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Application of new urban tunneling method in Baikoh tunnel excavation
Tunnelling and Underground Space Technology incorporating Trenchless Technology Research
Tunnelling and Underground Space Technology 20 (2005) 151–158
www.elsevier.com/locate/tust
Application of new urban tunneling method in Baikoh tunnel excavation H. Kimura a, T. Itoh b, M. Iwata
c,*
, K. Fujimoto
c
a
c
Ibaraki Prefectural Department of Public Works Office, Mito, Japan b Tukuba New Town Development Co. LTD., Tukuba, Japan Kajima Corporation, Civil Engineering Design Division, 5-30, Akasaka 6-chome, Minato-ku, Tokyo 1078502, Japan Received 20 September 2002; received in revised form 10 November 2003; accepted 12 November 2003 Available online 6 January 2004
Abstract This paper reports a new mountain tunneling method which yielded good results in Baikoh Tunnel construction. Since tunnels in urban areas, in many cases, are driven through soft ground with groundwater and in locations close to various utilities and structures, maintaining the structural stability of not only the tunnels but also such nearby existing structures is of utmost importance. In order to solve these problem, two methods have been developed: an auxiliary method to restrict displacements which consists of the special jet grouting for foot piles and the long steel pipe forepiling, and a boring method for groundwater drainage which does not cause adverse effects on tunnel construction. Using these methods in combination, the new mountain tunneling method is a useful means for constructing tunnels with shallow overburden in soft ground in urban areas. This method is called the new urban tunneling method. This method is cost-effective compared with other tunneling methods, especially for short urban tunnels with shallow overburden. Therefore, this method will be used in many future tunnel projects in urban areas subjects to various restrictions. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Urban tunnel; Mountain tunneling method; Forepiling; Foot pile; Drainage system
1. Introduction Mito City, the capital of Ibaraki Prefecture lying in the northern part of the Kanto Plain, is located about 100 km northeast of Tokyo. Traffic congestion is chronic in the city center due to the concentration of several major roads including three national highways. To solve this problem, Ibaraki Prefectural Government prepared a plan to construct a two-lane tunnel with a sidewalk immediately below National Highway Route 349, which runs through the city center (Fig. 1). During the preliminary design stage, three construction schemes were compared: cut and cover method, shield tunneling method, and mountain tunneling method. The cut and cover method was dropped from consideration since it might disrupt traffic on nearby *
Corresponding author. Tel.: +81-35561-2185; fax: +81-35561-2155. E-mail address: [email protected] (M. Iwata).
0886-7798/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.tust.2003.11.007
roads and cause environmental problems such as vibration and noise. The shield tunneling method is a reliable construction method, but would not be costeffective because of the short tunnel length (Koyama, 2003). In this trial estimation, shield tunneling method costed more than three times of mountain tunneling method. Although requiring auxiliary works because of the shallow overburden and the proximity to existing structures, the mountain tunneling method was the most feasible and thus finally selected for this project. The Baikoh Tunnel has a total length of 607 m: the 139 m southern section was constructed by the cut and cover method, since the vehicular traffic near this location was not expected to be affected by the construction, while the remaining 468 m section by the mountain tunneling method with auxiliary works (Fig. 2). This report covers the latter section. The geology through which the tunnel was excavated is shown in Fig. 3, and the typical cross section of the construction in Fig. 4.
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H. Kimura et al. / Tunnelling and Underground Space Technology 20 (2005) 151–158
methods were not effective for draining groundwater from the overlying aquifer (Dg1).
2. Site condition 2.1. Geology The geology along the tunnel route is shown in Fig. 3, and the properties in each layer are in Table 1. The characteristics of the layers in which the tunnel was constructed are summarized as follows. Fig. 1. National highway route 349.
The characteristics of the tunnel are summarized as follows (Fujimoto et al., 2002): 1. Large cross sectional area (102–104 m2 ) 2. Shallow overburden (6–11 m) 3. Proximity to many structures and underground utilities 4. Construction in unconsolidated ground with high groundwater levels As shown in the longitudinal geologic section in Fig. 3, near the center of the cutting face was soft and impermeable diluvial clay (Dc2), which was overlain and underlain by two diluvial aquifers (Dg1 and Dg2), both consisting of sand and gravel. The presence of layer Dc2 posed two difficult problems. First, the supports for the top heading of the tunnel erected on this soft clay could subside due to excavation and this might cause large surface settlement. Second, since this layer was impervious, conventional
2.1.1. Upper gravel layer (Dg1) Layer Dg1 lies along the top heading of the tunnel and consists of two types: one is gravel stratum which contains clay with low permeability, and the other is gravel stratum with high permeability. The gravel strata are mainly distributed near the boundary between layers Dg1 and Dc2, and their thicknesses and permeability are not uniform. Therefore the average permeability of layer Dg1 considerably varies depending on the location. The groundwater level is 2–4 m above the bottom of layer Dg1, and is the highest at the midst of the plateau. There are three groundwater flow directions: from northwest to southeast, and toward the cliffs located at the northern and southern sides of the plateau (Fig. 2). 2.1.2. Lower clay layer (Dc2) Forming the main part of the tunneling medium, layer Dc2 is an overconsolidated soft clay layer with an N -value of less than 10.
