Grouting techniques for the unfavorable geological conditions of Xiang’an subsea tunnel in China

Dingli Zhang,Qian Fang,Haicheng Lou

Key Laboratory for Urban Underground Engineering of the Education Ministry,Beijing Jiaotong University,Beijing 100044,China

Grouting techniques for the unfavorable geological conditions of Xiang’an subsea tunnel in China

Dingli Zhang,Qian Fang*,Haicheng Lou

Key Laboratory for Urban Underground Engineering of the Education Ministry,Beijing Jiaotong University,Beijing 100044,China

A R T I C L EI N F O

Article history:

Received 4 May 2014

Received in revised form

16 June 2014

Accepted 3 July 2014

Available online 16 September 2014

Subsea tunnel

One of the major challenges during subsea tunnel construction is to seal the potential water inf l ow.The paper presents a case study of Xiang’an subsea tunnel in Xiamen,the f i rst subsea tunnel in China.During its construction,different grades of weathered geomaterials were encountered,which was the challenging issue for this project.To deal with these unfavorable geological conditions,grouting was adopted as an important measure for ground treatment.The grouting mechanism is f i rst illustrated by introducing a typical grouting process.Then the site-specif i c grouting techniques employed in the Xiang’an subsea tunnel are elaborated.By using this ground reinforcement technique,the tunneling safety of the Xiang’an subsea tunnel was guaranteed.

?2014 Institute of Rock and Soil Mechanics,Chinese Academy of Sciences.Production and hosting by Elsevier B.V.All rights reserved.

1.Introduction

The geological conditions of subsea tunnels are ratherdiff i cult to accurately explore because of the close proximity of an inf i nite amount of seawater.Therefore subsea tunnels are greatly affected by the geological uncertainties and are more risky than most other tunneling projects(Palmstr?m,1994;Chen et al.,2013).During construction of the Seikan tunnel in Japan,the longest and deepest operational rail tunnel in the world,four disastrous water inf l ow accidents occurred.These accidents caused serious construction delays,enormous f i nancial losses and 34 fatalities(Anderson and Roskrow,2003).The excavation of the Vardo subsea tunnel in Norway was performed through some very unstable faults.At one of these faults,sliding took place and started to develop upwards towards the sea bed(Palmstr?m,1982).During construction of the Oslofjord subsea tunnel in Norway,an unexpected 15—20 m wide cleft f i lled with gravel material was detected,only 15 m in front of the tunnel cutting face.Tremendous efforts were made to deal with this unfavorable condition(Berggren,2000).In this paper,a case study of the Xiang’an subsea tunnel in Xiamen,China is presented. During its construction,different grades of weathered rocks were encountered.Grouting techniques were employed to address these unfavorable geological conditions.The grouting mechanism is f i rst illustrated by introducing a typical grouting process and specif i c grouting measures adopted during the subsea tunneling are elaborated.

2.Grouting mechanism

Grouting is critically important in tunnel engineering.The compaction grouting mode or hydro-fracture grouting mode can be used depending on the local geological conditions of the given project(Soga et al.,2004).To date,the majority of grouting applications have adopted the soil fracture technique(Essler et al., 2000).However,as grouting is a complex process,both permeation grouting and compaction grouting can be observed in a fracture grouting dominated process.Many factors may affect the eventual grouting modes used,such as the grout material,grouting pressure,soil type,and stress state of the ground.A typical relationship between the grouting pressure and grouting rate during the grouting process in a loose soil is shown in Fig.1.A conceptual model of grouting,which consists of the following f i ve stages,can be built up accordingly.

(1)Stage 1:backf i ll grouting and permeation grouting

A grouting pipe is f i rst f i lled with pressurized grout,and the grout enters the ground from holes preset along the pipe.The injected grout f i lls the cavities in the loose soil;this is referred to as backf i ll grouting.Meanwhile,the injected grout may also permeate the voids between the soil particles,which is referred to as permeation grouting(Nonveiller,1989).Due to the effects of backf i ll grouting and permeation grouting,the ground permeabilityis reduced and the ground is thus strengthened and stiffened.At this stage,the grouting pressure and the grouting rate can be reduced gradually.Generally,the duration of this stage is very short,lasting for only tens of seconds.

Fig.1.Grouting pressure and grouting rate during a typical grouting process in a loose soil.

