## 5 Conclusions  
In this study, induced trench installation (ITI) and embedded trench installation (ETI) models for large-diameter thermoplastic pipes subjected to high fill stresses were investigated by numerical analysis. The study evaluated the effects of crushed stone, sand, and clay backfill on the stresses and pipe deflections by considering the pipe diameter, rigidity, and thermoplastic material type (HDPE and PVC). The responses of such a system (i.e., pipe-EPS-backfill soil system) are controlled not only by pipe-EPS geometry and soil characteristics but also by the stiffness of the EPS material. However, in this study, EPS stiffness was not considered. As a result of the analyses, the following results were obtained:  
1. In ETI and ITI models in which the EPS material was used with thermoplastic pipe, positive soil arching increased regardless of the vertical pipe deflection. There was a decrease in the stress in the regions where the EPS Geofoam material was placed. Stress was uniformly distributed across the EPS Geofoam in the pipe crown; however, significant increases in stresses occurred from the edge point of the EPS. Creating a compressible zone on the pipe in the ITI model caused greater stress at the pipe spring line and pipe invert than in the ETI model. Thus, the importance of the geometry of the compressible region to be formed around the pipe was highlighted. It was determined that the ETI model reduced the stresses acting on the pipe and caused a more uniform stress distribution around the pipe;
2. The effects of diameter in HDPE pipes were investigated, and it was determined that the increase in pipe diameter increased the stresses acting on the pipe. Similar stress increases were determined in the ITI and ETI models. When the HDPE pipe diameter increased by approximately twice in the ETI model, there was an approximately 20%, 15%, and 35% increase in stress calculated at the crown, invert, and spring lines, respectively. The type of backfill soil around the pipe also affected the stresses. High stresses occurred in clay, sand, and crushed stone backfills, respectively;
3. The stiffness effect was examined in the PVC pipe, and higher stresses were observed on the pipes with higher rigidity. In the ITI and ETI models, the stress acting on the pipe decreased regardless of pipe stiffness. The stresses calculated in pipes whose stiffnesses differed by a factor of two were very close. Thus, it was determined that it is appropriate to use the lower rigidity pipe with EPS Geofoam (especially in the ETI model) under the higher fill stresses;
4. HDPE and PVC pipes were taken into account to examine the effects of pipe material on stress in thermoplastic pipes. It was determined that the stresses affecting the PVC pipe were higher than the HDPE pipe. This is because the stiffness of the HDPE pipe is lower than the PVC pipe. The HDPE pipe deflects more, causing further development of positive arching and resulting in less stress affecting the HDPE pipe;5. The use of EPS in the installation of thermoplastic pipes greatly affected the VAF and HAF values. When the thermoplastic pipe was buried according to the ETI method, active soil wedges at the sides of the pipe were induced, leading to a significant reduction in the horizontal stresses that act on the pipe wall. Reduction in horizontal stress in the pipe spring line was controlled by compression of the EPS zone that covers the side of the pipe;
6. The VAF and HAF values were compared. The highest values were determined in clay, sand, and crushed stone backfills when comparing the reference to the ITI and ETI models, respectively. With the increase in the H/D ratio in the ETI model, VAF and HAF values decreased to almost the same values for all three backfill types. This situation shows that clay backfill can be used instead of the sand and crushed stone material used as traditional backfill with the ETI model;
7.