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Introduction:

There are so many arts are available in our world for a long time due to various requirements. Among all of the above the tunneling is one of the oldest techniques for various reasons. There can be various reasons are available for the execution of the tunneling. The basic methodology of such kind of tunneling is to drill with the help of underground horizontal drilling mechanism. The need of the tunneling in this new era of technology related to the geological and engineering such kind of tunneling are made for various kind of requirements. Among all the requirements the most important one is the water tunneling or any transportation tunneling. For this case the tunneling mechanism with the various aspects related to the rock science and technology are given and discussed in a proper manner. Actually we did not used the rock science software for the virtual simulation but the usage of the mechanical calculation for the enhancement of different technique and the reduction of cost and all the engineering methods will help a lot.

The detail technological discussion is basically classified in to different sections like site verification and investigation, ground profile, ground characteristics and ground behavior this all things are discussed with the help of so many researches and the overview of different research paper. Such kind of assessment is necessary because the need of the different design and development aspects of the tunnels. For the current scenario we have also considered all these things and with the help of different subparts the detail folio report is done here. Just after the introduction we have discussed the selection of site. On the context of site selection, the main motive can be evaluated for the different properties of the soil and rock. The site selection will also be known as one of the important section for this kind of report formation. The next part of the report is based on the site selection and the various geological aspects.

The Site:

It is basically the selection of the position of the region where this project is being executed. The purpose can be anything but without compromising the purpose the main fundamental requirements should be taken into consideration at the time of the site selection. The reason behind it is, at the time of site selection we will get to know the various geological parameters without them it is difficult for us to implement this kind of project successfully. Here in this report we have considered mainly two geological aspects for the site those are topography and geological setting. A brief discussion is given in this part of the report to show the need of these two factors in rock boring and tunneling, how they are important in this kind of projects discussed here.

Fig: Map of the site for the stated tunnel

Topography:

Here, we are working on the base model of the Dinorwik diversion tunnel. It is basically made because of the transportation purpose for that reason the safety is the main issue and the construction should be kind of that where the factor of safety is much higher. At such condition we need to check the topography of that site, where we are going to make a tunnel underneath the soil. As we know the surface of the soil and the upper part of land is not an even surface for this if we make a tunnel under the surface the weight distribution will not be proper, the only reason is the topography of that particular place we are considering. With a very basic drawing we have shown the reason behind the uneven weight distribution. The topological map of the topography states the different surface heights of the ground and based on which the weight distribution occurs on the tunnel. The vertical pressure is the complete dead weight of the upper position. Since we have so many parts above the tunnel various region has different weighting. On the basis of these parameters the design and the construction work should be proceeding. And the wall thickness of the tunnel should be able to balance the vertical s well as horizontal loading so that any kind of hazardous situation may not occur.

Geological Setting:

• Ground profile:

Ground profile is one of the important topic needs to be consider at the time of performing any job related to the underground tunneling. The ground profile consists of several parameters like stratigraphy, structural geology and the hydrogeology. These all are related to the ground and soil condition of that particular site we are considering here. One by one in this part of the report we have discussed those topics to show the effectiveness of the parameters. How all these parameter affects the tunneling work is basically discussed here.

Stratigraphy:

In the engineering development of any construction work one of the most vital parameter is stratigraphy condition of the site of tunneling. The basic requirement on this context is the ground stability. This factor has the most effect in the shield tunnels. On considering this factor, we can determine the effectiveness of the boring mechanism at the time of construction of the tunnels for any purpose. Relatively the ground condition is broadly classified into four parts those are easy to cut easy to control, easy to cut hard to control, hard to cut easy to control and hard to cut hard to control. Without a brief study and testing the tunneling in a place will be very difficult because a study on this topic can determine the setting of the rock formation categories with their data and methodology. If we have the exact data related to the stratigraphy, then only the effectiveness will be there in the tunneling can be observe.

Structural geology:

On the context of the tunneling under the ground a study related to the geological structures is one of the important things which we need to consider. The geological structure is based on the structures related to the rocks under the ground and the tectonic parameters. Based on all of these the engineering team has to prepare the tunnel so that the stress concentration in the tunnel should be lesser. With the help of various research papers, we get to know that, the geological structure is completely based on the powerful tectonic forces exerted by the tectonic plates under the very deep of the ground. The movement of the tectonic plates causes various hazardous situations like folding and breaking of the rocks that causes the most effective failure like cracking, fatigue, displacement and all. The entire topic geological structure is based only on three topics these are fractures, faults and folds of the rocks. What are the effects of such kind of faults can be seen in any underground tunnel is described here?