Fig. 2. Plan of Baikoh tunnel.
Fig. 3. Longitudinal geological profile.
H. Kimura et al. / Tunnelling and Underground Space Technology 20 (2005) 151–158
153
3. Design 3.1. Measures for maintaining tunnel stability and preventing surface settlement For safe excavation of a tunnel in soft ground, stabilizing the cutting face and crown is essential. Minimizing the settlement and deformation of the tunnel is also important to prevent adverse effects on nearby structures such as underground utilities, roads, buildings, and residential houses. To achieve these objectives, the following measures were taken in the project.
Fig. 4. Typical cross-section.
2.1.3. Lower gravel layer (Dg2) Layer Dg2 is a very dense sand and gravel layer lying around the elevation of the invert. 2.2. Environment surrounding the project site The tunnel is located immediately beneath a busy traffic area where a national highway crosses several trunk roads in service including National Highway Route 50, and is in the proximity to many structures including buildings and residential houses (Fig. 1). In addition, as Fig. 4 shows, many utilities such as water and sewer pipes, gas pipes, and conduits for electricity and telephone cables, are running in the shallow depth areas beneath the roads. Although the clearance between the tunnel and private premises, in most part, was about only 10 cm, part of the curved sections overlapped private premises and was excavated beneath the pile foundations for buildings.
3.1.1. Auxiliary method for stabilizing crown Forepiling using high-stiffness long steel pipes has been widely used in constructing urban tunnels as an effective means to stabilize the crown at the cutting face and to restrict displacement ahead of the face. Forepiling has two types: long steel pipe and jet grouting. In this project, the long steel pipe forepiling was adopted, since forming forepiles by jet grouting would be difficult because of a large volume of groundwater seepage from layer Dg1. Urethane material was used in most sections for grouting because of the high resistance to dilution and low efflux rate. At locations with a small volume of seepage, however, cost-effective cement material (consumption: 10 l/min/grout hole) was used. 3.1.2. Auxiliary method for stabilizing cutting face In addition to shotcrete, a rigid protection was needed to stabilize the cutting face. At locations where clay was encountered during excavation, the face pile method employed jet grouting was adopted, since it showed an excellent performance during trial construction. 3.1.3. Auxiliary method for the settlement at the foot of top heading In order to restrict displacement ahead of the cutting face by forepiling using high-stiffness long steel pipes
Table 1 Soil properties Layer
Description
N values
Unit weight ct (kN/m3 )
Lm Dc1 Dg1
Loam Clay Gravel
26 25 >16
13 17 20
Cohesion cu (kN/m2 )
38 35 10
Angle of internal friction /u (°) 0 1.1 43
Modulus of deformation E (kN/m2 ) 15,000 25,000 77,000
Dc2 Ds2
Clay Sand
122 336
18 18
95 10
7.5 31
48,000 36,000
Dg2
Gravel
>15
21
10
45
140,000
Tm
Mudstone
>50
16
500
10
140,000
Permeability k (cm/s) – – 5.1 10 5 3.8 10 – 7.0 10 6 1.3 10 1.2 10 4 6.5 10 –
2
4
4
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Surface settlement (mm)
Fig. 5. Countermeasure schemes. 0 -10 -20 -30 Scheme A -40
Scheme B
-50 0
10
20 30 40 Distance from center of tunnel (m)
50
60
Surface tilt (1/1000rad)
0.0 -0.5 -1.0 -1.5 Scheme A -2.0
Scheme B
-2.5 0
10
20 30 40 Distance from center of tunnel (m)
50
60
Fig. 6. Surface settlement of analysis results.