(2)Stage 2:compaction grouting

The continuously injected grout expands the soil outwards and forms a grout bulb around the pipe.With the increase in the grouting pressure,the size of the grout bulb increases until the grouting pressure accumulates up to the fracturing pressure.The formation of the grout bulb displaces and compacts the surrounding soil(Wang et al.,2010).At this stage,the grouting pressure increases toa peak value and the groutingrate drops gradually.

(3)Stage 3:primary fracture grouting

Fig.2.Mechanism of grouting fracture initiation.

Fig.3.Layout of Xiang’an subsea tunnel.

When the grouting pressure reaches the f i rst peak value,a plane of fracture is formed in the soil by hydraulic fracturing(Goth?ll and Stille,2009).Assuming that grouting is performed in an isotropic elastic medium,the fracture plane is theoretically perpendicular to the secondary principal plane along the longitudinal direction of the pipe,considering that the f i rst principal plane is horizontal (Fig.2).The primary fracture is initiated by the tensile stress.Due to the grouting pressure inside the borehole,tensile stress occurs inthe tangential direction in the ground around the borehole.The maximum tensile stresses are located at two points in a crosssection of the borehole(points a and b in Fig.2)where the original tangential stresses are the lowest in compression(or the highest in tension)after borehole drilling.The initiation of the fracture changes the boundary conditions abruptly owing to the grouting effect and the injection pressure drops dramatically. Although the grouting pressure is reduced after fracture initiation, the fracture develops very fast due to the stress concentration at the tip of the fracture.The surrounding ground suffers continuous splitting until the pressure around the fracture tip is not high enough to split the ground any further.Meanwhile,with the development of the fracture,additional grout is required to f i ll the initiated fracture,leading to an incremental increase in the grouting rate.

Table 1Properties of different rock types in the project.

(4)Stages 4 and 5:secondary compaction grouting and secondary fracture grouting

When the grout f l ow is conf i ned,the grouting pressure starts to increase.With the increase in the grouting pressure,the injected grout can enlarge the width of the primary fracture.When the grouting pressure increases to a certain value,a secondary fracture may be initiated and then f i lled.More secondary fractures may be initiated due tothe subsequent grouting.The initiation location andpropagationpathofthesecondarygroutingfracturesvary signif i cantly.

Fig.5.Samples collected from a borehole in the weathered slot F1.

It should be noted that not all grouting applications necessarily consist of all the above-mentioned f i ve stages.For example,the permeation grouting effect in clay is very limited due to the low permeability of clay.The primary fracture and especially the secondary fracture do not always appear during a grouting process.In a subsea tunnel project,multiple boreholes are drilled for grouting in unfavorable geological conditions.In this case,the grouting serves as a method to strengthen the ground as well as to reduce the ground permeability.

Fig.6.Geological prof i le of weathered slot F1.

3.Project overview

The Xiang’an subsea tunnel is the f i rst subsea tunnel constructed in Mainland China.It connects Xiamen Island withXiang’an District,reducing the travel time by road from about 95 min to 15 min.This triple-tube tunnel project has two vehicular tunnels with an excavation cross-sectional area of 170 m2each,and one smaller-diameter service tunnel.Each of the vehicular tunnels serves as a three-lane one-way traff i c route.The total length of each tunnel is about 6.05 km,including a 4.2 km part under the seabed. The lowest point is 70 m below sea level.The drill-and-blast method was adopted for tunnel construction.The layout of this project is shown in Fig.3.The northernmost vehicular tunnel is referred to as the left line and the southernmost as the right line.

Table 2Properties of different rock types in the weathered slot F1.

Fig.7.Construction process and support parameters.

The undersea tunneling of this project encountered four weathered granite slots,naming F1,F4,F2 and F3 respectively,in the direction from Xiamen to Xiang’an.These types of unfavorable geological conditions could lead to sudden water inrush and were the most challenging parts in this project.The longitudinal geological prof i les of the left and the right lines are shown in Fig.4. The specif i c weight and saturated uniaxial compressive strength (UCS)of some typical rock samples are shown in Table 1.In thispaper,the grouting measures adopted for the treatment of the weathered slot F1 along the right line are outlined.

Fig.8.CRD excavation method.

Fig.9.Grouting and excavation process of weathered slot F1(unit:cm).

