MA Zhifu, YANG Changxian

(China Railway Design Corporation, Tianjin 300142, China)

0 Introduction

Recently, many projects have indicated that due to the poor diagenesis of Tertiary, the deformation of tunnel keeps to increase after excavation under the action of groundwater. For the long span tunnel, there are issues such as deformation control, water inrush and gushing mud, tunnel collapse, etc., which all become significant technical problems.

Zhang[1]has studied the main factors that may influence engineering properties of Tertiary sandtone in Lanzhou, and has also built quantitatively evaluated model, which has 5 grades to standardize the engineering properties; Li et al.[2]have studied the engineering properties of Tertiary expansive soil in Yuxi-Mengzi Railway, giving construction requirements to subgrade, bridge and tunnel; Qi[3]has used Tertiary water-rich sandstone as study object, proposed the technical idea of focusing on dewatering, grouting, strengthened support and quick closure; Gao[4]has studied Baidian Tunnel of Shanxi South Central Railway, which is located in Tertiary water-rich sand strata, and proposed construction technique of "Advance peripheral grouting + excavation face reinforcement and grouting + water releasing outside reinforced body + additional grouting during excavation + quick excavation and closure"; Yin[5]has analyzed Ganqing Tunnel of Datong-Xi′an passenger dedicated railway, which passes through Quaternary and Tertiary clay, sand and loess, proposed measures for stabilizing excavation face and advance consolidation; Based on Zhongtiaoshan Tunnel of Menghua Railway, Qin et al.[6]have studied the engineering geological characteristics of Tertiary strata, and proposed the construction requirements for tunneling; Zhang[7]has studied engineering geological characteristics of Sanchaling Tunnel of Lanzhou-Xinjiang High-speed Railway, which passes through Tertiary sulfate chemical deposit; Zhang et al.[8-14]have concluded the engineering geological characteristics of Tertiary weak consolidated sand strata

of Humaling Tunnel and Taoshuping Tunnel on Chongqing-Lanzhou Railway, and have also studied the design and construction technique such as tunnel dewatering, advance consolidation, construction technology for connection section between inclined shaft and main tunnel, etc. However, the researches above are basically on single stratum, which are very rarely on the case of Tertiary sand-mud interlayers.

Thus, this paper focuses on Shuangfeng Tunnel on Mudanjiang-Suifenhe Railway, which passes through Tertiary sand-mud interlayers with long distance under big overburden and rich groundwater, and systematically studies on the engineering geological characteristics, determines technical measures such as dewatering, advance consolidation, structure design and construction, effectively controlling tunnel deformation, realizing successful construction and safe operation.

1 Overview and risks of the project

The Shuangfeng Tunnel is 7.237 km in length, as key works of the expanding and rebuilding of Mudanjiang-Suifenghe Railway. The tunnel is located in hilly area, with very small terrain fluctuation, the maximum overburden is about 140 m. From the ground surface, there are basalt, granite, granodiorite, andesitic porphyrite, etc. in sequence, with soft Tertiary sandy mudstone strata in middle; there are 2.3 km section of this tunnel located in Tertiary sandy mudstone strata, passing through contact zones multiple times, with overburden of 100-120 m and high level ground water (shown in Fig. 1). This tunnel is designed as single-bore double-track tunnel for speed of 200 km/h, and the area of the excavation cross-section is about 145 m2.

Fig. 1 Sketch of longitudinal profile of tunnel passing through Tertiary sandy mudstone strata

In Tertiary sandy mudstone strata, due to the poor effect of consolidation and dewatering, large deformations occur to the tunnel under the high-pressure groundwater, followed by collapse and water-mud gushing (shown in Table 1).

Table 1 Statistics of large-scale water gushing and mud outburst

2 Engineering characteristics of sandy mudstone strata

2.1 Lithology and distribution pattern

The sandy mudstone strata belongs to Tertiary Pliocene series (N2d). See below for the specifics:

(1)Mudstone: ash black, greyish-green, pelitic texture, stratified structure, complete-strong weathered, soft, with organics and plant foliage. The drilled thickness is 0-40.9 m, and the mudstone is partly interbedded with sandstone.