We know the axis of the underground tunnel is always same on the context of vertical displacement of the tunnel. At the same time if the tunnel is a shield one then the problem is extensive for any failure. At the presence of any deformation of the underground soil and rock leveling due to the movement of tectonic plates, we can see vertical deformation with different deformed structure. At this situation the shield tunnel may get damaged. A clear deformation cross-sectional view is given below to show the reasons of failure.

Fig: geological structural change

The above figure shows the geological structural change and deformation due to the movement of the tectonic plates. If we can observe it carefully and assume a tunnel just between the layers then we will be able to get a clear view of the failure on the tunnel structure. So, this is the need of the geological structural study.

Hydrogeology:

For any kind of tunneling construction work one of the most important things has to be considered is the hydro geological factor of that particular place or site. The term hydrogeology stands for the water in the underground lining and the moisture content in the soil and the rocks. The moisture content has a effective role for the construction of a underground tunnel as well as the direct water level lining into a tunnel can make so many hazardous situations. For shield tunnels the effect of hydrogeology is not that much but the moisture content in the soil and direct water layer to a tunnel may damage the shield tunnel also. The determination of the hydrological factor in terms of geology there we have various topics to be considered. Those are, topographic location of the site, sealing of the tunnel, damaged zone due to excavation and geological setting of the project site. We have discussed all of them above one by one so that the clear understanding can be buildup on it.

The topographic location of the tunnel plays an important role on the context of hydrogeology. The slopping determines the standstill condition of the surface water. If the tunnel located at a place where the slope of the ground surface is not that much higher, we can observe the condensation will be higher than any other places. Considering this factor, the place divided into some categories like hill position, slope position and the valley position. The stagnant water in this positions are more in the valley position for the and for that position only we will face more problems to construct a tunnel.

In some of the saturated zones the tunnels are acts as a drainage unit. Due to such kind of effects the most important thing is the sealing of the tunnel in terms of its structure and concrete framing. The sealing is determined by the material used in the tunnel liner.

The excavation damaged zone is the positions in the tunnel where we can see some of the damages like fracture, fatigue and cracks in the liner of the tunnel. Such kinds of damages are also caused by some of the hydro geological characteristic of the soil. The main thing is that, for different positions the stress concentration becomes different and the exerted force on those positions will affect the tunnel liner and hence such damages are observed.

Last methodology for the occurrence of the hydro geological damages is the geological setting of the location. The geometrical configurations like different moisture content and different water soaking capacity.  If one can assume or determine the actual water passing capacity of the geological setting of the site, then the construction enhancement can easily be achieved. If there is any kind of default faults are available on the site, then the water intake and flow will be larger to the underground tunnel.

Ground characteristics:

To determine various aspects on the construction technology of an underground tunnel the ground characteristics is one of the superior part for understanding. There are several topics are available to determine the ground characteristics of the site. With the help of different parameters one can easily determine the ground characteristics. On the context of ground characteristics with the help of some research papers we have demonstrated this part of the report with some wording and mathematical model and calculation for the stress concentration. The topics involved in this section are intact rock behavior, Rock mass behavior, In situ stress condition, Induced stress condition and Discontinuity. Both of these points are discussed in this part of the report.

Intact rock behavior:

Intact rock this term is basically known as the un-fractured block of a rock between discontinuities in a rock mass. At the time of determining various technical aspects in the geological technology it is one of the important things we need to be considering here. When we need to determine the intact rock behavior then the most important things needed to be consider are, petro logical name of the rock, texture of the rock, grains of the rock, strength of the rock, density of the rock and deformability of it. Under consideration of all these things the intact rock behavior can be determined.

Rock type	Saturated m/c, %	Unit weight, kN/m3	Tensile strength, MPa	Compressive strength, MPa	Triaxial test results, MPa
					s3 = 5MPa	s3 = 10MPa	s3 = 15MPa
Nant Ffrancon slate	0.23	26.5		51.7 (^ to cleavage)
Graianog sandstone	1.5	25.2		35.8
Marchlyn slate	0.42	26.3		54.6
Bronllwyd grit	0.9	26.8	3.5	84.2	118	166	207
Llanberis slate (Purple Slate)	0.22	26.5		110 (^ to cleavage) 20 (30° to cleavage)

 

In the context of intact rock properties three things are measured these are deformation, strength and the failure. For measuring there are some assumptions are needed. Those are described here in this part of the assignment, obtaining the general impression of the property of the rock is a necessary one for finding out the rock mass.

