SOME EXPERIENCE FROM THE SOFT GROUND TUNNELLING IN URBAN AREA

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1 Seminar on The State-of-the-art Technology and Experience on Geotechnical Engineering in Korea and Hong Kong - 28 Mar Geotechnical Division, The Hong Kong Institution of Engineers SOME EXPERIENCE FROM THE SOFT GROUND TUNNELLING IN URBAN AREA Seung-Ryull Kim ESCO Consultant & Engineers Company Ltd Seoul, Korea Abstract: Poor geological conditions are often encountered during underground construction in urban areas. Additionally, buildings, underground infrastructures and other facilities are more densely situated along or under the streets of a city or town. Changes to groundwater levels or ground displacement caused by underground openings or deep excavations will have a negative impact on existing structures particularly those located in areas with poor subsoil conditions. This paper will address some valuable technical solutions concerning safe tunnelling in soft geological conditions. Successful case histories will also be presented with an emphasis on auxiliary measures. INTRODUCTION Generally, subway tunnels are constructed as near to the surface as possible. This is primarily because access to the system for passengers at stations will be more convenient and less expensive in terms of capital and also in terms of operation and maintenance taking into consideration facilities such as escalators or lifts. For this reason, construction is often conducted in relatively soft soil conditions such as alluvial deposits, clay and weathered soil. The two main methods employed in constructing subway systems in urban areas are the cutand-cover method and the bored tunnelling method. The former is used for relatively shallow runs following underneath streets while the latter is used primarily for deep construction or for routes that run underneath other structures or residential areas. Invariably, both methods result in lowering the groundwater table which will cause a significant amount of ground settlement in areas composed of compressible layers of soil like clay or silt in cases where the excavation needs to be performed below the in-situ groundwater table. The incurrence of lateral displacement is also a phenomenon common to both methods. For successful tunnelling adjacent to or beneath existing structures in urban areas, additional measures are often used to protect the structures from damage and to help ensure their stability. The following sections describe two types of measures that are frequently employed in urban area soft ground tunnelling and finally the case histories TUNNEL PROFILES, SUPPORT SYSTEMS AND DRAINAGE SYSTEMS Typical Tunnel Profiles and Support Systems Typical tunnel profiles used for the Seoul subway construction are schematically shown in Figure 1. The horseshoe shape was most frequently accepted in the various geological conditions with the groundwater drainage systems. The oval shape and circular shape were accepted for the poor geological conditions or the conditions with a higher groundwater head. 71

2 a) Horseshoe shape b) Oval shape c) Circular shape Fig. 1: Schematic diagram of subway tunnel profiles for single track Several different types of support systems were used relevant to the geological conditions. Typical support systems for the Seoul subway construction are illustrated in Figure 2 below. Support Pattern PD-2 PD-3 PD-4 PD-5 Ground class Round length 1.0~1.5m 1.2~1.8m 1.35~2.0m 1.5~2.5m Shotcrete lining 200~250mm 200mm 150mm 100mm SD35, D25 SD35, D25 SD35, D25 Rock Length L=3m L=3m L=3m Random Bolt No. 16~17EA/1.0m 12~13 EA/1.2m 9~10 EA/1.35m Steel Type H-125 H-125 H-100 Rib Spacing 1.0~1.5m 1.2~1.8m 1.35~2.0m Thickness of Concrete lining 300~400mm 300~400mm 300~400mm 300~400mm Sketch Fig. 2: Typical support systems for Seoul subway tunnels Tunnel Drainage Systems Generally, tunnel profiles depend on drainage systems after tunnelling, which are decided by evaluations of construction cost, maintenance cost for service period and impacts caused by the drawdown of groundwater and related settlement, ete. There are two different types of groundwater dealing method in tunnel drainage systems. One is a watertight tunnel and the other is a drained tunnel. The highest groundwater head considered as bearable for the watertight tunnel is limited to 7 bars. It means watertight tunnel drainage systems are not recommended where the groundwater head is more than 70 meters in Korea. In watertight tunnels, the entire periphery of concrete lining is enclosed by waterproofing membrane as shown in Figure 3a and no drainage from the tunnel is made. Thus, the concrete lining shall be subjected to the groundwater pressure corresponding to the water head. The watertight tunnels are the best option in the area where heavy groundwater inflow takes place 72

3 with reasonable low water head. Moreover, it also apply for the case that the drawdown of the groundwater table may give damages to the existing buildings and facilities above the tunnel. The drained tunnels are tunnels in which proper drainage system is provided for the water drainage throughout the lifetime of tunnels. The construction cost of drained tunnel is more economical as compared to watertight tunnel, while maintenance cost is higher. Drained tunnels are the best option in the region where the inflow of the groundwater is less and there is no hazard due to drawdown of groundwater table. a) Watertight tunnel b) Drained tunnel Fig. 3: Schematic tunnel profiles of watertight tunnel and drained tunnel MEASURES AGAINST GROUNDWATER Definitions of waterproofing are always subjective. Acknowledging that there are neither structures nor techniques capable of guaranteeing 100% watertightness, it is therefore a matter of implementing a number of control measures that will guarantee a high level of waterproofing. Cut-and-cover Tunnelling The cut-and-cover method refers to the provision of a shallow structure formed by first making an excavation from the surface in which the structure is later built. Backfilling and the restoration of the surface ground follow thereafter. There are many variations possible in construction sequences as well as the materials used for temporary and permanent structures. This type of construction in running tunnels is generally cheaper than providing the same facilities in bored tunnels and also avoids the need for tunnelling specialists, which are often in short supply. However, extensive surface disruption along the route is likely to occur during construction. This method also requires either the demolition of or complex temporary supports for existing structures and public utilities unless the construction takes place underneath open ground. The cut-and-cover method obviously results in a displacement of surrounding earth, vertically and horizontally, either through a removal of the earth, a release of naturally occurring stresses or the movement of groundwater. The magnitude of this ground movement depends largely on the nature of ground, groundwater conditions, the method of excavation and the use of retaining structures. This section will briefly describe some possible techniques that are effective in minimizing groundwater movement. 73

4 Figure 4 shows a typical cross section view of the cut-and-cover tunnelling method with a hydraulic cut-off scheme. There are two distinctive schema for hydraulic cut-off used in cutand-cover construction: one involves the placement of an impermeable barrier along the entire circumference of the excavation area; the other entails improving the watertightness of the surrounding ground using ground treatment methods such as permeation grouting, jet grouting, the casting of in-situ concrete piling, compaction grouting and ground freezing etc. Fig. 4: Typical cross section of cut-and-cover method with a groundwater cut-off scheme 1) Impermeable Cut-off Walls With the cut-and-cover method, the most common types of groundwater cut-offs achieved by installing an impermeable barrier into the ground are diaphragm walling and sheet piling, as shown in Figure 5. Both of these have been used extensively as cut-offs against water flow particularly in areas with relatively soft ground. To ensure the effectiveness of the cut-offs, the walls are sunk to an impermeable stratum or suitable cut-off level. Socketing into hard rock strata is possible if necessary, though it does present some challenges. In the case of the diaphragm wall, a plain concrete wall is satisfactory if it is used solely as a water cut-off. Equipment used for constructing these walls need to provide significant vertical clearance. Sheet piling techniques, on the other hand, often encounter difficulties when they are driven into ground that has a lot of gravel. Special care must be taken to prevent water leakage from the toe of the sheet pile. Fig. 5: Diaphragm wall and sheet pile construction 2) Concrete Piling Walls The different types of wall constructions for underground works are: 1) contiguous bored-pile walls; 2) true secant bored-pile walls; 3) pseudo-secant bored-pile walls. Two latter boredpiles as depicted in Figure 6a and 6b can be used to create watertightness in the cut-and-cover method. True secant bored-pile walls comprise a row of piles intersecting one another. Pseudo-secant bored-pile walls are composed of either a contiguous bored-pile wall with an immediate area behind the wall jet or watertight grouting to provide a good water seal. 74

