When surface soils are too weak, compressible or exposed to scour, engineers turn to piles. A pile is a slender structural element, made of concrete, steel or timber, that transfers loads to deeper, stronger ground. Piles support bridges, high-rise buildings, jetties, industrial plants and lightweight structures on bad ground. This article explains the main pile types, how pile capacity is designed from soil data, and how load testing confirms that a pile performs as expected.
How Piles Carry Load
Piles resist loads in two ways. End bearing is the resistance at the pile tip, strongest when the pile rests on dense sand, gravel or rock. Shaft (skin) friction is the shear resistance along the pile surface, mobilised in both clay (as adhesion) and sand (as friction). A pile that mostly relies on its tip is called an end-bearing pile, and one that relies on the shaft is a friction pile. Most real piles use both.
Ultimate capacity is the sum of the two parts: Qu = Qs + Qb. Shaft resistance usually mobilises at very small movement (about 0.5 to 1% of pile diameter), while full end bearing needs larger movement (about 5 to 10% of diameter), which is why settlement and capacity must be considered together.
Types of Pile Foundations
Driven piles are installed by hammering, vibrating or pushing the pile into the ground.
- Precast reinforced or prestressed concrete piles: durable and economical, common for marine and onshore work. They are heavy and can crack if hard driving occurs.
- Steel H-piles and tubular piles: easy to handle and splice, strong in bending, suitable for hard driving and long lengths. Corrosion protection is needed in aggressive environments.
- Timber piles: cheap and light, used for temporary or low-load works, but vulnerable to decay above the water table.
Bored (drilled) piles are formed by drilling a hole and filling it with reinforced concrete.
- Conventional bored cast-in-situ piles: use casing or bentonite slurry to support the hole. Diameters of 0.6 to 2.5 m are common, with large capacities and little noise or vibration.
- Continuous Flight Auger (CFA) piles: a hollow-stem auger is drilled to depth, then concrete is pumped through the stem as the auger is withdrawn. They are fast and quiet, and well suited to urban sites.
Screw piles and helical piles are steel shafts with helical plates that are rotated into the ground, useful for light loads, tension and low-headroom sites.
Micropiles are small-diameter (typically under 300 mm) grouted piles with high-strength steel reinforcement, used for underpinning, restricted access and difficult ground.
Selecting a Pile Type
Selection depends on soil profile, loads, access, environment and cost. Driven piles are good in sands and gravels but cause vibration and noise, which is a problem near existing buildings. Bored piles suit stiff clays, weathered rock and urban sites, but need careful control of concrete quality and base cleaning. CFA piles are efficient in cohesive and loose to medium-dense granular soils above rock. Rock sockets are used where piles must be anchored into bedrock. In black cotton soil, piles must pass through the active zone and be designed for uplift due to swelling.
Design Method: Capacity from Soil Data
Design begins with a ground investigation (boreholes, SPT or CPT) and laboratory tests to establish soil layers, strength and groundwater.
Piles in clay (total stress, α-method): shaft resistance is Qs = Σ α cu As, where cu is undrained shear strength, α is an adhesion factor (typically 0.3 to 1.0, decreasing with higher cu), and As is shaft area. End bearing is qb = Nc cu, with Nc ≈ 9 for deep piles.
Piles in sand (effective stress, β-method): unit shaft friction fs = β σ′v, with β = K tan δ, where K is the earth pressure coefficient, δ is the pile-soil friction angle and σ′v is vertical effective stress. End bearing is qb = Nq σ′v, with limits applied to unit values because resistance does not increase indefinitely with depth (critical depth concept). CPT-based methods and SPT correlations are also widely used.
Piles in rock: socket resistance depends on rock strength, roughness and socket length. Unit values are often related to unconfined compressive strength and RQD, with end bearing taken as a fraction of rock strength.
Factor of safety and limit states: allowable capacity is typically Qult / 2 to 3, with higher values where there is no load testing. Under Eurocode 7, characteristic resistances are divided by partial factors, with correlation factors that reduce when more tests are carried out.
