Case History of Excessive Groundwater Inflow during … EPBM was converted to closed mode and the high elasticity urethane grout was injected in the EPBM chamber, and consequently

Case History of Excessive Groundwater Inflow during TBM Tunneling and Excavating Highly Permeable Ground under High Porewater

Pressure

Joon-Shik Moon1a, Jaeyoung Kim2b and Gychan Jun3c

1Department of Civil Engineering, Kyungpook Naional University, 80 Daehak-ro, Buk-gu, Daegu 41566, Republic of Korea

2Geotech and Tunnel Division, KOTEM, 60-4 Gasan-dong, Keumchon-gu, Seoul, Republic of Korea

3Geotech and Tunnel Division, Korea Engineering & Construction, #809, 126 Guwollan-ro, Namdong-gu, Inchon 465-736, Republic of Korea

ABSTRACT

The tunnel experienced excessive and continuous groundwater inflow followed by sudden groundwater level drawdownduring shield TBM tunneling of about 53m depth when it encountered the limestone cavity network unexpectedly. The road subsidence and damage of adjacent structures were caused by 16m groundwater level drop and subsequent elastic ground settlement. The EPBM was converted to closed mode and the high elasticity urethane grout was injected in the EPBM chamber, and consequently the groundwater level was restored in 2 weeks. In order to find out the cause of unexpected excessive groundwater inflow, additional geotechnical investigation including ground borings and resistivity survey was carried out. In this study the stability of adjacent structures including subway station was evaluated through 3-dimensional numerical analysis. In addition, a ground reinforcement plan was proposed to minimize groundwater inflow and ground subsidence during excavation in a fractured zone under porewater pressure of 4bars and the suitability of the proposed measure was evaluated. Keywords: groundwater inflow; ground subsidence; limestone cavity; EPBM; 3-D numerical analysis; 1. Introduction

Because of the possibility of rapid increase of groundwater inflow and consequent

ground settlement when limestone cavities are encountered during the tunnel excavation in limestone formation, it is necessary to closely examine the geological structure of the limestone cavities and the possibility of encounters during geological

aAssistant Professor, E-mail: [email protected]

bPh.D., E-mail: [email protected]

cExecutive Director, E-mail: [email protected]

investigation. In this study, the behavior of groundwater flow and surrounding ground during limestone cavity encounters were discussed from the case study in which the groundwater inflow was increased rapidly to 670 ton/day while encountering the limestone cavity section during tunneling using a dual-mode EPBM.

In this case, the groundwater level dropped to GL-32m from GL-16m before tunnel excavation, and the ground settlement up to 23mm and damages of neighboring buildings were reported as the excess groundwater inflow during tunnel excavation in a limestone formation continued for more than one month. It was impossible to block the inflow of groundwater with the close-mode EPBM due to the ground water pressure (around 4 bars) exceeding the screw conveyor capacity (around 2~3 bars), and the construction was stopped. In the second stage, the high-elasticity urethane was injected twice into the chamber, and the groundwater level recovered to GL-17m in about two weeks.

In this study, the mechanism of excessive groundwater inflow during tunnel excavation in limestone formation was analyzed through additional ground survey and field measurement. The back analysis using 3D numerical analysis was also performed from groundwater level and inflow rate measurements and the behavior of groundwater flow and ground settlement with time were analyzed.

2. General conditions In this case study, excess groundwater was drained through the tunnel face in

limestone formation during the 1,873m–long cable tunnel excavation in at about GL-50m depth. The initial groundwater level of GL-16m was dropped to GL-32m, and ground subsidence and damage of adjacent buildings were reported. Fig.1 shows some pictures of building damages and subsidence of roads near NO.34+16 where the shield TBM equipment experienced high water inflow rate.

Fig. 1 Ground settlement due to excessive water inflow during tunneling

The schematic drawing of the shield TBM equipment used in this site is shown in Fig. 2. It is single-type EPB shield TBM composed of an excavation face plate with diameter of 3.55m, a 2-row tail seal, and a ribbon-type screw conveyor. A probe drilling equipment (4 holes for grouting upper 120° area with length of 20m) is installed for drilling and grouting in the fracture zone and weathered soil area.