(2)Sandstone: reseda, light yellow, poor diagenesis, completely weathered, rock core as sand, soft. The drilled thickness is 1.4-38.4 m, with partly pebbly sandstone.

(3)Argillaceous sandstone: light grey, reseda, sand-like structure, argillaceous cement, weathering completely, soft, thickness 3.8-5.0 m.

(4)Lignite: black, mainly deadwood and charcoal. The drilled thickness is 0.85-6.0 m.

After stereo probe by multiple geological prospecting such as drilling, seismic wave, electromagnetic wave CT, tomography, etc., the results indicate the uneven weathering of stratum. The sandstone is completely weathered, the mudstone is completely-strong weathered; with several faults, the distribution of the strata is extremely messy.

2.2 Groundwater situation

The groundwater in the tunnel area is basically bedrock fissure water, existing in fissure and weathered stratum, distributed 13.0 m underground, mainly supplied by rainfall and Quaternary phreatic groundwater. There is a large supply area on the right side of the tunnel located in sandy mudstone strata. In the section of DK464+350～DK467+370 where the tunnel passes through Tertiary sandy mudstone strata, the catchment area of groundwater is 5.22 km2. The seasonal variation of water level is 5.0-10.0 m. The water volume is affected by different seasons, with large water volume in raining season and snow-thawing season and small water volume in dry season.

2.3 Physico-mechanical properties of surrounding rock

See physico-mechanical properties of Tertiary mudstone in Table 2.

Table 2 Physico-mechanical properties of Tertiary mudstone

The Tertiary sandstone is pale yellow, with inhomogeneous distribution, serious weathered, poor diagenesis, no stability and developed groundwater.

2.4 Engineering features

(1)The sandy mudstone strata is with inhomogeneous distribution and poor diagenesis, which belongs to extreme soft rock.

(2)At the contact zone between the sandstone or sandy mudstone strata and the hard rock such as granite, granodiorite, andesitic porphyrite and basalt, etc., the ground water is very rich and well supplied.

(3)For mudstone, the joint fissure is developed, and the compressive strength is low; the mudstone is rich in hydrophilic mineral, with higher water absorption, poor permeability and weak-medium swelling; the mudstone is likely to crack once dehydration, easy to be soften and disintegrated, and its strength decreases dramatically.

(4)The sandstone is seriously weathered, as loose sand, and lacks of stability.

The permeability of Tertiary mudstone is poor, as aquiclude; and the sandstone is loose structure, as permeable layer; thus the stratum has a better permeability when containing sand. When the stratum is mainly mudstone, there is a difficulty in dewatering and draining, and water gushing is likely to happen with sandstone; when it is difficult in dewatering, the high water pressure will act on the tunnel structure, likely leading to deformation, collapse, and water/mud gushing.

3 Support structure design and construction method

3.1 Support structure design

For Tertiary sandy mudstone strata, there is a large volume of groundwater, with rich supply resource, therefore a series of measures such as dewatering and draining have been applied in construction. In order to ensure the tunnel structure safety in both the construction stage and the operation stage, the hydrostatic pressure is considered in the structure design, and the secondary lining can carry part hydrostatic pressure. Besides, for mud gushing and disturbed section, the support structure is considered to carry the loose load formed by mud gushing. The parameters of tunnel support in sandy mudstone are shown in Table 3.

Table 3 Parameters of tunnel support in Tertiary sandy mudstone strata

3.2 Selection of construction methods

With expanding deformation of tunnel constructed in Tertiary sandy mudstone strata, the temporary support may intrude into secondary lining because of delayed closure, and collapse might happen. Thus, the tunnel construction in such strata shall follow the principle of "quick excavation, quick support and quick closure".

