Steel Pipe Piles

Per API 2A-WSD Section 6.3.2, ultimate axial capacity Qu = Qs + Qt with safety factor 2.0 for static design.

Skin friction (Qs) per API 2A-WSD Section 6.3.3:

(1) Alpha method (clay): Qs = α × Su × As, where α = 0.5 for Su ≤ 25kPa, α = 0.5-0.25 for Su = 25-150kPa, α = 0.25 for Su ≥ 150kPa per API 2A-WSD Table 6.3.3-1

(2) Beta method (sand): Qs = β × σ'v × As, where β = 0.25-0.35 for low-density, 0.35-0.55 for medium, 0.55-0.65 for high-density sand per API 2A-WSD Table 6.3.3-2

Tip resistance (Qt) per API 2A-WSD Section 6.3.4:

(1) Cohesionless: Qt = Nq × σ'v × At, Nq = 8-50 per API 2A-WSD Figure 6.3.4-1 based on friction angle φ = 28-40°

(2) Cohesive: Qt = Su × Nc × At, Nc = 9.0 for deep foundations per Skempton (confirmed by API 2A-WSD Section 6.3.4.2)

Static load test acceptance per ASTM D1143/D1143M-07 Section 7.2: maintain test load at 200% design load for minimum 12h with creep ≤ 0.25mm/h over final 1h. Load-settlement curve per Section 8: Davisson offset limit = 0.15in + (D/120) for pipe piles.

Site investigation per ASTM D1586: SPT N-value every 1.5m to 2× pile embedment depth. CPT per ASTM D5778: minimum qc profile required for β method calibration.

Per API 2A-WSD Section 6.5.1, open-ended pipe pile capacity depends on plugging ratio PLR = (internal soil column length) / (pile penetration depth). Three plugging states control end-bearing contribution:

(1) Unplugged (PLR < 0.7): soil column height < 70% of penetration — internal friction Qs_internal contributes but no tip plug. Tip capacity calculated on steel annulus area only per API 2A-WSD Eq 6.5.1-1.

(2) Partially plugged (PLR = 0.7-0.95): soil compresses inside, internal friction develops but soil core does not fully engage at tip. Qplug = Qs_internal + 0.5 × Qt_plug per ISO 19901-4 Section 8.3.3.

(3) Fully plugged (PLR ≥ 0.95): soil column nearly full penetration — pipe behaves as closed-ended. Full cross-section tip resistance per API 2A-WSD Section 6.5.2: Qt = Nq × σ'v × (π × D²/4).

Plugging criteria per API 2A-WSD Table 6.5.2-1: plug will form when D₅₀ (mean particle size) / D (pile OD) ≥ 0.002 for sand. For ID > 900mm in loose sand, plug rarely forms — use unplugged calculation per Section 6.5.1.2.

Incremental filling ratio (IFR) per ISO 19901-4 Annex C: IFR = d(soil column)/d(penetration) — IFR ≤ 0.5 typically indicates plugging; IFR ≥ 0.9 means free soil entry. PDA measurements per ASTM D4945 confirm plugging state during final driving.

PDA per ASTM D4945 Section 5.1 measures real-time pile response during driving:

Measurements:

(1) Strain — force at pile head gages per Section 7.2 — strain resolution ±0.01%

(2) Acceleration — velocity integration per Section 7.3 — double integration for displacement

(3) Max compression and tension stress — per API 2A-WSD Section 6.6.3: compression ≤ 0.9Fy, tension ≤ 0.9Fy to prevent yield damage

(4) CAPWAP analysis per ASTM D4945 Annex A — signal matching of measured force vs velocity to derive shaft resistance distribution and tip bearing

Driving criteria per API 2A-WSD Section 6.6.2:

(1) Blow count / final set — refusal at 5mm per 10 blows for sand (Class 1), measured by PDA displacement transducer

(2) Hammer energy — computed from force × velocity integral per Section 8.1.2: EH ≥ 60kJ for Class 1 driving

(3) Pile impedance match — hammer energy transfer efficiency ≥ 40% per GRLWEAP analysis for diesel/hydraulic hammers

Restrike criteria per Section 6.6.5: perform restrike ≥ 24h after end of initial driving. Setup ratio SR = capacity_restrike / capacity_EOID — sand SR = 1.0-1.15, clay SR = 1.5-4.0 per API 2A-WSD Figure 6.6.5-1. Minimum restrike blows: 10-20 blows or until final set rate < 1mm/blow.

Per API 2A-WSD Section 6.4.3, negative skin friction (NSF) is the downward drag force from consolidating soil surrounding the pile shaft. Neutral plane (NP) per ISO 19901-4 Section 9.5 — the depth where soil settlement equals pile settlement; NSF acts above NP, positive friction below.