and to minimize ground deformation, it was necessary to establish a bottom structure strong enough to transfer the reaction forces to the bearing ground. In this project, the settlement of the primary supports for the top heading was predicted because of the presence of a clay layer at the bottom. The gravel layer Dg2 below the invert was dense enough to serve as bearing ground. In order to transfer the reaction forces of the supports for the top heading to this gravel layer, foot piles with large diameter were employed. During the design stage, FEM analyses were carried out to compare the foot pile method with other alter-
natives in terms of effect on restricting ground surface settlement and influence on structures in the vicinity. Although the nearby roads, utilities, buildings and houses vary in the extent of deterioration depending on the year of construction, the allowable ground surface settlement and the allowable building/house tilt were set to 30 mm and 1/1000 rad, respectively. Scheme ‘‘A’’ in Fig. 5 is a common foot pile arrangement built from the top heading. The analysis results in Fig. 6 show that the maximum ground surface settlement is 40 mm and the maximum building tilt 2.2/1000 rad, both considerably larger than the al-
H. Kimura et al. / Tunnelling and Underground Space Technology 20 (2005) 151–158
Measures for controlling groundwater can be roughly categorized into two groups: cutoff and drainage. Cutoff would take a long period and require large costs. On the other hand, drainage might cause consolidation settlement of the clay layer Dc2. However, since this layer had been over-consolidated, the adverse effects on the structures above the tunnel and nearby underground structures were considered to be negligible. Drainage was therefore selected as the method for controlling groundwater in the gravel layer Dg1. Using deep wells was dropped from consideration, since the locations for installation were limited because of the presence of many buildings on the ground and sufficient effect was not expected because of the large variation in the permeability coefficient of the gravel layer Dg1. A method to drain groundwater through horizontal pipes driven from inside the tunnel was therefore adopted. The horizontal drainage pipes were placed parallel to and outside the forepiles, in order to drain groundwater ahead of forepiles. Each drainage pipe consisted of an exterior steel pipe 139.8 mm in diameter and 12.5 m in length (same size as that used for
lowable values. This indicates that rigid foot piles bearing the reaction forces transferred from the longpipe forepiles and primary supports, needed to be provided in the ground before tunnel excavation. In order to fulfill this requirement, a new method (Scheme ‘‘B’’) was developed. The concept of this method is to construct continuous foot piles from the ground surface by the special jet grouting (hereafter called the ‘‘Xjet’’: a jet grouting method capable of constructing underground piles with uniform diameter and quality regardless of soil conditions), and combine them with long steel pipe forepiles to carry loads from the primary supports. This scheme was found to be the only measure satisfying the allowable ground settlement and building/house tilt. 3.2. Measures for groundwater Since the excavation of layer Dg1, which lay along the upper half of the tunnel and contained groundwater, could cause blowout of sand and groundwater and this could result in the collapse of the cutting face and excessive ground surface settlement, measures to prevent this problem were taken.
Steel pipe(139.8mm dia., L=12.5m)
3.5m Filter zone
Injection pipe
12.5m
Cement hardening agent Drainage pipe Vinyl chloride pipe(50mm dia., L=16m)
Fig. 7. Horizontal drainage pipe. Drainage pipe 50mm dia., L=16.0m Steel support H-250 @1.0m
Long steel pipe forepiling 139.8mm dia., L=12.5m Horizontal jet grouting 400mm dia., L=12.0m
Shotcrete t=300mm Lining concrete t=500mm
Ground water flow Dg1 Dc2
SL
Invert concrete t=600mm
155
Foot pile by X-jet 2,000mm dia., @1.7m (mm)
Fig. 8. Standard cross section.
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H. Kimura et al. / Tunnelling and Underground Space Technology 20 (2005) 151–158
forepiles), and an inner vinyl chloride pipe (50 mm in diameter and 16.0 m in length) fitted with a screen covering 3.5 m from the front end. To prevent the seepage of groundwater, the voids between the ground and the exterior pipes and those between the exterior and interior pipes were filled with cement hardening agent (Fig. 7). The number of drainage pipe was changed depending on the volume of seepage at the time of steel pipe forepiling. However, in the event that a gravel layer with a high permeability was encountered during excavation, enormous groundwater seepage from the crown and the cutting face beyond the capacity of the pipes would likely occur. In order to prevent this, continuous foot piles by Xjet of upstream side were extended up to 1 m above the bottom surface of the gravel layer Dg1, for the section where layer Dg1 was lying near the crown (Fig. 8). 3.3. The new urban tunneling method Considering the above results, the standard cross section shown in Fig. 8 was selected for the Baikoh Tunnel constructed by the New Urban Tunneling Method (Itoh et al., 2001). The construction procedure is as follows: 1. Prior to the top heading excavation, large diameter foot piles are placed by using the X-jet from the ground surface. Each foot pile is connected to form a continuous wall. 2. Artificial ground arch is formed by arranging the long steel-pipe forepiles at the cutting face and the continuous foot piles. 3. The top heading excavation is performed through the solid ground arch mentioned above. 4. After lower bench excavation, invert concrete is constructed to carry out the early enclosure. 5. Finally, the concrete lining is placed.
