ATS11-02113

Geological evidences of collapse zones in TBM tunneling; a case study of

Ghomroud water conveyance tunnel, IRAN

Mahdi Zolfaghari, Ehsan Mokhtari, Massoud Morsali

Sahel consultant engineers, No. 55, Ardakani street, Resalat highway,Tehran, Iran.

ABSTRACT

There are many factors such as equipments, management, personnel skills and ground condition that affected the TBM

performance and mechanized excavation. The adverse geological condition that encountered in the tunnel is one of the most

important parameters that affect the excavation process. Nature of the adverse geological conditions and fuzziness of them

cause to decrease the accuracy of their prediction. It seems there are some evidences that can lead us to detect the

problematic zones more exactly. To research the role of the geologic evidences in the collapse zones detection, the data

gathered from a water conveyance tunnel excavated in central Iran were considered and analyzed. The rock formations

along the tunnel path consist of metamorphic and sedimentary rocks aged from Jurassic to cretaceous. During the tunnel

excavation the adverse geological conditions several times cause to collapse of tunnel and subsequently sticking of TBM.

The parameters such as quartz content, fragment size and maximum fragment size of cuttings and amount of injected pea

gravel behind the lining were monitored during the excavation, especially in collapse zones. The mentioned parameters

have a variable rate along the tunnel path and these variations depend on the geologic condition. Quartz content of cutting

materials in the collapse zones are higher than surrounding ground of these zones and the fragment size and maximum size

of fragments in the collapse zones show an increasing trend relative to the normal condition of ground. Also, the injected

pea gravel in collapses decreases in respect of other parts of tunnel. The results of this study show that the monitoring of

variation in some geological parameters such as the amount of secondary minerals in cutting materials and the size of

cutting fragments, also the amount of injected pea behind the lining of tunnel can help us to better prediction of collapse

zones in the metamorphic rocks.

KEY WORDS:

TBM,collapsezone,quartzcontent,fragmentsize,cuttings.

1. INTRODUCTION

Mechanical excavation especially excavation with TBM

has many advantages over conventional drill and blast

methods. These advantages include lower cost and higher

advance rate than drill and blast excavation in most cases,

improved safety, minimal ground disturbances, elimination of

blast vibration, reduced ventilation requirements etc.

There are many factors such as equipments, management,

personnel skills and ground condition that affected the TBM

performance and mechanized excavation. Ground condition or

geology is one of the most important affecting factors in

mechanized excavation.

The effect of geologic condition on mechanized tunneling

can be grouped into two main categories; the first one is the

geologic condition that affects the machine choice and design.

Such condition determined before the choice of machine and

its designation. The second category consists of adverse

geological condition that encountered during the tunnel

excavation and this condition mostly is unexpected or

accepted. This means the second group of geological

condition has an adverse effect on excavation but we accepted

the presence of this condition.

In tunneling projects to determine the situations of these

adverse condition, site investigation studies and surface

geological surveys conducted before the beginning of

excavation and geophysical studies and probe hole drilling

performed during the excavation. Nature of the phenomena

and fuzziness of the geological problems cause to decrease the

accuracy of these determinations, so in real cases prediction of

the adverse geologic condition is a rough approximation to

what that happen.

To overcome this problem and reduce the fuzziness it is

necessary to monitor all the excavation process and ground

condition during the work. It seems there are some evidences

that can lead us to detect the problematic zones more exactly.

Many researchers have studied geological parameters that

affect the excavation and tunneling in difficult geological

condition. One of the first researches in this field has been

carried out by Deere, D.U. (1981) who studied the effect of

adverse geological condition on TBM tunneling. Lombardi G.,

Panciera A. (1997) showed the TBM tunneling problems in

squeezing ground condition and Barla G. and Barla M. (1998)

researched the tunneling in different adverse ground condition

such as fault zones and squeezing grounds. Tseng Y.Y. et al

(1998) also researched the mechanized tunneling in difficult

ground. Barla G. (2000), Barton N. (2000), Shang Y. (2004),

M. Sharifzadeh et al (2006), Mirmehrabi et al (2008), studied

the effect of adverse geologic condition on TBM tunneling in

recent years.

