# Experimental Study on a Granular Material-Filled Lining in a High Ground-Stress Soft-Rock Tunnel
## 6 Conclusions  
Given the structural stress characteristics of a high ground-stress soft-rock tunnel lining, an experimental study of yielding-pressure and relief-pressure support of granular material-filled lining was carried out. The conclusions are described below.  
(1) From the analysis of the confined compressive strain law, it is concluded that the final compressive strain of 8 mm soil is the highest. This can provide the maximum compression deformation space and approximately 50% of the compression space. The following conclusions are drawn from the law of load reduction: 8 mm soil has the highest load reduction effect, and its maximum load reduction rate can reach 71%.  
(2) By analyzing the test of a sand-filled simply supported beam, it is concluded that the maximum reduction rate of deflection of sand-filled lining is 36.7%, and the maximum reduction rate of pressure is 78%. Through the analysis of the energy released by the sand-filled lining, it is concluded that the discharge of granular material can unload the load of surrounding rock applied to the secondary lining structure and release energy. This dramatically reduces the damage to the secondary lining caused by the surrounding rock.  
(3) The granular material-filled lining support can provide uniform and controllable surface support force. It can also reduce stress concentration, homogenize surrounding rock pressure, and avoid excessive development of the plastic zone. It can also achieve a better yielding-pressure effect. By discharging particles, the partial pressure of the surrounding rock is unloaded, resulting in better relief-pressure performance. The uncontrollable deformation of the tunnel's surrounding rock is transformed into a controllable state. This reduces the stress in the surrounding rock behind the lining and ensures the long-term service of the lining structure in a lower-stress surrounding-rock environment.  
(4) The difference between this study and previous studies is that the innovative support method of granular-filled lining yielding and relief pressure is proposed, which can not only improve the stability of surrounding rock by yielding- and relief-pressure effects but also reduce the damage of surrounding rock to the secondary lining structure so that the force of the support system tends to be more reasonable. This support method is more adaptable to high ground-stress soft rock. This research offers innovative prospects for improving tunnel stability and provides a better design idea for tunnel support.  
Future work: Strain rate or loading rate is a key parameter that affects the strength and deformation response of granular and bound materials such as natural soils, rocks, and cemented soils [44]. This aspect was not properly considered in the present study, and the authors will consider the effect of strain/loading rate on the deformation behavior of the lining and also consider the study of incorporating the loading rate of different geomaterials [45; 46; 47; 48]. The above sections will serve as the authors' future research work.  
Formal analysis, J.L. ([PERSON]) and B.W.; carrying out experiments, X.X. and J.L. ([PERSON]); data curation, Y.W. and L.S.; writing--original draft preparation, J.L. ([PERSON]) and B.W.; writing--review and editing, J.L. ([PERSON]) and B.W. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Natural Science Foundation of China (No. 52079071) and the Key Projects of National Natural Science Foundation of China (51439003).  
Not applicable.  
All data generated or analyzed during this study are included in this published article.  
The authors declare no conflict of interest.