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Problem Statement

Updated:

October 5, 14:53 EDT – Added Q/A info
October 27, 18:04 EDT – Submission form below

Questions and responses relevant for all teams are found here.

Problem

A 3m radius circular tunnel is to be driven at a minimum depth of 60 m in weak rock. One horizontal fault is located at a depth of 56 m. The horizontal stresses are between 0.4 to 0.6 times the vertical stress. The water table is almost at the ground surface, but the fault also acts as a barrier, i.e., no water is present below the fault.

Properties

The properties (along with coefficient of variation provided in brackets) for the rock are as follows:

Property	Value	COV
Unit weight	26 kN/m³
Cohesion	750 kPa	0.3
Tensile strength	75 kPa	0.3
Friction	30 deg	0.1

Resistance to slip on the fault follows a Coulomb shear strength criterion

Property	Value	COV
Cohesion	0 kPa
Friction angle	25 deg	0.1

Criterion

Find the minimum depth for the tunnel in meters such that factor of safety is > 1.5 and probability of slip on the fault < 5%.

Alternate Scenario

What is the optimal location of tunnel if another horizontal fault is located at 67m below the ground surface with similar properties as the other fault.

Property	Value	COV
Cohesion	0 kPa
Friction angle	25 deg	0.1

What are the possible solutions (other measures that can be taken) to meet the desired factor of safety and probability of failure criteria.

What kind of construction method would you use for this tunnel?

Report

The submitted report should address the following topics:

Problem Definition
Figure representing the geological/geotechnical model.
Overall approach, including assumptions.
Define and justify the analysis methodology.
How is variability of rock mass strength addressed?
Numerical Modeling procedure and Results
Final Design Selection – specifying minimum / optimal tunnel location
What are the key risks to the proposed solution?
How could risk be mitigated?