In this paper, some geologic evidences before the entering

the collapsible ground condition are surveyed and the relation

between these evidences and occurred geologic hazards in the

tunnel are researched.

2. GEOLOGICAL SETTING AND PROJECT

DESCRIPTION

In this research, the data gathered from a water conveyance

tunnel excavated in central Iran were considered and

analyzed. The Ghomroud water conveyance tunnel is one of

the components of a water management system in central Iran

(Figure 1).

This involves a 36 km tunnel from the Dez River in

Lorestan province to the Golpayegan dam reservoir in Esfahan

province. The tunnel was divided into different parcels that

two parts of the tunnel with about 18 km length excavated by

Ghaem Construction Co., a subsidiary of Khatam Corp. This

segment constructed using a 4.5 m diameter double shield

TBM at a grade of 0.134% and finished with a concrete

segmental lining to a diameter of 3.8 m.

Figure 1- satellite photo from the tunnel path

The tunnel is located tectonically in Sanandaj–Sirjan belt in

Iran plate. This zone consists of a series of Jurassic-cretaceous

metamorphic and sedimentary rocks that has been formed

during the clash of Arabian plate and central Iran plate. As a

result of clash a wide trusted and folded belt has been formed

called Sanandaj-Sirjan zone.

The rock formations along the tunnel path consist of four

main categories. These formations that age from Jurassic to

cretaceous are as follows:

I. Limestone formation: this formation consists of

massive and thick bedded limestone and dolomite.

II. Slate and shiest: foliated metamorphic shiest and

slates embrace the most parts of tunnel path. This

formation is faulted and oriented in different

directions. Also the most of the schistose rocks

contain organic components.

III. Quartzite and quartz veins: because of the geologic

setting of study area and presence of tectonic

activities such as faulting and intrusion of plutonic

rock in this zone many secondary quartz veins with

igneous source have been injected into the

discontinuities formed by faults. These veins can be

seen randomly in different parts of Jurassic

formations. The maximum thickness of these veins

reaches up to 80 m. Their strength reaches over 100

MPa. The origin of these veins is mainly from

pegmatite formations in the area.

Based on the surface geologic studies bore holes data and

as built geologic maps, a geologic section of tunnel path was

drawn that can be seen in figure 2.

Table 1 – collapse zones along the tunnel path

NO. Chainage of collapse zones Lithology 1 2250 – 2265 Graphite shiest, shiest

2 2525 – 2545 Graphite shiest, shiest

3 3145 – 3215 Slate, shiest

4 4670 – 4730 Slate and shiest

5 5235 – 5490 Slate, shiest and Graphite shiest

6 5650 – 5700 Slate, shiest and Graphite shiest

7 6360 – 6400 Slate, shiest, sandstone

As can be seen in figure 2 there are many faults that affect

the tunnel. These structural defects cause to decrease of rock

mass engineering properties and subsequently to take place

tunnel collapses at different zones. The chainage of main

collapse zones along the tunnel are as bellow:

Figure 2- geological cross section of tunnel path

2. 1. Methods

During the tunnel excavation several times the adverse

geological condition such as fault zones and grounds with

weak rock masses caused to collapse of tunnel roof and walls

and subsequently sticking of TBM. Before the first and second

collapse the presence of quartz veins in the parent rock was

reported and quartz content of waste materials increased

(figure 3&4). Such conditions in two collapses excite this idea

in mind that there are some geological evidences of zones with

collapse potential.

Figure 3 – quartz veins in rock mass at the first collapse of tunnel

Figure 4 – Quartz chips taken from cuttings

With this idea all of tunneling process before the entering

the collapse zone was reviewed and geology related

parameters such as waste materials, ground convergence and

machine parameters have been checked.

One of the most important parameters is the cutting type

and size. The type of cuttings show the geological formation

and rock type that embedded the tunnel and change in the rock

cuttings means the change in host rock condition. Size of

cuttings or chips size is the other important parameter.

Chips are formed between two cutters. The size of chips is

controlled mainly by the distance of cutters, the spacing and

orientation of rock joints, rock strength and brittleness of rock

(Cong and Zhao, 2009). During the excavation by means of

TBM chips size changes depends on variation of joint spacing

and rock mass condition. So the change of chips size is one of

the important parameters that can be noticed in collapse zones.