Comparison of the rock property with a previously obtained rock property is needed, so that the general terminology of the rock property in terms of the load capacity can be obtained.

Generation a rock mass criterion for the perfect rock is needed for the general parameters of the rock like induced stress and in situ stress conditions. These two stress conditions are stated below in this report.

Rock mass behavior:

For the construction of an underground tunnel the rock mass calculation with the help of various manual methods is necessary. The involved considerations are induced stress conditions and the in situ stress consideration of that particular geological region. With the help of the discontinuity data given for the assessment we can find out the general parameters related to the rock mass criteria.

With the help of the dip/ dip direction and average spacing in the table given below we can find out the basics regarding the rock mass. As we have done all the practical test data those are sufficient for finding out the all the parameters and from there also we will be able to check the stress consideration. The table related to the rock mass and the mathematical calculation given below.

Rock type	Discontinuity set	Dip, °	Dip direction, °	Average spacing, m	Comments
Nant Ffrancon slate	N1 N2 N3	0 – 25 70 75	075 140 230	0.7 0.3 0.5	All are tight, straight, planar, smooth, persistent and clean
Graianog sandstone	Q1 Q2 Q3	25 85 85	315 030 120	3 1.5 2	All are wavy, stepped, rough, persistent, open, discoloured to clean. Glacially loosened at outcrop
Marchlyn slate	M1 M2 M3	30 60 75	070 130 225	0.3 0.3 0.5	All are tight, straight, planar, smooth, persistent and clean
Bronllwyd- grit	BG1 – Bedding NW limb – Bedding SE limb BG2 BG3	50 – 90 50 – 60 25 80	260 140 198 035	0.3 1.0 1.0	All are tight, straight, planar, smooth, persistent and clean Spacing reduced near faults Note well developed refracted cleavage in fine grained parts of the graded beds
Llanberis slate (Purple Slate)	P1 P2 P3	80 25 85	130 320 035	0.15 0.5 0.25	Parallel to cleavage, tight, straight, planar, smooth, persistent, clean Tight, straight, planar, smooth, persistent, clean Tight, straight, planar, smooth, persistent, clean

 

There in the table mainly four types of rocks are described but in the mathematical modeling here we have shown only for the first variant of rock,

Intact rock strength = 85 MPa, Rating =7

RQD = 77%, Rating = 17

The findings of such kind of parameter in case of tunnel design are due to the evaluation of the state or the behavior that suggest the geological structure of the rock mass. The failure in terms of fracture, fatigue and cracking can be evaluated by this kind of simulation. As the table given above shows all the detail parameters related to the rock mass behavior.

The findings of such kind of parameter in case of tunnel design are due to the evaluation of the state or the behavior that suggest the geological structure of the rock mass. The failure in terms of fracture, fatigue and cracking can be evaluated by this kind of simulation. As the table given above shows all the detail parameters related to the rock mass behavior.

In situ stress condition:

In situ stress is one of the main topics for the designing and the construction of underground tunnel. It is related to the rock mass quantity and the direction of the compression which is applied to the rock at any specific location. It is really very important to understand the geological condition of the rock at any particular location before initiation of the construction work. In situ stress condition is designed for finding out the various challenges occurred in construction. At any kind of tectonic movement, the vertical and horizontal ground failure may occur.

For the calculation of the in situ stress consideration we need to check the vertical weight of the rock column on which the stress concentration is calculating. As we have the unit weigh of the rock is about 26.5kN/m3 for the Nant Ffrancon slate and the calculation is based on this only. In terms of load it can be stated as 0.0265MN/m3 and the vertical stress can be stated as 26.5 MPa. This stress can be estimated with the help of a very simple relationship, it is given as,

Fig: Relationship between vertical stress and depth (After Brown and Hoek 1978).

The measurement of the vertical stress in different cases of the vertical stress for any kind of civil engineering projects we can assume a graph for the relationship between the depth and the vertical stress.