5 (a) Contiguous piled wall (b) True secant piled wall (c) Pseudo-secant piled wall (d) Concrete pile walls Fig. 6: Types of bored-pile walling Soil-cement walls can also be formed by jet grouting. Jet grouting, as illustrated in Figure 7, uses a high-velocity thin jet of water. As the jet rotates the water cuts through the soil and disintegrates it. In this process, the soil is simultaneously dislodged and mixed with the grout. The grout does not penetrate the soil either by permeation or splitting; simply, it becomes thoroughly mixed in-situ with the water and soil within the cavity created by the spinning water jet. No grouting pressure is applied. The objective here is to create a series of soilcement column walls and the main role of the jet grouting is to strengthen the ground. Additionally, the nature of its watertightness serves to provide reliable cut-off walls. It is recommended to set up a proper process for spoil treatment when applying this technique. Fig. 7:. Schematic diagram of jet grouting and completed jet columns at construction site Bored Tunnelling In unstable geological conditions that are pervious and weak soil condition, mass ground treatments often serve as adequate solutions for tunnels where mechanized tunnelling is not feasible. Figure 8 illustrates a typical example of such a situation. To construct shallow tunnels in an urban area, treatment from the ground surface is always preferred since it avoids most of the difficulties concerning groundwater. It also allows the ground treatment to be carried out in advance and, therefore, independently of tunnelling operations. However, this requires: 1) careful investigation and localization of underground facilities/infrastructures; 2) temporary restrictions to surface traffic; 3) additional dead drilling length through overburden. 75

6 Fig. 8; Ground treatment along tunnel periphery from the ground surface If the ground to be treated is cohesionless and has a greater permeability, drilling from below the groundwater table will tend to produce a large ingress of water and eventually soil erosion. The use of stuffing boxes at grout hole heads is necessary to avoid these problems. Drilling operations, therefore, become significantly more complicated when this situation occurs at the tunnel face. Where groundwater is a problem, the pervious soil around the tunnel must be completely sealed off at the treated zone. Ground treatment from the tunnel face is usually carried out through a succession of different steps. Figure 9 illustrates a typical example of this kind of treatment. Fig. 9: Cross section and profile of typical ground treatment from tunnel face Apart from the difficulties related to construction described above, treatment from the tunnel face also presents several other difficulties: 1) limited working space; 2) possible standing time of tunnelling; 3) increased work and costs involved, compared to vertical ones. Consequently, this type of treatment is usually carried out only when: 1) the depth is excessive; 2) access from the ground surface is impossible, or; 3) greater accuracy is needed for installing freezing tubes, for example. Permeation Grouting around the Underground Opening We can consider permeation to have been achieved once the fluidity of the grout and the grouting pressure have caused the grout to penetrate voids in the ground while removing the water without displacing soil or widening existing fissures. 76

7 (a) Numerical simulation (b) Monitored results from Seikan tunnel Fig. 10: Relationship between the size of grouting zone and the groundwater infiltration Perfect permeation is a complete substitution, replacing the free water in the ground with the grouting material without distorting the structure of the ground. This type of ground treatment is referred to as permeation grouting. Grouting materials should be selected according to how well they can penetrate a particular ground condition. Chemical grouting is preferred to ensure the cut-off walls function properly. With this technique, the key issue is the formation of a homogeneous cut-off wall at the designated location and depth. Figure 10 shows the relationship between the width of a grouted zone and the amount of water inflow into a tunnel. We can clearly see that the decrease in groundwater infiltration according to increases in the size of the grouted zone are most significant up to a thickness of about two meters, beyond which the amount of infiltration decreases at a diminishing rate. It is interesting to note from these results that using a grouted zone with a greater thickness results in only a small reduction in the amount of water infiltration. GROUND STRENGTHENING THROUGH INJECTION METHODS To minimize ground displacement vertically or horizontally, ground-strengthening techniques are often required. In the case of cut-and-cover tunnel construction, not only are stiffer temporary support systems needed, the strengthening of the ground adjacent to existing structures is also crucial for safe excavations in areas with poor geological subsoil conditions. When problems concerning serious ground runs are expected during the excavation of a bored tunnel, ground strengthening should be carried out to form a continuous zone of stabilized soil surrounding the axis of the future tunnel opening. Cut-and-cover Tunnelling In the case of cut-and-cover tunnelling, one of the ground strengthening methods that is generally accepted is the grouting technique. There are actually several different types of grouting techniques currently available, each with their own applications and benefits. They are commonly classified as: compaction grouting, fracture grouting, chemical grouting, permeation grouting and jet grouting. Compaction grouting entails a thick mortar mix that acts as a radial hydraulic jack, creating bulbs or lenses, thus displacing and compressing the surrounding ground to some extent. Chemical grouting is one of the types of permeation grouting with a relatively low-viscosity grout. This type of grouting is distinguished by the mechanism in which the grout fills the voids in the ground without causing any substantial change to the original volume or structure of the ground area. Hydrofracturing grouting fragments the ground using water in order to increase the total stresses through a wedging action of successive thin grout lenses, filling 77

8 unconnected spaces or fissures and possibly consolidating the soil under the pressure of the injection. Jet grouting provides a valuable solution for a wide range of problems when conventional injection methods are unsuitable, unsafe or too expensive. Jet grouting is among the newer generation of ground treatment methods and its popularity is growing rapidly around the world. Its development largely came through a need to be able to treat fine-grained soils that are normally untreatable using permeation grouting and for which significantly high amounts of strength are required. This method complies quite well with the stringently environmentally controlled circumstances that chemical grouts cannot meet. Fig.11: Schematic presentation of ground strengthening using the grouting technique Bored Tunnelling Weak ground conditions where serious ground runs have occurred should be strengthened prior to heading. Under such circumstances, grout could be injected so as to form a zone of stabilized ground around the excavated surface of the tunnel. As this stabilized zone becomes larger and stronger, surface settlements become smaller. Finite Element studies of this matter, conducted by many researchers have revealed that a structurally competent stabilized soil zone around a tunnel acts as a compression ring and prevents stress changes from being felt in the soft soil beyond. This, in turn, reduces the effects of compression in the untreated soil and thus limits the amount of settlement at the ground surface above the tunnel. However, it is interesting to see in Figure 12 how decreases in the surface settlements achieved by increases in the zone size are most significant up to a thickness of about three meters. Use of a larger strengthened zone produces only a small further reduction in surface settlements. For this reason, a thickness of three meters could be considered optimum. The use of greater thicknesses is primarily useful only as an added safety feature, due to the fact that the placement of grout in the tunnel face is difficult to be achieved with a great measure of accuracy. Fig. 12: Correlation between stabilized zone thickness and surface settlement 78

