Pile Foundation Reason for Piles Types of Piles Capacity Prediction Methods Load Tests

Reasons for Piles Large Structural Loads Settlement Intolerant Structures Addition to Pile Supported Structure Low Strength Soils at or near Ground Surface

Reasons for Piles Piles transfer loads: To suitable bearing strata through toe resistance (end-bearing piles) To strata in which pile is embedded through shaft resistance (friction pile) Through a combination of both shaft and toe resistance (most common)

Types of Piles Timber OK for capacities less than approx 25 tons OK for length less than 60 ft Protect from rotting above groundwater table

Types of Piles Concrete Appropriate where homogeneous soil conditions allow driving to a specific length Not appropriate in the upper Midwest due to variable soil conditions (variable length)

Types of Piles H-Piles Good choice when driving to bedrock or deep penetrations Limited uplift capacity

Types of Piles Pipe Piles Higher capacity per unit length that H-piles May not drive as deep as H-piles

Capacity Prediction Methods Static Dynamic Formulae Wave Equation Analysis of Piles (WEAP) Dynamic Measurement and Analysis

Static Capacity Prediction Estimates probable capacity range for a given length of pile, or probable length for a given capacity Requires exploratory borings to about 10 – 15 feet below deepest anticipated penetration Groundwater, qu, SPT

Static Capacity Methods B – Nt Method Inputs include qu, SPT N values, location of groundwater, unit weights Overburden stress major controlling factor, soil strength next Takes soil/pile set-up into account Variations of + 25% should be expected

Static Capacity Methods Qu = Qp + Qs Qu = Ultimate capacity of pile Qp = point capacity Qs = frictional resistance along shaft

Point Capacity in Sands Qp = Ap qp = Ap q’ N*q < Ap ql Ap = area of pile tip q’ = effective vertical stress at tip N*q = bearing capacity factor (F13.9) ql = limiting point resistance ql (kN/m2) = 50 N*q tan f

Point Capacity in Sands For homogeneous sands (L = Lb) qp (kN/m2) = 40 Ncorr L/D < 400 Ncorr

Point Capacity in Clays For undrained, saturated clays (f = 0) qp = 9 cu Ap

Frictional Resistance Qs = S p DL f p = perimeter of pile section DL = incremental length of constant p & f f = unit frictional resistance at depth d

Frictional Resistance in Sands f = K s’o tan d K = earth pressure coefficient and varies with depth and pile type K ~ 1.0 to 1.8 (1 – sin f) s’o increases with depth to about L = 15D d ~ 0.5 to 0.8 f fav (kN/m2) = 1 to 2 Ncorr

Frictional Resistance in Clays l Method fav = l (s’o + 2 cu) l varies with depth of penetration (F13.12) Qs = p L fav

Frictional Resistance in Clays a Method f = a cu a = empirical adhesion factor (F13.14) Qs = S f p DL = S a cu p DL

Static Capacity Examples

Dynamic Formulae Theoretically unjustifiable, purge from practice ENR Method is a common example

WEAP Analysis Predicts dynamic behavior of pile driving by modeling driving assembly/pile/soil system Estimates penetration resistance required for a given end-of-initial-drive (EOID) in the form of a graph

WEAP Analysis Provides estimate of probable capacity at EOID When set-up is considered, provides embedment-dependent penetration resistance criteria Provides design- and/or construction-phase flexibility w.r.t. selection of hammer/pile combinations May allow use of lower FS

WEAP Input Parameters Pile Properties Dimension (known) Material properties (known) Efficiency of driving assembly (assumed) Resistance distribution (assumed) Shaft and toe: quake and damping (assumed)

Dynamic Measurement and Analysis Performed in Field with a Pile Driver Analyzer (PDA) At pile head, strain measured to determine force, acceleration measured to determine velocity Provides estimate of toe resistance, magnitude & distribution of shaft resistance, ultimate capacity

Dynamic measurement and Analysis CAse Pile Wave Analysis Program (CAPWAP) Performed on PDA measurements Selects appropriate WEAP inputs to match predictions with measurements Can be very cost-effective May allow for lower FS

Load Tests Provides a proof test for design assumptions Most valuable if pile fails (plunges) Wait at least 30 days after EOID to allow for set-up Load Pile to 250% of design load

Load Test Readings Applied axial compressive load Head deflection Deflection of reaction piles Deflection of pile at depth Strain in pile section at depth

Load Test Instrumentation Telltales Measures pile head movement relative to specific location in pile Backcalculate average load in pile above telltale based on known elastic modulus

Load Test Instrumentation Strain Gauges Vibrating-wire strain gauge most common Measured strain used to calculate load at specific depths Estimate load distribution in pile shaft