Application of freezing method to recover tunnel accident in complex stratum of Nanjing subway.

Ping, Yang ; Feng-Bin, Zhu

By the application of the freezing method to handle several engineering accident in Nanjing subway's complicated stratum, this paper presented the freezing design method, and analyzed the monitoring result and pattern of freezing-induced ground surface deformation in the sites. It is suggested that bore-unloading could counteract a portion of frost heave deformation and the grouting was to be the key measure to minimize the thaw settlement. Guidelines are provided for the application of the freezing method to deal with related engineering accident and construct subway in complicated stratum.

INTRODUCTION

The main stratum which the shield tunnel of Nanjing subway line #1 traversed are peaty clay, silty soil and fine sand. The peaty clay possesses high compressibility and easily fluid. Silty soil and fine sand with high water content and strong penetrability easily bring about spurting water and gushing sand. Because of the soft and weak stratum, several methods of improving soil have been used at the entrance of Zhang Fuyuan southern shield tunnel (Yang et al., 2003) and in five out of six corridors of line #1, but all was failing, only the freezing method succeeds to reconstruct and guarantee the safety.

HANDLING THE ACCIDENT AT SHIELD ENTRANCE

Permeability grouting and soil mixing methods for soil improvement were used at the entrance of Zhang Fuyuan southern shield tunnel .When punching concrete wall, a great deal of flow-sand twice gushes from both sides around the cave for shield, the quantity of which estimated to be about 110[m.sup.3]. The ground surface's settlement is about 1.5m in the 20[m.sup.2] area of the east, and about 1m in the 1.5[m.sup.2] zone of the south west. The settlement of the whole region near shield entrance is larger (Fig.1). In order to insure the pipelines and the transportation being in normal usage and safety, and to guarantee the shield machine safely passing doorway, the freezing method was applied to improve the stratum.

[FIGURE 1 OMITTED]

Frozen wall and freezing parameter

By experiment, obtained the compression strength of frozen silty soil and fine sand is 9.95MPa and 9.57MPa at -10 [degrees]C, and the tension strength is 1.81MPa and 1.72Mpa, respectively. The thickness of frozen wall can be calculated by empirical formula:

e = kB x D x p / [2[sigma].sub.c] 1/2 (1)

where e is frozen wall's thickness; k is safety coefficient, adopting 3; B is empirical coefficient, adopting 1.2; D is excavation diameter, 6.5m; p is the total water and earth pressure, 0.217Mpa; [[sigma].sub.c] is the compression strength of frozen soil.

The thickness of frozen wall is calculated to be 0.5m. The overlapping length between the frozen wall and concrete wall which is around the cave for shield is usually 1-2m according to experience, and taking the larger water-earth pressure into account, the width and depth of frozen wall are designed to be 9.5m and 18.5m, respectively. The freezing depth above the cave is 3m. The zone from 3m above the cave to the ground surface is not frozen so as to reduce freezing-induced influence on ground surface.

Freezing pipes are set up perpendicular to ground surface. Because there have been soil mixing piles in the zone out of frozen wall ,which can bear lateral earth pressure, the design of the frozen wall should be calculated according to the demand of sealing water and according to the distance between soil-cement mixing piles and concrete wall. The thickness of 0.5m is deemed enough. It is more difficult to freeze soils near the concrete wall due to the large thermal conductivity of concrete. Freezing pipes should be lay near the concrete wall, And should take measures to preserve heat of the wall.

The layout of freezing pipes and frozen wall are shown in Fig.1. the 21 freezing pipes with diameter of 108mm and 2 temperature holes were laid out with the bottom at a depth of 18.5m. The distance between two pipes is 450mm and the distance between the freezing pipes and concrete wall is 200mm. The brine temperature should come to -22 [degrees]C after freezing for 7days and start excavation of the cave of shield after freezing for 15 day. The lowest temperature of recycling brine is -24 [degrees]C ~ -28[degrees]C and the average temperature of the frozen wall is no more than -10[degrees]C. When excavating the cave for shield, the temperature at interface between the frozen wall and concret wall is less than -3 [degrees]C.

Monitoring and analysis of the temperature and deformation

In order to monitor the temperature and displacement during the freezing, measured temperature holes C1and C2 have been set up in frozen wall. And there are totally 24 points of monitoring displacement on the ground surface. Fig.2 shows the temperature change of the temperature holes with freezing time. C1 is near the concrete wall, where there is direct exchange of heat with hot atmosphere(20 [degrees]C).So its temperature descends slowly, but is basically uniform at various depth. The temperature come to -7 [degrees]C after freezing for 15 days. Whereas, the C2's temperature descends quickly to a lowest value of -16.5 [degrees]C as it is adjacent to the side of mixing piles. Moreover, because the hole leans to freezing pipes with the increase of the depth, the temperature discrepancy at different depth is very large. Such as ,the temperature at the depth of 3m is only -8 [degrees]C, while it is -16.5 [degrees]C at 9m. The figure also shows it is read for excavation after freezing for 15 days

[FIGURE 2 OMITTED]

Fig.3 shows that the measured points 15# and 18#, which lie in the flow sand area east to the frozen wall(Fig.1). It appears that quite large initial settlements before freezing, and there is frost heave deformation of 4mm when the temperature descends, but the total settlement is 3-4mm at last. The settlement at the point 2# and 19# are not too large, with a maximum value of 5mm at 2# and only 1mm at 19# . This is because point 19# lies in the accident-induced subsiding zone. According to the results measured at 24 points, it is found that the maximal frost heave deformation is no more than 5mm.