The mini-bench excavation method is adopted for the construction of the tunnel in Tertiary sandy mudstone strata with rich groundwater based on advance consolidation and machinery capability, etc. This construction method requires the invert and the secondary lining to be installed closely following the excavation (shown in Fig. 2).

3.3 Site tests and analysis

The displacement monitoring data obtained at 151 locations of Shuangfeng Tunnel, including locations in mud/water gushing section, those in Tertiary mudstone section, those in sandy mudstone strata interlayer section, and those in contact zone with other strata, are used for analysis. For each stratum above, cross-sections are picked for stress monitoring. The analysis results show that:

(1)In mudstone stratum, the maximum convergence deformation value of the tunnel is 69 mm, and the maximum settlement of crown is 181 mm; in sandy mudstone strata interlayers, the maximum convergence deformation value of the tunnel is 45 mm, and the maximum settlement of crown is 230 mm; in contact zone between sandy mudstone strata and other stratum, the maximum convergence deformation value of the tunnel is 41 mm, and the maximum settlement of crown is 525 mm. The deformations all indicate that the settlement of crown is larger than the convergence; When the temporary support is not closed in time, the deformation of the tunnel in sandy mudstone strata may increase continuously and intrude into the secondary lining. Thus, the deformation control in this stratum shall follow the principle of "quick excavation, quick support and quick closure", and technical measures shall be taken to ensure that the inner contour of the cross-section is smooth and continuous.

Fig. 2 Layout of excavation method with mini-bench (unit: m)

(2) The measured maximum pressure of surrounding rock is 753.49 kPa, the maximum contact pressure between the temporary support and the secondary lining is 1 190.52 kPa, and the distribution of the surrounding rock pressure and the contact pressure is asymmetric. The stress tends to be stable after temporary support closure and secondary lining construction, which indicates that the surrounding rock pressure and the contact pressure are sensitive to the geological difference and lateral disturbance, and stress redistribution is very obvious during tunnel construction.

4 Advance consolidation

As Tertiary sandy mudstone strata is of weak diagenesis, with poor stability and high level groundwater with rich supply, the tunnel excavation may result in geological hazard such as collapse, deformation, quick-stand and water/mud gushing, and having a negative effect on construction safety and progress. As a consequence, the deformation of the tunnel in this strata shall be controlled and the stability of the surrounding rock shall be ensured. The key to ensure the tunnel construction safety is to reduce water pressure and consolidate the strata ahead of the tunnel face and surrounding the tunnel in advance.

4.1 Purpose and method of pre-grouting

Grouting techniques combining high pressure splitting, compacted grouting and reinforcement bar in grouting holes are adopted according to the distribution of Tertiary sandy mudstone strata and the groundwater revealed by excavation.

Based on the site machinery capacity and the tunnel construction method, full-face pre-grouting is applied for the strata ahead of the excavation face, using the upper excavation face of the tunnel as the working face. The pre-grouting holes are shown in Fig. 3.

Fig. 3 Diagram of pre-grouting hole position (unit: cm)

4.2 Design parameters of pre-grouting

Based on site test of grouting technique, the grouting parameters for Tertiary sandy mudstone strata are determined as Table 4.

4.3 Selection of grouting materials

According to site grouting test result and engineering characteristics of Tertiary sandy mudstone strata with high pressure water, cement-water glass grout with short gelation time is selected for the holes of the outer ring to seal the groundwater, and rapid-hardening high-strength sulphoaluminate cement grout is selected for the holes of the inner rings as consolidation material to ensure sufficient strength for excavation. The parameters of the grouting materials are listed in Table 5.

Table 4 Grouting parameters

Table 5 Grouting materials parameters

4.4 Analysis of grouting consolidation result

After pre-grouting consolidation, the construction of the tunnel in Tertiary sandy mudstone strata with high pressure water indicates that the excavation face and the surrounding sandy mudstone strata are basically anhydrous; during excavation, the tunnel face is stable, the deformation is controlled with no collapse, which ensures tunnel construction safety.