Figure 5 shows the chips size of the cuttings from the tunnel

in metamorphic shiest and slates.

Figure 5-Cuttings of metamorphic shiest and slates from the tunnel

The machine depending parameters such as trust pressure

or torque of cutter head also change in different geological

zones but because of dependency of the machine parameters

on the human decisions, in this research these parameters are

not noticed.

The pea gravel injection is one of the parameters that

depend on ground condition. In squeezing ground and

collapse zone that the ground be closer to the machine shields

the pea gravel injection decrease in a considerable amount.

The mentioned parameters were monitored during the

excavation, especially in collapse zones. The normal cutting

size based on the observations during the excavation is in the

range between 2 to 10 centimeters. In two first tunnel

collapses the percent of abnormal chips size with length less

than 2 cm increased in matrix of waste materials. Also the

maximum size of chips in these situations changes from the

normal condition.

Figure 6A shows the variations of percent of fragments and

chips with size less than 2 cm. this figure also shows the

position of collapse zones. The maximum chips size variation

along the tunnel path has been shown in figure 6B.

The presence of quartz vein in collapse zones and increase

in quartz content of cuttings has been shown in figure 6C.

Also the variation of the amount of injected pea gravel behind

the tunnel lining can be seen in figure 7.

Figure 6 – variation of geological evidence for collapse zone determination along the tunnel path: A- Percent of fragments with length less

than 2 cm. B- Maximum sizes of fragments. C- Quartz content (%) of cuttings.

Figure 7 – variation of Injected pea gravel along the tunnel path

3. RESULTS

Monitoring the mentioned parameters such as quartz

content, fragment size and maximum size of cuttings, also

injected pea gravel behind the tunnel lining show a variable

rate along the tunnel path and these variations depend on the

geologic condition. An overall look at the parameters variation

along the tunnel path reveals that there are some relative

differences in the amount these parameters in collapse zones

and other parts of tunnel.

Quartz content of cutting materials in the collapse zones

are higher than surrounding ground of these zones and the

fragment size and maximum size of fragments in the collapse

zones show an increasing trend relative to the normal

condition of ground.

Also, the injected pea gravel behind the lining of tunnel in

collapses decreases in respect of other parts of tunnel.

The variations of the mentioned parameters together along

the tunnel path have been shown in figure 8.

Figure 8 – The variation of quartz content, fragment size and maxium size os cuttings and injected pea gravel along the tunnel path

4. DISCUSSION AND CONCLUSIONS

The increase in quartz content of cuttings can be seen

almost in the entire collapse zones and this phenomenon is

related to the secondary quartz that re-crystalizes in openings

of fracture and fault zones. The source of such silica and other

secondary minerals is the plutonic activity in the region.

In the fractured and fault zones due to frequency of

fractures and low quality of rock mass the collapse potential

increase and in the most cases collapse and rock falls are

unavoidable.

In fractured and crushed zones (the zones by high potential

of collapse), there are unsystematic discontinuities and

fractures that form rock blocks in different sizes. During the

excavation in such zones these rock blocks will be released in

the cuttings and consequently the maximum size of fragments.

By increasing the fracture density in collapse zones, the

fragment size of cuttings mainly controlled by the spacing and

orientation of the fractures and joint, more than cutter head

characteristics. In this condition commonly the average of

fragment size of cuttings increases.

The amount of injected pea gravel behind the tunnel lining

depends on many factors from operational condition to

geological situation. The squeezing ground, falling blocks,

karts, groundwater condition, and ground collapse are some of

the geologic phenomenon that affects the pea gravel injection.

Closing to the zones with collapse potential because of the

very deformable rock mass in collapsible zones around the

tunnel the ground come closer to the lining and the amount of

injected pea gravel reduce in a considerable rate. When the

ground is collapsed the free space behind the lining

completely is filled with collapse materials. In such condition

the pea gravel injection is almost impossible.

The results of this study show that in different geological

condition with monitoring the excavation process it's possible

to predict the adverse geological condition using some

geological and operational parameters. In similar geological

condition to this tunnel, geological parameters such as

secondary minerals in fractures or mineral veins (e.g. calcite

and quartz), size of cuttings fragments can be used to

determine the collapsible zones locations.

5. REFERENCES

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