Marsh gas in shallow soils and safety measures for
只怕不再遇上tunnel construction
Yi-Qun Tang *,Wei-Min Ye,Yu Huang
Department of Geotechnical Engineering,Tongji University,Shanghai 200092,PR China
Received 9October 2001;accepted 8August 2002
Abstract
Marsh gas is a danger for tunnelling through soils in Shanghai.Predrilling can assess the hazard and help to vent the gas.Additional measures include tunneling with an airproof shield,adequate ventilation,special precautions against ignition and education of the workers.
D 2002Elsevier Science B.V .All rights reserved.
Keywords:Marsh gas;Tunnel safety;Shield;Confluent sewerage works;Shanghai
1.Introduction
The Huangpu River and Suzhou Creek have been polluted for ages,which have affected the daily life of people and economic development in Shanghai.Shanghai Confluent Sewage Project (Phase 1),funded by World Bank and executed by the Chinese govern-ment,is one of the largest basic facilities of the city since its founding.This project is making full use of the old sewer system of the city.Interceptor mains and link sewers were built so that the sewage will be drained directly into the Yangtse River after treatment.The sewage will be diluted into the sea by the large volume of water of the Yangtse River.Thus the pollution of the Huangpu River and Suzhou Creek will be minimal.The use of a shield in tunnel construction in shallow soil layers containing marsh gas is a first in urban construction in China.Tunnel construction
through these conditions is hazardous and should be reported in the literature,so others can learn from this experience.
2.Engineering geological and hydrogeological conditions 2.1.Soil properties
There are three kinds of soil layers along the axial direction of the tunnel construction (Fig.1).The permeability and main physical properties of different layers of soil are listed in Table 1.2.2.Hydrogeological conditions
According to the geological investigation,the mucky clay layer is an aquifuge and both the upper and lower ones are aquifers.The upper layer is of phreatic water,whereas the lower one is a confined
0013-7952/02/$-see front matter D 2002Elsevier Science B.V .All rights reserved.PII:S 0013-7952(02)00207-7
*Corresponding author.
E-mail address:y.huang@163 (Y .Huang).
www.elsevier/locate/enggeo
Engineering Geology 67(2003)373
–378
aquifer.The confined water pressure is about 0.15MPa,the outflow rate 2.97m 3/day and the pumping radius 15.26m.No hydraulic conductivity exists between the two aquifers due to the thick mucky clay aquifuge.As the underlying silty clay layer contains clay particles,it is low permeable and thus contribute to negligible change of the water level.However,this layer is important not only as an aquifer,but also as the main layer containing marsh gas.
3.Cause of formation,composition and distribution of marsh gas
3.1.Cause of formation and composition
The outlet (No.9.1)is located at the mouth of the Huangpu River flowing into the Yangtse River (Fig.2).For several hundred years,the river embankment has been collapsing into the water by river erosion with much reeds,leaves and debris.An archaeological investigation has also proved that some ancient build-ings,at least 500years old,had sunk into the bottom of river due to the changing of the coastline of the Yangtse River.Therefore,an organic enrichment zone has come into being under the water outside Gaoqiao.
The Yangtse River has also brought large quantities of organic materials from upstream.As the flow slows down due to seasonal changes,these organic materials have been deposited at the estuary of
the river and buried into riverbed.At the suitable temperature and pressure,the anaerobic decomposition of these or-ganic materials turned into peat and marsh gas.The main composition of the marsh gas is CH 4and CO 2.When the marsh gas cannot diffuse into the atmos-phere,it will remain and accumulate in the soil and become gas pockets.The mucky clay layer forms an impermeable aquifer for gas and water.The under-lying silty sand with pore space becomes a good gas reservoir.Thus,the area near outlet No.9.1,about 100–1000m from the coastline is rich in marsh gas.Certain amounts of marsh gas has accumulated even in the mucky clay layer with silty sand in-between.Sample tests indicate that the main composition is CH 4,about 87.03–98.7%of the total;the rest is CO 2and N 2,about 1.43–12.97%.3.2.Distribution
In the shallow soil layer at outlet No.9.1and nearby,there are gas pockets in which the quantity of marsh gas is small and with low pressure.The
source
Fig.1.Geology profile and tunnel location.