Table 2 Typical excavation cycle Days 1 Face piling Long steel pipe forepiling Drainage boring Top heading excavation Bench excavation
2
3
4
5
6
7
5. Measurement results 5.1. Ground surface settlement The ground surface settlement along the center of the tunnel is shown in Fig. 9. Since the gravel layer expected did not appear at the foot of the upper section, the settlement for the pipe roof section (up to 62 m from the portal) exceeded 60 mm. On the other hand, the surface settlement for the first half (62–270 m from the portal) for the section constructed by the new urban tunneling method was about 22 mm and that for the latter half (270 m from the portal to the end point) was about 16 mm. Here, the first half refers to the section where groundwater seepage was encountered during excavation, and the latter half refers to the section where seepage was minimal. The cross-sectional distribution of the ground surface settlement is shown in Fig. 10. These results show that the settlement was generally within the predicted range. Fig. 11 shows the relationship between the surface settlement with the distance the settlement measuring point and cutting face. Ground surface settlement started when the cutting face reached about 20 m from the measuring point, increased to 5–10 mm when the face reached immediately below the measuring point and then stopped increasing when the face moved away to a distance of about 25 m. 5.2. Building tilt
4. Construction The maximum tilt for three buildings expected to be adversely affected by tunnel excavation ranged from 0.5/ New Urban Tuneling Method
Piperoof TD62
TD270
TD468 Surface settlement (mm)
Since there were few utilities in the vicinity of the section up to 62 m from the northern portal, and a gravel layer hard enough to serve as a bearing stratum was expected to lie near the top heading, a pipe roof method and a conventional foot pile method were adopted. The remaining section was built by the new urban tunneling method because of proximity to many utilities and buildings. The excavation was carried out, after placing foot piles from ground surface, in 9 m cycles which comprised five activities: face piles, long steel pipe fore piles, drainage boring, top heading excavation and bench excavation. On average, the excavation for each cycle which has 9 m in length, took seven days. The typical excavation cycle is shown in Table 2.
0 -20 -40 -60 -80 -100 500
Settlement
400
300
200
100
TD: Tunnel distance from the northern portal (m)
Fig. 9. Observed longitudinal ground surface settlement.
0
H. Kimura et al. / Tunnelling and Underground Space Technology 20 (2005) 151–158
157
Surface settlement (mm)
0
-10 Calculated value TD209(R) TD209(L) TD329(R) TD334(L)
-20
-30 0
10
20 30 40 Distance from tunnel center (m)
50
60
Fig. 12. Seepage volume from drainage pipes.
Fig. 10. Comparison between observed settlement and calculated settlement in transverse section.
Lowering of ground water level (m)
1000 to 0.8/1000 rad and was smaller than the allowable value of 1.0/1000 rad. No major damage to the buildings was observed during visual inspection. 5.3. Groundwater The section, up to 320 m from the portal, was excavated while drilling one to four horizontal holes for drainage depending on the volume of groundwater seepage during long steel pipe forepiling. The remaining section was driven without drainage holes, since the seepage rate decreased down to 2 L/min/hole (Fig. 12). The typical relationship between groundwater level measured by groundwater monitoring wells and the distance of the cutting face from them is shown in Fig. 13, and the change in groundwater level in Fig. 14. As shown in these figures, decrease in groundwater level of wells located at the downstream side, when the cutting face was moving away from them, ranged between 2 and 3 m. The groundwater level observed at wells of upstream side, however, was almost unchanged regard-
Well of the upstream side Well of the downstream side
0.0
1.0
2.0
3.0 -50
0
Fig. 13. Relationship between lowering of groundwater and tunnel progress.
less of the distance from the face. This indicates that continuous foot piles are quite effective for controlling groundwater. Using horizontal boring for drainage in combination with the X-jet cutoff wall method reduced the seepage from the cutting face to lower than 40 L/min. As a Observed values from TD 62 to TD270 Observed values from TD270 to TD468 Fitting curve
20
Surface settlement (m)
10 0 -10 -20 -30 -40 -30
-20
-10
50
Distance from heading (m)
0
10
20
Distance from heading (m) Fig. 11. Relationship between ground surface settlement and tunnel progress.
30
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H. Kimura et al. / Tunnelling and Underground Space Technology 20 (2005) 151–158
Fig. 14. Lowering of groundwater level.
result, the cutting face demonstrated neither soil inflow nor face collapse.
6. Conclusions The results of using the newly developed tunneling method in the construction of the Baikoh Tunnel are summarized as follows: 1. Ground surface settlement due to tunnel excavation was about the same as the predicted value, 17 mm, and smaller than the allowable value, 30 mm. 2. The maximum building tilt was smaller than the allowable value, 1/1000 rad. No major damage to buildings was observed during visual inspection.
3. Foot piles constructed by X-jet showed an excellent performance in groundwater cutoff. 4. The newly developed drainage boring method prevented groundwater seepage and enabled safe excavation. Thus, the new urban tunneling method developed in the Baikoh Tunnel, which is composed of X-jet, long steel pipe forepiling, face piling by jet grouting, and boring for groundwater drainage, was proved to be very effective in minimizing surface settlement and controlling groundwater. This method, which can form a solid ground arch at the cutting face, is cost-effective compared with other tunneling methods, especially for short urban tunnels with shallow overburden. Because of these characteristics, this method will be used in many future tunnel projects in urban areas subjects to various restrictions.
References Fujimoto K., et al., 2002. Challenges in construction of an urban tunnel: design and construction of Baikoh tunnel. Proceedings of AITES-ITA DOWNUNDER 2002 CONGRESS. Itoh T., et al., 2001. A new urban tunneling method adopted to the soft ground with high groundwater level. Proceedings of the International Symposium on Modern Tunneling Science and Technology (IS-KYOTO2001). Koyama, Y., 2003. Present status and technology of shield tunneling method in Japan. Tunnelling and Underground Space Technology 18, 145–159.