9 Figure 12 has as its assumption that the grouting zone completely surrounds the tunnel opening. This situation is not possible where soil layers that cannot be treated with grout are located in the face area. Such a discontinuous grouting zone might be accepted even in a homogeneous ground. In cases where only the soil above the springline of the tunnel is grouted in the homogeneous ground condition, ground movements have been found to be greater than those within a continuous zone that has a lesser thickness but uses the same area extent of grouted soil. This means that, for a given amount of grouting, a continuous zone of grouting around the tunnel opening is more effective in controlling ground movement. Figure 13 shows the effectiveness of the shape of grouted zone qualitatively using numerical simulations. Fig. 13: Effect related to the shape of a grouted zone in homogeneous ground SAFE TUNNELLING IN SOFT GROUND Cut-and-cover Tunnelling It is essential to provide protection for adjacent existing structures when a new excavation reaches a depth at which a loss of bearing capacity, settlements, or lateral movements to the existing properties could occur. New construction may employ the cut-and-cover method when the excavation depth is not sufficient to utilize tunnelling method. This type of work requires the installation of some kind of earth retaining structures. The retaining structure may be constructed with: 1) sheetpiling; 2) soldier beams with lagging; 3) in-situ concrete piles, and; 4) diaphragm walls. The most commonly applied system in which to hold the retaining wall in place is a bracing system with wales and struts, shown in Figure 14a. Construction costs associated with shallow cut-and-cover tunnels are less than those for bored tunnels; however, this cost goes up dramatically as depth is increased. Furthermore, this technique also tends to have significantly disruptive effects in particularly congested urban environment. When cut-and-cover excavation is made in sandy soil and clay, the stability of the bottom against such things as boiling (Figure 14b) and heaving (Figure 14c) should be attended to for matters of safety. To prevent boiling in sandy soil, the hydraulic gradient should be reduced by lowering the groundwater table behind the retaining wall structure or by increasing the length of the waterflow path. To avoid a base instability of the excavation in a soft clay stratum, soil along the bottom area needs to be sufficiently strengthened. 79

10 (a) Wales and struts (b) Boiling (c) Heaving Fig. 14: Schematic diagram of temporary structures for cut-and-cover tunneling Schema for Reinforcement ahead of Bored Tunnel Face Reinforcement ahead of a tunnel face is generally obtained by creating a structurally strong ground to improve stability in the span of tunnel. In bored tunnelling, the most versatile technique used to reinforce the ground for this purpose is jet grouting, which forms an umbrella-like arch ahead of the face. When a jet-grouting umbrella is created, the columns are designed to overlap each other so that they provide an archway around the tunnel. They are usually reinforced using steel pipes installed inside the columns. The size of the steel pipes usually depends on the nature of ground conditions and the construction equipment used. Each set of jet-grouting umbrellas should also properly overlap longitudinally along the tunnel to ensure the stability of the face. Thus, the overlapping distance is determined according to a length of advance, i.e. round length, and a bench height to be excavated at once. Fig. 15: Cross-section and profile of typical jet-grouting umbrella configuration This technique can be applied when the overburden is too thin for other supporting techniques or when it is necessary to control to a high extent the possible subsidence of the surface. Since the jet-grouting arch is normally installed at the crown area of the tunnel, sub-vertical columns are often placed at the wall area to support the jet-grouting arch. The arch supports the soil during the excavation and homogenizes the stresses, which will act on the final supports. The steel pipe umbrella (often called pipe roofing ), or forepoling, is an umbrella of steel pipes or beams with a truncated conical shape set on the crown of the future tunnel. These techniques are also used to control ground run at the face. Fiberglass pipes are also sometimes installed to reinforce the face. However, it should be clearly understood that use of grouting, a jet-grouting umbrella or any other auxiliary measures is not a substitute for good construction procedures. 80

11 Fig. 16: 3-D view of steel pipe umbrella arch installed ahead of tunnel face Excavations and Support Systems for Bored Tunnelling The excavation progresses through a cycle of excavation, mucking and installing supports. This cycle of work should be well organized in such a way as to produce the greatest length of completed tunnel in the shortest possible time. To achieve this target, the smallest number of operations possible as well as stabilization structural elements should be employed while guaranteeing the permanent stability of the work without creating dangerous conditions for the workers, other people or surrounding facilities located near the site. Excavation methods can be roughly divided as follows: 1) conventional drill and blast method; 2) conventional cyclic method using excavation machines (roadheader, high impact hammer, mechanical excavator, etc.), and; 3) the TBM or shield method. Mechanized tunnelling (using TBM and shield machine) is not dealt with in this paper. Drill and blast excavation and machinery excavation can be carried out with full face heading, half face heading or multiple bench heading (preferably crown, bench, or invert). In soft ground conditions, the full face heading may not be applicable because of the ground s relatively short stand-up time. Thus, the bench excavation is quite common for soft ground tunnelling in urban areas. The distance and interval between working cycles can be chosen freely depending on the stand-up time of ground and the requirements for completing the support system. The multiphased side gallery technique has also proven to be especially suitable for urban areas with a shallow overburden and poor subsoil conditions. Fig. 17: Schematic diagram of bench heading 81

12 Perhaps the most important thing related to the excavation and support system for tunnelling in poor ground conditions is the rapid completion of the tunnel lining. Therefore, the smaller the excavated area is, the faster the completion of the lining. Figures 17 and 18 depict the bench heading and the multi-phased side gallery heading, respectively, in weak ground conditions. Besides these two excavation methods, the ring-cut, the central diaphragm wall cut (CDcut) and top heading subdivision are frequently used for soft ground tunnelling. Sidewall galleries are the best excavation method for the safety of the tunnel face and for controlling the settlement of ground surface. Fig. 18: Schematic diagram of multi-phased side gallery heading The ring-cut is also a common excavation method for stabilizing the tunnel face. If the minimization of ground settlement is a major concern during excavation, the CD-cut is perhaps the best alternative. Top-heading subdivision is applied at a relatively large cross-section of the tunnel with relatively good ground conditions. (a) Ring-cut with supporting core (b) Central diaphragm wall cut (CD-cut ) Fig.19: Examples of excavation method Tunnel support is constructed of materials brought in following the excavation. This system of support is mainly composed of shotcrete, rock bolt and steel ribs. Shotcrete is primarily used as a temporary application prior to a final liner being installed or as a local solution to instabilities in a rock tunnel, however, it can also be used as a final lining. The thickness of the shotcrete, spread at one time, is about 10cm. The next layer should be sprayed within an hour. The order of installing the rock bolt depends on how one is to achieve the arching effect of the tunnel periphery. One method is to install it during the line of excavation at a small over break in a place with good ground conditions. Another way is to install it at the first layer of shotcrete to defend against tunnel deformation after two or three excavation cycles. Steel ribs are self-supported before the hardening of the shotcrete and increase the solidity of the shotcrete support. Steel ribs need less connections and their shape is safe against the applied load. 82

13 Fig. 20: Installation of various supports Instrumentation and monitoring plays a key role in verifying whether the support system is su fficient. More importantly, it also serves as a warning if the support system is not performing as intended, or is in danger of collapse. Particularly, it is essential to prevent the existing struc tures and underground facilities from damage or even collapse as a result of the tunnelling in an urban area. All of the data collected by the cor rect instrumentation and monitoring system will p rovide good communication between the engineer and the contractor, and thus facilitate decision m aking during the construction. Instrumentation schema for the urban tunnelling should be well designed to monitor the movements of ground and structures, in particular the movement (i.e. settlement) of existing structures above or nearby the tunnels, or any deformation of the tunnels themselves (i.e. tunnel lining convergence). Monitoring is initiated prior to construction in order to establish a baseline Fig. 21. Typical cross-section of tunnel with instrument from which movements associated with the construction can be compared. During construction, monitoring schedules will be carefully coordinated with excavation and the initial line sequencing of the tunnels. In general, monitoring is done very frequently during the excavation, when most ground and lining movements are taking place. During the installation of the waterproofing membrane and the final cast-in-place of the concrete lining, monitoring is less frequent as most ground and structural movements have already occurred. During tunnel excavation, data from the instrumentation and monitoring system is reviewed daily by the tunnel s engineer and compared to predictions. Data on building settlement can be used by the contractor to implement a compensation-grouting program, which is very effective in keeping building settlement within acceptable limits. In addition, monitoring data is compared with threshold limits on lining convergence. In the event that convergence threshold limits are exceeded, the contractor can quickly implement a contingency plan to install additional support. 83

