# Deep Foundation Systems ## Table of Contents - [Introduction](#introduction) - [When Deep Foundations Are Required](#when-deep-foundations-are-required) - [Pile Types by Installation Method](#pile-types-by-installation-method) - [Driven Piles](#driven-piles) - [Bored Piles](#bored-piles) - [Continuous Flight Auger (CFA) Piles](#continuous-flight-auger-cfa-piles) - [Micropiles](#micropiles) - [Pile Load-Carrying Mechanisms](#pile-load-carrying-mechanisms) - [End-Bearing Piles](#end-bearing-piles) - [Friction Piles](#friction-piles) - [Combined End-Bearing and Friction](#combined-end-bearing-and-friction) - [Pile Design](#pile-design) - [Ultimate Pile Capacity](#ultimate-pile-capacity) - [Pile Capacity Formulas](#pile-capacity-formulas) - [Factor of Safety](#factor-of-safety) - [Pile Cap Design](#pile-cap-design) - [Pile Group Arrangements](#pile-group-arrangements) - [Pile Cap Sizing and Reinforcement](#pile-cap-sizing-and-reinforcement) - [Group Effects](#group-effects) - [Efficiency of Pile Groups](#efficiency-of-pile-groups) - [Settlement of Pile Groups](#settlement-of-pile-groups) - [Negative Skin Friction](#negative-skin-friction) - [Causes](#causes) - [Design Implications](#design-implications) - [Lateral Load Capacity](#lateral-load-capacity) - [Short vs Long Piles](#short-vs-long-piles) - [Design Methods](#design-methods) - [Raking Piles](#raking-piles) - [Pile Testing](#pile-testing) - [Integrity Testing](#integrity-testing) - [Static Load Tests](#static-load-tests) - [Dynamic Load Tests](#dynamic-load-tests) - [Selection Criteria](#selection-criteria) - [Comparison of Pile Types](#comparison-of-pile-types) - [Factors Influencing Selection](#factors-influencing-selection) - [Practical Notes for Architects](#practical-notes-for-architects) - [Related Topics](#related-topics) - [References](#references) --- ## Introduction Deep foundations transfer building loads through weak or compressible near-surface soils to stronger strata at depth, or distribute loads over a large depth through skin friction. They are required when shallow foundations cannot provide adequate bearing capacity or when settlement on near-surface soils would be unacceptable. For the architect, the choice of pile type and layout has significant implications for cost, programme, site logistics, noise and vibration impacts, and coordination with basement structures. Deep foundations predominantly take the form of piles — slender structural elements driven or bored into the ground — though other systems such as caissons and barrettes exist for special applications. ## When Deep Foundations Are Required Deep foundations are typically necessary when: - Near-surface soils have **inadequate bearing capacity** for the imposed loads - Excessive **settlement** would occur on compressible strata (soft clay, peat, loose fill) - Competent bearing stratum lies **below the practical depth** of shallow foundations (typically >3m) - **Lateral loads** (wind, seismic, earth pressure) cannot be adequately resisted by shallow foundations - **Scour** from water flow could undermine shallow foundations (bridges, waterfront structures) - The structure is **sensitive to differential settlement** (precision equipment, connected structures) - **Uplift forces** from wind, buoyancy, or overturning must be resisted - Adjacent excavation or tunnelling could affect shallow foundations - The site contains **deep fill, contaminated ground, or cavities** that must be bridged ## Pile Types by Installation Method ### Driven Piles Piles installed by driving with a hammer (drop hammer, hydraulic hammer, vibratory driver): **Precast concrete piles:** - Typically 250-450mm square or octagonal cross-section - Lengths: 6-25m (joined sections can extend further) - Reinforced or prestressed - Advantages: High quality (factory made), can be inspected before driving, robust - Disadvantages: Noise and vibration during driving, handling length constraints, difficult in hard strata **Steel H-piles:** - Standard UC or H-pile sections - Can be driven through dense soils and weak rock - Easy to splice for long lengths - Good for end-bearing on rock - Susceptible to corrosion (loss of section thickness: 0.01-0.05mm/year depending on soil) **Steel pipe piles:** - Circular hollow sections, open or closed-ended - Open-ended: soil enters the tube (plugging behaviour at capacity) - Closed-ended: displaces soil like a solid pile - Large diameters available (up to 2m+ for marine applications) **Timber piles:** - Traditional