NSF calculation per API 2A-WSD Eq 6.4.3-2: F_nsf = β × σ'v × As, where β = 0.10-0.25 for normally consolidated clay and 0.15-0.30 for fill material. For soft sensitive clay (Su < 25kPa), α-red = 0.3-0.5 × Su (reduced from α = 1.0 for positive skin friction per API 2A-WSD Section 6.4.3.2).

Trigger criteria per API 2A-WSD Section 6.4.3.1: NSF must be considered when surface settlement exceeds 10mm or when fill height > 1m within pile influence zone (≈ 3 × pile group width).

Peat/organic layers per Eurocode 7 Section 7.3.2.2: compression index Cc ≥ 0.3, settlement up to 200mm per meter of peat thickness — NSF can reach 50-80% of positive shaft capacity.

Mitigation per API RP 2A Section 6.4.3.3:

(1) Bitumen coating — reduces NSF adhesion to 5-10kPa per tested interface per ASTM D5321; shear strength ≤ 10kPa for bitumen temperature range -10°C to +40°C

(2) Sleeved pile section — separates shaft from surrounding soil for top 5-10m

(3) Pre-drilling/jetting — reduces driving resistance but does not eliminate NSF

Design acceptance: per API 2A-WSD Section 6.4.3.4, NSF load combined with axial load must not exceed structural capacity with safety factor ≥ 1.5. For large groups, group NSF = 0.5-0.8 × individual NSF due to arching per API 2A-WSD Figure 6.4.3.

Pile splices must develop 100% pipe strength:

Splice types:

(1) Full-penetration groove weld — most common, develops 100%

(2) Partial-penetration groove — not recommended for tension

(3) Sleeve weld — full circumference required

Groove weld design:

(1) Full V or U groove — complete root penetration

(2) Back-gouging — remove root flaws for complete fusion

(3) Multi-pass welding — fill groove completely

Strength verification:

Tensile test of splice joint (full pipe vs spliced pipe)

Acceptance: ≥ 100% of pipe minimum yield

Xenith Steel welders hold AWS D1.1 certification.

Per API 2A-WSD Section 6.6 and EN 12699 Section 7.5, driving criteria are soil-dependent:

Sand and gravel (Driving Class 1 per API 2A-WSD Table 6.6-1):

(1) Blow count criteria per API 2A-WSD Section 6.6.2.1: final set ≤ 10mm per 10 blows (EH ≥ 60kJ hammer energy). Refusal at 5mm per 10 blows with max stress ≤ 0.9 × yield per Section 6.6.3.

(2) Driving stress limits per API 2A-WSD Eq 6.6.3-1: compression stress σc ≤ 0.9Fy (Fy = min yield), tension stress σt ≤ 0.9Fy during hammer rebound. Measured by PDA per ASTM D4945 Section 7.5 — max stress recorded at pile head gages.

(3) Setup factor per API 2A-WSD Section 6.6.4: sand gains 0-15% capacity in 24h (negligible setup); restrike after ≥ 24h confirms final capacity per API 2A-WSD Section 6.6.5.

Clay (Driving Class 2):

(1) Sensitivity ratio St = Su_undisturbed / Su_remolded per ASTM D4648 — high sensitivity clay (St > 8) gains 100-300% setup over 7-30 days per API 2A-WSD Figure 6.6.5-1

(2) Pore pressure dissipation: Blow counts may drop 30-50%, then rise as pore pressure dissipates. Drive must continue to 2× estimated depth per API 2A-WSD Section 6.6.2.2 to bypass remolded zone.

(3) Soil adhesion on pipe wall: per API 2A-WSD Table 6.6-2, adhesion stress τ_ad = 5-30kPa for clay end-bearing (reduces driveability).

Rock (Driving Class 4):

(1) Predrilling per API 2A-WSD Section 6.6.6.1: socket depth = 1-3 × OD into competent bedrock (UCS ≥ 50MPa per ASTM D7012). Minimum rock socket length for end-bearing: 1.5 × OD when UCS = 50-100MPa.

(2) Rock shoe (drive shoe) per EN 12699 Section 7.4.2: hardened steel tip with 30-45° bevel, wall thickness 1.5× pipe WT. Maximum driving stress with rock shoe ≤ 0.75Fy per API 2A-WSD Section 6.6.6.2.

(3) Wave equation analysis (GRLWEAP) per ASTM D4945 Annex A: required for all Class 4 driving — match hammer energy, cushion stiffness, and pile impedance within 25% of measured values per Section A.3.