14 Fig.22: Ground surface settlement trough as a result of tunneling CASE HISTORIES To ease surface traffic congestion, most of the world s largest cities have constructed subway networks and continue to extend them. These networks obviously result in technical or construction-related interference between existing structures above and below the ground and the newly constructed subway systems. In addition to the construction of passenger transportation tunnels, the construction of water and sewage tunnels as well as utility tunnels for electric and communications cables also present a number of problems. Due to the social demands concerning the use of underground structures such as waste isolation, as well as recreational facilities and other strategic spaces, difficulties related to construction will continue to arise more and more in near future. The types of underground spaces we are discussing require large cross-sectional excavations, possibly through difficult ground conditions. The future of tunnelling requires more refined and sophisticated monitoring and excavation technologies together with superior construction measures and techniques to address various site-specific conditions. With some of these challenges in mind, this lecture will present a systematic survey of some of the technical solutions and case histories in soft ground tunnelling. Brief case histories will be provided, reflecting a broad spectrum of technical solutions employed in soft ground tunnelling, and we will look at how these techniques are used both in designing tunnels and selecting appropriate tunnelling methods. The case histories described in this section primarily reflect Korean tunnelling experiences. Choice of Support and Groundwater Cut-off in Highly Permeable Alluvial Deposits (a) Site Characteristics Construction lot 910 of Seoul s subway line 9, as shown in Figure 23, adopted the cut-andcover method for the very permeable alluvial deposit consisting of silty sand and gravel. The tunnel route was designed to be located underneath the street. The excavation went to a depth of approximately 27m adjacent to 14-storey apartment and office buildings. The alluvial deposit appearing at the surface in the concerned area was about 20m thick. This layer was then underlain by weathered soil and rock, as illustrated in Figure 24. The groundwater table was located about 9m below the surface. An appropriate construction measure was needed to protect the buildings and even the temporary earth retaining structures from the possible damage or collapse. 84

15 Fig. 23: Location of the construction lot 910 Fig. 24: Subsoil profile Fig. 25: Surface conditions of construction site (b) Issues and Technical Solutions Ground run and boiling during the excavation in the alluvial deposits were expected. Particularly, some transportation of soil due to a high seepage force was likely during the excavation in the alluvium, which could possibly lead to severe ground deformation. To prevent against these undesirable circumstances, the groundwater cut-off method and a costeffective earth retaining structure were employed. Fig. 26: Soldier piles with cast in-situ concrete wall and lagging A temporary earth retaining structure consisting of soldier piles, wales and struts with lagging and a cast in-situ concrete wall was employed to prevent the displacement of ground. Additionally, SGR (Space Grouting Rocket system) grouting was done, a type of permeation grouting for the groundwater cut-off to eliminate the piping phenomenon in sandy soil. SGR grouting was originally developed in Japan, and uses various types of chemical grouts having different gel-times (short gel-time, long gel-time, etc.) and strengths. Grouts are permeated into the voids in the soil using low pressure, preferably less than 5kPa, generated through the rocket in the inducement space. This technique is quite effective for groundwater cut-off. 85

16 For this project, one row of SGR grouting columns placed at 80cm intervals along the back of the retaining structure was applied, achieving more than 20cm of overlapping. Other earth retaining methods such as SCW and diaphragm walls were not considered because of the limited workspace available on the ground surface. (c) Lesson Learned The excavation was fulfilled in an urban area where with adjacent buildings without having to temporarily close down the street above the tunnel. These surrounding conditions made it difficult to choose an auxiliary technique and excavation method. At this construction site, SGR (which requires a smaller working space than any other method) could cut off the groundwater inflow and could meet the control value of the ground movement together with the application of soldier piles and a bracing system with a cast in-situ concrete wall. The excavation work was successfully carried out without causing any damage to pre-existing buildings. Ground Strengthening Technique with Root Piling and Grouting (a)site Characteristics Construction lot 512 belongs to Seoul s subway line 5, and was characterized with a shallow overburden of alluvial deposit and a high level of groundwater. The tunnel was to be a dualtrack tunnel undercrossing the Anyangchun, one of the tributaries of the Han River, which is a good source of groundwater. The minimum thickness of the overburden was about 17m and the groundwater table was just 3-7m below the surface. Due to the poor ground conditions and shallow overburden, the design methodology for constructing this tunnel had to be updated with stability as the main consideration. The subsoil at the site mainly consists of four layers: fill material, alluvial deposit, weathered soil and rock, and bedrock (soft to hard rock) from the surface. The alluvial deposits were silty to sandy soil. Hydraulic conductivity of alluvium varied to cm/sec. This layer s deformation modulus was in the range of 100 MPa. Figure 28 shows the subsoil profile of the site. Fig. 27: Location of the construction lot 512 Fig. 28: Subsoil profile of the site (b) Issues and Technical Solutions The weak ground above the tunnel crown area needed to be improved and measures addressing the groundwater inflow were also required for safe construction. The approximately 8m thick alluvial layer in the crown area was reinforced with 25mm diameter re-bars installed at one meter intervals along a rectangular pattern with the cement milk grouting as demonstrated in Figure 29a. LW grouting was carried out along the edge of the 86

17 tunnel except the inverted area. All the ground treatment activities were conducted at the ground surface since this area was open space. (a) Cut-off/reinforcement method with re-bars (b) Profile of reinforcement plan Fig. 29: Ground reinforcement method above crown Due to the ground s short stand-up time, the upper half section of the tunnel was excavated in three parts with a supporting core as depicted in Figure 30. To minimize the settlement of the shotcrete lining, an H- shape steel beam (250 x 250mm), referred to as the load distribution beam (LDB) in Figure 30, was placed longitudinally at both footing areas of the shotcrete lining. (c) Lesson Learned The successful completion of the tunnel Fig. 30: Excavation method/auxiliary technique revealed that reinforcement using vertical reinforcing bars installed from the ground surface with the cement milk grouting allowed for safe tunnel construction even through alluvial deposits. Also, the divided excavation in the upper half section of the tunnel increased the unsupported stand-up time of alluvial deposits and also made it possible to reduce crown settlement by installing LDB at the footings of the shotcrete linings. Excavation and Ground Reinforcement Technique for Tunnelling in Alluvial Deposit (a) Site Characteristics Construction lot 912 is on Seoul s subway line 9, currently under construction. A bored tunnel is scheduled to be built in the alluvial deposits spread over the area near the Banpo stream, one of the tributaries of the Han River. The depth of the tunnel is about 18m below the surface and the entire soil up to the springline of the tunnel consists of sandy gravel. In the design stage, the groundwater table was assumed to be situated at about 1.5m below the surface. For this reason, cut-off grouting was planned along the periphery of the upper half with a thickness of about 3-5m. Located along the tunnel route are 14-storey apartment buildings and 5-storey shopping centers. In addition, several different sizes of box culverts for sewage and cables are also embedded above the tunnel. 87