The weathered slot F1 is located from YK8+324 to YK8+460 along the right line with a total length of about 136 m.It is about 1756 m away from the tunnel portal on the Xiamen side.In this section,the overburden thickness above the tunnel crown to the sea bed varies from 30 m to 35 m,and the sea depth ranges from 10 m to 18 m.Samples collected at various depths from the same borehole before construction of the tunnel are shown in Fig.5.Both completely weathered and highly weathered granite were identifi ed.A probe drilling method in conjunction with the tunnel seismic prediction(TSP)method was adopted when the cutting face of the tunnel approached the weathered slot F1 identi fi ed by the previous geological site investigation.It was revealed that the weathered slot F1,a bowl-shaped slot,was mainly composed of weathered granite,weathered monzonite and weathered diabase. These weathered rocks are classi fi ed as completely weathered, highly weathered and slightly weathered(Fig.6).The typical physico-mechanical properties of different rock samples of the weathered slot F1 are shown in Table 2.

A center cross diaphragm(CRD)sequential excavation method was adopted for this project when the tunnel construction was performed in the unfavorable geological conditions under sea.The construction process and support parameters are shown in Fig.7.A picture of the CRD excavation method taken during construction is shown in Fig.8.

4.Ground grouting for subsea tunneling through the weathered slot F1

4.1.Construction process for weathered slot F1

During tunnel construction through the weathered slot F1,the CRD excavation method was employed.Full-face grouting was adopted to reinforce the ground before excavation.The construction process for the weathered slot F1 along the right line consisted of four rounds.The f i rst two and f i nal rounds of tunneling were mainly performed through completely and highly weathered granite formations.The third roundwas excavated through the core of the weathered slot,where the geological conditions were much worse.The construction process is illustrated as follows(Fig.9):

(1)The f i rst round of excavation started on 20 June 2007.During this round,a total length of 25 m of rock was full-face reinforced by grouting,and then a 20 m long tunnel was excavated and supported.

Fig.11.Marithan injection samples.

(2)The second round of excavation started on 22 October 2007 with a total length of 30 m of rock full-face reinforced by grouting.Subsequently a 25 m long tunnel was excavated and supported.

(3)The third round of excavation began on 21 March 2008.At this time,40 m of the rock was full-face reinforced by grouting,and a 35 m long tunnel was excavated and supported.

Fig.10.Layouts of grouting boreholes.

Table 3Laboratory test results of some ordinary CS grout mixes.

(4)The fourth round of excavation started on 18 July 2008;41 m of rock was full-face reinforced by grouting,and then the weathered slot F1 was excavated through with support.

It should be noted that in the f i rst two rounds the full-face grouting was performed through the upper bench and lower bench,respectively,where the upper bench of the tunnel was f i rst grouted and excavated,followed by the lower bench.In order to improve the construction eff i ciency,grouting was only performed through the upper bench in the f i nal two rounds,during which the full-face ahead of thetunnel cutting facewasreinforced bygrouting from the upper bench f i rst and then the ground was excavated sequentially.The layout of the grouting boreholes in the second round and the third round is shown in Fig.10.A total of 237 and 216 grouting boreholes were drilled in the f i rst two rounds and f i nal two rounds,respectively.

4.2.Grouting techniques used for the weathered slot F1

A chemical grouting product called Marithan and a grout mixture composed of ordinary Portland cement(P.O 42.5R)and sodium silicate(Na2O·3SiO2,30—45 Baume degrees)were selected for injection when tunneling through most sections of the weathered slot F1.Marithan is a two component polyurethane product, which has high adhesive strength and outstanding mechanical properties.It can create a bond with the rock and can remain intact through the lifetime of the project.When the product is injected into the ground,the low-viscosity mixture remains liquid for several seconds and penetrates easily into the f i ssures,where it expands,sets and seals the threaded zone.Some Marithan injection samplesobtained afterexcavation are shown in Fig.11.The Portland cement-sodium silicate grout(CS grout)is a type of dualcomponent grout,which has the properties of early strength, early setting time,low bleeding and low shrinkage.Moreover,it is cost-effective,and its setting time is controllable.The initial setting time and UCS of some typical CS grout mix designs are shown in Table 3.Generally,the setting time of this dual-component grout increases with increasing water-cement ratio and decreases when the cement-sodium silicate ratio increases.The grout strength increases rapidly at f i rst,and then slows as time elapses.The grout strength increases with a decrease in thewater-cement ratio and an increase in the cement-sodium silicate ratio.The optimum grout mix design should be adaptively adjusted according to site-specif i c conditions based on the laboratory test results.During a typical grouting process,the CS grout was f i rst injected into the ground using a double packer that is movable within a sleeve pipe known as a tube à manchette(TAM)from the base of the borehole to the surface.The Marithan product was injected to the end of the borehole(Fig.12).The CS grout was injected toreinforce the groundand reduce its permeability.The Marithan injection mainly served to solve the water inf l ow problem.