Tunnelling and Underground Space Technology 20 (2005) 151–158
www.elsevier.com/locate/tust
Application of new urban tunneling method in Baikoh tunnel excavation H. Kimura a, T. Itoh b, M. Iwata
c,*
, K. Fujimoto
c
a
c
Ibaraki Prefectural Department of Public Works Office, Mito, Japan b Tukuba New Town Development Co. LTD., Tukuba, Japan Kajima Corporation, Civil Engineering Design Division, 5-30, Akasaka 6-chome, Minato-ku, Tokyo 1078502, Japan Received 20 September 2002; received in revised form 10 November 2003; accepted 12 November 2003 Available online 6 January 2004
Abstract This paper reports a new mountain tunneling method which yielded good results in Baikoh Tunnel construction. Since tunnels in urban areas, in many cases, are driven through soft ground with groundwater and in locations close to various utilities and structures, maintaining the structural stability of not only the tunnels but also such nearby existing structures is of utmost importance. In order to solve these problem, two methods have been developed: an auxiliary method to restrict displacements which consists of the special jet grouting for foot piles and the long steel pipe forepiling, and a boring method for groundwater drainage which does not cause adverse effects on tunnel construction. Using these methods in combination, the new mountain tunneling method is a useful means for constructing tunnels with shallow overburden in soft ground in urban areas. This method is called the new urban tunneling method. This method is cost-effective compared with other tunneling methods, especially for short urban tunnels with shallow overburden. Therefore, this method will be used in many future tunnel projects in urban areas subjects to various restrictions. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Urban tunnel; Mountain tunneling method; Forepiling; Foot pile; Drainage system
1. Introduction Mito City, the capital of Ibaraki Prefecture lying in the northern part of the Kanto Plain, is located about 100 km northeast of Tokyo. Traffic congestion is chronic in the city center due to the concentration of several major roads including three national highways. To solve this problem, Ibaraki Prefectural Government prepared a plan to construct a two-lane tunnel with a sidewalk immediately below National Highway Route 349, which runs through the city center (Fig. 1). During the preliminary design stage, three construction schemes were compared: cut and cover method, shield tunneling method, and mountain tunneling method. The cut and cover method was dropped from consideration since it might disrupt traffic on nearby *
Corresponding author. Tel.: +81-35561-2185; fax: +81-35561-2155. E-mail address: [email protected] (M. Iwata).
0886-7798/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.tust.2003.11.007
roads and cause environmental problems such as vibration and noise. The shield tunneling method is a reliable construction method, but would not be costeffective because of the short tunnel length (Koyama, 2003). In this trial estimation, shield tunneling method costed more than three times of mountain tunneling method. Although requiring auxiliary works because of the shallow overburden and the proximity to existing structures, the mountain tunneling method was the most feasible and thus finally selected for this project. The Baikoh Tunnel has a total length of 607 m: the 139 m southern section was constructed by the cut and cover method, since the vehicular traffic near this location was not expected to be affected by the construction, while the remaining 468 m section by the mountain tunneling method with auxiliary works (Fig. 2). This report covers the latter section. The geology through which the tunnel was excavated is shown in Fig. 3, and the typical cross section of the construction in Fig. 4.
152
H. Kimura et al. / Tunnelling and Underground Space Technology 20 (2005) 151–158
methods were not effective for draining groundwater from the overlying aquifer (Dg1).
2. Site condition 2.1. Geology The geology along the tunnel route is shown in Fig. 3, and the properties in each layer are in Table 1. The characteristics of the layers in which the tunnel was constructed are summarized as follows. Fig. 1. National highway route 349.
The characteristics of the tunnel are summarized as follows (Fujimoto et al., 2002): 1. Large cross sectional area (102–104 m2 ) 2. Shallow overburden (6–11 m) 3. Proximity to many structures and underground utilities 4. Construction in unconsolidated ground with high groundwater levels As shown in the longitudinal geologic section in Fig. 3, near the center of the cutting face was soft and impermeable diluvial clay (Dc2), which was overlain and underlain by two diluvial aquifers (Dg1 and Dg2), both consisting of sand and gravel. The presence of layer Dc2 posed two difficult problems. First, the supports for the top heading of the tunnel erected on this soft clay could subside due to excavation and this might cause large surface settlement. Second, since this layer was impervious, conventional
2.1.1. Upper gravel layer (Dg1) Layer Dg1 lies along the top heading of the tunnel and consists of two types: one is gravel stratum which contains clay with low permeability, and the other is gravel stratum with high permeability. The gravel strata are mainly distributed near the boundary between layers Dg1 and Dc2, and their thicknesses and permeability are not uniform. Therefore the average permeability of layer Dg1 considerably varies depending on the location. The groundwater level is 2–4 m above the bottom of layer Dg1, and is the highest at the midst of the plateau. There are three groundwater flow directions: from northwest to southeast, and toward the cliffs located at the northern and southern sides of the plateau (Fig. 2). 2.1.2. Lower clay layer (Dc2) Forming the main part of the tunneling medium, layer Dc2 is an overconsolidated soft clay layer with an N -value of less than 10.