Table 1
The permeability and other physical properties Layer Type
Depth (m)Field moisture (%)Saturation (%)V oid ratio Permeability (10À6cm/s)4silty clay interbedding silty sand 2–828.0–29.290.7–93.00.84–0.94 5.32–38.35mucky clay intercalating silty sand 9–14.545.1–55.795.7 1.319.556
silty sand
3–5
29.3
89.5
0.87
3.60
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374
of information about the marsh gas in the shallow layer is as follows.
1.The outflow of marsh gas at the sinking of the well No.1.
2.Sudden release of marsh gas during excavation.The marsh gas burns with light blue colour,but the amount is not large.As this layer is above the tunnel,it has little effect on the tunnel construction although it contains a small amount of marsh gas.Therefore,it could be neglected generally.According to the drilling report at No.9.1,marsh gas forms pockets in layers 4,5and 6in the direction of the tunnel axis (Fig.3).
In layer 4,silty clay is interbedded by silty sand,and gas and water were ejected from Bore Z21at the elevation of À10.42m,to a height of about 1.0m with a duration of about 20s.An investigation of marsh gas was made when well No.1was sunk to the layer 3.From 22April to 30May 1991,gas bubbles came out of the lower end of caisson.When it sank to the elevation of À11.0m,a large amou
nt of marsh gas was released at the corner No.2of the caisson.Then about 25m 3of water flowed into the space of the gas pocket,yet it was not full.This suggested that the volume of marsh gas pocket was large.
Layer 5is a mucky clay one intercalated with thin layers of silty sand in which marsh gas was mainly stored.For example,when Bore Z20was drilled to the elevation of À16.4m,gas and water were ejected from the hole to a height of about 5m above the ground.During the drilling of Bore Z83,415m away from well No.1,marsh gas was found at the depth of 19m with a maximum pressure of up to 0.06MPa measured in field.The discharge of gas was about 1.1m 3/h on the average and lasted 25h.
In layer 6of silty sand,marsh gas was found ejecting out with a strong pressure from the elevation of À21.75to À27.45m in 24boreholes.The maximum pressure measured in field was 0.540–0.598MPa,with a maximum height of 15.70m.
An
Fig.2.Plan of the outlet No.
9.1.
Fig.3.Profile of marsh gas distribution.
Y.-Q.Tang et al./Engineering Geology 67(2003)373–378375
observation of gas volume was made for21h in Bore Z5and it was145m3.
天下有情人According to data obtained from boreholes,a zone of gas pockets had been formed in the direction of the tunnel axis in layer5.Moreover,it contained a large amount of marsh gas with strong pressure.It was a serious threat to the safe driving of the shield.
4.Danger to tunnel construction
4.1.Criteria of marsh gas content in the tunnel
Table2(Chinese Ministry of Coal Engineering, 1974)shows the criteria for the content of marsh gas relative to catching a fire exploding in the marsh gas is mixed with air,9.5%CH4is the most tolerant mixture when it explodes.Hence,for safety during tunnel construction,the content of marsh gas should be controlled less than1%by adequate ven-tilation.
4.2.Ways of marsh gas accumulation into the tunnel
There are four ways for marsh gas to enter the tunnel:
1.from the working face
2.from the joint of the knife edge and the shield shell
3.from the clearance at the end of shield
4.from joints in the lining.
The larger the difference of gas pressure between that in the soil and that in the tunnel,the more the marsh gas will enter the tunnel.4.3.Potential hazard of fire and explosion in tunnels
According to data obtained from drilling,layer5is the mucky clay intercalating with silty sand and contains much marsh gas.Its pressure was measured in the field to be up to0.06MPa and it was ejected to a height of15.70m during the first drilling.Its ejecting duration was from several seconds to15 days.Under such a condition,if the lining of the tunnel or the shield is not air tight,the content of marsh gas will soon build-up to5%and an explosion is possible.The marsh gas released from the cutting face of the shield was relatively less dangerous, because it was easier to monitor and safety measures could be taken in time.However,the marsh gas accumulated in the inner part of the tunnel was the most dangerous.Marsh gas is colourless and tasteless, and easy to be neglected usually.
