INTRO TO GRLWEAP

WAVE EQUATION ANALYSIS PROGRAM (WEAP)

Deep Foundations Course

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.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:• 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)• 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/Lcompression) 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)– 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– 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.– 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).

Assign ram Assign ram velocity and velocity and

Flow Diagram RLWEAPBearing Graph

Model hammer &Model hammer &driving system & driving system & pilepile

Distribute RuDistribute RuSet Soil ConstantsSet Soil ConstantsTime IncrementTime Increment

Increase Ru Increase Ru InputInput

analyze pile/soilanalyze pile/soil

Model PileModel Pile •• Pile stressesPile stresses•• Energy transferEnergy transfer•• Pile velocitiesPile velocities

Choose first Ru Choose first Ru Calculate BlowCalculate BlowCountCount

Output Output

IncreaseIncreaseRRuu??

NN

Bearing Graph:Vulcan 506; HP 12x53; Clay/Sand

Bearing Graph Comparison

Bearing Graphs from Formulas and Wave Equation

3000350040004500

Cap

acity

in k

N

Ru-Gates

0500

10001500200025003000

0 5 10 15 20

Blows/25 mm

Cap

acity

in k

N

Ru-Gates

ENR - inferred

GW-Sand

GW-Clay

Comparison Pile Capacity Estimates from different Methods

At Blow Count of At Blow Count of 55 88 blows/25 mm:blows/25 mm:

GatesGates 19501950 22502250 kNkN

ENRENR 19301930 25702570 kNkNENRENR 19301930 25702570 kNkN

GWGW--SandSand 17501750 19801980 kNkN

GWGW--ClayClay 13901390 15801580 kNkN

Note: GW = GRLWEAP

Blow count comparison

Ultimate CapacityUltimate Capacity 1335 1335 1780 1780 kNkN

Nominal Safe Cap:Nominal Safe Cap: 75 75 100 100 tonstons

GatesGates 1.91.9 3.83.8 BlBl/25mm/25mmGatesGates 1.91.9 3.83.8 BlBl/25mm/25mm

ENRENR 3.03.0 4.44.4 BlBl/25 mm/25 mm

GWGW--SandSand 2.4 2.4 5.35.3 BlBl/25 mm/25 mm

GWGW--ClayClay 4.3 4.3 14.314.3 BlBl/25 mm/25 mm

Inspector’s Chart

Driveability Graph

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– 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 Quake (in)

Quake (mm)

Shaft Quake All Soil Types All Types 0.10 2.5 Toe Quake All Soil Types, Soft Rock Open ended pipes 0.10 2.5

In dry soils, or in very dense or hard soils

Displacement Piles of diameter D or width D D/120 D/120 dense or hard soils diameter D or width D D/120 D/120

In submerged soils or in loose or soft soils

Displacement Piles of diameter D or width D D/60 D/60

Hard Rock All Types .04 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 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.

Soil Type Damping Factor s/ft

Damping Factor s/m

Shaft Damping Non-cohesive soils 0.05 .16 Cohesive soils 0.20 .65

Recommended Smith Damping Values (recommended option)

Cohesive soils 0.20 .65 Toe damping In all soil types 0.15 0.50

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.