Fig. 2. Plan of Baikoh tunnel.
Fig. 3. Longitudinal geological profile.
H. Kimura et al. / Tunnelling and Underground Space Technology 20 (2005) 151–158
153
3. Design 3.1. Measures for maintaining tunnel stability and preventing surface settlement For safe excavation of a tunnel in soft ground, stabilizing the cutting face and crown is essential. Minimizing the settlement and deformation of the tunnel is also important to prevent adverse effects on nearby structures such as underground utilities, roads, buildings, and residential houses. To achieve these objectives, the following measures were taken in the project.
Fig. 4. Typical cross-section.
2.1.3. Lower gravel layer (Dg2) Layer Dg2 is a very dense sand and gravel layer lying around the elevation of the invert. 2.2. Environment surrounding the project site The tunnel is located immediately beneath a busy traffic area where a national highway crosses several trunk roads in service including National Highway Route 50, and is in the proximity to many structures including buildings and residential houses (Fig. 1). In addition, as Fig. 4 shows, many utilities such as water and sewer pipes, gas pipes, and conduits for electricity and telephone cables, are running in the shallow depth areas beneath the roads. Although the clearance between the tunnel and private premises, in most part, was about only 10 cm, part of the curved sections overlapped private premises and was excavated beneath the pile foundations for buildings.
3.1.1. Auxiliary method for stabilizing crown Forepiling using high-stiffness long steel pipes has been widely used in constructing urban tunnels as an effective means to stabilize the crown at the cutting face and to restrict displacement ahead of the face. Forepiling has two types: long steel pipe and jet grouting. In this project, the long steel pipe forepiling was adopted, since forming forepiles by jet grouting would be difficult because of a large volume of groundwater seepage from layer Dg1. Urethane material was used in most sections for grouting because of the high resistance to dilution and low efflux rate. At locations with a small volume of seepage, however, cost-effective cement material (consumption: 10 l/min/grout hole) was used. 3.1.2. Auxiliary method for stabilizing cutting face In addition to shotcrete, a rigid protection was needed to stabilize the cutting face. At locations where clay was encountered during excavation, the face pile method employed jet grouting was adopted, since it showed an excellent performance during trial construction. 3.1.3. Auxiliary method for the settlement at the foot of top heading In order to restrict displacement ahead of the cutting face by forepiling using high-stiffness long steel pipes
Table 1 Soil properties Layer
Description
N values
Unit weight ct (kN/m3 )
Lm Dc1 Dg1
Loam Clay Gravel
26 25 >16
13 17 20
Cohesion cu (kN/m2 )
38 35 10
Angle of internal friction /u (°) 0 1.1 43
Modulus of deformation E (kN/m2 ) 15,000 25,000 77,000
Dc2 Ds2
Clay Sand
122 336
18 18
95 10
7.5 31
48,000 36,000
Dg2
Gravel
>15
21
10
45
140,000
Tm
Mudstone
>50
16
500
10
140,000
Permeability k (cm/s) – – 5.1 10 5 3.8 10 – 7.0 10 6 1.3 10 1.2 10 4 6.5 10 –
2
4
4
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H. Kimura et al. / Tunnelling and Underground Space Technology 20 (2005) 151–158
Surface settlement (mm)
Fig. 5. Countermeasure schemes. 0 -10 -20 -30 Scheme A -40
Scheme B
-50 0
10
20 30 40 Distance from center of tunnel (m)
50
60
Surface tilt (1/1000rad)
0.0 -0.5 -1.0 -1.5 Scheme A -2.0
Scheme B
-2.5 0
10
20 30 40 Distance from center of tunnel (m)
50
60
Fig. 6. Surface settlement of analysis results.