5 Water pressure reducing by water releasing

From top to bottom, the tunnel is covered by basalt, soft sandy mudstone strata, granite, granodiorite, andesitic porphyrite, etc. The tunnel passes through Tertiary sandy mudstone strata with high water pressure and the contact zone between sandy mudstone strata and other strata. The distribution of groundwater is complicated. The working condition of Tertiary sandy mudstone is rapidly worsened under the action of the groundwater, resulting in extremely poor stability. For Tertiary sandy mudstone strata, water releasing is needed to ensure safety during construction and operation of the tunnel.

5.1 Key technique of water releasing

5.1.1 Cavity release

After tunneling into Tertiary sandy mudstone strata, two large-scale water/mud gushing incidents occurred, forming an irregular water storage cavity (as shown in Fig. 1). In order to reduce the water pressure on the tunnel construction, water releasing holes are drilled from the ground surface toward the temporary support of the tunnel, as shown in Fig. 4.

Fig. 4 Layout of water releasing form the cavity (unit: m)

The diameter of the water releasing hole is 89 mm, with mm steel pipe (50 mm in diameter and 5 mm in thickness) installed in the hole to drain the water to the central drainage ditch. Below the groundwater level, the steel pipe is drilled with holes with 10-16 mm diameter, 15 cm spacing and quincuncial arrangement. The steel pipe is double wrapped, with geotextile on the inner side and 80-120 carbon fiber filter mesh on the outer side. In order to ensure safety of the tunnel during operation, channels are constructed along the temporary support of the tunnel to guide the water into the central drainage ditch, then the water can be drained out of the tunnel.

3 water releasing holes are drilled. At the beginning, the rate of water releasing through the holes is large; when it is stable, the rate is about 35-50 m3/h, indicating that there is a balance between the water supply and the water releasing.

5.1.2 Advance water release

When completely-weathered sandstone strata with rich water are predicted ahead of the excavation face, advance water release holes are arranged based on the advance geology forecast holes and the pipe roof holes, as shown in Fig. 5.

Fig. 5 Layout of advance water releasing holes (unit: m)

The diameter of the advance water releasing holes is not less than 130 mm, the length of the advance water releasing holes is no less than 25 m, and the final hole position is 5 m outside the excavation contour. The horizontal overlapped length of the advance water releasing holes of two cycles is no less than 5 m. More advance water releasing holes can be drilled based on the volume of water released. Perforated steel pipe wrapped (108 mm in diameter and 6 mm in thickness) with 2 layers of 100 carbon fiber filter mesh is installed in the advance water releasing hole to drain the water to the central drainage ditch.

During the drilling of the advance water releasing holes, water gushes out of the holes; after the drilling is completed, the water volume decreased gradually. The advance water releasing holes are effective in probing the geological conditions ahead of the tunnel face so as to ensure the stability of the excavation face and avoid water/mud gushing.

5.1.3 Radial water releasing

When the deformation of the temporary support is not convergent, with wet stained on the surface and partly water seepage, radial water releasing is needed.

One radial water releasing cross-section with 2 water releasing holes is set every 5 m in longitudinal direction. The lengths of the radial water releasing holes are normally 10 m. The other parameters of radial water releasing are the same as those of advance water releasing.

As the strata are complicated, the water released volume through the radial water releasing holes is small, which cannot form continuous drainage. However, the radial water releasing holes can release the support deformation to some extent.

5.1.4 Water releasing through parallel gallery

When Shuangfeng Tunnel enters Tertiary sandy mudstone strata, a by-pass parallel gallery is arranged to speed up the tunnel construction. The distance between the centerline of the parallel gallery and that of the main tunnel is about 30 m. The parallel gallery is used for geological probing and water releasing[15].

