JP2936100B2 - Gas detection method and device for shield excavation - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas detecting method and a gas detecting apparatus for excavating a muddy water or mud pressure shield used for excavating a shield tunnel.

[0002]

2. Description of the Related Art During excavation work on shield tunnels, there have been cases in the past where free gas accumulated in the stratum suddenly erupted, leading to serious disasters. In the shield method, the importance of combustible gas detection systems and It is strongly pointed out that there is a need to establish a method for predicting the gas content of the ground in front of the shield in advance.

In the mud shield construction method, gas components are extracted in real time and continuously from the circulating mud during excavation,
Prototype experiments have been conducted on a method and an apparatus for quantitatively detecting the formation gas content, and a patent is pending (Japanese Patent Application No. 7-208).
No. 91, unpublished). According to this apparatus, it has been confirmed that the quantitative evaluation of the gas content of the ground and the advance prediction of the high gas concentration formation are possible.

[0004]

However, the apparatus for which a patent is currently applied is applicable only to the muddy water shield method, and cannot be applied to the mud pressure shield method, which accounts for a majority of the shield method.

[0005] That is, in the case of the mud pressure shield construction method, water and other additives are added to soil excavated from the ground to form mud, and the soil is discharged by earth pressure or a screw conveyor. Since there is no fluidity, it is not possible to adopt a method of attaching a gas trap device to the muddy water pipe and directly detecting the dissolved gas amount from the wastewater flow as in the invention of the patent application.

An object of the present invention is to provide a highly accurate gas detection method and apparatus which can be applied to both the muddy water shield method and the mud pressure shield method by a novel method.

[0007]

In order to solve the above-mentioned problems, a gas detection method according to the present invention is characterized in that in a shield excavation method, earth and sand excavated by a rotary excavation cutter are taken into a mud sampling cylinder, The mud sample extruded from the sampling cylinder is led to a gas extraction cylinder or gas extraction means such as a gas trap cylinder,
In this gas extraction means, the concentration of the gas and air extracted from the mud sample containing hot water is measured by a gas analyzer, and the excavation volume per unit time (M ³ / min) and the mud It is characterized in that the addition amount per unit time (M ³ / min) of the aqueous solution added to the excavation is measured, and the gas concentration in the excavation layer is measured from the following relational expressions (1) and (2). Formation gas concentration (%) × geological drilling volume (M ³ / min) = mud gas concentration (%) × [solution amount (M ³ / min) + geological drilling volume ^{(M 3 / min)] ...} (1) Measurement Gas concentration (%) × mixed gas amount (M ³ ) = mud gas concentration (%) × amount of mud sample (M ³ ) (2) Further, the gas detector of the present invention is a rotary excavator for a shield excavator. A gas extraction cylinder or gas that can take a mud sample excavated from the soil and a mud sample extruded from the cylinder, take a mud sample extruded from the sample cylinder, supply hot water and air, and create a mixed gas by reducing the internal pressure. It is characterized in that it comprises a gas extracting means comprising a strap or the like, and a gas analyzer for measuring the concentration of a fixed amount of mixed gas supplied from the gas extracting means. The gas extraction cylinder is connected to the mud sampling cylinder via a valve via a valve.The gas extraction cylinder is provided with a hot water pipe and an air pipe, and the gas extraction cylinder is made by depressurizing with a gas extraction piston. It is preferable to connect to a gas analyzer for measuring the concentration of the mixed gas via a gas pipe. A mud suction pipe and a mud discharge pipe are respectively connected to the mud sampling cylinder via a three-way ball valve, a hot water pipe is connected to the three-way ball valve on the discharge pipe side, a three-way ball valve on the suction pipe side, and a sample. Between the sampling cylinder and the suction port connected to the sampling cylinder, a circulation loop was formed in which the gas trap cylinder provided with air piping and a circulation pump were interposed, and the gas trap cylinder was formed inside the loop. It is preferable to connect to a gas analyzer for measuring the concentration of the mixed gas via a gas pipe.

According to the present invention, for example, shield excavation
While traveling a meter, a mud sample in the excavation layer was collected several times, a gas in the sample was quickly extracted to form a mixed gas with air, and the gas concentration was measured with a gas analyzer. The gas concentration in the excavation layer can be measured and detected with high accuracy using the equation (2).