and to minimize ground deformation, it was necessary to establish a bottom structure strong enough to transfer the reaction forces to the bearing ground. In this project, the settlement of the primary supports for the top heading was predicted because of the presence of a clay layer at the bottom. The gravel layer Dg2 below the invert was dense enough to serve as bearing ground. In order to transfer the reaction forces of the supports for the top heading to this gravel layer, foot piles with large diameter were employed. During the design stage, FEM analyses were carried out to compare the foot pile method with other alter-
natives in terms of effect on restricting ground surface settlement and influence on structures in the vicinity. Although the nearby roads, utilities, buildings and houses vary in the extent of deterioration depending on the year of construction, the allowable ground surface settlement and the allowable building/house tilt were set to 30 mm and 1/1000 rad, respectively. Scheme ‘‘A’’ in Fig. 5 is a common foot pile arrangement built from the top heading. The analysis results in Fig. 6 show that the maximum ground surface settlement is 40 mm and the maximum building tilt 2.2/1000 rad, both considerably larger than the al-
H. Kimura et al. / Tunnelling and Underground Space Technology 20 (2005) 151–158
Measures for controlling groundwater can be roughly categorized into two groups: cutoff and drainage. Cutoff would take a long period and require large costs. On the other hand, drainage might cause consolidation settlement of the clay layer Dc2. However, since this layer had been over-consolidated, the adverse effects on the structures above the tunnel and nearby underground structures were considered to be negligible. Drainage was therefore selected as the method for controlling groundwater in the gravel layer Dg1. Using deep wells was dropped from consideration, since the locations for installation were limited because of the presence of many buildings on the ground and sufficient effect was not expected because of the large variation in the permeability coefficient of the gravel layer Dg1. A method to drain groundwater through horizontal pipes driven from inside the tunnel was therefore adopted. The horizontal drainage pipes were placed parallel to and outside the forepiles, in order to drain groundwater ahead of forepiles. Each drainage pipe consisted of an exterior steel pipe 139.8 mm in diameter and 12.5 m in length (same size as that used for
lowable values. This indicates that rigid foot piles bearing the reaction forces transferred from the longpipe forepiles and primary supports, needed to be provided in the ground before tunnel excavation. In order to fulfill this requirement, a new method (Scheme ‘‘B’’) was developed. The concept of this method is to construct continuous foot piles from the ground surface by the special jet grouting (hereafter called the ‘‘Xjet’’: a jet grouting method capable of constructing underground piles with uniform diameter and quality regardless of soil conditions), and combine them with long steel pipe forepiles to carry loads from the primary supports. This scheme was found to be the only measure satisfying the allowable ground settlement and building/house tilt. 3.2. Measures for groundwater Since the excavation of layer Dg1, which lay along the upper half of the tunnel and contained groundwater, could cause blowout of sand and groundwater and this could result in the collapse of the cutting face and excessive ground surface settlement, measures to prevent this problem were taken.
Steel pipe(139.8mm dia., L=12.5m)
3.5m Filter zone
Injection pipe
12.5m
Cement hardening agent Drainage pipe Vinyl chloride pipe(50mm dia., L=16m)
Fig. 7. Horizontal drainage pipe. Drainage pipe 50mm dia., L=16.0m Steel support H-250 @1.0m
Long steel pipe forepiling 139.8mm dia., L=12.5m Horizontal jet grouting 400mm dia., L=12.0m
Shotcrete t=300mm Lining concrete t=500mm
Ground water flow Dg1 Dc2
SL
Invert concrete t=600mm
155
Foot pile by X-jet 2,000mm dia., @1.7m (mm)
Fig. 8. Standard cross section.
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forepiles), and an inner vinyl chloride pipe (50 mm in diameter and 16.0 m in length) fitted with a screen covering 3.5 m from the front end. To prevent the seepage of groundwater, the voids between the ground and the exterior pipes and those between the exterior and interior pipes were filled with cement hardening agent (Fig. 7). The number of drainage pipe was changed depending on the volume of seepage at the time of steel pipe forepiling. However, in the event that a gravel layer with a high permeability was encountered during excavation, enormous groundwater seepage from the crown and the cutting face beyond the capacity of the pipes would likely occur. In order to prevent this, continuous foot piles by Xjet of upstream side were extended up to 1 m above the bottom surface of the gravel layer Dg1, for the section where layer Dg1 was lying near the crown (Fig. 8). 3.3. The new urban tunneling method Considering the above results, the standard cross section shown in Fig. 8 was selected for the Baikoh Tunnel constructed by the New Urban Tunneling Method (Itoh et al., 2001). The construction procedure is as follows: 1. Prior to the top heading excavation, large diameter foot piles are placed by using the X-jet from the ground surface. Each foot pile is connected to form a continuous wall. 2. Artificial ground arch is formed by arranging the long steel-pipe forepiles at the cutting face and the continuous foot piles. 3. The top heading excavation is performed through the solid ground arch mentioned above. 4. After lower bench excavation, invert concrete is constructed to carry out the early enclosure. 5. Finally, the concrete lining is placed.