18 Fig. 31; Location of the construction lot 912 Fig. 32: Surface condition of construction site The sandy gravel layer, which presents many challenges in safe tunnelling, spreads along the entire route concerned. The alluvial layer has more or less a consistent thickness and is placed atop the banded biotite gneiss of the bedrock formation. The permeability coefficient for this layer is 1.0x10-2 to 2.9x10-5 cm/sec. Photographs of the features of excavated surfaces in the alluvial deposit are shown in Figures 33 and 34. Fig. 33: Ground condition at portal and tunnel face Fig. 34: Excavated surface in alluvial deposit (b) Issues and Technical Solutions As seen in Figure 34, ground run is quite common after the excavation unless the excavated surface is properly protected or reinforced. Furthermore, adequate measures against the groundwater are essential here for successful tunnel construction. (a) Excavation method with jet grouting piles (b) Dewatering scheme Fig. 35: Ground strengthening techniques and dewatering schema 88

19 It was revealed from the construction of a cut-and-cover tunnel that a cut-off wall constructed along the periphery of the excavation line was not very successful in keeping the groundwater at its original level when the excavation was carried out. As a result, gradual lowering of the groundwater level was unavoidable. Ground deformation stemming from ground run or groundwater infiltration should be prevented. In order to strengthen the weak ground around the tunnel face, horizontal jet grouting columns and dewatering techniques are applied prior to excavation. A jet grouting column reinforced with a steel pipe (11.4cm diameter) is installed every 60cm along the excavation line at the crown area to form a jet grouting umbrella arch. This arch is then supported by the sub-vertical jet columns to reduce settlement. The length of the jet column umbrella arch is 13.4m and each set of arches overlap by 7.1m longitudinally. Fiber reinforced pipe (FRP) grouting was also applied to help stabilize the tunnel face. The groundwater level was continually lowered as well by applying dewatering systems from the tunnel face. Excavation was made using the ring cut method, as shown in Figure 35. (c) Lesson Learned An important lesson was learned regarding excavating tunnels under a shallow, weak soil overburden. It was possible to control displacement at the tunnel crown by applying horizontal jet grouting as a stiffer auxiliary supporting method. FRP grouting made the tunnel face stable, and was applied to help control displacement at the tunnel face. Passing under Piled Foundation of the Bridge Piers (a) Site Characteristics Some portion of the construction lot 515, which belongs to Seoul s subway line 5, was designed to pass along and under the flyover bridge seen in Figure 36. Bridge piers were built on a group of pre-cast concrete piles. The shortest distance between pile tip and the excavation line of the tunnel was estimated to be about 5.25m, as illustrated in Figures 37. Fig. 36: Location of the construction lot 515 Fig. 37: Tunnel under the pier foundation The site investigation revealed that the pile tips were located in the alluvium and the entire tunnel face lies also within this weathered rock formation. The degree of this rock formation s weathering varies from a completely weathered state to a moderately weathered state. Subsoil conditions were complex as the thickness of the alluvial layer ranged from 12-15m. Tunnel depth was around m while the groundwater level rested 10-15m below the surface. The alluvium was underlain by the weathered soil layer and weathered rock layer. 89

20 (b) Issues and Technical Solutions The flyover bridge plays a very important role for public transportation. Therefore, any instability or structural damage done to the bridge resulting from the construction of the tunnel needed to be avoided. Although pile tips were set apart at a reasonable distance from the excavation line of the tunnel, the intermediate ground between the pile tips and the tunnel needed to be reinforced. In addition, an excessive lateral displacement of each pile should be restrained to protect the piers from any damage. Thus, the ground above the tunnel was strengthened using steel pipe roofing and forepoling prior to heading. The diameter and length of each steel pipe were 52mm and 12m, respectively. Lateral spacing of pipes was set to 40cm and each pipe roof umbrella overlapped 6m longitudinally. To prevent the occurrence of unwanted lateral displacement of piles, the ground surrounding the pile group was strengthened by grouting (see Figure 39). To enhance the effects of this strengthening, the volume of grouting soil to be strengthened was enclosed by the cut-off grouting of SGR. To reduce settlement of the ground at the crown area, a central diaphragm wall excavation (CD-cut) was constructed, as illustrated in Figure 38. Fig. 38: Profile of mini pipe roof Fig. 39: Curtain grouting injection (c) Lesson Learned The tunnelling was quite successful, resulting in only a small amount of settlement. The maximum amount of settlement measured was just 24mm. Forepoling applied between the steel pipe umbrella arch was effective in preventing the ground underneath from falling down. The length of the forepoling was about 2.5 times the round length. The central diaphragm wall excavation (CD-cut) method proved to be a practical method in this situation. It was particularly helpful in reducing the crown settlement and preventing any invasion of the ground at the face. During construction, the support systems were adjusted according to the results of the FEM analysis and monitored field data. Grouting applied around the footings may act as band element for pile group. Passing under Buildings in Soft Ground (a) Site Characteristics Seoul s subway line 9 passes through weak ground conditions to the south of the Han River, as shown in Figure 40a. A 748m-long bored tunnel in construction lot 906 was designed to pass underneath several buildings, as depicted in Figure 40b. Tall buildings such as an apartment building and office building have two underground levels. The vertical distance between the office building foundation pile (PHC, 400mm) heads and the tunnel crown was about 15m. The apartment building stood on a reinforced mat foundation. To protect this 19- storey apartment building along with its parking area, soil cement grouting was applied to reinforce the subsurface ground. The vertical distance between the reinforced soil zone and 90

21 tunnel crown was around 11m. The principal task in this project was to secure the stability of the existing office tower and apartment building. (a) Location of the construction lot 906 (b) Subsoil profile of the site Fig. 40: Location of the construction lot 906 and subsoil profile The subsurface geology of the site mainly consisted of four layers, namely alluvium, weathered soil, weathered rock, and gneiss as bedrock of variable strength, joint spacing and degree of weathering. The N-value of weathered soil is 9/30 to 50/11 and the RQD was very low (less than 0-17%). The full face soil section consisted of weathered rock at the lower side and weathered soil at the upper side. The ground water table sat at a depth of m below the surface. (b) Issues and Technical Solutions The stability of the buildings above the tunnel was the main challenge of the project in this are a. Mindful of the stability of existing structures, the tunnel was excavated essentially using the ring cut method. An ITC machine was employed for a quick and safe excavation. After the e xcavation, first layer of shotcrete was spread, followed by a steel support (H-pile 125mm). After completing the steel support, a second layer of shotcrete was then added. Once the shotcrete was installation, the tunnel face was reinforced using glass reinforced plastic pipe (GRP) 22mm in diameter and 12m in length. Before excavation of the upper half, the tunnel crown and walls were reinforced using the multi step pipe grouting technique. This technique uses steel pipes with a diameter of 114mm, a thickness of 8mm and Fig. 41: Auxiliary measures for safe tunnelling a length of 15m were installed at transverse intervals of 50cm along the entire outer periphery of the upper half (a total of 37 pipes). To support this jet grouting arch, a single row of subvertical steel pipes (73mm in diameter; 4m in length) were installed with cement grouting at both ends of the arch. Before commencement of the excavation on the lower half, three rows of GRP (22mm in diameter, 6m in length) inclined toward the lead direction were installed every four meters, while 4mlong rock bolts were installed perpendicular to the excavated surface. In addition, the invert 91

