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ECGD 4122 – Foundation Engineering Lecture 2

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## ECGD 4122 – Foundation Engineering Lecture 2

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**Faculty of Applied Engineering and Urban Planning**Civil Engineering Department 2nd Semester 2008/2009 ECGD 4122 – Foundation Engineering Lecture 2**Revision of Soil Mechanics**• Soil Composition • Soil Classification • Groundwater • Stress (Total vs. Effective) • Settlement • Strength**Soil: A 3-Phase Material**Air Water Solid**The Mineral Skeleton**Solid Particles Volume Voids (air or water)**Air**Water Solid Three Phase Diagram Idealization: Three Phase Diagram Mineral Skeleton**Water**Solid Fully Saturated Soils Mineral Skeleton Fully Saturated**Dry Soils**Air Solid Dry Soil Mineral Skeleton**Air**Water Solid Partially Saturated Soils Mineral Skeleton Partly Saturated Soils**Air**Water Solid Three Phase System Va Wa~0 Vv Vw Ww WT VT Ws Vs Volume Weight**Weight Relationships**• Weight Components: • Weight of Solids = Ws • Weight of Water = Ww • Weight of Air ~ 0**Volumetric Relationships**• Volume Components: • Volume of Solids = Vs • Volume of Water = Vw • Volume of Air = Va • Volume of Voids = Va + Vw = Vv**Volumetric Relationships**• Volume Components: • Volume of Solids = Vs • Volume of Water = Vw • Volume of Air = Va • Volume of Voids = Va + Vw = Vv**Specific Gravity**• Unit weight of Water,w • w = 1.0 g/cm3 (strictly accurate at 4° C) • w = 62.4 pcf • w = 9.81 kN/m3**Specific Gravity, Gs**• Iron 7.86 • Aluminum 2.55-2.80 • Lead 11.34 • Mercury 13.55 • Granite 2.69 • Marble 2.69 • Quartz 2.60 • Feldspar 2.54-2.62**Example: Volumetric Ratios**• Determine void ratio, porosity and degree of saturation of a soil core sample Data: • Weight of soil sample = 1013g • Vol. of soil sample = 585.0cm3 • Specific Gravity, Gs = 2.65 • Dry weight of soil = 904.0g**134.9cm3**243.9cm3 109.0cm3 1013.0g 585.0cm3 341.1cm3 904.0g Example Air Wa~0 W =1.00 Water 109.0g Solid s =2.65 Volumes Weights**Air**134.9cm3 W =1.00 Water 243.9cm3 109.0cm3 585.0cm3 Solid s =2.65 341.1cm3 Volumes Example**Soil Unit weight (lb/ft3 or kN/m3)**• Bulk (or Total) Unit weight = WT / VT • Dry unit weight d = Ws / VT • Buoyant (submerged) unit weight b = - w**Fine-Grained vs. Coarse-Grained Soils**• U.S. Standard Sieve - No. 200 • 0.0029 inches • 0.074 mm • “No. 200” means...**Sieve Analysis (Mechanical Analysis)**• This procedure is suitable for coarse grained soils • e.g. No.10 sieve …. has 10 apertures per linear inch**Hydrometer Analysis**• Also called Sedimentation Analysis • Stoke’s Law**Soil Plasticity**• Further classification within fine-grained soils (i.e. soil that passes #200 sieve) is done based on soil plasticity. • Albert Atterberg, Swedish Soil Scientist (1846-1916)…..series of tests for evaluating soil plasticity • Arthur Casagrande adopted these tests for geotechnical engineering purposes**liquid**(pea soup) Liquid limit Plasticity Index plastic (pea nut butter) Plastic limit semi-solid (cheese) Shrinkage limit solid (hard candy) Atterberg Limits • Consistency of fine-grained soil varies in proportion to the water content**Liquid Limit (LL or wL)**• Empirical Definition • The moisture content at which a 2 mm-wide groove in a soil pat will close for a distance of 0.5 in when dropped 25 times in a standard brass cup falling 1 cm each time at a rate of 2 drops/sec in a standard liquid limit device**Engineering Characterization of Soils**Soil Properties that Control its Engineering Behavior Particle Size coarse-grained fine-grained • Soil Plasticity Particle/Grain Size Distribution Particle Shape**Clay Morphology**• Scanning Electron Microscope (SEM) • Shows that clay particles consist of stacks of plate-like layers**Soil Consistency Limits**• Albert Atterberg (1846-1916) Swedish Soil Scientist ….. Developed series of tests for evaluating consistency limits of soil (1911) • Arthur Casagrande (1902-1981) ……Adopted these tests for geotechnical engineering purposes**Arthur Casagrande (1902-1981)**• Joined Karl Terzaghi at MIT in 1926 as his graduate student • Research project funded by Bureau of Public Roads • After completion of Ph.D at MIT Casagrande initiated Geotechnical Engineering Program at Harvard • Soil Plasticity and Soil Classification (1932)**Plastic Limit (PL, wP)**• The moisture content at which a thread of soil just begins to crack and crumble when rolled to a diameter of 1/8 inches**Plasticity Index ( PI, IP )**• PI = LL – PL or IP=wL-wP • Note: These are water contents, but the percentage sign is not typically shown.**Groundwater**U = porewater pressure = wZw**Stresses in Soil Masses**P X X Area = A = P/A Soil Unit Assume the soil is fully saturated, all voids are filled with water.**Effective Stress**• From the standpoint of the soil skeleton, the water carries some of the load. This has the effect of lowering the stress level for the soil. • Therefore, we may define effective stress = total stress minus pore pressure ′ = - u where, ′ = effective stress = total stress u = pore pressure**Effective Stress**′ = - u • The effective stress is the force carried by the soil skeleton divided by the total area of the surface. • The effective stress controls certain aspects of soil behavior, notably, compression & strength.**Effective Stress Calculations**′z = iHi - u where, H = layer thickness sat = saturated unit weight U = pore pressure = w Zw When you encounter a groundwater table, you must use effective stress principles; i.e., subtract the pore pressure from the total stress.**Compressibility & Settlement**• Settlement requirements often control the design of foundations • This chapter provides a general overview of principles involved in settlement analysis • The subject will be dealt with in greater detail in Chapter 7.**Increase in Vertical Effective Stress**• Due to a Placement of a fill • Due to an external load