Table 2 Typical excavation cycle Days 1 Face piling Long steel pipe forepiling Drainage boring Top heading excavation Bench excavation
2
3
4
5
6
7
5. Measurement results 5.1. Ground surface settlement The ground surface settlement along the center of the tunnel is shown in Fig. 9. Since the gravel layer expected did not appear at the foot of the upper section, the settlement for the pipe roof section (up to 62 m from the portal) exceeded 60 mm. On the other hand, the surface settlement for the first half (62–270 m from the portal) for the section constructed by the new urban tunneling method was about 22 mm and that for the latter half (270 m from the portal to the end point) was about 16 mm. Here, the first half refers to the section where groundwater seepage was encountered during excavation, and the latter half refers to the section where seepage was minimal. The cross-sectional distribution of the ground surface settlement is shown in Fig. 10. These results show that the settlement was generally within the predicted range. Fig. 11 shows the relationship between the surface settlement with the distance the settlement measuring point and cutting face. Ground surface settlement started when the cutting face reached about 20 m from the measuring point, increased to 5–10 mm when the face reached immediately below the measuring point and then stopped increasing when the face moved away to a distance of about 25 m. 5.2. Building tilt
4. Construction The maximum tilt for three buildings expected to be adversely affected by tunnel excavation ranged from 0.5/ New Urban Tuneling Method
Piperoof TD62
TD270
TD468 Surface settlement (mm)
Since there were few utilities in the vicinity of the section up to 62 m from the northern portal, and a gravel layer hard enough to serve as a bearing stratum was expected to lie near the top heading, a pipe roof method and a conventional foot pile method were adopted. The remaining section was built by the new urban tunneling method because of proximity to many utilities and buildings. The excavation was carried out, after placing foot piles from ground surface, in 9 m cycles which comprised five activities: face piles, long steel pipe fore piles, drainage boring, top heading excavation and bench excavation. On average, the excavation for each cycle which has 9 m in length, took seven days. The typical excavation cycle is shown in Table 2.
0 -20 -40 -60 -80 -100 500
Settlement
400
300
200
100
TD: Tunnel distance from the northern portal (m)
Fig. 9. Observed longitudinal ground surface settlement.
0
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157
Surface settlement (mm)
0
-10 Calculated value TD209(R) TD209(L) TD329(R) TD334(L)
-20
-30 0
10
20 30 40 Distance from tunnel center (m)
50
60
Fig. 12. Seepage volume from drainage pipes.
Fig. 10. Comparison between observed settlement and calculated settlement in transverse section.
Lowering of ground water level (m)
1000 to 0.8/1000 rad and was smaller than the allowable value of 1.0/1000 rad. No major damage to the buildings was observed during visual inspection. 5.3. Groundwater The section, up to 320 m from the portal, was excavated while drilling one to four horizontal holes for drainage depending on the volume of groundwater seepage during long steel pipe forepiling. The remaining section was driven without drainage holes, since the seepage rate decreased down to 2 L/min/hole (Fig. 12). The typical relationship between groundwater level measured by groundwater monitoring wells and the distance of the cutting face from them is shown in Fig. 13, and the change in groundwater level in Fig. 14. As shown in these figures, decrease in groundwater level of wells located at the downstream side, when the cutting face was moving away from them, ranged between 2 and 3 m. The groundwater level observed at wells of upstream side, however, was almost unchanged regard-
Well of the upstream side Well of the downstream side
0.0
1.0
2.0
3.0 -50
0
Fig. 13. Relationship between lowering of groundwater and tunnel progress.
less of the distance from the face. This indicates that continuous foot piles are quite effective for controlling groundwater. Using horizontal boring for drainage in combination with the X-jet cutoff wall method reduced the seepage from the cutting face to lower than 40 L/min. As a Observed values from TD 62 to TD270 Observed values from TD270 to TD468 Fitting curve
20
Surface settlement (m)
10 0 -10 -20 -30 -40 -30
-20
-10
50
Distance from heading (m)
0
10
20
Distance from heading (m) Fig. 11. Relationship between ground surface settlement and tunnel progress.
30
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Fig. 14. Lowering of groundwater level.
result, the cutting face demonstrated neither soil inflow nor face collapse.
6. Conclusions The results of using the newly developed tunneling method in the construction of the Baikoh Tunnel are summarized as follows: 1. Ground surface settlement due to tunnel excavation was about the same as the predicted value, 17 mm, and smaller than the allowable value, 30 mm. 2. The maximum building tilt was smaller than the allowable value, 1/1000 rad. No major damage to buildings was observed during visual inspection.
3. Foot piles constructed by X-jet showed an excellent performance in groundwater cutoff. 4. The newly developed drainage boring method prevented groundwater seepage and enabled safe excavation. Thus, the new urban tunneling method developed in the Baikoh Tunnel, which is composed of X-jet, long steel pipe forepiling, face piling by jet grouting, and boring for groundwater drainage, was proved to be very effective in minimizing surface settlement and controlling groundwater. This method, which can form a solid ground arch at the cutting face, is cost-effective compared with other tunneling methods, especially for short urban tunnels with shallow overburden. Because of these characteristics, this method will be used in many future tunnel projects in urban areas subjects to various restrictions.
References Fujimoto K., et al., 2002. Challenges in construction of an urban tunnel: design and construction of Baikoh tunnel. Proceedings of AITES-ITA DOWNUNDER 2002 CONGRESS. Itoh T., et al., 2001. A new urban tunneling method adopted to the soft ground with high groundwater level. Proceedings of the International Symposium on Modern Tunneling Science and Technology (IS-KYOTO2001). Koyama, Y., 2003. Present status and technology of shield tunneling method in Japan. Tunnelling and Underground Space Technology 18, 145–159.