material, limited to light loads and short lengths - Must be maintained permanently below the water table to prevent decay - Diameters: 200-400mm; lengths: 6-15m ### Bored Piles Piles formed by boring a hole in the ground and filling it with reinforced concrete: **Rotary bored piles:** - Diameters: 450-2400mm (up to 3000mm for special applications) - Depths: up to 60m+ - Casing or drilling fluid (bentonite) used to maintain borehole stability - Base can be enlarged (under-reamed/belled) in stable cohesive soils - Advantages: No vibration, any diameter and depth, can be founded on rock - Disadvantages: Slower than driving, spoil disposal, groundwater can cause problems, quality depends on workmanship **Secant and contiguous piled walls:** - Interlocking (secant) or closely spaced (contiguous) bored piles forming a retaining wall - Used for basement construction — combine retaining and foundation functions - See [[Basement Construction]] for detailed treatment ### Continuous Flight Auger (CFA) Piles CFA piles are formed by drilling a hollow-stem continuous flight auger to the required depth, then pumping concrete through the stem as the auger is extracted. - **Diameters:** 300-1200mm (commonly 450-750mm) - **Depths:** up to 25-30m - **Advantages:** Fast installation (15-30 minutes per pile), low vibration, low noise, no casing or bentonite required, suitable for most soil conditions - **Disadvantages:** Cannot penetrate hard rock or dense gravel, depth limited by auger torque, quality assurance relies on monitoring equipment - **Most common pile type** in UK and European commercial construction due to speed and economy ### Micropiles Small-diameter piles (typically 100-300mm), usually drilled and grouted with a central reinforcing bar or tube. - **Capacity:** 100-1000 kN per pile - **Applications:** Underpinning existing foundations, restricted-access sites, sites sensitive to vibration, temporary works, supplementary piling - **Advantages:** Can be installed in confined spaces with small rigs, low vibration - **Disadvantages:** Limited capacity per pile (many piles needed for heavy loads), cost per kN of capacity is higher ## Pile Load-Carrying Mechanisms ### End-Bearing Piles Transfer load primarily through the pile base to a competent stratum (dense sand, gravel, rock). The pile acts as a column, carrying the load through its shaft to the bearing stratum. - Suitable when a strong bearing layer exists at a definable depth - Pile length is determined by the depth to the bearing stratum - Settlement is generally small (governed by elastic compression of the pile and base deformation) ### Friction Piles Transfer load primarily through skin friction (shaft resistance) along the pile length. Used where no distinct hard bearing layer exists but the soil has reasonable shear strength throughout. - Load capacity increases with pile length (more shaft area) - Settlement is larger than for end-bearing piles - Common in deep deposits of stiff clay (e.g., London Clay) ### Combined End-Bearing and Friction Most piles derive capacity from both shaft friction and end bearing. The relative contribution depends on soil conditions and pile type. In typical UK conditions (glacial till, London Clay), shaft friction provides 60-80% of the capacity. ## Pile Design ### Ultimate Pile Capacity **Qult = Qs + Qb** Where: - Qs = shaft resistance = Σ (qs,i × As,i) — sum of shaft friction over all soil layers - Qb = base resistance = qb × Ab - qs,i = unit shaft friction in layer i - As,i = shaft area in layer i (π × D × Li) - qb = unit base resistance - Ab = base area (π × D²/4) ### Pile Capacity Formulas **In cohesive soils (undrained):** - Shaft friction: qs = α × cu (where α = adhesion factor, typically 0.3-1.0) - Base resistance: qb = Nc × cu,base (where Nc ≈ 9 for deep piles) **In granular soils:** - Shaft friction: qs = Ks × σ'v × tan(δ) (where Ks = lateral earth pressure coefficient, δ = pile-soil friction angle) - Base resistance: qb = Nq × σ'v,base (where Nq depends on the friction angle φ') **Eurocode 7 approach:** Allows design based on ground test results (calculated capacity with partial factors), pile load test results, or a combination. Model factors (ξ values) depend on the number and type of test profiles. ### Factor of Safety | Design Method | Typical FoS | Notes | |---|---|---| | Working stress design (traditional) | 2.0-3.0 | Global FoS on calculated capacity | | EC7 (with load tests) | Partial factors | ξ₁, ξ₂ on test results; γt on resistance | | EC7 (calculated) | Partial factors | γb on base (1.25-1.6), γs on shaft (1.0-1.3) | | ASCE/IBC (with load tests) | 2.0 | On tested capacity | ## Pile Cap Design ### Pile Group Arrangements Pile caps connect groups of piles to a single column. Standard arrangements: | Number of Piles | Arrangement | Minimum Cap Size (for 600mm dia piles) | |---|---|---| | 1 | Single pile (used only if laterally restrained) | N/A — ground beam provides restraint | | 2 | Line pair | ~2.4m × 1.2m | | 3 | Triangle | ~2.4m × 2.1m | | 4 | Square | ~2.4m × 2.4m | | 5 | Square + centre | ~2.4m × 2.4m | | 6 | 2×3 rectangle | ~3.6m × 2.4m | **Pile spacing:** Minimum 3 × pile diameter centre-to-centre (to avoid group interaction reducing individual pile capacity). Edge distance from pile centre to cap edge: ≥ pile diameter. ### Pile Cap Sizing and Reinforcement Pile cap design is typically based on the **strut-and-tie model** (truss analogy): - Compression struts from the column to each pile - Tension ties (reinforcement) across the bottom of the cap connecting pile positions - Cap depth governed by the angle of the compression strut (typically 40-60° to horizontal) Minimum depth: often 500-1000mm for caps with 2-4 piles. Reinforcement is placed as bundles or bars centred over pile positions. ## Group Effects ### Efficiency of Pile Groups When piles are closely spaced, the stress zones around individual piles overlap, reducing the group capacity below the sum of individual pile capacities: **Group efficiency η = Qgroup / (n × Qsingle)** Where η ≤ 1.0 for friction piles in clay. For end-bearing piles on rock or dense sand, η ≈ 1.0. For friction piles at 3D spacing, η is typically 0.65-0.85 in clay. ### Settlement of Pile Groups Pile groups settle more than single piles because the stressed zone extends deeper. The equivalent raft method treats the pile group as an equivalent shallow foundation at a depth of 2/3 of the pile length (for friction piles) and calculates settlement using shallow foundation methods. Group settlement can be 2-5 times the single pile settlement, depending on pile spacing and group size. ## Negative Skin Friction ### Causes Negative skin friction (downdrag) occurs when the soil surrounding the pile settles more than the pile, dragging the pile downward. This imposes an additional load on the pile and its base. Common causes: - **Consolidation of soft clay** under fill or embankment surcharge - **Recent fill** that is still consolidating under its own weight - **Lowering of the water table** causing consolidation of compressible soils - **Removal of overburden** followed by reloading (e.g., basement excavation then building construction) ### Design Implications - Negative skin friction converts shaft friction from a **resistance** to an **applied load** over the settling portion of the pile - The neutral plane (where pile and soil settlements are equal) determines the division between positive and negative friction zones - Pile structural capacity must resist the sum of the applied load **plus** the downdrag force - Pile base must support the full load (applied + downdrag) without exceeding bearing capacity - Coating piles with bitumen or using slip layers can reduce downdrag forces ## Lateral Load Capacity ### Short vs Long Piles - **Short piles** resist lateral loads by rotating as a rigid body, mobilising passive soil resistance along their length - **Long piles** (most structural piles) form a plastic hinge in the pile shaft; the soil below the hinge zone provides fixity The transition between short and long behaviour depends on the pile-soil relative stiffness. ### Design Methods - **Broms' method:** Hand calculation method for ultimate lateral capacity of single piles in cohesive or cohesionless soils - **p-y curve method:** Models the soil as a series of non-linear springs along the pile length. The most widely used method for detailed design (e.g., LPILE software) - **Eurocode 7:** Requires lateral load design considering soil-structure interaction ### Raking Piles Raking (inclined/battered) piles resist lateral loads through their axial capacity component: - Rake angle: typically 1 in 6 to 1 in 3 (9.5° to 18.4°) - Advantages: Efficient lateral load resistance using pile axial capacity - Disadvantages: More complex installation, greater ground disturbance, cannot accommodate settlement without bending forces - Not recommended in seismic zones (raking piles are prone to failure at the pile head during differential ground