1-25-00, Settlement and Consolidation Ref: Principles of Geotechnical Engineering, Braja M. Das, 1994 Coastal Engineering Handbook, J.B. Herbich, 1991 Topics: Effective Stress Compression and Consolidation Process Settlement Categories Preconsolidation Condition Calculation of Primary Consolidation Settlement Time Rate of Consolidation - Effective Stress Total stress at point A below the soil surface is equal to (1) the stress carried by the water in the continuous void spaces (i.e. pore water pressure, u) and (2) the stress carried by the soil solids at their points of contact (i.e. the soil skeleton). Approximately the effective stress () satAwHHH where totalws satVWW au 1 where a is the fractional surface area of solids (usually a1 for all practical conditions) so u rearranging and subtracting the pore pressure (wAHu, i.e. hydrostatic pressure) HHHHHHAwAsatAw, where wCompression of soil layers due to stress increase by construction of foundations or other loads. Compression is caused by: 1. Deformation of soil particles 2. Relocation of soil particles 3. Expulsion of water or air from void spaces Consolidation - Process the reduction of bulk soil volume under loading due to flow of pore water. For saturated soils, any increment of loading (, called surcharge) will be initially taken up by the pore pressure and result in consolidation until a new equilibrium is reached where the soil solids (or skeleton) takes up the added load. surcharge: u AHAHFor cohesive soils: at t = 0, u; t=, For non-cohesive soils: water drains faster and the load is transferred immediately Categories: 1. Immediate settlement - elastic deformation of dry soil and moist and saturated soils without change to moisture content a. due to high permeability, pore pressure in clays support the entire added load and no immediate settlement occurs b. generally, due to the construction process, immediate settlement is not important 2. Primary consolidation settlement - volume change in saturated cohesive soils because of the expulsion of water from void spaces a. high permeability of sandy, cohesionless soils result in near immediate drainage due to the increase in pore water pressure and no primary consolidation settlement occurs 3. Secondary compression settlement - plastic adjustment of soil fabric in cohesive soils Preconsolidation Condition 1. normally consolidated - present effective overburden pressure = maximum pressure the soil has been subjected to in the past (pc) 2. overconsolidated - present effective overburden pressure maximum pressure the soil has been subjected to in the past (pc) elog pNormally consolidatedelog pOverconsolidatedpcMaximum past loadNon-linear rebound when load is removedPreconsolidation pressure determination (Casagrande, 1936) 1. Establish point a at which e-log p has minimum radius of curvature 2. Draw horizontal line from a (line ab) 3. Draw tangent to curve at a (line ac) 4. Draw line ad to bisect angle bac 5. Project the straight-line portion of gh back to intersect ad at f 6. Abscissa of point f is the preconsolidation pressure, pc log pVoid ratio, eahbcdfpcgCalculation of Ultimate Primary Consolidation Settlement Saturated clay soil layer of thickness H, cross-sectional area A, existing overburden pressure po, increase in pressure p, and resulting ultimate primary consolidation settlement S Change in volume is SAV , change in volume is equal to the change in volume of the voids (definition of settlement) and by the definition of the void ratio: SveVVSAV Using the initial void ratio and total volume (eo and Vo) gives ooo SeAH eVV11Combining and rearranging gives eeAHeVSAVoS1 oeeHS1Compression index Cc = slope of the e-log p curve: ooc pppeppeeC loglog 1221 oooc ppp eHCSlog1 For thick clay, more accurate to divide multiple layers Consider depth to 2B for square foundation (BxB) or 4B for strip foundations (BxL), B is the width Compression vs. Swell Index normally consolidated clays (pc = po + p) ioiiooic pppeHCSlog1overconsolidated clay (pc po + p) use the Swell Index (Cs) ooos ppp eHCSlog1NOTE: use of Cc vice Cs is conservative most marine soils are overconsolidated - sedimentation increases the surcharge on the soil, but subsequent erosion removes much of the load underconsolidated clay (pc po + p) (RARE CONDITION) coococoS ppp eHC pp eHCSlog1log1(partially on both curves) If the e-log p plot is known, can simply find e over the appropriate range of pressures and use oeeHS1(works for all conditions) Good condition! Compression index determination 1. Graphically from laboratory e-log p plot, use “virgin c