Fig. 2 Schematic drawing of the shield TBM equipment used in this site

If the chamber pressure of close-mode shield TBM is sufficiently managed to

correspond to the earth pressure and the water pressure, there will be almost no problem with the ground settlement. However, the capacity of the shield TBM equipment used in the site is 2~3 bars, which is only around 50% of the expected water pressure of 4 bars. Also, it is considered that the probe drilling facility for grouting the upper 120° will not be able to sufficiently cope with the inflow of high pressure water from the side and the bottom.

3. Geological Condition The limestone cavity zone appears as a narrow-width cavity network along the

discontinuity as shown in Fig. 3(a), or as a large-scale cavity with a width of tens to hundreds of meters as shown in Fig. 3(b) (Lee and Sun, 2010). The narrow-width cavity network type is a structure in which the limestone is dissolved due to groundwater flow along the discontinuities formed by the stress. It is considered that the limestone formation in this case has a narrow-width cavity network shape developed mainly along the discontinuities.

(a) narrow-width cavity network (b) large-scale cavity with a width of along the discontinuity tens to hundreds of meters

Fig. 3 Cavity zone in limestone formation

The limestone cavities were found at GL-23~45m depth and relatively good quality

rock mass was encountered at the tunnel depth (GL-50~55m) during Phase I geotechnical investigation as shown in Fig. 4. In the limestone formation, there are 2~3 joint sets dipping 10~30° and 60~80°, and narrow-width cavities are expected to distributed along joints. The limestone cavities was found at 5~10m above the tunnel crown, and the possibility of encountering cavities during tunneling was overlooked in tunnel design. However, the development depth of the narrow-width cavity network is generally irregular according to the groundwater flow condition and discontinuity structure, and the tunnel suddenly encountered a high-dipping cavity at No.34+04 as shown in Fig. 5.

(a) Location of boreholes

(b) BH-12 (c) BH-13

Fig. 4 Rock cores obtained from geological investigation during design (Phase I)

Current TBM Location (No.34+16)

Fig. 5 Irregular development depth of limestone cavity

Further drilling and electro-resistication tomography were performed to better

understand the geological conditions of fractured zone after occurrence of excessive groundwater inflow. Based on the results of the drilling survey, electrical resistivity tomography, and BIPS, both narrow-width cavity network type and large-scale cavity type were found in the project area. The limestone cavities were classified into 2 types, in which empty cavities (Type I) and cavities filled with clayey silt (Type II). Fig. 6 shows a comprehensive longitudinal profile reflecting the results of the additional (Phase II) geotechnical investigations. Additional drilling investigation was conducted adjacent to the tunnel face experienced excessive groundwater inflow (No. 34+04), and the limestone cavity distribution and development depth (NBH-4) differed significantly from the 1st-phase geological investigation performed during tunnel design (BH-14) as summarized in Table 1.

Fig. 6 longitudinal profile reflecting the results of the additional geotechnical investigations

Unexpected encounter of limestone cavity

TBM

boring

Highly weathered rock

Unweathered rock(Gneiss)Limestone

Weathered

Weathered soil

Soil

Alluvium

Narrow-widthcavity network

(0.1~1.8m)Fractured

+Cavity zone

Fractured zone(clay filled)

Moderately weathered rock

Table 1 Comparison of limestone cavity development understood during 2 phases of geological investigations

Boring No. Drilling Depth limestone cavities (m)

BH-14 (Phase I)

58m 24.3∼24.5, 26.3∼26.7, 29.4∼30.2, 31.4∼31.7, 32.5∼

33.0, 37.7∼38.2, 38.8∼40.2

NBH-4 (Phase II)

72m

- Type I : 16.0∼16.6, 19.1∼21.0, 36.0∼36.6, 37.2∼37.55,

42.3∼42.6, 43.4∼44.3, 69.7∼72.0 :

- Type II: 21.0∼21.6, 21.6∼26.1, 39.05∼39.1

4. Numerical Modeling Fig. 7(a) shows the plan view of the 3-D numerical model (105m × 100m). The

numerical model covers the two 10-story buildings where structural damage was reported and a subway station near the current TBM location for investigate the influence of ground movement due to groundwater level drawdown. Fig. 7(b) is a geological cross-sectional view of the study area including a numerical analysis section based on geological investigation results. The bottom level of the numerical model depth is set to 17.0m (= 5 × tunnel diameter) below the tunnel invert in order to minimize the effect of boundary condition after tunnel excavation.

