Grlweap Driven Piles
INTRODUCTION TO WAVE EQUATION...
Deep Foundations Course
INTRO TO GRLWEAP WAVE EQUATION ANALYSIS PROGRAM (WEAP)
What does GRLWEAP do? • Simulates the driving process of a pile • When the hammer strikes the top of the pile a wave propagates along the pile length. Note that the particles within the pile willl oscilate and forces will change along the pile and with time. • GRLWEAP computes: – Number of blows to drive the pile into the ground a certain distance. – Stresses during driving. – Pile capacity as a function of blows per unit penetration
BACKGROUND WEAP • Idea started in the 1940s (E.A.L. Smith) • Idea: – To generate a Bearing Graph using a model of the pile and the soil – Compute stresses during driving.
• Advances post - E.A.L. Smith: – Models are now more precise and realistic (particularly for pile driving hammers - diesel, hydraulic, vibratory, etc). – Development of soil models and the associated parameters that are more reliable (basded on field measurements and the PDA test). – Assess “driveability”
What it models?
What do we need? • • • • • •
Representative soil profile, Pile type Anticipated installation depth Design capacity (Qd) Ru = Qd x FS Estimate dynamic soil parameters to model soil resistance (Damping, quake) • Select candidate hammers based on loca practice and contractors
WEAP Analysis Typical uses: • Graph of Ru versus #blows/L • Graph of driving stresses (tension and compression) as a function de #blows/L
What to check with WEAP? • Check #blows/L for desired Ru : • If # of blows is very high: – >100 blows/ft for friction piles – >100 blows/ft for end bearing piles – Try a more powerful hammer (GRLWEAP)
• # de blows too low: – < 24 blows/ft for friction piles – Installation QC can be difficult and imprecise – Try using a less powerful hammer (less energy).
What to check with WEAP? • Check induced stresses to ensure a safe pile installation (integrity of pile) • If compression stresses are too high (and #blows/L are acceptable): – Use smmaller hammer – Reduce stroke of hammer or drop height (if hammer allows adjustments) – Use a thicker Pile cushion – Use a softer Pile cushion material
What to check with WEAP? • Induced stresses (continued): • If tension stresses are too high (and #blows/L are low); concrete piles: – Increase thickness of Pile cushion, – Reduce stroke of hammer, – Try a different hammer with a heavier ram.
What to check with WEAP? • Induced stresses (Continued): • If tensile stresses are too high (and #blows/L are also high); concrete pile: – Analyze a new hammer with a heavier ram
What to check with WEAP? • Induced stresses (Continued): • If both the induced stresses and the #blows/L are high or excessive: – Try a pile with a larger cross section (if feasible), – Use a pile material of higher strength,
IMPORTANT • Must compare what you analyzed with GRLWEAP with actual site/construction conditions! – Actual pile dimensions. – Driving system used (hammer, helmet, cushions) – sizes, types, materials, etc. – Energy and field operation of hammer.
• Usually WEAP is complemented with field measurements using the PDA (another class).
Flow Diagram RLWEAP Bearing Graph Input Model hammer & driving system & pile
Choose first Ru
Distribute Ru Set Soil Constants Time Increment Assign ram velocity and analyze pile/soil • Pile stresses • Energy transfer • Pile velocities Calculate Blow Count
Increase R u? N
Bearing Graph: Vulcan 506; HP 12x53; Clay/Sand
Bearing Graph Comparison
Capacity in kN
Bearing Graphs from Formulas and Wave Equation 4500 4000 3500 3000 2500 2000 1500 1000 500 0
Ru-Gates ENR - inferred GW-Sand GW-Clay
10 Blows/25 mm
Comparison Pile Capacity Estimates from different Methods 5
At Blow Count of
Note: GW = GRLWEAP
Blow count comparison
Nominal Safe Cap:
Bl/25mm Bl /25mm
Bl/25 Bl /25 mm
Bl/25 Bl /25 mm
Bl/25 Bl /25 mm
SUMMARY • GRLWEAP is based on Smith’s model with important extensions such as: – Realistic hammer models – Non-linear spring models for interfaces and slacks – Alternative soil models – Residual stress analysis
• The wave equation analysis works with “Static Resistance to Driving” (SRD) plus a Damping or Dynamic Resistance • Important analysis options include Driveability and Inspector’s Chart
Recommended Quake Values Soil Type
Pile Type or Size
All Soil Types
All Soil Types, Soft Rock In dry soils, or in very dense or hard soils In submerged soils or in loose or soft soils Hard Rock
Quake Quake (in) (mm) 0.10 2.5
Open ended pipes 0.10 Displacement Piles of D/120 diameter D or width D Displacement Piles of D/60 diameter D or width D All Types .04
2.5 D/120 D/60 1.0
DAMPING • •
The Damping option screen can be entered by using the pull down menu Options, General Options, and then Damping. The damping options include are those for the Soil, Hammer and Pile. In general, skin damping is computed according to Smith as Rd=Rs(js)v, where Rs is the static resistance at a certain time, js is the Smith damping factor and v is the pile velocity, all at one particular pile segment. GRLWEAP also offers the viscous Smith damping option: Rd=Ru(js)v, with Ru being the ultimate static resistance. Since Ru (js) is constant, this approach is equivalent to a third GRLWEAP option, the Case damping, where Rd=jc(EA/c)v, with EA/c being the pile impedance, as long as the damping constants are calculated appropriately. The first option is the most commonly used one; the second one leads to somewhat more corrective capacity results. For the third, experience or measurement results are needed to find the proper damping factor. GRLWEAP also offers two more Soil Damping Options, which are based on the exponential relationship proposed by Gibson and Coyle. Certain changes of this method were important for good agreement of computed pile top force and velocity with measured values. This led to the last Soil Damping Option which was described by Rausche.
Recommended Smith Damping Values (recommended option)
Damping Factor s/ft 0.05
Damping Factor s/m .16
In all soil types
Soil Type Shaft Damping Non-cohesive soils Toe damping
Other Damping options: Coyle and Gibson Damping: The damping is calculated using a non-linear approach, Rd = jRuvn, where n is a damping exponent according to Gibson and Coyle. This is a research option. Rausche Damping: Damping is calculated using a non-linear approach, Rd = jRa vxn (v/vx), where Ra is the activated capacity, vx is the maximum velocity, both occurring during the hammer blow, and n is the damping exponent according to Coyle and Gibson. This is a research option. Damping Exponent: Enter the exponent of the non-linear damping approach. Recommendations are 0.18 and 0.20 for clay and sand, respectively. Only required for either Coyle and Gibson or Rausche damping, a default of 0.20 is activated if no entry is made.