5.1.4.1 Precipitation well in parallel gallery

For radial water releasing and advance water releasing of the parallel gallery, please refer to the main tunnel. When sandy mudstone strata and other contact zone are located below the floor of the parallel gallery, precipitation wells are constructed on the floor of the parallel gallery to relieve water pressure on the support structure.

The precipitation wells are perpendicularly constructed on the floor of the parallel gallery. The depth of the precipitation well is normally 8-10 m. One precipitation well is constructed every 30 m in longitudinal direction. The water is discharged directly into the side ditches of the parallel gallery. The parameters of the precipitation wells are the same as those of the advance water releasing holes.

After the precipitation wells are constructed, water gushing appears in the precipitation wells in some ways, which effectively relieve the water pressure on the support structure, ensuring safety of the parallel gallery.

5.1.4.2 Water releasing holes drilled from the parallel gallery toward the crown of the main tunnel

For water releasing holes drilled from the parallel gallery toward the crown of the main tunnel, refer to Fig. 6.

The spacing of the water releasing holes is 2-5 m. According to the spatial location relation between the gallery and the main tunnel, the length of the water releasing holes is not less than 30 m. The diameter of the water releasing holes is 89 mm. Perforated steel pipe (50 mm in diameter and 5 mm in thickness) is arranged inside the water releasing hole. The steel pipe is double wrapped, with geotextile on the inner side and 80-120 carbon fiber filter mesh on the outer side. The water from the water releasing holes are discharged to the side ditches of the parallel gallery.

Fig. 6 Layout of water releasing holes drilled from parallel gallery toward crown of main tunnel (unit: m)

The water releasing holes are mainly arranged in sections with mud gushing risk, with 20 holes in total, uneven water volume for each hole, accumulative volume being about 40-50 m3/h. The water releasing holes can relieve water pressure on the support structure to some extent.

5.1.5 Ground surface dewatering

Water releasing from inside the tunnel disturbs the tunnel construction in some ways. For sections with unsatisfactory water releasing effect, dewatering wells can be arranged on the ground surface according to the tunnel construction.

The ground surface dewatering wells are located 8 m outside the tunnel structure on both sides, constructed vertically from up to down. The diameter of the ground surface dewatering wells are not less than 600 mm, and the ground surface dewatering wells shall reach the floor of the tunnel. Steel pipes are arranged inside the well. Above the water level, the hot rolled seamless steel pipe (273 mm in diameter and 6 mm in thickness) shall be used, and below the water level, the bridge type filter pipe (273 mm in diameter and 7 mm in thickness) shall be used, welded by socket. The filter pipe shall be triplicate wrapped by geotextile, 80-120 carbon fiber filter mesh and bamboo curtain splint respectively. The 0.5-1 cm gravels shall be filled between filter pipe and hole as inverted filter. After construction of well, the water pump shall be arranged to discharge water to the ground surface.

12 dewatering wells are constructed on site, and 5 of them with stable water volume, with each one as 11-27 m3/h; the rest wells turn out to be off and on, with water volume less than 10 m3/h. The accumulative stable water volume is 120-130 m3/h. The time-history curve of water yield of dewatering well is shown in Fig. 7.

(a) Right side

(b) Left side

Fig. 7 Time-history curve of water yield in dewatering well on surface (in 2015)

From Fig. 7, the water volume (per hour) slowly decreases and tends to be stable; there are a lot of differences for different positions, which indicates the uneven groundwater distribution in these stratums. The largest groundwater volume is at DK466+480～+500, indicating that these sections are located in mud gushing range, the stratum is soft, with rich groundwater. During excavation, besides ground surface dewatering and in-tunnel dewatering, strong tunnel deformation control measures shall also be taken to ensure safety during tunnel construction.

5.2 Comprehensive analysis of result of dewatering and water releasing

After water releasing in Tertiary sandy mudstone strata, besides real-time monitoring, the water yield is also be asssessed by arranging observation wells (the technical parameters of the observation wells are the same as those of the water releasing holes for the cavity). The time-history curve of variation in water level is shown in Fig. 8.