movement) ## Pile Testing ### Integrity Testing Low-cost tests to verify pile shaft quality: - **Sonic echo (low strain):** Hammer strikes the pile head; reflected waves reveal defects (necking, bulging, breaks). Cost: minimal. Tests 100% of piles - **Cross-hole sonic logging (CSL):** Ultrasonic pulses between tubes cast into the pile reveal concrete quality. For large-diameter bored piles - **Thermal integrity profiling (TIP):** Measures heat of hydration to detect cross-section variations ### Static Load Tests The gold standard for pile capacity verification: - **Maintained load test:** Load applied in increments, maintained for specified durations (typically 1.5-3× design load). Settlement measured at each stage - **Constant rate of penetration (CRP):** Pile pushed at constant rate until failure - **Bi-directional (Osterberg) cell:** Hydraulic jack cast into the pile separates shaft and base resistance. Eliminates the need for kentledge or reaction piles - Cost: significant (requires reaction system); typically 1-3 per project - Required by most codes for large projects ### Dynamic Load Tests - **High-strain dynamic testing (PDA):** Measures force and velocity at the pile head during a hammer blow. CAPWAP analysis derives static capacity - Faster and cheaper than static tests - Good correlation with static tests when calibrated - Can test many piles economically - Typically used for driven piles, but can be adapted for bored piles with a drop hammer ## Selection Criteria ### Comparison of Pile Types | Criterion | Driven Precast | Bored | CFA | Micropile | |---|---|---|---|---| | Diameter range | 250-600mm | 450-2400mm | 300-1200mm | 100-300mm | | Typical capacity | 500-3000 kN | 1000-15000 kN | 500-5000 kN | 100-1000 kN | | Noise / vibration | High | Low | Low | Very low | | Installation speed | Fast | Slow | Very fast | Moderate | | Spoil production | None | Large volume | Moderate | Small | | Quality assurance | Good (prefabricated) | Depends on workmanship | Relies on monitoring | Good | | Cost per kN | Low-moderate | Moderate-high | Low | High | | Suitable in rock | H-piles only | Yes (rock socket) | No | Yes (drilled) | ### Factors Influencing Selection 1. **Ground conditions:** Soil type, groundwater, obstructions, contamination 2. **Structural loads:** Magnitude, type (compression, tension, lateral) 3. **Adjacent structures:** Sensitivity to vibration and ground movement 4. **Access:** Rig size, headroom, working platform requirements 5. **Environmental constraints:** Noise, vibration, spoil disposal, contamination 6. **Programme:** Speed of installation vs curing time 7. **Cost:** Direct cost, programme cost, risk-adjusted cost ## Practical Notes for Architects 1. **Pile layout affects column grid freedom.** Pile caps for large groups (4+ piles) can be 2.5-3.5m across — this may conflict with basement planning 2. **Piling rigs require adequate access.** CFA rigs need approximately 3-4m clear width and 10-15m headroom. Low-headroom rigs are available for restricted sites at premium cost 3. **Vibration from driven piles** can damage adjacent historic or sensitive buildings — bored or CFA piles are often mandated in urban areas 4. **Piling mat:** A crushed-stone working platform (300-600mm thick) is required for piling rigs — this affects site levels and earthworks 5. **Programme:** Piling is on the critical path — delays in piling delay the entire project. Allow 4-8 weeks for a typical piling contract (depending on number of piles) 6. **Pile protrusion above cut-off level** must be broken down to expose reinforcement for connection to pile caps — coordinate cut-off levels with ground beam and slab levels 7. **Coordinate pile positions with basement walls and services** — piles cannot be moved once installed, so early coordination is essential ## Related Topics - [[Shallow Foundation Design]] - [[Pile Foundation Types]] - [[Basement Construction]] - [[Soil Mechanics for Architects]] - [[Load Path and Load Combinations]] ## References - EN 1997-1: Eurocode 7 — Geotechnical Design: General Rules - EN 1536: Execution of Special Geotechnical Works — Bored Piles - EN 12699: Execution of Special Geotechnical Works — Displacement Piles - ICE Specification for Piling and Embedded Retaining Walls, 3rd Edition - Tomlinson, M.J. and Woodward, J., *Pile Design and Construction Practice*, CRC Press - Fleming, K. et al., *Piling Engineering*, CRC Press --- #structures #foundations #deep-foundations #piles #bored-piles #driven-piles #CFA #micropiles