PDA Workshop: Stresses, Integrity, Energy and Capacity Ir Richard C L Yu IEM 2 Sep 2013
Dynamic Pile Monitoring For each blow determine Pile driving stresses Pile integrity Hammer performance Capacity at time of testing Working through some examples Stresses Outline Compression, Tension, Bottom Pile Integrity Beta Examples Hammer Performance and Energy Measurements Capacity Methods - RSP, RMX, RSU
Dynamic pile-top measurements provide information throughout pile length and resistance effect FMX, CSX = FMX/A Force (Stress) Maximum at gage location FMX = ε av E A ; CSX = FMX/A or ε av E CSI is the individual high gages reading Strain transducer Ensure that pile top is not overstressed
Compressive Stress at the Sensors BN595; yielding Pile force at any location From this morning: Downwards traveling waves from the hammer combine with the upward traveling wave reflected at L/c t = 0 L/c 2L/c L Downward Wave Upward Wave
CTN, TSN Maximum Computed Tension force in first period 2L/c after impact CTN = F (t 2 ) + F (t 3 ) min where t 1 <t 3 <t 2 TSN = CTN / A Related: CTX, TSX max. computed tension stress over complete record. +C -T Tension Stress Calculation Wave-Up toe top t 3 Point of max tension Tension Stress Distribution Max. Tension Wave Up
CTX, TSX Maximum Computed Tension force (stress) throughout record considering downward tension wave. W d, max-tension + W u, min-comp. CTX = Max CTN TSX = CTX / A -T +C Tension Stress Calculation Wave-Down 2L/c Max. Tension Wave Down Min. Comp. Wave Up
CFB, CSB Computed Force at Pile Base CFB CSB = R toe = F t 2 + F t 1 - R shaft = CFB / A F To avoid overstressing at bottom when driving to hard bearing layer F CF B
Max CFB is with a time delay due to toe quake CFB = RTL SFT = WD1 + WU2 SFT = 2549 + 1609 702 = 3456 Codes: Allowable Driving Stresses Steel piles 90% of yield strength for steel Concrete piles Compression: (85% of c.strength) - prestress Tension : prestress + (50% of t.strength) Tension (RR): 70% of yield of reinforcement
Stresses Outline Compression, Tension, Bottom Pile Integrity Beta Examples Hammer Performance and Energy Measurements Capacity Methods - RSP, RMX, RSU Pile Damage: BTA, LTD Pile damage causes a tension reflection before 2L/c The time at which the tension reflection arrives at the gage location indicates the distance to the damage: LTD = t damage (c / 2) LTD The extent of the damage is quantified with the damage factor BTA ( β )
Length to damage or damage location Impedance Reduction Approximate Solution no Skin Friction Z 1 F d,1 F u,1 β = Z 2 / Z 1 β = (F d,1 + F u,1 )/(F d,1 - F u,1 ) Z 2 F d,2 F u,1 is the reflection wave that indicates the extent of the impedance reduction (damage)
t 1 F d,1 = ½(F t1 +Zv t1 )= 2863 t 3 F u,1 = ½(F t3 -Zv t3 ) = -650 Beta = {2863 + (-650) } / {2863 (-650) } = 0.63 BTA interpretation Integrity - BTA method for uniform piles Looks for local decrease in wave-up β < 80 major damage β < 60 complete break Possible false causes purposely non-uniform pile bending, noise, phase (VT) wrong 2L/c (2 x length / wavespeed) Soils resistance unloading for long friction piles ALWAYS confirm by visually look at data top break toe
26m long Steel Pipe Pile
Good pile square prestressed concrete piles, about 24m long. Damaged pile Static Bending + Dynamic Stresses
large bending force between F1 and F2 Usually from hammer misalignment Bending stresses Difference between F1 and F2
Stresses Outline Compression, Tension, Bottom Pile Integrity Beta Examples Energy Measurements Capacity Methods - RSP, RMX, RSU Energy Why is it important? Contractor productivity To install pile to design depth (capacity) Quality control tool (blow count criteria)
PE = WH KE = ½ m v i 2 m = W/g v i = (2gH) (ideal case, no losses) v i = (2gH) e e = efficiency, <1 Can be measured with HPA or proximity switches?
From this morning Work = Force x distance = F x EMX = F d x = F d x (dt/dt) = F v d t Energy EMX = maximum of E ( t ) ETR = EMX / (max hammer rated E) STK = ½*g*( T/2 ) 2-0.1(m) ; STK is open end Diesel hammer stroke; T is time between blows ETH = EMX (Wr x STK) diesels only
F+ Rebound F+ V+ V- Energy from hammer to pile EMX Energy from pile to hammer DMX E (t) = F v dt Temp. Compression = DMX - DFN DFN Net measured efficiencies (steel piles) 0% 20% 40% 60% 80% 100% 60% hydraulic drop hammers 100% 40% air or steam hammers 70% 30% diesel hammers 60% 20% cabled drop hammers 80% Efficiencies on concrete piles are lower ~ 10%
Effects of diesel hammer pre-ignition on energy transfer: Stresses Outline Compression, Tension, Bottom Pile Integrity Beta Examples Energy Measurements Capacity Methods - RSP, RMX, RSU
From this morning Total Resistance Sum of downward wave at impact and upward wave at toe reflection Case Method Case damping factor applied to toe velocity Rs = (1-Jc)F d,1 + (1+Jc)F u,2 Case Method Static Capacity 5,450 kn 5.75 m/s 5,550 kn 0.0 m/s t 1 t2 RTL = WD1 +WU2 ~ 5450 + (5550/2) = 8225 R static = RTL J C [2F d1 RTL] R static = 8225 0.4(2*5450 8225) R Static = 8225 1070 = 7155 kn
RMX - RXi Calculates R static at all times after the first velocity peak Selects the maximum R static for J C = i 2L/c t 1 t 2
RSU, RUI Case Method Capacity Corrected for Early Unloading What Case Damping Factor to choose? Almost always, we choose damping factor based on correlation with CAPWAP or static load test. J typically varies between 0.4 for clean sands and 1.0 for clays.
An Example: PDA Capacity Results End of Driving PDA Capacity Results Restrike after 1 month
Summary Compression and Tension Stress Measured near top Calculated elsewhere in the pile Integrity - Beta is calculated in real time Energy - PDA can measure energy transferred to pile Capacity The Case Method provides many real time estimates of capacity (EOD or Res) One test result is worth a thousand expert opinions Werner Von Braun Father of the Saturn V rocket END