In case of finding out the horizontal stress it is much more difficult than finding out the vertical load. The reason behind it is, at the time of calculating the horizontal stress we are unable to get the vertical normal component of load. The stress concentration in horizontal loading will be,

Fig: Relationship between horizontal stress and depth (After Sheorey 1994)

These plotting shows the relationship between In situ stress concentrations on the horizontal direction to the depth, and undoubtedly this relationship can be used worldwide application for determining the in situ stress concentration.

Fig: World stress map (www.world-stress-map.org.)

Induced stress condition:

Fig: Illustration induced stresses in an element of rock close to a horizontal tunnel

We have already discussed about the in situ stress concentration into the tunnel. It is basically the stress concentration when the tunnel boring is done. At this condition the in situ stress condition is applicable in the tunnel. But when the tunnel is excavated and we get the opening portion in the tunnel at this time we can experience the induced stress condition. As shown in the figure the induced stress condition is the tunnel can be finding out by this method, in this method with the help of a unit cube in a surface we can check all the in situ stress and the principal induced stress is the total of the all stress in the tunnel.

Discontinuity:

Fig: Showing discontinuity in different layer (rasidfaridi.com)

The term discontinuity is basically related to the general geological behavior of the underground soil and rock property. It is basically an assumed separation surface that shows the changes in the physical or chemical characteristics in a soil and rock mass. In the mechanical aspects discontinuity is the plane of weakness but an integral discontinuity is defined as a strong bonding in the soil and the rock material over a surface. The integral discontinuity can be transferred into mechanical discontinuity due to the change in material and chemical property of the soil and the rock.

The study of discontinuity is important because if we go downward into a underground tunnel then the material property will change by increasing the depth and in different changes of the soil and rock property we can observe there geological setting is different. Since the material property has so many effects on the stress and strain concentration in a material for that reason at the time of underground tunnel boring the study of discontinuity is very much important. It shows the separation between two individual layers. And the separation between two layers means there is a different stress concentration in two different layers. Both the in situ and induced stress will be changed when there is a discontinuity is observed. Let us assume there is a discontinuity in two different layers and the tunnel is just made at that position at this condition for two different geological setting of the layers the stress concentration will be different and the liner of the tunnel will experience different loads and fracture and fatigue or any kind of permanent deformation may be there.

Ground behavior:

When there is a construction is going on for an underground tunneling then we all knows the internal parameters of the soil and rock is one of the most common topics to be followed by the engineering and surveying team. Along with this, the surface or ground condition is necessary one. The behavior in terms of the property of the ground and its surface configuration can be determined by geological survey. With the help of the topological map we can also determine the ground behavior. The necessity of the ground behavior can be determined by the two failure issue those are ‘stress induced failure’ and the ‘structurally controlled failure’. For both of these two failures there is some relationship between the ground behaviors to these failures. This is discussed in this part of the report.

Stress induced failure:

Stress is the material property, for all kinds of material there one can observe a stress concentration due to the external force exerted on it. The stress induced failure is the failure which is caused by over stress concentration in a material. If there is an over stress concentration available in a material, then after elastic zone there is a failure may occur on that material. For our case study we are considering the underground tunnel. The failure will be seen in most of the time due to the vertical loading stress on the tunnel. The vertical loading will be there due to the presence of the soil and rocks over the tunnel. What is the weight of the upper part of the tunnel depends on the ground or surface behavior? If the upper ground surface is a plane surface, then the stress concentration will be same throughout the tunnel. But is there is a hilly region on the ground surface then we can observe a different load distribution on the tunnel and hence a different stress concentration will be there. Due to an uneven ground surface the thickness of the soil and rock at the upper position will be different and that means we can observe an uneven weighting on the tunnel. Such kind of dissimilar stress concentration may cause a fracture, fatigue or crack in the tunnel liner. Sometime if there is an uneven surface on the ground then in some of the places a concentration of water can be observed. That may cause a wet condition of the soil and the rocks just above the tunnel. In this scenario we also get some deformation in the material property as well as in the physical behavior of the rocks there might be some changes. Considering this issue a failure may occur in the tunnel.