(a) plan view (b) geological cross-sectional view

Fig. 7 Size of 3-dimensional numerical model

Phase IPhase II

105m

100m

BLDG 1(10-story) BLDG 2

(10-story)

Subway Station

29.1m

21.7m

TBM Location(No. 34+16)

BLDG 1(10-story)

BLDG 2(10-story)

Narrow-width limestone cavity zone

Large-scale limestone cavity + fractured zone

TBM Location(No. 34+16)

Excavation

PileFoundation

Tunnel

Numerical Analysis

Num

erica

l Analy

sis

(17m

belo

w tunnel in

vert

)

Fig. 8 shows the measured groundwater level from the day of occurrence of excessive groundwater leakage (D+0). The initial groundwater level was located at the GL-16m and it was lowered continuously down to GL-32m (D+32days). If the ground settlement measurements are made after occurrence of an event as this case history, it is insufficient to analyze the behavior of the surrounding ground due to the tunnel excavation by performing back-analysis based on the displacement measurements. Therefore, the back-analysis using the numerical method was conducted based on the continuously measured groundwater level change and groundwater inflow rate with time.

Fig. 8 Variation of groundwater level with time

Fig. 9 shows the 3D modeling of the geological formation and adjacent structures including the cable tunnel, the existing subway concrete box structure, and adjacent buildings using ABAQUS program. Each stratum and limestone cavity zone was modeled by taking into account both phases of geological investigation results. The Mohr-Coulomb elasto-plastic failure criterion was applied, and three-dimensional 8-node stress-pore pressure coupled element(CBD8P) was used to simulate groundwater level drawdown and consequent ground settlement due to TBM tunnel excavation. Table 2 summarizes the geotechnical properties used for numerical analysis.

(a) 3-D modeling of geological formation (b) 3-D modeling of adjacent structures

Fig. 9 3D modeling of the geological formation and structures using ABACUS

GL-15m

GL-20m

GL-25m

GL-30m Excessive Groundwater Inflow(670ton/day)

High ElasticityUrethane Grouting(316ton/day)

(distance from tunnel face=15.1m)

(distance from tunnel face=43.3m)

(distance from tunnel face=78.7m)

Point 1

Point 2

Point 3

BLDG 1(10-story)

BLDG 2(10-story)

Alluvium

Weathered Soil

HighlyWeathered Rock

ModeratelyWeathered Rock

Unweathered Rock

InitialGW(GL-16m)

Narrow-widthCavity Network

Limestone Cavity Zone

BLDG 2(10-story)BLDG 1

(10-story)

Subway Concrete Box Structure

Shield TBMTunnel

Table 2 Summary of geotechnical properties used for numerical analysis

Elastic

Modulus (MPa)

Poisson’s Ratio

Cohesion (MPa)

Friction Angle (°)

Unit Weight (kN/m3)

Permeability (cm/sec)

Fill 14 0.31 0.00 27 18.0 1.17×10-3

Alluvium 16 0.31 0.00 29 18.0 6.58×10-4

Weathered Soil 38.5 0.34 0.02 31 19.0 1.64×10-4

Highly Weathered Rock

200 0.34 0.10 32 21.0 -

Moderately Weathered Rock

2,300 0.27 0.36 34 24.0 8.16×10-5

Unweathered rock 11,000 0.22 1.00 40 26.0 7.28×10-5

The limestone cavity in this study area is filled with soil and gravel, but no field tests

for geotechnical characteristics was conducted. In this study, the strength, deformation, and hydraulic properties of limestone cavity zone filled with soil were determined referring to relevant research papers (Farid, 2013; Poulos, 2013). The deformation coefficient of the limestone cavity zone filled with soil was assumed to be similar to that of soft rock mass (E=2,300MPa), highly weathered rock mass (E=200MPa), or composite of highly weathered rock and soft rock mass (E=600MPa).

5. Numerical analysis results 5.1 Goundwater level

In order to obtain reliable results from the numerical analysis, the hydrological

characteristics, such as permeability coefficient of the limestone cavity zone is essential. Through the trial and error method for various permeability coefficients of the limestone cavity zone, it was found that the numerical analysis results and actual measured groundwater level change with time were similar when the permeability coefficient is 1.0× 10-3cm/sec.

