Preliminary Engineering Geological Assessment and Design of a Rock Tunnel Harvard Case Solution & Analysis

Preliminary Engineering Geological Assessment and Design of a Rock Tunnel Case Study Help

Ground Characterization

In this section, we are going to discuss the tunnel ground characterization in depth ordetail.In this section, we are going to analyze various behaviors and conditions related to the tunneling process.

Intact rock behavior

Table 1: Rock Material Test Result

In-ground characterization, the most basic and important parameter is the intact rock behavior. Intact rock denotes the “un-fractured blocks between discontinuities in atypical rock mass.” In the tunneling process,the rock material strengths are mostly defined by the intact rock strength. The above-mentioned table is the test result of rock material and these rocks are extracted from the site area or the tunnel site so that we can analyze them and predict or conclude that whether the tunnel site is safe or not for tunnel design. This parameter can be varied according to the sites because in this world there are various types of soil and these soils, directly and indirectly,affects the rocks strength, design, as well as behavior.

Rock mass behavior plays an important role inthe tunneling process because it defines whether the rocks mass is capable ofwithholding the stress or not. By examining the tunnel site area we get the table 2 data and by analyzing the table 2 (Discontinuity data) and figure 1 (Discontinuity spacing classification)we can conclude that the tunnel site area can easily withhold the stress for a certain limit. This parameter can also vary according to the sites because in this world there are various types of soil and these soils, directly and indirectly,affects the rocks strength, design, as well as behavior. The best part of this parameter is that if we analyze this incorrect way or in a precise manner than it can increase the length of the tunnel but if there is a slight error present than it can easily reduce the life of the tunnel, therefore, for this parameter the requirement of precision is essential.

In situ stress condition

The definition of the in situ stress is simple, within a rock mass formation this stress is the local as well as natural stress of that rock mass. Rock disrupting is the common parameter or component in all the measuring techniques of the situ stresses (Vick, 1974).The disturbance response is measured in various forms which include displacements, hydraulic pressure, strain, and many more. By assuming various scenarios or by various assumptions related to the “rock’s constitutive behavior” we analyzed the rock disturbance. In the analysis, the disturbance process is also accounted for. For tunneling process rock formation properties related to the situ stress and various factors are necessary (Deere, 1981). Before the failure evaluation or rock stress analysis the in-depth knowledge of the situ stress is required or essential.In several ways properties of the rock, mass is affected by the situ stress which includes.

• “Stronger rock can sustain higher in situ stress.”
• “Stress concentration decreases with displacements.”
• “In situ stresses normal to discontinuities with a large aperture will be low.”
• “Effective stress is reduced by increasing pore water pressure.”
• “Hydrostatic in situ stresses tend to close discontinuities.”
• “Stress field alters the permeability of rock mass.”
• “Stresses cause normal to minimum rock fracturing at principal stress directions.”
• “High stress causes rock mass to fracture and its quality to deteriorate.”
• “Rock bursts in highly stressed rock masses affect excavation methods.”
• “Ideal cavern shape is controlled by an in situ stress field.”
• “Tectonic stresses, erosion, topography, and other factors also affect the stress””
• “In situ stress varies with depth.”
• “Discontinuities control magnitudes and directions of in situ stress fields.”
• “A highly variable in situ stress field exists in the fractured rock mass.”

Induced stress condition

Similar to the in situ stress condition, induced stress condition also plays an important role in the tunneling process. During the analyzing phase, if the induced stress condition is precisely calculated or measured than as a consequence of this we can face a huge loss in terms of money and time and similarly, the lifetime of the tunnel also reduced due to the induced stresses (Cherns, Wheeley, & Karis, 2006).

Discontinuities

The simple definition of the discontinuity is that it is a plane or a surface that faces the change in rock mass or soil characterization in terms of physical or chemical properties. In some cases or parts, discontinuity is denoted by joints (Ajalloeian, 2012). There are various example of discontinuities available which includes.

• “Bedding”
• “Schistosity”
• “Foliation”
• “Joint”
• “Cleavage”
• “Fracture”
• “Fissure”
• “Crack”
• “Faultplane”

Ground behavior

This is the last section for analyzing the ground or site of the tunnel and in this section, we discuss some of the failures we can face during tunneling or after the tunneling which includes.

Stress-induced failure

There are several reasons available for the failure of the induced stress. When excavated the most important requirement for the rock is its stability and we know that unloading intensity and initial stress level are the main parameters for the surrounding rock failure (SüleymanDalgıç, 2002). Therefore, for rocks, we always have to consider the joints and cracks because we know that these continuities become a major reason for the rock mass failure under induced stress which directly failed the induced stress.

Structurally controlled failure

There are various reasons for the structurally controlled failure and stress is the main parameter for the failure because if the stress increases from a certain point then it directly or indirectly affects the most parameters which affect the structure. Similarly, soil also plays an important role in the failure of structure because if the soil is not analyzed properly then it can easily damage the tunnel after a time.

Excavation and Support Recommendations

For the excavation of the tunnel, there are various ways for the excavation which includes:

• “Cut and cover”
• “Boring machines”
• “Clay kicking”
• “Shafts”
• “Sprayed concrete techniques”
• “Pipe jacking”
• “Box jacking”
• “Underwater tunnel”
• “Land tunnel”
• “Temporary way”
• “Enlargement”

The recommended excavation method is the boring machine method because it reduces the cost of tunneling as well as time but this is used for railways and underpass, therefore, we have to reduce the size of the boring machine so that it works according to our requirement (Ozfirat, 2015).

Conclusion

The conclusion of the above discussion or project is, tunneling is the most complex project because there are numerous factors involves in the development of the tunnel which include soil, rock, stress, strain, area, discontinuity, and many more. Similarly, there are various methods for the excavation of the tunnel and from those methods, the best one is the boring method because it reduces the time as well as the cost of the tunneling.

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