Driven Pile Foundation Types
Driven piles are deep foundation elements that are installed by driving prefabricated piles into the ground using impact hammers, vibratory hammers, or hydraulic jacks. The pile material is selected based on the soil conditions, the required capacity, and the durability requirements for the specific application. Steel H-piles are wide-flange steel sections that are driven to bear on competent soil or rock layers. The steel pile sections can be spliced by welding or bolting to achieve the required length. The corrosion resistance of steel piles in aggressive soil environments is improved by increasing the section thickness or by applying protective coatings. The allowable stress design of steel piles follows the AISC specification with adjustments for the driving stresses that the pile must withstand during installation. The steel pile provides high capacity in a relatively small cross-section, making it suitable for applications where space is limited or where the piles must penetrate through dense soil layers to reach bearing strata.
Precast concrete piles are reinforced concrete elements that are cast in a factory or at the project site and driven after the concrete has achieved adequate strength. The pile cross-section is typically square or octagonal, with the reinforcement designed to resist the driving stresses and the service loads. The prestressing of concrete piles using high-strength steel strands applies compression to the concrete that counteracts the tensile stresses from handling, driving, and service loading. The prestressed pile can be manufactured in lengths up to 60 feet for transportation to the site. The pile sections are spliced using mechanical connections or welding of steel end plates to achieve the required total length. The durability of concrete piles in corrosive environments is superior to steel piles because the concrete cover protects the reinforcement from corrosion, provided the cover thickness is adequate for the exposure conditions.
Timber piles are the oldest type of driven pile, used for thousands of years in foundation construction. Timber piles are cut from straight tree trunks with the natural taper providing increasing cross-section toward the butt end. The pile tips may be fitted with a steel point to facilitate driving through dense soils and to protect the wood from damage during driving. The length of timber piles is limited by the available tree height, typically 40 to 60 feet. The piles must be driven below the water table to remain submerged and prevent decay from exposure to air and insects. The creosote treatment of timber piles extends the service life by protecting the wood from decay and marine borers. The load capacity of timber piles is limited by the wood strength, with typical capacities of 20 to 40 tons per pile. Timber piles are economical for light to moderate loads in permanent submerged applications and for temporary construction support.
Pile Driving Equipment and Methods
Impact pile hammers deliver energy to the pile top through a falling ram that strikes the pile cap. The hammer energy is determined by the ram weight and the stroke height. Diesel hammers use the energy of diesel fuel combustion to lift the ram, providing high energy output and operating independently of external power sources. The hammer energy can be adjusted by changing the fuel setting. Hydraulic hammers use hydraulic fluid to lift the ram and provide more controlled energy delivery than diesel hammers, with lower noise levels and no exhaust emissions. The hammer energy and stroke are precisely controlled by the hydraulic system. The pile cushion at the top of the pile protects the pile from damage and distributes the hammer impact over the pile cross-section. precast prestressed concrete pile manufacturing and handling. pile driving analyzer for dynamic load testing. static load test interpretation using davisson offset method. The cushion material of plywood, micarta, or aluminum is replaced periodically as it compresses and loses effectiveness.
Vibratory hammers use rotating eccentric weights to generate vertical vibrations that reduce the soil resistance at the pile tip and along the pile shaft, allowing the pile to penetrate under its own weight and the weight of the vibratory driver. The vibratory method is effective for installing piles in granular soils where the vibrations densify the soil around the pile. The frequency and amplitude of the vibrations are adjusted to match the soil conditions and the pile characteristics. The vibratory hammer is also used for extracting piles by reversing the vibration direction. The vibratory method produces lower noise levels than impact hammers and is preferred in urban areas with noise restrictions. The impact hammer is used to proof-test piles installed by vibratory methods and to achieve the final penetration resistance required by the design specifications.
The pile driving analyzer monitors the driving process by measuring the strain and acceleration at the pile top during each hammer blow. The PDA sensors transmit the data to a computer that computes the pile capacity using the Case method or CAPWAP analysis. The PDA testing provides real-time capacity estimates that guide the driving termination criteria. The wave equation analysis models the pile driving system as a series of masses and springs and predicts the pile stresses and capacities for different hammer and cushion configurations. The GRLWEAP software is the standard wave equation analysis program used in the pile driving industry. The dynamic load testing using the PDA provides capacity verification for production piles without the cost and time required for static load testing.
Pile Capacity Determination
The ultimate capacity of a driven pile is the sum of the end bearing resistance at the pile tip and the skin friction along the pile shaft. The end bearing resistance depends on the bearing capacity of the soil or rock at the pile tip elevation. The skin friction depends on the soil shear strength along the pile shaft and the pile surface characteristics. The pile capacity increases with time after driving in clay soils due to the dissipation of excess pore water pressures generated during driving. This setup effect can increase the pile capacity by 50 to 100 percent over the weeks following installation. The setup effect is measured by restrike testing where the pile is re-driven after a waiting period and the capacity is compared with the end-of-driving capacity.
The static load test is the most reliable method for determining the pile capacity. The test load is applied to the pile top using a hydraulic jack reacting against a weighted beam or anchor piles. The test load is applied in increments, and the pile head settlement is measured at each increment. The load-settlement curve is plotted and interpreted using methods such as the Davisson offset method that defines the failure load as the load at which the settlement exceeds the elastic compression of the pile plus an offset of 0.15 inches plus the pile diameter divided by 120. The static load test typically loads the pile to 200 percent of the design load to verify the factor of safety. The test pile is instrumented with strain gauges and telltales to measure the load distribution along the pile shaft and to separate the end bearing and skin friction components of the total capacity.
The dynamic load test using the PDA provides capacity estimates that are calibrated against static load tests on comparable piles at the same site. The CAPWAP analysis of the PDA data models the soil resistance distribution along the pile and provides capacity estimates that agree with static load tests within 10 to 20 percent when properly calibrated. The dynamic testing is faster and less expensive than static testing, allowing a larger percentage of the production piles to be tested. The typical testing program for a driven pile project includes static load tests on 1 to 2 percent of the piles and dynamic tests on 5 to 10 percent of the piles. The combination of static and dynamic testing provides reliable capacity verification at reasonable cost.
Pile Group Effects and Design
Pile groups transfer the superstructure loads to the foundation through multiple piles connected by a reinforced concrete pile cap. The interaction between closely spaced piles reduces the group capacity compared to the sum of the individual pile capacities. The group efficiency factor accounts for this reduction and depends on the pile spacing, the pile arrangement, and the soil type. The minimum center-to-center spacing of piles is typically 3 pile diameters for end-bearing piles and 3 to 4 diameters for friction piles. The group settlement of friction piles in clay soils is greater than the settlement of a single pile under the same average load because the stress bulbs from adjacent piles overlap in the soil below the pile tips.
The design of pile groups considers the distribution of the column loads to the individual piles in the group. The piles under a column are arranged symmetrically around the column centerline to distribute the load evenly. The eccentricity of the column load relative to the pile group centroid creates unequal pile loads that must be considered in the design. The overturning moment from lateral loads is resisted by the piles in the group through a combination of axial tension and compression in the piles and lateral resistance from the pile bending and the soil reaction. The pile cap must be designed to transfer the column loads and moments to the piles and to resist the punching shear and flexural forces from the pile reactions. The reinforcement in the pile cap is designed for the critical sections at the column face and at the pile locations.