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below. In the shield method to which the present invention is applied, the formation gas observed during excavation is mainly methane gas, and this methane gas exists in a state of being dissolved in water in the formation, and its gas component is mud together with crushed earth and sand. Mixed in.

Therefore, in the present invention, excavated earth and sand excavated at the front portion of the excavator, free gas is expelled from the collected earth and sand (mud soil sample), and the gas is analyzed, and the gas concentration is analyzed by a gas analyzer. By doing so, the gas concentration in the excavation layer is detected.

The principle of implementing the above method will be described. As shown in FIG. 3, the solubility of methane gas in pure water is:
It tends to increase with increasing pressure and decrease with increasing temperature. For example, if the pressure around 20 meters underground is 3 atm and the temperature is 15 ° C, the solubility under that condition is
It is about 0.1 in terms of volume ratio in terms of atmospheric pressure.

From the above, if the volume ratio of water in the stratum is assumed to be 40%, under the same conditions (that is, at a pressure of 3 atm and a temperature of 15 ° C.), the volume under the atmospheric pressure of the gas concentration contained in the stratum is assumed. The ratio corresponds to 4%, above which the supersaturated gas components are released from the water to create free gas in the formation.

FIG. 3 shows that depressurization and heating are very effective for completely removing and quantifying gas components mixed in a mud sample, which is performed as one step of the present invention. It is easily understood from.

In the present invention, the formation gas concentration is measured from the gas concentration analysis value in the mud sample. In order to accurately determine the formation gas concentration from the gas concentration analysis value in the mud sample, Geological excavation volume per unit time (M ³ / min)
In addition, it is necessary to accurately measure the addition amount (M ³ / min) of the aqueous solution added to the mud per unit time in advance. Here, the following relational expression is established. Formation gas concentration (%) × geological drilling volume (M ³ / min) = mud gas concentration (%) × [solution amount (M ³ / min) + geological drilling volume ^{(M 3 / min)] ...} (1)

In the analysis of the gas concentration in the mud, the mud sample is collected in a state where the stratum pressure is applied in order to prevent gas emission in the mud sample. It is important to pay attention to the amount and the amount of the sample collected.

The collected mud sample is diluted with hot water so that gas is easily released. Further, in order to promote the liberation of gas, it is necessary to take measures such as decompression, heating and stirring.

The gas released from the mud is mixed with air and diluted to form a mixed gas. In this case, it is important to keep the mixed gas amount constant regardless of the amount of free gas. The gas concentration of the produced mixed gas is measured by an analyzer such as a gas chromatograph.

The following relational expression holds between the measured gas concentration value and the mud gas concentration. Measurement gas concentration (%) x
Mixed gas amount (M ³ ) = mud gas concentration (%) × amount of mud sample (M ³ ) (2)

Therefore, by measuring the concentration of the mixed gas of gas and air in the collected mud sample, the formation gas concentration can be obtained by using the relational expressions (1) and (2).

A description will be given with reference to FIGS. In FIG. 4, a standing hole 2 is opened to a predetermined depth from the ground surface 1, and a shield excavator 3 is carried into the ground through the standing hole 2, and the shield excavator 3 is moved laterally from a lower portion of the standing hole 2. The propulsion excavates the shield tunnel 4 by the mud pressure method.

The body of the shield machine is known, and the ground is excavated by a rotary cutter 7 rotating about a shaft 6 at the front side of a front partition wall 5 (shown in FIGS. 2 and 3), and a jack propulsion at the rear is performed. The shield excavator 3 is propelled. Then, a steel or reinforced concrete segment assembly lining is completed behind the shield machine 3.

At the front end of the shield machine 3, a water supply channel 10 and a mud discharge channel 11 led from a mud treatment device 8 installed on the ground surface 1 are guided, and the ground is turned into mud and turned. Excavate with the cutter 7. 86 is an excavation control and monitoring device, and 87 is a data recording device.

In the shield machine 3, the gas detection device 12, which is a main part of the present invention, is disposed behind the front partition 5.

The first embodiment of the present invention will be described with reference to FIG. 1. The gas detecting device 12 comprises a sampling device 14 and a gas analyzing device 15.

The sampling device 14 has a sampling cylinder 16, a gas extraction cylinder 17, and a hot water injection cylinder 18 as main components. A suction cylinder 20 connected to the tip of the sample collection cylinder 16 via a first valve 19 passes through an opening 21 formed in the front partition wall 5 from the rear to the front, and a sample suction port 22 at the tip is connected to a shield excavator. It is located between a rotary cutter (not shown) and the front partition 5,
A mounting plate 23 supporting the suction tube 20 is fixed to the rear side of the front partition 5.