Fig. 10 shows the numerical analysis results of groundwater level change and groundwater inflow rate from the day of occurrence of excessive groundwater inflow (D+0) to D+32 days when the TBM extraction chamber is filled with high elasticity urethane grout. The groundwater inflow rate was calculated by multiplying the flow velocity from the tunnel face by the tunnel face area. The numerical analysis estimates the groundwater inflow rate of 667ton/day on the day of occurrence of excessive groundwater inflow (D+0). The excessive groundwater inflow rate lasted for a month and the inflow rate on D+32 days was 641ton/day, and consequently the groundwater level dropped to GL-32m. It was found that the numerical analysis could estimate similar to the measured groundwater inflow rate and groundwater level change with time.

(a) D+0 (666.7 ton/day) (b) D+32days (640.73 ton/day)

Fig. 10 Numerical analysis results of groundwater level change

Fig. 11 compares the measured groundwater level and the numerical analysis results in this study. The groundwater level is lowered down from the point near the tunnel face (point 1) as measured in the field, and the groundwater level change with time obtained from numerical analysis showed a similar tendency to the measured results.

It was found that the rate of groundwater level drawdown is faster than the measured value in the adjacent point of the tunnel face (point 1). As the groundwater flows into the tunnel, the porewater pressure decreases mainly around the tunnel in the beginning, and the influence range gradually increases to the upper part and groundwater level drawdown occurs. The porewater pressure decrease makes increase of effective stress, and consequently the propagation of influence is slowing down due to reduction of the permeability of granular soil. On the other hand, in the numerical analysis the permeability coefficient is assumed to be constant, and the propagation of influence is relatively faster and consequently the rate of groundwater level drawdown is faster than the measured value.

(a) Location of groundwater level measurements

(c) measured groundwater level (d) groundwater level estimated form numerical analysis

Fig. 11 Comparison of measured groundwater level and numerical analysis results

Initial Groundwater Level GL-16m

Lowered GroundwaterLevel GL-16m

Initial Groundwater Level GL-16m

Current Groundwater Level GL-16m

Point 1

Point 2

Point 3

Point 1 Point 2 Point 3

-35

-30

-25

-20

-15

0 5 10 15 20 25 30 35

Gro

un

dw

ate

r Ta

ble

be

low

GL

Days

Point 3

Point 2

Point 1

-35

-30

-25

-20

-15

0 5 10 15 20 25 30 35

Gro

un

dw

ate

r Ta

ble

be

low

GL

Days

Point 3

Point 2

Point 1

5.2 Ground surface settlement Fig. 12 shows the ground surface settlement measured from the day of occurrence

of excessive groundwater inflow (D+0) to D+32 days when the TBM extraction chamber is filled with grout. The measured subsidence of road surface was less than 10mm for the first 17 days (from D+0 to D+17) even though 7~10m of groundwater level drawdown occurred during that period. It is reported that the surface settlement increases faster from D+17days and the settlement of the asphalt pavement surface is 16mm ~ 23mm at D+32 days.

(a) Location of ground surface settlement measurements

(b) Variation of ground surface settlement with time

Fig. 12 Measured ground surface settlement

Fig. 13 shows the ground surface settlement estimated from the numerical analysis.

Various elastic modulus values (200MPa, 600MPa, 2,300MPa) were used for limestone cavity zone in the numerical model in order to investigate the effect of elastic modulus on the settlement. The subsidence is increased as the elastic modulus of limestone cavity zone is decreased. However, the increase of ground surface settlement is less than 9mm even if the elastic modulus of limestone cavity zone is decreased by more than 10 times from 200MPa to 2,300MPa. It could be concluded that the settlement is more sensitive to the groundwater level drop than the stiffness of the ground.

Point 36

BLDG 2

Point 37

Point 38

BLDG 1

Point 36

Point 37

Point 38

Excessive Groundwater Inflow and Groundwater

Level Drawdown

Sett

lem

ent (m

m)

Surface settlement starts

Fig. 13 Ground settlement for various elastic modulus values of limestone cavity zone

Fig. 14 shows the change of surface settlement with time assuming the elastic

modulus of limestone cavity zone is 200MPa. It was found from the numerical analysis that the surface settlement increases rapidly for about 10 days fthe day of occurrence of excessive groundwater inflow (D+0), and gradually converged from thereafter. The point 38 where the maximum settlement of 118mm occurred, is near the borehole BH-15 where weathered soil was found down to a depth of 64m.

