Construction Loads And Earth Pressures On Large Shield Tunnel Lining With Unconventional Overburden

As more and more urban underground space are developed,the overburden of sheild tunnels is becoming shallower or deeper.But,there are fewer studies of the impact of construction loads on the shallow and deep overburden shield tunnels and the earth pressure distribution of them.So,it is necessary to carry out related studies to analyze the impact of construction loads on the shallow and deep overburden shield tunnels and the erath pressure distribution of them to provide theoretical supports and technical guidances for the design and construction of shield tunnels.In-situ monitoring were carried out to measure construction loads and earth pressures on two shallow and deep overburden shield tunnels.The time history and the distribution of construction loads and pressures on the lining and inner force in the lining during tunnel construction and post-construction were obtained.It was found that tail brush pressure had great influence on segment safety performance of shallow shield tunnel and current design method is conservative for deep tunnels.Some valuable suggestions for shield tunenl desgin and construction are presented based on related tests and numerical analysis comparing with the in-situ monitoring data.The main contents and related conclusions of this paper are shown as follows:(1)In-situ monitoring were carried out to measure construction loads and earth pressures on two shallow and deep overburden shield tunnels.The time history and the distribution of construction loads and pressures on the lining and inner force in the lining during tunnel construction and post-construction were obtained.It was found that shield tail brush pressure,especially steel plate brush pressure was the largest load acting on the shallow shield tunnel,which had great influence on segment safety performance of segments.And measured earth pressure,segment axial forces and bending moments were all smaller than theoretical values.That is to say,current design method is conservative for deep tunnels.(2)Loading test was carried out for common used shield tail brushes:wire brush and steel plate brush,and load–displacement relation curves were obtained.The distribution of axial force and bending moment caused by tail brush pressure was analyzed through numerical analysis.A continuous fictitious spring support with a coefficient of subgrade reaction k_r in the normal direction,which had no stiffness on the tension side,was used to model the effect of supporting a tunnel ring inside of the shield machine.Results showed that for large shield tunnel,the pressure caused by steel plate brush was larger than that by wire brush.Axial force and bending moment became larger as the decrease of the gap between shield and segment.The maximum bending moment casued by steel plate brush was more than 1000 kN·m,which had great influence on the safety of segment.(3)Model tests were carried out to study the earth pressures acting upon shield tunnel lining given simultaneous backfill grouting under different depths and grouting pressures in sand and clay grounds.The time history and the distribution of earth pressures on the lining were obtained.Test results showed that the distribution of earth pressures is uniform with the effect of grout.The ratios of earth pressures on the tunnel to that at the top of the tunnel in different layers lied between 0.95-1.2.The final earth pressure mainly depended on soil type and overburden,and was influenced little by grouting pressure.The earth pressure at tunnel crown was smaller than whole overburden when the overburden was more than 2D in sand and 3D in clay.(4)The relationship of axial force and bending moment caused by earth pressure and tail brush pressure was compared through numerical analysis to analyze the impact on the shield segment caused by tail brush pressure.It was found that for the shield tunnel whose diameter was larger than 10 m,when the overburden was smaller than 25 m,steel plate brushes should not be used.The distance between segments and shield shell should be also no less than 10mm during shield advanding to ensure the safety of shield tunnel segments.Even if wire brushes were used in this case,the distance between segments and shield shell should be also no less than 10 mm during shield advanding.(5)Based on in-situ monitoring data,the shield tunnel load system was analyzed considering the ground reaction,vertical earth pressure and coefficient of lateral earth pressure to study the earth pressure distribution of deep overburden shield tunnel.It was found that,when the overburden of shield tunnel was more than 2D,Terzaghi's loosening earth pressure can be used to calculate the vertical earth pressure in sand layer and the coefficient of lateral earth pressure can be 0.1-0.2 larger than the recommended values of geological exploration report.In clay layer,the coefficient of lateral earth pressure can be 0.1larger.For the design of shield tunnel lining,the load system of Chinese standard can be adopted and lateral soil reaction can be ignored by setting ground springs aroud the lining.