By pulling the piston rod 24 of the sampling cylinder 16 and retreating the sampling piston 25, the mud excavated by the shield excavator is removed from the sample inlet 22.
About 200 cc inside the sampling cylinder 16.
It is inhaled and collected as a mud sample 27. The piston rod 24 is provided with a position detection sensor 26 for that purpose, and the amount of sample taken is measured by the amount of movement of the piston rod 24. The piston rod 24 is provided with a load sensor 28 for monitoring the suction force.

The sampling cylinder 16 has a second valve 29.
Of the mud sample 27 in the sampling cylinder 16 by closing the first valve 19, opening the second valve 29 and pushing the piston rod 24 , Exactly 100cc.
Is extruded into a gas extraction cylinder 17. Here, the gas extraction piston 30 is a piston that requires a particularly precise operation. The gas extraction piston 30 is provided on the piston rod 31 to accurately detect the suction of the mud sample 27 of 100 cc. A sensor 32 is provided.

The distal end of the hot water injection cylinder 18 is connected to a gas extraction cylinder 1 by a pipe 34 through a third valve 33.
7 is connected. This hot water injection cylinder 18
100 cc of hot water of about 80 ° C. sucked from a hot water tank 37 through a pipe 36 via a fourth valve 35 is injected from a nozzle 38 to the mud sample 27 in the gas extraction cylinder 17 to dilute and simultaneously stir. At this time, the piston rod 31 of the gas extraction piston 30 has a mud sample of 100 cc.
200 indicating that a total of 100 cc.
Move to the cc. scale position.

Next, the gas extraction piston 30 performs a forced decompression suction operation in the gas extraction cylinder 17 to give a vacuum degassing effect to the mud sample inside the cylinder, and holds the state for about 3 minutes. . At this time, in order to enhance the stirring effect between the mud sample 27 and the hot water in the gas extraction cylinder 17, vibration may be applied from outside the cylinder using, for example, an ultrasonic vibrator (not shown).

After a lapse of 3 minutes, the piston rod 31 of the gas extraction piston 30 stops the operation of forced decompression suction in the gas extraction cylinder 17 and then the fifth valve 40 for air injection.
Is opened to supply air into the gas extraction cylinder 17, and the air and the reduced pressure in the cylinder are used to dilute the gas in the gas extraction cylinder 17 and move on to the operation of producing a mixed gas. The fifth valve 40 for air injection is closed until the amount of the mixed gas reaches 10 cc., That is, when the piston rod 31 reaches the position of the scale of 210 cc.

The gas extraction cylinder 17 has a sixth valve 4
1, a water tank 43 is connected by a pipe 42,
Further, a gas analyzer 15 composed of a gas chromatograph is connected by a pipe 45 via a seventh valve 44. The hot water injection cylinder 18 has an injection piston 48, a piston rod 82, and a position detection sensor 83, and is connected to the water tank 43 by a pipe 47 via an eighth valve 46.

Then, the sixth valve 41 is opened and the water tank 43 is opened.
When water is injected into the gas extraction cylinder 17 from the cylinder, the mixed gas stored in the cylinder due to the pressure is discharged to the seventh position.
The gas analyzer 15 is connected via a pipe 45 with the valve 44 opened.
The gas concentration is measured by the gas analyzer 15. The supply of the mixed gas to the gas analyzer 15 can be replaced by a method of gently pushing down the gas extraction piston 30.

The mixed gas amount required for gas analysis is about 2 cc. However, it is desirable that the length of the pipe 45 reaching the gas analyzer 15 is short, and if it is long, the mixed gas amount of 10 cc is further increased. Need to be done. The amounts of mud samples and gases mentioned above are only reference numbers and can be changed.

When the mixed gas stored in the gas extraction cylinder 17 has been discharged, the mud sample 27 remaining in the sampling cylinder 16 and the gas extraction cylinder 17 is removed by the operation of each of the pistons 25 and 30 to the front of the shield excavator. It is discharged into the mud in front of the partition 5.