Fig. 14 Change of surface settlement with time (elastic modulus of limestone cavity zone=200MPa)

Using the empirical equation (1) proposed by De Beer (1965), the accuracy of the

three-dimensional numerical analysis results was examined. Fig. 15 shows that De Beer (1965) empirical equation (equation (1)) estimates 119.2mm of ground settlement when the groundwater level drawdown is 16m from GL-16 to GL-32. The calculated settlement value is similar to the predicted value from the numerical analysis at point 38. It should be noted that the empirical equation is for 1-dimensional settlement and assumes that the groundwater level drawdown and effective stress changes are consistent in the horizontal direction.

(

) (1)

70

80

90

100

110

120

0 500 1000 1500 2000 2500

Gro

un

d S

ettl

emen

t (m

m)

Elastic Modulus of Limestone Cavity Zone (MPa)

Point 36

Point 37

Point 38

Sett

lem

ent (m

m)

Converge

Point 36

Point 37

Point 38

Rapid Settlement for 10 days

where, S=ground settlement (m); P0 : initial effective vertical stress (kN/m2); ΔP=increment of effective vertical stress due to groundwater level drawdown (kN/m2); N=N-value; and H=thickness of soil layer (m)

Fig. 15 Ground settlement estimation using De Beer (1965) empirical equation

The cumulative settlement of 76mm~118mm estimated from numerical analysis and the settlement of 119mm calculated by the empirical equation is 5~6 times larger than the measured value of 16mm ~ 23mm. Also, the amount of settlement increase with time obtained from numerical analysis was different from the measured value. The asphalt pavement and the soil layers are assumed to have a unified behavior as a continuum in the numerical analysis and the surface settlement occurs immediately when the ground movement occurs in the soil layers. On the other hand, the asphalt pavement layer has its own stiffness and behaves separately from the upper soil layer, and the surface settlement is relatively small even if the volume reduction in soil layers occurs due to groundwater level drawdown in the soil layer.

Since there is a larger porewater pressure change in the vicinity of the tunnel, the ground deformation around the tunnel is larger. In the numerical continuum model, the large deformation around the tunnel is directly transferred to the surface and causes immediate settlement. As the groundwater flows into the tunnel, the porewater pressure decreases mainly around the tunnel in the beginning, and the influence range gradually increases to the upper part and groundwater level drawdown occurs. In the numerical model, since the ground is assumed to be a continuum, the increase of effective stress due to porewater pressure reduction and the ground deformation immediately appear as ground subsidence. However, even if large volume decrease occurs around the tunnel in the site, the ground settlement is relatively small because the tunnel depth is around 50m below the ground surface and some volume decrease is compensated by loosening of the dense granular soil above the tunnel.

In other words, it is reasonable to understand that the estimated ground settlement from numerical analysis is the long-term maximum settlement that can occur when no action is taken after the short-tem ground settlement due to excessive groundwater inflow. In this case history, the shield TBM was switched to closed-mode and high

Groundwater Level GL(-) m

Gro

und S

ett

lem

ent

(mm

)

elasticity urethane grout was injected to block groundwater inflow, so that the groundwater level was restored in a short period of time. As a result, it is considered that the long-term maximum ground settlement will not actually occur.

Table 3 shows the estimated maximum settlement, minimum settlement, differential settlement, and angular distortion for adjacent buildings and a subway station from numerical analysis, which occurred as the groundwater level dropped from GL-16m to GL-32m. Based on the numerical analysis results and the field stability inspection, damages found in the structures would not affect the functional performance. The three-dimensional numerical analysis is based on the groundwater level drawdown and inflow rate change for 40 days from the day of occurrence of groundwater level drawdown. Sinse the the groundwater level was restored in a short period of time and the measured ground settlement converged during that period, additional ground settlement is likely to be small. However, if additional groundwater leakage occurs again, there is a possibility of additional behavior in the adjacent structures. Therefore, ground reinforcement grouting will be needed before excavating the geological vulnerable zones including fractured zones and the deep weathered soil section. Also, continuous and appropriate management of field measurement should be conducted during the excavation in order to ensure the stability of the structures.

