When Deep Foundations Are Required

Shallow foundations become impractical when: adequate bearing soils exist only at depth, total or differential settlement from shallow foundations is unacceptable, the structure is sensitive to lateral loads (bridges, tall buildings), or soft soils at the surface would cause unacceptable long-term settlement. Deep foundations transfer load to deep, competent strata through skin friction (side resistance) along the shaft and end bearing at the tip.

Pile Types and Selection

  • Steel H-piles (HP): high capacity in dense sand/rock; low displacement; good driveability; HP14×89 is a common profile. Subject to corrosion in aggressive soils.
  • Precast concrete piles: durable; square (250–600 mm) or octagonal; prestressed for tensile capacity during driving. Requires careful handling and a minimum driving energy cushion.
  • Steel pipe piles (open or closed end): high capacity; open-end pipe driven and later cleaned and filled with concrete (cast-in-steel-shell, CISS). Common in offshore and waterfront applications.
  • Drilled shafts (bored piles, caissons): large diameter (450–3000 mm) drilled and cast-in-place. No vibration; good in sites with existing structures; excellent for high lateral loads; inspectable.
  • Auger cast-in-place (ACIP / CFA): continuous flight auger drilled to refusal, then grout pumped as auger withdraws. No casing, no vibration, low noise; limited to soils without boulders.

Static Pile Capacity — General Equation

Q_ult = Q_s + Q_p = (Σ fs·As) + (qp·Ap)

where Q_s = total skin friction (kN), Q_p = tip resistance (kN), fs = unit skin friction at each layer (kPa), As = pile perimeter × layer thickness (m²), qp = unit end bearing (kPa), Ap = pile tip area (m²). Net capacity must account for pile self-weight and, for uplift, pile weight aids resistance.

Capacity in Clays — α (Total Stress) Method

The α-method (Tomlinson, API RP2A) uses undrained shear strength Su:

fs = α · Su

where α = adhesion factor. α = 1.0 for Su < 25 kPa; decreases to α ≈ 0.5 at Su = 75 kPa; α ≈ 0.4 for very stiff clays (Su > 150 kPa) due to stress relief during installation. End bearing in clay: qp = 9·Su (Skempton).

Capacity in Sands — β (Effective Stress) Method

The β-method (Meyerhof, Burland) uses effective stress:

fs = β · σ'v

where β = K · tanδ (K = lateral earth pressure coefficient after pile driving, δ = pile-soil interface friction angle). Typical β ranges: 0.25–0.40 for loose to medium sand; 0.35–0.60 for dense sand. End bearing in sand: qp = Nt · σ'v ≤ q_limit, where Nt ranges from 20 to 120 depending on relative density and pile type (limiting tip resistance is typically 4–12 MPa to avoid overestimating).

Nordlund Method for Driven Piles in Sand

FHWA's preferred method for driven pile capacity in granular soils (FHWA-NHI-16-009). Accounts for pile taper, volume of soil displaced (soil displacement coefficient Cf), and interface friction based on pile material. The method provides unit friction Ks·σ'v·sinδ integrated over depth, with Ks values tabulated against φ' and pile type. Nordlund is considered more reliable than simple β-method for tapered or H-piles in layered sands.

Drilled Shaft Design

Per FHWA publication FHWA-NHI-10-016 (Brown et al.):

  • Side resistance in clay: fs = α · Su with α = 0.55 for Su < 190 kPa; reduce by 50% in the top 1.5 m and bottom 1 diameter (1D) of the shaft where disturbance is greatest.
  • Side resistance in sand: fs = β · σ'v with β = 1.5 − 0.135√z for z in meters (Reese and Wright), not to exceed 200 kPa.
  • Tip resistance: only use if tip cleanliness can be verified (air lift or cleaning bucket) and shaft diameter > 900 mm. In soft to medium rock: qt = 3 · qu (unconfined compressive strength, where qu is rock mass strength).
  • Construction method matters: dry construction (no casing, no slurry) gives highest quality; slurry displacement method requires careful inspection of shaft cleanliness at bottom.

Pile Group Efficiency

Piles in a group interact through overlapping stress zones. Group capacity: Q_group = η · n · Q_single where η = group efficiency factor:

  • For piles in sand: η ≈ 1.0 when spacing s ≥ 3D (typical assumption); Converse-Labarre formula provides theoretical η.
  • For piles in clay: take the lesser of (1) η·n·Q_single or (2) block failure capacity = 2·Df·(B+L)·Su + B·L·9·Su (where B, L are group dimensions in plan).
  • Minimum pile spacing: 3D center-to-center for driven piles (AASHTO LRFD Section 10.7.1.2); 2.5D for drilled shafts (FHWA).

Group settlement often governs over individual pile capacity — treat the pile group as an equivalent pier and calculate settlement using consolidation theory for the deeper soil layers below the pile tip elevation.

Pile Installation Verification

Wave equation analysis (WEAP): models the stress wave generated by hammer impact to predict driving resistance and predict pile damage. Provides driving criteria (blows per foot at required resistance) before installation. Does not replace load testing but is required by AASHTO for design verification.

Dynamic monitoring (PDA / CAPWAP): strain gauges and accelerometers attached to pile during driving or high-strain dynamic testing post-driving. CAPWAP signal matching provides static capacity estimate with accuracy ±20%. Per AASHTO LRFD Article 10.5.5.2.3, resistance factor φ_dyn = 0.65 for CAPWAP vs φ_dyn = 0.40 for pile formula alone.

Static load test (ASTM D1143): most accurate capacity determination; resistance factor φ_stat = 0.75–0.80. Required for major bridge foundations; frequency may be one test per site or per soil zone.

Negative Skin Friction (Downdrag)

When soft soil surrounding a pile consolidates (due to fill placement, drawdown, or adjacent loading), it settles faster than the pile and creates downward drag force along the upper portion of the shaft. This downdrag force must be added to structural load when checking pile structural capacity, but is not added when checking geotechnical bearing capacity (it is actually mobilizing skin friction, not reducing it, at depth below neutral plane).

AASHTO LRFD Article 10.7.3.7: load factor for downdrag = 1.80 (it's treated as a permanent load with high variability). Mitigation: bitumen coating on pile surface, preboring, or installing pile after fill consolidation is complete.