22 area was also strengthened with cement grouting to prevent heaving or weakening of the ground. ITC machine excavation 1 st layer shotcrete steel rib installation 2 nd layer shotcrete face grouting completion of excavation 3 rd layer shotcrete pipe installation side wall drilling side wall pipe installation steel rib connection final shotcrete Fig. 42: Construction Sequences In the longitudinal direction the steel pipe umbrella arch installed in the upper half of tunnel o verlapped by 11m longitudinally. Auxiliary measures adopted for safety are all illustrated in F igure 41. After the full face excavation, 600mm thick concrete linings were installed. The exc avation steps and supporting methods used during construction of the tunnel are shown in Fig ure 42. Feedback analysis was made using the settlement records obtained from the in-situ inclinometer installed horizontally at the tunnel face. From the results of this analysis, GRP grouting was recommended to reinforce the ground at the tunnel face. Compared to other grouting methods, the main advantage of GRP is that the grouted area and pipe can be cut easily without any loosening of the ground. By applying this additional support measure, crown settlement was controlled within an acceptable limit along with the safe advancement of the tunnel. (c) Lesson Learned The rock mass at the tunnel face was a heavily weathered rock. Moreover, the rock cover above the excavation line in the crown area was mostly less than 2m thick. Weathered soil appeared immediate beyond this weathered rock cover with a high hydrostatic pressure level. These geological and geometrical arrangements were enough to present significant challenges. Consequently, for the construction to be a success, every step of the construction process required a great deal of extra attention and added alertness. Crown settlement as a result of the arching effect of the tunnel face induced by steel pipe grouting and careful excavation was measured by a horizontal in-situ inclinometer system. Immediate settlement was unable to be 92

23 measured by the delayed instrumentation system, but turned out to be about 52.2% of the actual settlement of the tunnel Cutting of Foundation Piles and Bridge Protection in the Gravely Ground (a) Site Characteristics Part of construction lot 912 crosses the estuary of the Banpo stream, one of the tributaries of the Han River. Several piers belonging to three flyover bridges are founded on the riverbed with pile foundations. Fig. 43: Location of the construction lot 912 Fig. 44: Tunnel under the pile foundation The water level is greatly influenced by that of the Han River, since this location is the area w here the Banpo stream joins the river, as shown in Figure 43. The vertical alignment of the tunnel in this area was designed to hit the piles of the pier foundations. Sixty-seven foundation piles needed to be cut during the tunnel construction, as illustrated in Figures 44, 45 and 49. Foundation piles are expected to lay their entire shank within gravel-like soil. This alluvial deposit, as illustrated in Figure 46, rests unconformably atop of the Precambrian age gneiss bedrock. Its typical gradation curve is shown in Figure 47. The average value of hydraulic conductivity in the alluvial layer was approximately 8cm/sec. Fig. 45: Cross section of the tunnel under the piers Fig. 46: Ground composition 93

24 (b) Issues and Technical Solutions Two particularly demanding issues from a technical perspective were presented at the design stage. One is the safety of tunnel construction in gravel-like ground conditions with a large source of groundwater supply; the other is the protection of the flyover bridges from the possible settlement of the piers. The challenge becomes even greater when the pile tips are exposed and cut in the soft ground conditions when the tunnel is excavated. Fig. 47: Gradation curve for alluvium Intensive subsoil investigations were conducted to determine the exact subsoil conditions and status of pile foundations. After comprehensive analyses and discussion, the foundation system was changed from the pile foundations to footings by applying intensive jet grouting in the foundation area, which applied grouting along the full depth of the piles to be cut and a substantial aerial extent prior to tunnel excavation. This intensive grouting could prevent a large amount of groundwater infiltration. In addition, conservative schema for excavation and supports were also followed in order to eliminate any possible source of instability concerning the tunnel and flyover bridges. Figure 48 shows the sequence of construction. Comprehensive instrumentation and monitoring systems were activated to check the impact of the tunnel construction on existing structures. Emergency measures were also established during the construction. steel pile tip exposure size confirmation steel pile cutting steel support installation elastic pad installation Fig. 48: Pile cutting and support installation procedure (c) Lesson Learned Jet grouting turned out to be quite successful. Neither significant ground settlement nor groundwater infiltration was observed. The monitored amount of settlement was negligible while the steel pile tips were being cut. 94

25 Fig. 49: Steel pile visible at tunnel face during excavation Construction of Station Tunnel under the Old Shopping Mall and the Subway in Weak Ground (a) Site Characteristics Station 923 is located under the Gangnam underground shopping mall (GSM) and Seoul s subway line 3, which opened in April, 1979 and October, 1985, respectively. The dimensions of the 27-year-old GSM are 26m (width) x 4.2m (height) x 620m (length). Both underground structures were built using the cut-and-cover method. GSM lies under the street adjacent to the open space of the express bus terminal and Seoul subway line 7 also passes near this site, as shown in Figure 50. Thus this area is one of the major human traffic areas in the Gangnam District. A cross section of Station 923 and the existing underground structures are illustrated in Figure 51. Fig. 50: Location of the construction lot 913 Fig. 51. Cross section and subsoil profile The subsurface geology of the construction site is composed of three primary features: 1) fill material; 2) alluvium; 3) weathered rock formation of gneiss from the ground surface. The alluvial layer persists up to a depth of about 18m, where the crown of the station tunnel is located (see Fig. 51). The shortest distance between the existing subway and the new station tunnel is about one meter. Ground condition visible in the steel pipe face placed along the excavation line at the crown area is shown in Fig

26 The fill material at the ground surface is generally composed of compact-to-dense sand and gravel, while alluvium consists of beds of silt, sand and gravel. The size of the gravel in the alluvium varies from 5-20cm and the gravel volume ratio ranges from 30-50%. The weathering zone of the bedrock (gneiss) has a relatively narrow band, which ranges in thickness to about one meter on top of the gneiss bedrock formation, while the bedrock has variable strength and fracture/joint spacing and an RMR value of (RQD: 50~100%). The groundwater table exists about 9.5m below the surface. Hydraulic conductivity of the alluvium is 1.34x10-5 to 1.22x10-3 cm/sec. (b) Issues and Technical Solutions. Since the new station tunnel was constructed underneath the existing GSM and some portion of Seoul subway line 3, the stability of those existing structures was top priority. To protect these structures from any possible damage, the Cellular Arch Method (CAM) was used. CAM consists of 13 steel pipes installed along the periphery of the tunnel at the crown area, as shown in Figure 53. The pipes are 2m in diameter, 2cm in thickness, and 159m in length. Fig.52: Ground condition at the face of steel pipe (2m diameter) Fig. 53: Steel pipes and launching gallery Fig. 54: Cross section of station 923 tunnel The pipe installation gallery was first constructed at one end of the station tunnel, and then pipe was jacked from one end to the other, one by one, by connecting 3m-long pieces of pipe. After completing the pipe installation, support beams were installed every five meters longitudinally. Pipes and support beams were all filled with reinforcement and concrete. Once the cellular arch was completed, sidewall galleries were excavated for the construction of a concrete wall to support the cellular arch. After this series of procedures was completed, as shown in Figure 55, the central part was excavated. A detailed account of the construction process is presented in Figure 55 and the final 3-D view and cross section view of the station tunnel are shown in Figure

27 Fig. 55: Process of the Cellular Arch Method(Seoul Subway- A World Class System, 2005) For the pipe jacking to form the cellular arch, a rectangular launching gallery (see Figure 56) was constructed. Since a cut-and-cover excavation could not be made due to the underground structures, this gallery was also constructed using the pipe jacking method in a transverse direction of the station tunnel. Fig. 56: Construction of launching gallery (c) Lesson Learned Many constraints related to the urban nature of this site had to be dealt with during the design and construction phases, as the tunnel was to pass underneath the existing underground structures. The challenges faced in the construction of Station 923 included very unfavorable geological and topographical conditions, and excavation under a shopping center, subway and water mains. There was a low overburden, porous soil, and a very shallow subway tunnel below the running line. Safe construction was achieved by using the pipe jacking method and CAM. This method helped control any ground deformation for this challenging tunnel section but did impose a number of intensive labor demands. Tunnelling under the Underpass using the Underpinning Method (a) Site Characteristics This tunnel is located in the vicinity of the Gimpo domestic airport and is a part of the cutand-cover section of construction lot 902, which belongs to Seoul subway line 9 (see Figure 57). The subway route crosses two underpass structures that cannot be closed because of their 97

