# 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/m^{3} | |

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?

Ideally, the report should be less than 8 pages, but there is no formal limit.

**Resources**

Problem-specific FLAC2D templates are available here,

**Submissions**

Please submit your report to this form.