Each piston 2 after discharging the mud sample 27
It is necessary to clean the insides of 5, 30 in preparation for the next suction of mud sample 27. For this reason, water is supplied from the water tank 43 into the hot water injection cylinder 18, water is injected from the nozzle 38 to wash the inside of the gas extraction cylinder 17, and then the gas extraction piston 30 is operated to operate the gas extraction cylinder 17. The water is transferred into the sampling cylinder 16 and discharged out of the front bulkhead 5 by the operation of the sampling piston 25. If necessary, the above operation is repeated again to sufficiently clean both pistons 25 and 30.

The first valve 19 and the second valve 29 are hydraulically operated ball valves.
Necessary opening and closing operations are performed in accordance with inhalation and release of the mud sample 27 into the sections 5 and 30. Third valve 33, fourth valve 35, fifth valve 40, sixth valve 41, seventh valve 4
Each of the fourth and eighth valves 46 is an electromagnetic valve, and each valve opens and closes a supply pipe of air, gas, and water as described above.

In the gas concentration measurement, the time required from one sampling of mud to one gas analysis is about 5 minutes. Although formation gas analysis can be performed several times per meter of shield excavation length, taking the average value of gas analysis values of two or three times from the viewpoint of "prior prediction" sufficiently satisfies the purpose. Of course,
The number of analyzes per hour can be pre-programmed.

In the present invention, the sampling piston 25,
All piston operations of the gas extraction piston 30, the hot water injection piston 48, and the first to eighth valves 19, 29,
Opening and closing operations of ...
Controlled by a control program, all processes are monitored by sensors (not shown) attached to each unit. If an abnormality occurs, the process stops at that location, notifies the abnormality, and performs a recovery operation in some cases.

As an example, the role of the load sensor 28 mounted on the piston rod 24 for sampling will be described. Gravel, small gravel, sand, etc. are mixed in the mud, and the suction port 22 of the mud sample 27 has a screen for preventing suction of gravel, etc. When the opening of the piston rod 24 becomes insufficient due to the closing of the valve or the intrusion of the sample collection cylinder 16 and the sliding of the ball valves as the first and second valves 19 and 29, the opening of the valves becomes insufficient. Abnormality occurs in the load. In this case, the abnormality of the piston rod 24 is detected by the load sensor 28, the valve is opened again as an operation outside the process, and each piston repeats the operation of discharging and sucking the mud to try to remove foreign substances, and the abnormality of the load is eliminated. In this case, it is controlled to return to the original process. Known means may be adopted for this configuration, and detailed description will be omitted.

In the illustrated example, the sampling device 14
Are shown in a horizontal and vertical arrangement, but the arrangement of these cylinders may be changed in any direction.

Next, FIG. 2 shows a second embodiment of the present invention. In the second embodiment, the device is reduced in size and weight by replacing two of the three cylinders in the first embodiment with a small pump, and in place of the gas extraction cylinder 17. The major difference is that the use of the small volume gas trap cylinder 57 changes the method of extracting the gas component while circulating the mud liquid diluted with warm water by the circulation pump 51.

Referring to FIG. 2, the sampling cylinder 56 has a suction pipe 49 and a discharge pipe 50 for the mud sample 27.
Are independently provided and connected to the sampling cylinder 56 via first and second three-way ball valves 52 and 53, respectively. The discharge port 55 penetrates the front partition wall 5 of the shield machine 3 and is located on the front side thereof.

When collecting the mud sample 27, the first three-way ball valve 52 is opened, the sampling piston 58 is operated, and the 200 cc mud sample 27 is
Then, the first three-way ball valve 52 is closed, the second three-way ball valve 53 is opened, and the entire amount is once discharged. Then, 100 cc. Is collected again. The purpose of this operation is to discard the remaining mud in the suction pipe 49 at the time of the previous gas concentration measurement and to collect a fresh mud sample 27.

When the collection of the mud sample 27 is completed, the two three-way ball valves 52 and 53 are switched to cut off the communication with the external mud side, and then the third valve 60 and the fourth valve 61 on the hot water injection pump 59 side are opened. Then, hot water is pumped from the hot water tank 62 into the sampling cylinder 56. The hot water injection volume is
It is measured by the amount of movement of the sampling piston 58 by the piston position zero detection sensor 63, and is about 100 cc.

When the injection of the hot water into the sampling cylinder 56 is completed, the second three-way ball valve 53 and the third valve 60 on the side of the hot water injection pump 59 are closed.
While switching the first three-way ball valve 52 on the one side,
The fifth valve 64 and the sixth valve 65 are opened.