28 importance. The length of the tunnel passing under the underpass is 45m and the distance between these two structures is about 12-13m, as shown in Figure 58. Fig. 57: Location of the construction lot 902 and general view of construction site Subsoil at the construction site is composed of dense, silty sand near the surface, underlain by a substantially thick layer of weathered soil. Most of the subway route passes through the weathered rock. The permeability coefficient is 1.5~ cm/sec, except for the alluvial deposits. The groundwater table sits at a depth of 11 Fig. 58: Subsoil profile meters. The underpinning method was selected after various alternatives were considered. The construction period was also considered as an important parameter during the selection process. Ground conditions above and at the excavation depth are so highly unfavorable that a support system was necessary to carry the overburden load. Figure 58 shows a profile of the subsoil conditions. Fig. 59: View of the underpinning (b) Issues and Technical Solutions Any damage to the underpass should be avoided during and after tunnel construction. For this reason, the total load of the underpass structure and the soil beneath it was supported using steel beams. Moreover, lateral movements of the soil, which would eventually lead to settlement of the underpass when the movement takes place, should be tightly controlled by tie bars, as shown in Figure 60. Before the transverse excavation was made, soil adjacent to the underpass was strengthened by SRC (Slime Reused Column) grouting (1m diameter; CTC = 800mm, 2 rows). The tunnel area was excavated using a sequential excavation method with 98

29 a downward extension of the support beams as the excavation progressed. Figure 60 illustrates the construction process. (c) Lesson Learned This underpinning method has proven that it can still be applicable, even if it is one of the more traditional methods for undercrossing construction. However, the construction process involved is so complicated and the workspace was rather limited due to the necessarily heavy support systems. These obviously invite a certain amount of undesired quality assurance. SRC grouting (plan) installation of tie-rod(section A-A) steel pipe jacking sequential excavation construction of tunnel structure backfilling Fig. 60: Tunnel excavation method and process Underwater Tunnelling with Pre-grouting (a) Site Characteristics Construction lot 518, which crosses the Han River bed and has a shallow overburden, was one of the most challenging tunnel construction lots in the Seoul subway network. The tunnel was a parallel set of single tubes (A= 48.2 m 2 ) set at a distance of 30m from center to center, as shown in Figure 62. As seen in Figure 61, this tunnel links the west area (Gimpo Airport) and the east area (Godeok-dong and Geoyeo-dong) to the city center. This was the first tunnel constructed under the Han River with a total of 1.58km lying under the water. The excavation 99

30 was conducted using the conventional drill and blast method and a cross section can be seen in Figure 63. Fig. 61: Location of the construction lot 518 Fig. 62: Subsoil profile of the site (b) Issues and Technical Solutions Underwater tunnelling using the conventional drill and blast technique is quite challenging not only because of unfavorable or weak ground conditions, but also the nature of the water source above the tunnel. In the event of a tunnel collapse, this water would rush through the tunnel and result in a large number of lives lost as well as significant economic loss. For this reason, a relatively conservative construction scheme is generally preferred to cover the uncertainties of the surrounding ground. To avoid the difficulties due to severe groundwater infiltration into the tunnel during Fig. 63: Tunnel profile and temporary drains the excavation, pre-grouting was performed as schematically depicted in Figure 64. Liquid waterglass (LW) was used in the grouting. The entire section of tunnel beyond the excavation line in the weathered rock (25m injection length; 5m bulk head; injection pressure: MPa) was grouted. In addition to grouting, mini pipe roof grouting at the crown area was also implemented to help prevent the ground from caving-in during excavation (50mm outside diameter; 15m length; 5-8m reiteration length). Reinforced concrete lining with a thickness of 500mm was placed to act against the high external water pressure. Temporary drainage pipes were installed along the tunnel s inverted area to drain water from the tunnel during the construction stage. For emergency purposes, flood control gates were also installed at both ends of the underwater tunnel section. 100

31 Fig. 64: Schematic description of the pre-grouting for water cutting-off (c) Lessons Learned The tunnel excavation under high water pressure was performed safely by adopting LW grouting for groundwater cut-off placed along the entire periphery of the tunnel and mini pipe grouting for ground reinforcement at the crown area. However the waterproofing system of the tunnel was unsatisfactory because of some damage of membrane incurred by various reasons during the construction. Single Arched Station Tunnel with a Shallow Overburden (a) Site Characteristics The large underground section of Noksapyoung Station, as depicted in Figure 66, was constructed as part of Seoul s subway line 6 (construction lot 606). The heavy volume of daily traffic and a large number of commuters did not allow the cut-and-cover method to be used for this large station tunnel. A drill and blast technique with a divided excavation was eventually applied. The shape of this station was the first of its kind to be built in Korea. The total length of the station tunnel is 167.5m. The Fig. 65: Location of the construction lot 606 overburden depth was around 17-24m, as shown in Figure 67. The rock mass in the station area was composed of slightly weathered gneiss slanting to the southeast. The rock mass was also cut by a small-scale fault and crushed zone. However, almost all of the Noksapyoung Station was to be constructed in a rock mass of fair-to-good conditions. 101

32 Fig. 66: Sections of Noksapyoung station Fig. 67: Subsoil conditions and cross sections of tunnel (b) Issues and Technical Solutions Ensuring stability during the excavation was perhaps the most important technical issue for such a large section of tunnel being built in an urban area. A sequential excavation method (4- benches, 3-divisions per bench) as shown in Figure 68 was employed to help cope with the less than favorable conditions and high strength concrete was used for the tunnel lining to enhance structural soundness. Excavated surfaces were reinforced using shotcrete ( mm thickness) and rock bolts (5-6m in length). (a) Cross-section of station tunnel (b) Profile (c) Plan Fig. 68: Tunnel construction procedures 102

33 To verify the stability during the excavation process, a numerical analysis was done using the 3-D Finite Element Method (FEM). The FEM model of the station s structure with a vertical ventilation shaft (analyzed area was 84m below surface; 140m in length along the z- axis; 124m in a cross section Fig. 69. Finite Element Model along the y-axis), as shown in Figure 69, and the analysis simulated the excavation process (see Figure 70). Ground displacement and support pressure were checked at each step of the excavation. 1 st step 2 nd step 3 rd step 4 th step Fig. 70: Process of FEM analysis It was estimated through the FEM analysis that the maximum stress developed in the shotcrete lining was about 1.36 MPa at the final stage of excavation, well below the allowable limit (8.4 MPa). The maximum total displacement was about 10mm. (c) Lesson Learned The sequential excavation method proved to be successful in the safe construction of a shallow cover single-arch tunnel. After the station was successfully completed, the design methodology regarding a single-arch tunnel for a large section was updated. Two-arch and Three-arch Station Tunnels (a) Site Characteristics Myeongdong Station is situated next to Hoehyeon Station, and both belong to Seoul s subway line 4 (construction lots 415 and 416, as shown in Figure 72). 103 Fig 71 : Site view of single arched tunnel

