Chapter 8 Civil Engineering Part 2: Geotechnical Engineering 4th Edition Chapter 8 Civil Engineering Part 2: Geotechnical Engineering

Part 2: Geotechnical Engineering The stability of a structure is no better than its foundations. Ask the people who built this  Exploring Engineering

Trouble in the Basement … Exploring Engineering

Properties of Soils Exploring Engineering

sat = (Ws + Wp)/V = dry + n w Wet Soil The mechanical properties of soils depend how much the soil’s pores are water filled sat = (Ws + Wp)/V = dry + n w where Ws and Wp are the weight of dry soil and the weight of water in the pores. The corresponding weight densities are dry and w. V is the volume of the soil. The term, n, is the volume fraction of the pores in the soil. Exploring Engineering

Dry Weight Densities of Different Soils Wet Soils Dry Weight Densities of Different Soils Soil Type Dry Density (lbf/ft3) Porosity Wet density Lbf/ft3 Sand 94.8 0.375 118 Clay 74.9 0.550 109 Silt 79.9 0.425 106 Water 62.4 — Exploring Engineering

Effective Stress Principle Pressure Is What You Feel when Your Fingers Press with Force Fn on the Contact Patch of Area A Exploring Engineering

Effective Stress Principle Soil section in next slide being analyzed for stress on the horizontal plane (dashed line) at depth z. This force is actually distributed uniformly over the bottom face creating a constant pressure (or stress).

Effective Stress Principle ’ =  - pp = (sat - w)z is the stress at depth z FBD Effective stress on layer of wet soil Exploring Engineering

Example Two top layers of a soil sample taken from a potential building site are shown in the next figure. If the thicknesses of the layers are HA = 2.50 ft and HB = 2.00 ft, determine: (a) the effective stress at a depth of ZA = 2.00 ft in Layer A (b) The effective stress at a depth of ZB = 3.00 ft in Layer B. Assume both layers are fully saturated with weight densities of γA = 78.6 lbf/ft3 and γB = 91.2 lbf/ft3. The weight density of water is γw = 62.4 lbf/ft3.  Exploring Engineering

Example Continued Need: Find the values of effective stress at depths of 2.00 ft and 3.00 ft. Know: The depths of interest are ZA = 2.00 ft and ZB = 3.00 ft; the layer thicknesses are HA = 2.50 ft and HB = 2.00 ft; the weight densities of saturated soil and water are γA = 78.6 lbf/ft3, γB = 91.2 lbf/ft3, and γw = 62.4 lbf/ft3; both soil layers are fully saturated. Exploring Engineering

Example Continued Exploring Engineering

Example continued How: Since all of the soils above z = ZA are of a uniform compositions and fully saturated, use effective stress principle. With two layers of soil above z = ZB generalize the effective stress equation for the weights z = ZB in the free-body diagram. (1) draw the free-body diagram of the soil section, (2) find Fn from the summation of forces in the vertical direction, (3) substitute Fn into the definition of pressure to get total stress, (4) subtract pore pressure from total stress to get effective stress. Exploring Engineering

Example continued Solve: a) ’ =  - pp = (sat - w)z = (78.6 – 62.4)(2.00) [lbf/ft3] [ft] = 32.4 lbf/ft2 Generalize the equation for the effective stress ’ = AHA + BHB - wz = (78.6)(2.50) + (91.2)(3.00 - 2.50) – (62.4)(3.00) [lbf/ft2] = 54.9 lbf/ft2

Summary of Part 2 Geotechnical Engineering Structures stand on soil whose properties are variable, particularly if wet Starting with a FBD, stress in wet soil can be estimated from the weight of dry soil and wet soil at any layer The effective stress equation allows the calculation of stress at any layer of wet soil knowing its porosity Exploring Engineering