A circulation loop 70 is formed between the fifth valve 64 and the sixth valve 65 by connecting the circulation pump 51 and pipes 66 and 67 with a gas trap cylinder 57 interposed in the loop. The circulation loop 70 is filled with water.

Next, the circulation pump 51 is operated to circulate the mud sample 27 diluted with warm water in the sampling cylinder 56 in the circulation loop 70. During this time, the sampling piston 58 exerts a forced depressurizing effect on the mud sample 27 in the circulation loop 70.

The gas components in the mud sample 27 are sequentially stored in the upper part of the gas trap cylinder 57 while circulating in the circulation loop 70. Next, the air injection valve 71 is opened and the gas trap cylinder 57 creates a mixed gas of the extracted gas component and air. At this time, the mixed gas amount is adjusted so as to be always constant. The circulation pump 51 is stopped during the process of producing the mixed gas. The circulation pump 51 uses a small-sized slurry pump.

Next, by opening the water valve 72, the gas trap cylinder 57 is filled with water. At this time, by opening the gas discharge valve 73, the mixed gas in the gas trap cylinder 57 is subjected to gas analysis through the pipe 84. Apparatus 1
5, where the concentration of the mixed gas is measured.

When the mixed gas has been discharged from the gas trap cylinder 57, the water valve 85 is opened and the hot water injection pump 59 is operated to send water from the water tank 74 into the sample collection cylinder 56, and the sample collection piston 58 is moved. In operation, the mud sample 27 in the sampling cylinder 56 and the circulation loop 70 is washed with water and discharged out of the front partition 5. After the cleaning, the circulation loop 70 is filled with fresh water.

As described above, also in the second embodiment of the present invention, the mud sample 27 of excavated earth and sand is taken into the sampling cylinder 56 by the shield excavator, and the sampling cylinder 56 and the gas trap cylinder 57 are in the path. The gas is released from the mud sample 27 by circulating in a circulation loop 70 incorporated in the sample while adding hot water, a mixed gas with air is produced in a gas trap cylinder 57, and the mixed gas is supplied to the gas analyzer 15. The gas concentration is measured, and the formation gas concentration in the shield excavation can be quickly and accurately measured by using the relational expressions (1) and (2) described above.

In FIG. 2, the sample collection piston rod 75 has a power cylinder
6, the power cylinder 7 is screwed with the female screw portion of the rotary cylinder 78 rotated by the drive motor 77.
6 driven. In addition, a foreign substance in the mud sample 27 taken into the sample collecting cylinder 56 is detected with such a strength that the rear end of the piston hits the load sensor 80. Further, the position detection linear encoder 81 is provided so that the position of the sampling piston 58 is detected.

[0053]

As described above, according to the present invention,
It is possible to measure the formation gas concentration in real time while excavating underground with a shield excavator and to predict in advance the danger of encountering an abnormal gas concentration formation, greatly contributing to the safety of construction and the safety and health of workers Is what you do. Further, the present invention can be applied to not only the mud type but also the mud type shield excavation, and has the effect of further improving the accuracy of measuring the formation gas concentration in comparison with the conventional device.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a second embodiment of the present invention.

FIG. 3 is a graph showing the solubility of methane gas in pure water.

FIG. 4 is an explanatory diagram showing a place where a gas detection device is installed in a shield tunnel excavation work.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Ground surface 2 Vertical hole 3 Shield excavator 4 Shield tunnel 5 Front partition 6 Shaft 7 Rotating cutter 8 Mud treatment device 10 Water supply channel 11 Mud discharge channel 12 Gas detector 14 Sampling device 15 Gas analyzer 16 Sampling cylinder 17 Gas extraction cylinder 18 Hot Water Injection Cylinder 19 First Valve 20 Suction Cylinder 21 Opening 22 Sample Inlet 23 Mounting Plate 24 Piston Rod 25 Sampling Piston 26 Position Detection Sensor 27 Mud Sample 28 Load Sensor 29 Second Valve 30 Gas Extraction Piston 31 Piston Rod 32 Precision Movement amount detection sensor 33 Third valve 34 Piping 35 Fourth valve 36 Piping 37 Hot water tank 38 Nozzle 39 Hot water 40 Fifth valve 41 Sixth valve 42 Piping 43 Water tank 44 Seventh valve 45 Piping 46 Eighth valve 47 Piping 48 Hot water injection piston 49 Suction pipe 50 Drain pipe 51 Circulation pump 52 First three-way ball valve 53 Second three-way ball valve 54 Inlet 55 Drain 56 Sampling cylinder 57 Gas trap cylinder 58 Sampling piston 59 Hot water injection pump 60 3 valve 61 4th valve 62 Hot water tank 63 Piston position zero detection sensor 64 5th valve 65 6th valve 66 Piping 67 Piping 70 Circulation loop 71 Air injection valve 72 Water valve 73 Gas exhaust valve 74 Water tank 75 Piston rod 76 Power cylinder 77 Drive motor 78 Rotating cylinder 80 Load sensor 81 Position detection linear encoder 82 Piston rod 83 Position detection sensor 84 Piping 85 Water valve 86 Excavation control and monitoring device 87 Data recording device