Pile Groups and Group Effects
Piles are rarely used alone. Groups of piles are connected by a pile cap, and their behaviour differs from single piles:
- Group efficiency: in clay, a block failure of the whole group can govern, and capacity may be less than the sum of single piles. Typical minimum spacing is 2.5 to 3 times pile diameter.
- Group settlement: the stressed zone below the group is deeper and wider, so group settlement is larger than single-pile settlement. It is often estimated with an equivalent raft method.
- Lateral loads: wind, earthquake, vessel impact or earth pressure require analysis of pile-soil interaction (p-y curves or subgrade reaction methods) and structural design for bending moments.
Special Design Issues
Negative skin friction (downdrag): when soil around a pile settles more than the pile (for example, due to fill on soft clay), it drags the pile downward and adds load. This must be added to the design load or reduced using bitumen coatings or sleeves.
Uplift: swelling soils, buoyancy and overturning cause tension in piles; capacity is generally lower than in compression.
Durability: concrete cover, sulphate-resisting cement and steel corrosion allowances are specified according to ground and groundwater chemistry.
Buckling: rarely governs, except in very soft soils or where piles are exposed above ground or scour level.
Pile Load Testing
Because pile capacity prediction carries uncertainty, load testing is used to verify design and construction.
Static load test (ASTM D1143, BS EN ISO 22477-1): a maintained load test applies compressive load in increments using a hydraulic jack against a reaction system of kentledge (dead weight) or anchor piles. Piles are normally tested to 150% to 200% of working load, and settlement is recorded against time and load. Results give load-settlement behaviour, ultimate capacity and creep. It is the most reliable method but is slow and expensive. Tension and lateral static tests follow similar principles.
Bi-directional (Osterberg cell) test: a hydraulic cell is cast into the pile and pushes the upper and lower parts against each other. This allows very high loads without reaction frames and separates shaft and base resistance.
Dynamic load test (ASTM D4945), also known as PDA testing: a heavy drop weight strikes the pile head while strain gauges and accelerometers measure force and velocity. Signal matching (such as CAPWAP) estimates capacity and driving stresses. It is quicker and cheaper than static testing, so more piles can be tested.
Rapid load testing (Statnamic): a longer-duration pulse than dynamic testing, giving results closer to static behaviour.
Integrity testing: the Pile Integrity Test (PIT) or low-strain test (ASTM D5882) taps the pile head with a small hammer and analyses reflected waves to detect necks, cracks or voids. Cross-hole sonic logging (CSL) (ASTM D6760) sends ultrasonic signals between tubes cast into large bored piles to detect defects in the concrete. These tests check quality, not capacity.
A typical specification tests a small percentage of working piles (often 1 to 2%) statically, a larger share dynamically, and all or most piles for integrity.
Common Problems and How to Avoid Them
- Unexpected ground conditions: add boreholes at pile locations for critical structures.
- Poor base cleaning in bored piles, giving soft toe and settlement.
- Concrete defects: use tremie pipes, control slump and over-pour to remove contaminated concrete.
- Hole collapse: use casing or properly managed bentonite.
- Driving damage: use correct hammer energy and pile cushions, with driving records for every pile.
Conclusion
Piles are a powerful solution for weak, variable or reactive soils, and their performance depends on matching pile type to ground, estimating capacity from reliable soil data, accounting for group effects and special problems, and verifying results through testing. A well-planned programme of static, dynamic and integrity tests turns design predictions into confirmed performance and gives owners confidence in the foundation for the life of the structure.
References and further reading
- Tomlinson, M. and Woodward, J. Pile Design and Construction Practice
- Fleming, K., Weltman, A., Randolph, M., Elson, K. Piling Engineering
- Poulos, H.G. and Davis, E.H. Pile Foundation Analysis and Design
- EN 1997-1, Eurocode 7; BS EN 1536 (bored piles); BS EN 12699 (displacement piles)
- ASTM D1143, D4945, D5882, D6760
- FHWA (2016). Design and Construction of Driven Pile Foundations