4.4.Observed events of marsh gas during tunnel construction
4.4.1.Case1
Before construction,bore holes were drilled at No.
9.1to let marsh gas vent at regular intervals.During shield driving,penetration had also been made.Yet, accidents still took place.It proved that the marsh gas pocket along and near the axis of the tunnel is very dangerous to tunnel construction in soft soil and great care must be taken(Table3).
4.4.2.Case2
In the construction of another drainage tunnel, marsh gas and water burst into the tunnel through the lining joints when the shield was being driven at the bottom of the Yangtse River.The underlying silty sand was disturbed and became hollow.The tunnel settled down so quickly that it fractured.The marsh gas content was6%.The salvage of the emergency lasted20h,but failed.The accident was a great loss though no person was dead.
5.Safety measures against marsh gas
The following safety measures must be taken for the construction of a tunnel with the shield in soils containing marsh gas.咬耳朵mv
Table2
Dangers of marsh gas(Chinese Ministry of Coal Engineering,1974) Content(%)Condition Remarks
1–4nonexplosive,
flammable
faint blue flame
5–6lower limit
for explosion
From5–6to
14–16
explosive
小莫 騒麦歌曲14–16upper limit
>14–16nonexplosive,胡彦斌 潇湘雨
nonflammable mixed with fresh air,
may catch fire and explode
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5.1.Predrilling to expel marsh gas prior to tunnelling
Vertical boreholes can be drilled in advance to vent marsh gas in the axial direction of shield driving. 5.2.Drilling probe holes ahead during shield driving
During shield driving,probe holes into the face release marsh gas into the tunnel atmosphere with adequate ventilation pipes as shown in Fig.4.
5.3.Airproof measures in the shield tunnel
Airproof measures for the shield and joints can prevent marsh gas from entering the tunnel.Espe-cial
ly,after the tunnel has entered the silty sand layer, grouting at the lining and joints can decrease the soil void and improve the obturation of the tunnel.It can also reduce the differential settlement of the tunnel caused by the release of marsh gas.
5.4.Abundant ventilation
Abundant ventilation is the most important method to reduce marsh gas accumulation in a tunnel.Fresh air may be pumped at the rate of4.0m3/min into the tunnel to minimize the hazard.The specific gravity of marsh gas is approximately0.544and lighter than air. According to a test in Circle305,it took only10min or more for marsh gas to form a layer with a content of2.5%at the top of the tunnel.When the length of this layer was10m or over,it is the most dangerous and quite easy to burn or explode with a small spark. To prevent marsh gas from accumulating in the tunnel,the best way is to increase the flow of air to dilate the gas.
5.5.Marsh gas monitoring inside the tunnel
Special monitoring and testing programme should be implemented to control the marsh gas intrusions. Automatic alarm monitors should be installed at the top of the shield in the tunnel.
5.6.Safety measures adopted during construction 1.All workers should be educated about the dangers
of marsh gas and fire
prevention.
Fig.4.Venting marsh gas in shield driving.
Table3
Tunnel encounters of marsh gas
Date Circle number Danger Emergency measures 5September1992292Marsh gas inburst from the shield end,(1)increase ventilation;
the content was0.65%at the working surface.(2)grouting
20:008September–
6:0010September1992301–310Marsh gas and water entered through a joint and the shield;
the content was2.5%at the working face and increase to
(1)occluding the shield
end by sponges
5.1%on9September.(2)polymer grouting
(3)increase ventilation
(4)emergency lasted32h
15:1028March1993931Marsh gas and water inburst from the shield end;
the content was1%at the working surface;(1)occluding the shield by sponges
the settlement of tunnel was25mm.(2)grouting polymer
(3)enhance aeration
(4)emergency lasted20h
15:006April19931007Marsh gas and water inburst from the shield end;
the content was2.8%at the working surface;(1)occluding the shield by sponges
the settlement of tunnel was56mm.(2)grouting polymer
(3)increase ventilation
(4)emergency lasted9h Y.-Q.Tang et al./Engineering Geology67(2003)373–378377
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