Settlement Criteria  f f f for clays, silty clays, plastic silts: Chapter 5 (short term) Chapter 7 (long term, i.e., consolidation)  i i i in this module SANDS (including gravelly sands, silty sands and non-plastic silts) are considered

The concern?  i i i in most cases the maximum allowable settlement will not be reached before shear failure at a factor of safety of 3  t t t the main concern is with narrow footings  s s s settlement in sands is rapid, occurring almost entirely during construction and initial loading      dead load + max. live load are considered to estimate settlement

Maximum Allowable Settlement Footing on Sand ~ 25 mm  t t t this makes it likely that any differential settlement between footings will be less than 20 mm Raft on Sand ~ 50 mm  c c c corresponds to differential settlement between footings less than 20 mm

Pre-Construction Treatment  f f f for loose sand deposits, compaction prior to construction is recommended (vibroflotation, for example)  f f f for clays, if possible, surcharging with fill and vertical drains for several years prior to construction will reduce the ultimate settlement of the structure as most of the consolidation will have taken place

Plate Bearing Test  u u u used to simulate a foundation  a 1.5 m square test pit is dug Sowers & Sowers, 3 rd Ed.

Plate Bearing Test (Cont’d)  t t t then a 1 foot square (300mm x 300mm) steel plate is loaded in increments and the corresponding settlements measured Peck, Hanson, Thornburn, 2 nd Ed.

Plate Bearing Test Results  a load-settlement curve is produced Sowers & Sowers, 3 rd Ed.

Modulus of Vertical Subgrade Reaction, K v  K v is taken from straight line portion of this curve McCarthy, 6 th Ed. A plate = Area of plate Q plate = Plate Load S plate = Plate settlement

Design K v Values *These are for the case where the water table is at a depth greater than 1.5B. If the water table is at the base of the foundation use 0.5Kv. Use linear interpolation for intermediate locations of the water table. Condition Relative Density, % Representative Values of Dry Unit Weight (kN/m 3 ) Values of K v * (1000 kN/m 3 ) Loose< 35< Medium Dense Dense Very dense> 85> McCarthy, 6 th Ed. B 1.5B D DwDwDwDw

Settlement Calculation For cohesionless soils where D < B < 6.1m: where S = expected foundation settlement (m) Q = column load (kN) B = footing width (m) Kv = modulus of vertical subgrade reaction (kPa/m or kN/m3)* *Remember, when calculating K v from plate test data, plate area (A plate ) is 0.09m 2 !

Beware!The Plate Bearing Test results are extrapolated for the design of the foundation! Craig, 6 th Ed.

Standard Penetration Test  part of a standard bore hole investigation  split barrel sampler is advanced by dropping a 64 kg hammer 760 mm  N-Values are the number of blows (hammer drops) to advance sampler 300 mm McCarthy, 6 th Ed.

Split Barrel Sampler  N N N N-Value = “Standard Penetration Resistance” Peck, Hanson, Thornburn, 2 nd Ed. McCarthy, 6 th Ed.

Sample Bore Hole Log McCarthy, 6 th Ed.

North American Equations for B < 1.2m for B > 1.2m where Sa = allowable settlement (mm) qa = allowable bearing pressure (kPa) B = footing width (m) N = average corrected standard penetration resistance  a a a a number of corrections are applied to N-Values (pore water pressure, overburden stress…see Craig)

Water Table Correction water table correction:  w w w we will be using average corrected N-Values  T T T Terzaghi and Peck proposed a correction, Cw to the allowable bearing pressure, qa to reflect the depth of water table D w = D + B D DwDwDwDw B B