34 These stations were built underneath existing structures including the Chungmu underground shopping center and the Daehanjeonsun building. Myeongdong Station is a 167.5m-long two-arch tunnel, while Hoehyeon station is meters long and is a three-arch tunnel, as shown in Figure 73. The vertical distances between structures (building) and the tunnel crown is about m through weathered rock. Myeongdong Station was constructed in a fair-to-good quality rock mass. The upper part of the tunnel was situated in soft rock while the lower part was in hard rock. Hoehyeon Station was constructed in soft rock with weathered soil above the rock. The weathered soil layer was approximately 7m thick. Fig. 72: Location of the construction lot 415 and 416 (a) Myeongdong subway station (b) Hoehyeon subway station Fig. 73: Location and cross sections of two-arch subway station and three-arch subway station (b) Issues and Technical Solutions Stability and safety were the most important and technically demanding issues in constructing such a large section tunnel, especially in an urban area. These tunnels were 104

35 constructed using a sequential excavation method, as detailed in Figures 75 and 77. With the two-arch tunnel construction method, the middle part of the tunnel was excavated first and concrete columns were constructed before the excavation of the side areas. For the three-arch tunnel excavation, the side parts of the tunnel were excavated first, followed by the construction of concrete columns and lining and, later, the middle part of the tunnel. Fig. 74: Myeongdong subway station on service Fig. 75: Construction sequence for two-arch tunnel (Myeongdong station, subway line 4) Both methods utilize reinforced concrete supports in the excavated areas prior to moving on to the next step in the sequence of excavation. Excavation of Myeongdong Station was accomplished from both sides of the tunnel, the starting and ending points. To help with ground stability at the crown area of the tunnel, the excavation was performed using forepoling ( = 32mm; 2m in length). 105

36 Fig. 76: Hoehyeon subway station on service Fig. 77: Construction sequence for three-arch tunnel (Hoehyeon station, subway line 4) Hoehyeon Station was the first three-arch subway station built in Korea. It was completed in 1985 with an excavated width of 21.4m and a height of about 7.55m. The station is located under the Toegyero, sharing a boundary with Namdaemun market, as seen in Figure 76 For safety reasons, the Hoehyeon station tunnel was excavated with a bench length restricted to 15m for an early ring closure during construction. However, this bench length was not enough for the equipment to operate easily and it was hard to maneuver trucks and boring machines during the construction work. 106

37 On the basis of numerical analysis and measured results, the bench length was increased to 30m for those areas where there was no existing structure above the surface. The original bench length of 15m was maintained for those areas that passed directly under an existing structure. During the construction of these tunnels, a problem with water leakage occurred due to a damaged waterproofing membrane. To address this leakage problem, drainage pipes with a diameter of 150mm were installed longitudinally along the tunnel (see Figure 78). To reduce the number of connection joints in the waterproofing membrane, the membrane was cut as few times as possible. In order to verify the stability of the Fig. 78: Measure for water leakage structure during the excavation, an FEM numerical analysis was conducted. To consider the influence of being in such close proximity to buildings in the case of Hoehyeon Station, a 2- dimensional Finite Element Model was used, as shown in Figure 79. The analysis showed that both the maximum settlement and the maximum inclination appear at the building corner close to the tunnel. However, these values fell within the allowable limits for settlement. No serious problems related to deformation occurred during the tunnel construction and the actual displacement and support pressure monitored also verified this. (a) Soil-structure model (b) Analysis result of excavation sequence Fig. 79: Finite Element Mesh and simulation of excavation process (c) Lesson Learned An important lesson was learned during the excavation of these large section station tunnels through the adoption of a sequential excavation methodology. The drill and blast technique was selected as an appropriate construction method. Forepoling and LW grouting were used to secure the stability of existing structures. To solve the leakage problem, drainage pipes were installed while the waterproofing membrane was protected from potential damage that might occur during the welding of membrane and fabrication of the reinforcement. After the successful completion of these tunnel stations, the design methodology behind similar twoand three-arch tunnels was updated and applied to other sites along Seoul s subway lines. Some other cases involving such large cross-sectional areas of a station tunnel can be seen in Figures 80 and

38 Euljiro station (Line No. 5) 920 Station (Line No. 9) Yeouido station (Line No. 5) Fig. 80: Two-arch tunnel construction at other sites Daejeon station Fig. 81: Three-arch tunnel construction at other site CONCLUDING REMARKS Some measures frequently employed for the soft ground tunnelling in urban area are addressed in this paper. Those are the matters related to the measures against groundwater and the strengthening of ground for safe tunnelling without causing any possible tunnel collapse and damages on existing structure and facilities. Several successful case histories which are mostly technically demanding tasks are presented in a systematic manner. It can be concluded from this comprehensive review on the soft ground tunnelling that either the stable tunnelling or tunnel collapse is possible in the very similar geological conditions. Thus, human factors play significant role for the safe tunnelling. Consequently, the unique tunnelling technique may not be executed even if it exists. ACKNOWLEGEMENT This paper is a slightly modified version of the author s lecture which was made at the Training Course of 2006 World Tunnel Congress in Seoul. REFERENCES H. Burger(editor) et. al.(1993), Options for Tunnelling, Developments in Geotechnical Engine ering, 74, Elsevier Science Publishers B. V., Netherlands. J.T.Edwards. (1990), Civil Engineering for Underground Rail Transport. Butterworths, pp.51-53, 65-67, 73-77, M. Kobayashi., S. Seki., I. Iwamura., H. Nagai.(2003), Design and construction of urban tunnel beneath operation railway. Proc. of the ITA world tunnelling congress 2003, Amsterdam, the Netherlands, volume2, pp

39 Office of Subway Construction of Seoul Metropolitan Government (1987), Construction Report for Seoul Subway Line No. 3-4, Lot 415, 416, pp (in Korean). Office of Subway Construction of Seoul Metropolitan Government (1994), Design Report for Seoul Subway Line No. 6, Lot 6-6, pp , (in Korean). Office of Subway Construction of Seoul Metropolitan Government (1996), Design Report for Seoul Subway Line No. 5, Lot 5-12, pp. 3-15, (in Korean). Office of Subway Construction of Seoul Metropolitan Government (1996), Design Report for Seoul Subway Line No. 5, Lot 5-15, pp , (in Korean). Office of Subway Construction of Seoul Metropolitan Government (1996), Design Report for Seoul Subway Line No. 5, Lot 5-18, pp (in Korean). Office of Subway Construction of Seoul Metropolitan Government (1996), Design Report for Seoul Subway Line No. 5, Lot 5-36, pp. 3-15, (in Korean). Office of Subway Construction of Seoul Metropolitan Government (1997), Design and Construction of The Han River under-riverbed Tunnel(1), pp (in Korean). Office of Subway Construction of Seoul Metropolitan Government (1998), Construction Report for Seoul Subway Line No. 5, pp (in Korean). Office of Subway Construction of Seoul Metropolitan Government (2001), Design Report for Seoul Subway Line No. 9, Lot 910, pp (in Korean). Office of Subway Construction of Seoul Metropolitan Government (2002), Design Report for Seoul Subway Line No. 9, Lot 913, pp (in Korean). Office of Subway Construction of Seoul Metropolitan Government (2004), Design Report for Seoul Subway Line No. 9, Lot 912, pp (in Korean). Office of Subway Construction of Seoul Metropolitan Government (2004), Modified Design Report for Seoul Subway Line No. 9, Lot 902, pp (in Korean). Office of Subway Construction of Seoul Metropolitan Government (2005), Modified Design Report for Seoul Subway Line No. 9, Lot 902, pp (in Korean). Office of Subway Construction of Seoul Metropolitan Government (2005), Seoul Subway-A World-Class System, pp Office of Subway Construction of Seoul Metropolitan Government (2006), Modified Report for Seoul Subway Line No. 9, Lot 912, pp (in Korean). W. Arnold et. al.(1985), Der Tunnel, Deutsche Bundesbahn Bundesbahndirektion Stuttgart, Germany 109

40