Fig: Stress concentration related to the ground behavior

Structurally controlled failure:

There are some of the failure may occur in the tunnel due to some mismatch or defect in the structural part of the tunnel. Structural part is based on the design and the construction. If there is any kind of damage is happening in the tunnel, then we can say it is only due to the structural defect. At the time of designing of the tunnel the designing engineering team should find out an enhancement in the designing so that the chances of failure can be reduced. If the design team is able to make the optimal design, then the failure can be seen only by the defective construction. The structurally checking up should be there in the tunnel liner. And the time of concrete reinforcement for making the tunnel liner there may have some gap between the concrete and that leads to the failure of the tunnel. Since we are discussing about the ground behavior so we need to consider this thing carefully, the reason behind it is, when the dead load of the soil and rock will be there on a structurally defective part we may observe such kind of damage like fatigue, crack and many more.

Excavation and support recommendation:

The underground tunneling construction has basically two steps one is the boring of the tunnel and another one is the excavation. Here the discussion is completely based on the excavation from the tunnel. At the time of boring in a long tunnel the wastage soil and rock materials are poured into the tunnel. This mass body helps the hollow tunnel to get a support before excavation, but at the time of excavation of the tunnel the internal part becomes completely hollow, and there is a chance of the failure in the soil setting inside the tunnel.

At this scenario the implementation of excavation support is necessary. Before the construction structural walling throughout the tunnel such kind of supports are really very necessary for making a healthy risk less working environment inside the tunnel. After construction completed those supporting structure make a possibility for getting an extra strength in the tunnel. For a tunneling work there are two types of supporting are available one is the inside support and another is the outside support. The inside support is used in shorter distanced tunnel and the external support used in longer distanced tunnel. The reason behind it is, if we use an inside we cannot access the tunnel easily under construction period, and we will have to be remove all of them for final construction. In case of external support in tunnel boring it is more efficient than internal supports. The external support also makes kind of a sealing throughout the tunnel and it is permanent one, there is no need to remove all of them before or after final construction. The recommendations are,

1. For any kind of short length underground tunnel we need to implement some kind of inner supports.
2. For a long span underground tunnel, we need to implement support to the inside wall and the supports will be shown from tunnel inside portion.
3. At the time of excavation, we do not excavate the tunnel suddenly as a whole; we should excavate the tunnel part by part by making support on the inner walls.
4. With the help of calculation of the various stress conditions we have to make the support at the time of excavation because the support should sustain the load. Otherwise there will be no meaning of the support.
5. For long span tunnels the supports are permanent so we should use a material less corrosive compared to any other material.

These all are the basic recommendation for the excavation of an underground tunnel and support implementation.

Conclusion:

In this report we have done with the various technical and geological aspects of construction of a underground tunnel. The detail points are to be consider at the time of selection of site are topology and the geological setting of that location we have discussed in detail so that one can understand how these factors can affect the construction of the tunnel. After that the ground profile consists of different geological aspects like stratigraphy, structural geology and the hydrogeology both of these are also taken into consideration. These are the basic theory part of the assignment report. Along with it for the aspects of structural engineering behavior of the tunnel the rock behavior, rock mass, in situ stress condition and induced stress condition is the general requirement for calculating all the loads in the tunnel and its condition at the loading condition.

Along with this here we have also discussed the excavation procedure in the context of construction of a tunnel. How we will be able make a possibility for getting the most suitable option also discussed in this report with the possible recommendations.

Recommendation for future investigation:

Since, we have done with all the possible characteristics related to the construction of the underground tunnel. For any other suitable condition, we need to find out these all things with more detailing. The more detailing means the calculation of all these things in an engineering context. There are so many software is available for the simulation of the mechanical load testing of the underground tunnel like Rock science is one of the commonly used software. With the help of them we can see the visual conditions also for the particular tunnel and its design scheme can also be prepared. For any further investigation of the construction the recommended steps are,

1. A site visit can be done with the help of geological experts for finding out the different condition of the soil, terrain, rock and topography.
2. More critical mathematical analysis can also be performed for the exact simulation of the proposed model of the tunnel.
3. A designing in the software tool and the virtual static load simulation can be performed to get an enhancement in the optimal condition of the underground tunnel.
4. Along with the static load testing with virtual modeling some kind of dynamic loading condition can also be performed for seismic loading in the software tool.
5. Some lab testing report can be generating on the basis of the selection side rock to get their compressive and tensile test.

These are the recommended steps can be followed at the time of any further investigation of the underground tunnel. If we go through the steps, then only we will be able to create the most sustainable design and construction for any underground tunneling job.

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