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yukio Ibaraki 4-1-2605 Honnakayama, Funabashi-shi, Chiba (72) Inventor Risuke Kaneko 3-5-16 Koyama, Shinagawa-ku, Tokyo (72) Inventor Akito Hara 2-27-1 Minamiikuta, Tama-ku, Kanagawa Prefecture (72) Inventor Yukio Harada 1622-12 Sakata, Okegawa-shi, Saitama (72) Inventor Tanehisa Tsuruoka 1-13-4, Hattori-Toyocho, Toyonaka-shi, Osaka ( 72) Inventor Yu Saito 3-13-9, Nishikawaguchi, Kawaguchi City, Saitama Prefecture (72) Inventor Eishiro Mitida 2-2-20 Shioki, Ichikawa City, Chiba Prefecture 2-1007 Gyotoku Heimu (72) Inventor Hirotoshi Ozawa Shinagawa-ku, Tokyo 6-22-2 Toyomachi (56) References JP-A-8-193490 (JP, A) JP-A-7-217375 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) E21D 9/06 301 G01N 1/22 G01N 30/04

Claims (4)

(57) [Claims] In a shield excavation method, soil excavated by a rotary excavator is taken into a mud sampling cylinder, and a mud sample extruded from the sampling cylinder is extracted from a gas extraction cylinder or a gas trap cylinder. In this gas extraction means, the concentration of a fixed mixture of gas and air extracted from a mud sample containing hot water is measured by a gas analyzer, and the excavation volume per unit time (M ³ / min) and
Amount of aqueous solution added to mud per unit time (M ³ / mi
A gas detection method for shield excavation, comprising measuring n) and measuring the gas concentration in the excavation layer from the following relational expressions (1) and (2). Formation gas concentration (%) × geological drilling volume (M ³ / min) = mud gas concentration (%) × [solution amount (M ³ / min) + geological drilling volume ^{(M 3 / min)] ...} (1) Measurement gas concentration (%) × mixed gas quantity (M ³⁾ = mud gas concentration (%) × mud amount of sample (M ³⁾ ... (2) 2. A shield excavator, comprising: a cylinder for sampling mud of earth and sand excavated by a rotary excavator;
A gas extraction means comprising a gas extraction cylinder or a gas trap capable of taking a mud sample extruded from a sampling cylinder, supplying hot water and air, and creating a mixed gas by reducing the pressure inside the gas sampling means; And a gas analyzer for measuring the concentration of a fixed amount of a mixed gas supplied from a gas detector. 3. The gas extraction cylinder is connected to the mud sampling cylinder via a valve, and the gas extraction cylinder is provided with a hot water pipe and an air pipe.
The shield excavation according to claim 2, wherein the gas extraction cylinder is connected via a gas pipe to a gas analyzer that measures the concentration of a mixed gas produced by depressurizing the gas with a gas extraction piston. Gas detector. 4. A mud suction pipe and a mud discharge pipe are respectively connected to the mud sampling cylinders via three-way ball valves, a hot water pipe is connected to the three-way ball valve on the discharge pipe side, and a three-way ball on the suction pipe side. Between the ball valve and a suction port connected to the sampling cylinder, a circulation loop is formed in the loop through the gas trap cylinder provided with air piping and a circulation pump. The gas detector for shield excavation according to claim 2, wherein the gas detector is connected via a gas pipe to a gas analyzer for measuring the concentration of a mixed gas produced therein.