Deep foundations transfer structural loads past unsuitable surface soils to competent bearing material at depth. They are used when shallow foundations cannot achieve the required bearing capacity, settlement tolerance, or lateral resistance for a given project.
The three most commonly encountered deep foundation types in residential, commercial, and light industrial construction are micropiles, helical piers, and driven piles. Each has distinct installation characteristics, load capacity ranges, equipment requirements, and applicable conditions. Selecting the right system for a project starts with understanding what distinguishes them.
When Deep Foundations Are Required
Deep foundations are required when: surface soils are too weak or compressible to support the structural loads on a shallow spread footing; the site contains fills or organic material that will settle unacceptably under load; the structure requires resistance to uplift forces that shallow footings cannot provide; seismic design requires the foundation to reach stable material below potentially liquefiable surface soils; or existing shallow foundations are settling and need to be underpinned with elements that reach competent material.
The choice of deep foundation system depends on the soil profile, the applied loads, site access, vibration sensitivity of adjacent structures, and cost. A geotechnical investigation is required before deep foundation design can begin. Our engineering team works from geotechnical data to select and design the appropriate system for each project.
Micropiles
What they are
Micropiles are small-diameter (3 to 12 inches) drilled and grouted deep foundation elements. A hole is drilled to the required depth, a steel reinforcement element (rod, pipe, or tube) is inserted, and the annulus is filled with cement grout under pressure. The grout bonds to the soil or rock to develop the pile’s load capacity.
Load capacity
Micropiles can achieve working loads of 50 to 400 tons or more per pile depending on the diameter, grout design, and the strength of the bearing material. They carry load through a combination of skin friction along the grouted length and end bearing on competent material at the tip.
Advantages
Micropile rigs are compact. They can operate inside existing structures, under low headroom, on steep slopes, and in locations where conventional foundation equipment cannot reach. Micropiles generate minimal vibration and spoils compared to driven piles. They can be installed at angles (battered piles) to resist lateral loads as well as vertical loads.
Best applications
Restricted access sites. Underpinning of existing foundations. Sites with variable or obstructed subsurface conditions (fill, rubble, existing concrete). Projects requiring both compression and tension capacity. Seismic retrofit of existing structures.
Helical Piers
What they are
Helical piers (also called helical piles or screw piles) are steel shafts with one or more helical plates (helix-shaped flanges) welded at specific intervals along the shaft. They are installed by rotating the shaft into the ground using a hydraulic torque motor. The helical plates advance the pier into the soil with each rotation, similar to a wood screw threading into material.
Load capacity
Helical piers can achieve working loads of 25 to 200 tons per pier depending on shaft size, number of helices, and soil conditions. The capacity of a helical pier is closely correlated with the installation torque, which allows field verification of capacity during installation without additional load testing.
Advantages
Helical piers can be installed in almost any soil condition that the rotating equipment can penetrate, including clay, sand, and weathered rock. Installation is fast: most residential helical piers are installed in minutes per pier. The installation process is relatively quiet and low-vibration. The torque-to-capacity correlation provides immediate installation verification.
Limitations
Helical piers cannot penetrate hard rock or dense gravel without pre-drilling. They are most effective in cohesive and loose granular soils. They are not typically used in applications requiring very high individual pile loads, and they are not an effective solution in soils with large cobbles or boulders.
Best applications
New residential and light commercial construction on poor soil. Foundation repair and underpinning. Elevated decks, additions, and accessory structures. Solar ground mount foundations. Applications where immediate loading after installation is desired.
Driven Piles
What they are
Driven piles are structural elements that are forced into the ground by impact, vibration, or pressing. Common driven pile types include steel H-piles (wide-flange sections), open-end and closed-end steel pipe piles, precast concrete piles, and timber piles. A pile hammer mounted on a crane or excavator drives the pile incrementally with repeated blows until the pile reaches the required depth or refusal criteria.
Load capacity
Driven piles can achieve very high working loads, from 100 tons for smaller pipe piles to 500 tons or more for large-diameter steel pipe piles driven to rock. Load capacity is estimated from blow count data during driving and confirmed by dynamic load testing (PDA) or static load tests.
Advantages
Driven piles produce no spoils (the soil is displaced, not removed). They are well-suited for marine and waterfront applications. High individual pile capacities reduce the number of piles required for a given structure. For projects with large column loads, driven piles can be more economical than micropiles or helical piers at the required capacity level.
Limitations
Driven pile installation requires large, crane-mounted equipment that needs substantial site access and working room. The impact driving process generates significant noise and vibration, which restricts use near existing structures, buried utilities, and occupied buildings. Driven piles cannot be installed in areas with low headroom or restricted access.
Best applications
Large commercial and industrial foundations. Bridge and waterfront structures. Sites with good access and no vibration-sensitive constraints. Projects requiring very high individual pile loads that would require an impractical number of micropiles or helical piers.
Comparing the Three Systems
Access: Micropiles are the clear choice for restricted access. Helical piers require moderate access. Driven piles require substantial access and open working room.
Vibration: Micropiles produce minimal vibration. Helical piers are essentially vibration-free. Driven piles produce significant vibration and are not appropriate near sensitive structures without vibration monitoring and control.
Capacity range: Helical piers handle light to moderate loads. Micropiles handle moderate to heavy loads. Driven piles handle heavy to very heavy loads.
Verification: Helical pier capacity is verified by installation torque. Micropile capacity is verified by load testing when specified. Driven pile capacity is verified by dynamic load testing or static load tests.
Engineering Requirements for All Three Systems
PE-stamped deep foundation design is required for all three system types in virtually every commercial application and in most residential applications above certain loads. The design engineer determines pile depth, size, and spacing based on the geotechnical data and structural loads. Post-installation load testing is often specified for critical applications. Our permit design services include deep foundation engineering for all three system types. Start a project with our team to discuss system selection for your site conditions.
Frequently Asked Questions
How do I choose between micropiles, helical piers, and driven piles?
The primary selection factors are site access, applied loads, soil conditions, vibration sensitivity, and cost. Micropiles are best for restricted access, variable subsurface, and moderate to heavy loads. Helical piers are best for light to moderate loads, fast installation, and immediate loading. Driven piles are best for heavy loads, good access, and non-vibration-sensitive sites.
Do deep foundation projects require geotechnical investigations?
Yes. Deep foundation design requires a geotechnical investigation that includes soil borings, standard penetration tests, and laboratory analysis of soil samples to determine soil classification, strength parameters, and depth to competent bearing material. Without this data, pile depth and capacity cannot be accurately calculated.
What is a load test for a deep foundation?
A load test applies a controlled load to an installed pile to verify that it meets its design capacity. Dynamic load tests (PDA) are performed during driving or restrike for driven piles and can also be used on micropiles after installation. Static load tests apply a direct compression or tension load through a loading frame and measure settlement or movement. Load tests are specified when the design load is critical or when soil conditions are uncertain.
Can helical piers be used in clay soils?
Yes. Helical piers perform well in clay soils, where their capacity develops primarily through bearing on the helical plates. Installation torque in clay is a reliable indicator of installed capacity. Very stiff or hard clays may limit the depth achievable with standard helical pier equipment.
Are driven piles appropriate near existing buildings?
Generally not without significant precautions. Driven pile installation generates vibration and ground movement that can damage nearby structures, crack existing foundations, and disturb buried utilities. Pre-construction vibration surveys and monitoring during installation are required when driven piles are used near existing structures.
References
Federal Highway Administration. Design and Construction of Driven Pile Foundations.
Federal Highway Administration. Micropile Design and Construction. Publication No. FHWA-NHI-05-039.