[FIGURE 3 OMITTED]

THE APPLICATION OF FREEZING METHOD TO TUNNEL CORRIDOR

Corridors of tunnels of subway line #1 in Nanjing are almost located at depth of 14m to 17m below ground surface. The stratum consists of silt clay layer, silty soil and fine sand layer, which have the properties of high moisture content, high sensitivity, low strength, large voids ratio and big penetrability, They belong to unstable stratum. The construction of five corridors once used several methods , but all was failing, only the freezing method succeed to reconstruct them. The freezing method is very effective on improving soil in weak layer with rich water. Sometimes ,it may be unique method.

Taking only San-Zhang corridor for example, the layout of freezing pipes, freezing parameter and monitoring frost heave and thaw settlement in the site are presented below. The corridor with a diameter of 3.2m is at depth of 17.13m located in liquefiable fine sand layer. The corridor and pumping-pit are constructed in one operation (Fig.4).

[FIGURE 4 OMITTED]

Layout of freezing pipes and determining freezing parameters

The freezing pipes were in line with rectangle, and two upper angles of wall were changed into arc(Fig.5). Excavation outline of the corridor is a height of 4.24m and breadth of 4.16m. Frozen wall was designed according to frozen powder sand strength for the corridor located in powder sand layer ,and a design thickness of 1.6m and average temperature of -10 [degrees]C were obtained. The horizontal freezing pipes with 108mm diameter were installed in the direction of parallel to corridor axis, as shown in Fig.4 and fig.5. The detail parameters about freezing method applied to the corridor are shown in table 1.

[FIGURE 5 OMITTED]

Monitoring and analysis of frost heave and thaw displacement

Twelve monitoring points(1 to 12) of displacement were positioned on the ground surface in the direction perpendicular to the corridor axis(Fig.5).The frost heave and thaw displacement were obtained during different construction stages and shown in Fig.6.

[FIGURE 6 OMITTED]

Fig. 6 shows that the displacement above corridor axis is the biggest and it decreases with the increase of the distance from centerline. The deformation is divided into four stages. The first is drilling holes and installing pipes stage(25days), during which the settlement of ground surface is not big with a maximum value of 10.3mm. This is due to partial water and soils running off holes and the ground volume is lost a little. But grouting into holes is adopted for reducing the settlement, which is very effective. The second is freezing stage after installation of pipes (32 days), during which the frost heave deformation is small at the beginning of freezing for the temperature of most soils being above 0 [degrees]C, but the frost heave deformation increases greatly when soil temperature falls below zero degree. The reason is that original water and water migrating to the freezing front at 0 [degrees]C is changed into ice and the soil swells rapidly. The figure shows the maximum heave deformation is 16.5mm after taking the measure of drilling four unload holes. The third is maintained freezing stage, i.e. excavation and installing lining stage (35days), during which the heave displacement rate reduces with an increment of only 2.6mm. The final maximum frost heave is 19.1mm. The fourth is unfreeze stage, during which thaw zone is grouted with injecting-pipes placed in advance to prevent the large ground surface subsidence, As such, the maximum thaw settlement is 20.4mm. The deformations during every stage are satisfied with allowance deformation of structure.

The soil will be disturbed during improving stratum with freezing method, especially, when the unfreezing time lasts longer. The disturbance may influence and even damage tunnel lining and buildings and pipelines around the tunnel(Brown et al.,2003). Some research results show that freezing-induced deformation may be bigger. Moreover, thaw settlement was about two to three times larger than frost heave without taking measures of unloading and grouting (Konrad J.M.,2002). But the examples in this paper proved that drilling unload holes and grouting with injecting-pipes placed in advance can effectively reduce frost heave and thaw settlement in order to protect buildings and tunnel lining.

CONCLUSION

Freezing method has been effectively applied to recover tunnel accidents in complex stratum of Nanjing subway. The examples proved that the frost heave and thaw settlement in the technique can be completely controlled within allowance value by means of taking measures during different stages. For example, partial frost heave can be compensated by drilling unload holes and thaw settlement can be reduced by grouting. So freezing method should be adopted prior to other methods when recover accidents and improves soil of shield entrance ,corridors and pumping-pit in weak stratum with rich water.

REFERENCES

Brown, D.A. (2003), "Hull wastewater flow transfer tunnel: recovery of tunnel collapse by ground freezing", Geotechnical engineering, Vol.157,No.2,77~83

Konrad, J.M. (2002). "Prediction of freezing-induced movements for an underground construction project in Japan", Can.Geotech.J.,Vol.39,No.6,1231-1242

Yang, P. and She, C.G. (2003), "Application of artificial freezing method in Zhang Fuyuan station of Nanjing subway", Rock and Soil Mechanics , Vol.24,No.2,365-368

YANG PING & ZHU FENG-BIN

Faculty of Civil Engineering, Nanjing Forestry University, Longpan road 159#, Nanjing Nanjing, Jiangsu, China

able 1. Freezing parameters

Parameter name Value Parameter name Value

Design thickness of 1.6m Design space 0.6~0.84m
frozen wall of freezing
 pipes

Recycling brine -22~-25 [degrees]C Bottom space 1.0m
temperature of freezing
 pipes

Average temperature of -8~-10 [degrees]C Time of 25day
frozen wall forming sealed
 wall

Amount of measure 10 Positive 32day
temperature holes freezing time

Freezing pipes Amount 55 Maintainance 35day
 freezing time

Unload holes 4 Time of whole 95day
 Construction