Table 3 Estimated behavior of foundations of adjacent buildings and subway station from numerical analysis

Structure Elastic modulus of limestone cavity

zone (MPa)

maximum settlement

(mm)

minimum settlement

(mm)

differential settlement

(mm)

angular distortion

BLDG 1

2,300 14.6 1.3 13.3 1/2,765

600 18.7 3.8 14.8 1/2,484

200 27.2 9.5 17.6 1/2,095

BLDG 2

2,300 3.1 2.4 0.7 1/56,000

600 5.0 2.7 2.3 1/17,043

200 8.4 3.0 5.4 1/7,259

Subway Station

2,300 1.2 1.0 0.2 1/50,000

600 2.9 2.6 0.3 1/32,258

200 7.3 7.0 0.3 1/29,412

References Abdellah, W., Mitri, H, Thibodeau, D., and Moreau-Verlaan, L. (2014),"Stability of mine

development intersections – probabilistic analysis approach",Can. Geotech. J.,51, 184-195.

Barla, G. and Barla, M. (2000),"Continuum and discontinuummodelling in tunnel engineering", RudarskoGeoloskoNaftniZbornik, 12(1), 45-57.

Barla, G. (2016),"Applications of numerical methods in tunnelling and underground excavations: Recent trends",In Rock Mechanics and Rock Engineering: From the Past to the Future,29-40, Cappadocia, Turkey, 29-31 August 2016.

Barton, N., Lien, R., and Lunde, J. (1974),"Engineering classification of rock masses for the design of tunnel support",Rock Mechanics6(4), 189-236

Baskerville, C. A. (1994),"Bedrock And Engineering Geologic Maps Of New York County and Parts of Kings and Queens Counties, New York, and Parts of Bergen and Hudson Counties, New Jersey", U. S. Geological Survey, 1994.

Chen, M., Yang, S., Zhang, Y., and Zang, C. (2016),"Analysis of the failure mechanism and support technology for the Dongtan deep coal roadway",Geomechanics& Engineering,11( 3), 401-420.

Cording, E.J., Hendron, A.J. and Deere, D.U. (1971),"Rock engineering for underground caverns",Proc. Symposium on Underground Rock Chambers, Phoenix, Arizona. pp. 567-600. ASCE.

Cording, E.J., and Mahar, J.W. (1974),"The Effect of Natural Geologic Discontinuities on Behavior of Rock in Tunnels",Proceedings of the Rapid Excavation and Tunneling Conference, Las Vegas, Nevada, 1974.

Fuller, T., Short, L. and Merguerian, C. (1999),"Tracing the St. Nicholas Thrust and Cameron’s Line Through the Bronx NYC", Proc. Conf. Long Island Geology, SUNY, 1999.

Hadjigeorgiou, J. and Karampinos, E. (2017),"Use of predictive numerical models in exploring new reinforcement options for mining drives",Tunnelling and Underground Space Technology,67, 27-38.

Hashash, Y.M.A., Cording, E.J. and Oh, J. (2002),"Analysis of shearing of a rock", Int. J. Rock Mech. Min. Sci., 39(8), 945-957.

Oh, J., Cording, E.J., and Moon, T. (2015),"A joint shear model incorporating small-scale and large-scale irregularities",Int J Rock Mech Min Sci.,76, 78-87.

Sanders, J.E., and Merguerian, C. (1997),"Geologic Setting of a Cruise from the Mouth of the East River to the George Washington Bridge, New York Harbor", A Field Trip for Participants of the 36th U.S. Rock Mechanics Symposium, Columbia University, New York, June 29-July 2, 1997.

Shah, A. N., Wang, J., and Samtani, N. C. (1998),"Geological Hazards in the Consideration of Design and Construction Activities of the New York Area",Environmental & Engineering Geoscience, IV(4), 524-533.

Wang, T. and Huan, T. (2014),"Anisotropic Deformation of a Circular Tunnel Excavated in a Rock Mass Containing sets of Ubiquitous Joints: Theory Analysis and Numerical Modeling",Rock Mech Rock Eng.,47(2), 643-657.