is 202 MPa.