Fig.12.Grouting process in a borehole.

Fig.13.Grouting effects in water sealing.

4.3.Evaluation of grouting effects

Grouting mainlyserves toimprove thewater-tightness,strength and stability of the surrounding ground when it is used in a subsea tunneling project.These three effects are extremely important in guaranteeing the safety of the tunnel construction.The grouting effects of the Xiang’an subsea tunneling through theweathered slot F1 are elaborated below.

A total of 22 inspection boreholes were drilled after the third round of grouting was performed in the weathered slot F1.These boreholes were mainly used to determine water inf l ow after grouting,two of which were also selected to provide core samples. Due to the grouting reinforcement,the maximum water inf l ow from a single borehole was reduced from 50 m3/h before grouting to 0.6 m3/h after grouting(Fig.13).The subsequent tunneling could then be performed under dry or low leakage conditions.

Some representative mechanical properties of the rock samples obtained before and after grouting are shown inTable 4.We can see that the mechanical properties of the surrounding ground are effectively improved due to grouting.

Because grouting was carried out through multiple boreholes, the ground was fractured and the grout propagated into the induced cracks(or the existing discontinuities were f i lled by grouting).Therefore,both the integrity and stability of the surrounding ground were signif i cantly improved.The potential cavities during construction could be effectively controlled.Some photographs of the grouting fractures taken after excavation are shown in Fig.14.

Fig.14.Grouting fractures after excavation.

5.Conclusions

This paper presents a case study to illustrate the grouting techniques adopted in the Xiang’an subsea tunnel construction in China.First,a conceptual model of grouting is proposed to introduce the f i ve stages of a typical grouting process,i.e.backf i ll and permeationgrouting,compactiongrouting,primaryfracture grouting,secondary compaction grouting,and secondary fracture grouting.Then,the grouting techniques that were used during the construction of the Xiang’an subsea tunnel in highly weathered rock under the seabed are introduced.A cement grouting was used to reinforce and seal the rock,and the permeability was further reduced by Marithan injection.This reinforcement technique signif i cantly reduced the permeability,strengthened the ground and guaranteed stability of the excavation.It is noted that due tothe inherent uncertainties of the geotechnical and geological conditions and the complex interactions between the ground and grouting material,the grouting parameters(e.g.grouting pressure, grouting material,and the amount of grouting)employed during construction were mainly determined from f i eld tests.Some further studies should be performed to facilitate the determination of the grouting parameters.

Table 4Properties of rock samples before and after grouting.

Conf l ict of interest

The authors wish to conf i rm that there are no known conf l icts of interest associated with this publication and there has been no signif i cant f i nancial support for this work that could have inf l uenced its outcome.

Acknowledgments

The authors gratefully acknowledge the f i nancial support given by the State Key Program of National Natural Science of China (Grant No.51134001)and the Fundamental Research Funds for the Central Universities of China(Grant No.2012JBM081).

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Dingli Zhang obtained the Bachelor degree and M.Sc. degree from Shandong University of Science and Technology in 1985 and 1988,respectively;and his Ph.D.in Mining Engineering at the China University of Mining and Technology in 1995.After that he got the post doctoral title on Mining Engineering at the same university in 1998.In 2004,he got an Associate Professor position in the China University of Mining and Technology.He is now a Professor and the Head in School of Civil Engineering in Beijing Jiaotong University.He has worked with large engineering fi rms dedicated to tunnel services and design.His research interests cover risk management for urban tunnel projects, structural analysis of rock-support interaction,and subsea tunnel related topics.He is the chief scientist of National Key Basic Research Development Program(also known as“973 Program”)and the head of High Technology Research and Development Program(also known as“863 Program”). He has published over 100 academic papers and is the author of four monographs and four textbooks.He has got eight national invention patents.He has been invited for more than twenty keynotes both at home and abroad.

*Corresponding author.Tel.:+86 10 51688115.

E-mail address:qfang@bjtu.edu.cn(Q.Fang).

Peer review under responsibility of Institute of Rock and Soil Mechanics,Chinese Academy of Sciences.

1674-7755?2014 Institute of Rock and Soil Mechanics,Chinese Academy of Sciences.Production and hosting by Elsevier B.V.All rights reserved.

http://dx.doi.org/10.1016/j.jrmge.2014.07.005

Weathered rocks

Grouting

Water inf l ow