Analysis Sequential Excavation Method For Large Span

Modern tunnel design mandates the careful selection of appropriate techniques and technologies for all tunnel projects. The pivotal factor for success in tunnel construction lies in choosing the right excavation methods for extensive-span tunnel projects in soft soil materials. The choice of excavation methods during tunnel construction significantly impacts both the project timeline and associated costs. This study specifically concentrates on the selection of excavation techniques, sequencing, and temporary supports employed during the excavation phase for tunnels located in soft soil environments. Given the considerable diameter of the tunnel, exceeding 200 m2, and the presence of soft soil, traditional methods like the Full-face method and the Central Diaphragm (CD) and Side wall Drift (SD) approach are not feasible. Therefore, this study suggests the application of a Sequential Excavation Method (SEM) for the tunnel construction.


Introduction
In this modern era, infrastructure is developing very quickly.Limited land and high human awareness of environmental damage encourage the development of tunnel construction in Indonesia.The construction of high-speed railway tunnels, toll road tunnels and spillway tunnels on dams projects is evidence that tunnels have started to be in demand rather than having to carry out open cut excavations.
The effect of the tunnel construction process is the disruption of the rock in situ stress in the field so that it can cause a decrease in the elevation of the land surface above it.The larger the diameter of the tunnel, the greater the potential for land subsidence above it.Challenging rock mass conditions can heighten the effects of tunnel excavation.Thorough and precise engineering planning is eesential to motogate the consequences of tunnel construction.The choice of excavation methods during tunnel construction significantly influences the time and expenses involved.
Tunnel construction in urban environments is a complex undertaking due to its shallow depth, soft soil, proximity to populated areas, and the presence of delicate surface and subsurface structures.Ensuring the safe excavation of tunnels within these urban settings is a central concern throughout the design and construction phases [1].Sequenced excavation method (SEM) is a common practice in urban tunnel construction to improve tunnel face stability and minimize surface deformation and settlement [2].When the final two factors are ignored, there could be significant soil deformation around the tunnel's perimeter and possible harm to the established support system [3].
The tunnel design process and the selection of excavation methods used are very important in the success of tunnel construction.For this reason, the research in this paper focuses on the selection of excavation methods and excavation sequences that are suitable for use in constructing large crosssectional tunnels.

Description of Yogyakarta-Bawen Twin Tunnels Project
Yogyakarta-Bawen Twin Tunnel Project will be located in Magelang Regency, Central Java, Indonesia(Figure 2).This project is the largest tunneling project in Indonesia in length, large cross section and steps route.Tunnels with a net excavation area of 220 m and cross-sectional height and width of 3 lanes plus 1 emergency lane of 13.7 m and 20.2 m, respectively (Figure 2).The maximum tunnel cover soil depth is only 32 m up to the tunnel roof.

Geotechnical Investigation
Geological and geotechnical investigations were conducted to determine the soil conditions around the tunnel area.Some of the investigations carried out were geotechnical drilling, Standart Penetration Test (SPT), insitu test, geophysical test and laboratory test.The total number of geotechnical investigations as shown in Table 1.
Based on geotechnical investigations, it was found that the soil layers in the tunnel alignment are silt, clay and silt sand.Soil strength parameters obtained from in situ and laboratory tests are used for numerical modeling in order to determine the deformation that occurs due to tunnel excavation.the results of the soil parameters used for numerical modeling are described in Table 2.

Numerical Modeling
The tunnel excavation process is simulated in stages using Finite Element Method (FEM) modeling.
We can predict the value of stress and displacement distribution by using FEM application.In this study, 2D numerical analysis was performed on the section with the greatest overburden depth (32 m to the roof of the tunnel) using the RS2 Rocsience software.Figures 3 and 4 show the cross-sectional section that will be examined.

Sequential Excavation Method (SEM)
Throughout the tunnel project, the incorporation of engineering and technology is essential at every stage of design and construction.For tunnels situated in urban settings, the selection of excavation methods and sequencing plans typically hinges on intricate considerations involving factors like safety, cost, and project schedule [4].The accuracy of the excavation method selection is determined by a number of surrounding material-related parameters.The geotechnical features, tunnel section size, subsurface hydrology, in-situ and induced stresses, regional geology, structural geology, and weak zone characteristics are all included in this list [5].When plotting the values of span size and tunnel ratio while adhering to the constraint of the uniaxial compressive strength of the soil in relation to vertical stress [6] , the most appropriate excavation method appears to be the Side wall Drift (SD) (Figure 5).

Optimization Analysis In Tunnel Excavation Sequence
The use of the side drift construction technique, which incorporates reinforced concrete sidewalls, has found extensive application in challenging ground conditions to minimize the excavation cross-section in a single phase.This approach effectively mitigates the risk of instability at the working face, making it particularly suitable for shallow-depth tunnels [7].The side drift construction method effectively reduces the amount of deformation caused by tunnel excavation, as shown by numerical models and field observations.In this study, we compared 3 excavation sequence methods when using the same side drift construction method (Figure 6).With the same material properties and the same type of reinforcement used in the tunnel, we can see the results of the comparison of deformations resulting from each excavation sequence.

Result and Discusion
From the results of the deformation values that we get on the roof, floor and walls of the tunnel from the 3 tunnel excavation sequences in the numerical modeling (Figure 7), we can choose which excavation sequence is best used by looking at the smallest deformation value.Of the 3 numerical models compared, the smallest deformation results are shown in Figure 6 (c).Table 3 displays the specific outcomes of the deformation values obtained from the numerical modelling.

Conclusion
With a large diameter and area in Yogyakarta-Bawen tunnel, a suitable sequential excavation method is the side drift method.The order of excavation in the side drift method during tunnel construction affects the value of deformation caused by excavation.Of the 3 numerical models that have been compared, the results of the lowest deformation at the monitoring point of the roof, floor and wall of the tunnel is the Figure 6 (c).

Figure 3 .
Figure 3.Typical cross-section of the tunnel.

TunnelFigure 5 .
Figure 5. Determination of the method of excavation of the Yogyakarta-Bawen Tunnel.

Figure 6 .
Figure 6.Sequence of excavation using the proposed Side wall Drift (SD) Method for Jogja Bawen Tunnel.

Figure 7 .
Figure 7.The results of the total displacement from numerical modeling.

Table 1 .
List of the geological and geothecnical surveys at the tunnel location.

Table 2 .
Material properties around tunnel zone.

Table 3 .
Detail results of the deformation values from the numerical modeling.