# Thickness of Plank

**Situation**

A temporary earth retaining wall consists of wooden plank driven vertically into the ground. The wall is designed to resist 2.4 m height of soil.

Cross-sectional dimensions of the plank = 300 mm wide × 75 mm thick

Allowable bending stress of the plank = 10.4 MPa

Allowable shear stress of the plank = 0.8 MPa

Unit weight of retained soil = 17.3 kN/m

^{3}

Active earth pressure coefficient = 1/3

1. Calculate the maximum flexural stress.

A. 12.7 MPa | C. 8.6 MPa |

B. 14.2 MPa | D. 10.1 MPa |

2. Calculate the maximum shear stress.

A. 1.11 MPa | C. 0.99 MPa |

B. 0.33 MPa | D. 0.77 MPa |

3. Calculate the minimum thickness of the plank to prevent failure.

A. 90 mm | C. 110 mm |

B. 80 mm | D. 100 mm |

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