From Fig. 8, after all-around dewatering and water releasing measures in these stratums, the water level above the tunnel slowly decreases and tends to be stable, from 60 m to 43 m. On one hand, it indicates that the groundwater level is lowered, which is good for safe excavation and deformation control; on the other hand, it indicates that there is difficulty in dewatering for these stratums, and it is inevitable to work under water condition, thus, measures such as advance consolidation, strong support and quick closure shall be taken to ensure construction safety.

Fig. 8 Time-history curve of variation in water level (in 2015)

6 Post evaluation of structural safety

Based on the monitoring data of contact pressure between the temporary support and the secondary lining in 4 cross-sections (Tertiary mudstone, sandy mudstone strata interlayers, contact zone between sandy mudstone strata and others, and water/mud gushing disturbing section), with consideration of instrumental error, the load-structure model is used to analyze internal forces of the structure, with loading as 100% monitored contact pressure. The safety checking is carried out for lining structure based onCodeforDesignofRailwayTunnel[16]. The loadings are valued as Table 6, and the node numbers of model are shown in Fig. 9.

Table 6 Load applied on lining in the typical cross-sections kPa

The calculation result indicates that the checked cross-sections of structure (reinforced concrete) are all controlled by compression, and the safety factor can comply with the design requirement. In the above 4 cross-sections, the smallest safety factor occurs at the left skewback in mud gushing disturbing area, with the value being 3.19>2.4 (the axial force and bending moment are shown in Fig. 10). Besides, the secondary lining is good, with no cracks and decrustation, which indicates the good performance of the secondary lining, ensuring structure safety.

Fig. 9 Node numbering of calculation model

(a) Axial forces of secondary lining (unit: N)

(b) Bending moment of secondary lining (unit: N·m)

Fig. 10 Calculation diagram of force applied on lining structure in gushing mud disturbing section

7 Conclusions and discussion

Regarding Tertiary sandy mudstone strata with high water pressure of Shuangfeng Tunnel on Mudanjiang-Suifenhe Railway, the stratum is poor diagenetic, lack of stability, difficult in deformation control, and subjected to collapse, quick sand, water/mud gushing. According to research, the key techniques route of "stereo exploration, pressure reduction by water releasing, pre-grouting, supporting timely and overall monitoring" is established for this stratum, which ensuring the tunnel construction. The main conclusions and discussions are listed as below:

(1)The strengthened composite lining is used for the tunnel in Tertiary sandy mudstone strata with high pressure water. Mini-bench method, which requires lining closely following the excavation, is used for tunnel construction, achieving the aim of "quick excavation, quick support and quick closure", and very effective in deformation control.

(2)The method of combining high pressure splitting with compacted grouting, and the technique of grouting with reinforcement are used. The cement-water glass grout with short gelation time is used for the holes of the outer ring to block groundwater, and the rapid-hardening high-strength sulphoaluminate cement grout is used as consolidation material to ensure sufficient strength. Through grouting, the aim of consolidating, improving, and stabilizing Tertiary sandy mudstone strata is achieved, to bear the water pressure together with the support, ensuring tunnel construction safety.

(3)The all-round water releasing measures from the ground surface, in-tunnel and parallel gallery have been used to lower the groundwater level, which are very important in tunnel deformation control, avoiding water/mud gushing, ensuring tunnel construction safety and operation safety.

(4)Due to the uneven groundwater distribution in Tertiary sandy mudstone strata, there are a lot of difficulties in water releasing. When the water releasing result is not satisfactory and water is inevitable in construction, further measures combining advance consolidation with strong support and quick closure shall be taken to ensure construction safety.

Acknowledgements

The study was supported by the Key Project of Technology Research and Development of China Railway Corporation (2013G011-B), and the Key Project of Technology Research and Development of The Third Railway Survey and Design Institute Group Corporation (721332).

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