Increase the bearing capacity Benefits of Driven Piles
Driven piles, also known as displacement piles, are a commonly-used form of building foundation that provide support for structures, transfering their load to layers of soil or rock that have sufficient bearing capacity and suitable settlement characteristics. Driven piles are commonly used to support buildings, tanks, towers, walls and bridges, and can be the most cost-effective deep foundation solution. They can also be used in applications such as embankments, retaining walls, bulkheads, anchorage structures and cofferdams.
A driven pile is a long, slender column made of preformed material and having a predetermined shape and size that can be installed by impact hammering, vibrating or pushing it into the ground to a design depth or resistance. If the soil is particularly dense, pre-drilling may be required to enable the pile to reach the design depth. Driven piles are very adaptable and can be installed to accommodate compression, tension or lateral loads with specifications set according to the needs of the structure, budget and soil conditions.
Driven piles are a total engineering solution. The design, installation and quality assurance that are a part of each driven pile combine to eliminate guesswork and produce a known, reliable and cost-effective product that can accommodate a wide variety of subsurface conditions.
Driven piles consist of natural materials or pre-manufactured structural shapes built to precise tolerances utilizing high strength materials and reliable quality control. Their quality is consistent from the first pile to the last and can be seen and verified prior to installation.
Driven piles maintain their shape during installation. They do not bulge in soft soil conditions and are typically not susceptible to damage from the installation of subsequent piles. Many hollow-section piles can be visually inspected after installation to assure integrity. Most solid-section piles are uniform in section and can be dynamically inspected to verify integrity.
The pile driving process can be easily modeled prior to installation to determine adequate and economic equipment selection. Static or Dynamic testing can confirm load carrying capacities of installed piles. Dynamic testing can easily confirm proper hammer performance and its effect on the pile. Many modern hammers have impact velocity measurement devices permanently installed, providing a very high level of quality control.
Driven piles are usually the most cost-effective deep foundation solution. There are no hidden extra costs or added expenses for site clean-up. The wide variety of materials and shapes available for driven piles can be easily fabricated or specified for high structural strength, allowing them to be driven by modern hammers to increased working loads thus requiring fewer piles per project, resulting in substantial savings in foundation costs.
Pile capacity is easily verified by either static or dynamic pile testing. Capacity per pile or pile length can be easily optimized to provide exactly the required capacity (including safety factors) to minimize foundation costs. Testing also eliminates the uncertainty of bearing capacity estimates based on static analysis. There is no need to be overly conservative and thus wasteful to protect against failure.
As an additional benefit, driven piles often gain capacity after installation. Shaft soil strength usually increases with time after pile installation is complete to provide additional load capacity. This phenomenon, called "setup", can result in substantial foundation cost savings when considered in the design and confirmed by testing. The incorporation of setup into the foundation design results in fewer piles and/or shorter piles driven with lighter equipment. The reduction in time, labour and materials provide substantial cost savings to the owner.
Driven piles are installed to accommodate compression, tension or lateral loads. Piles can be selected to meet the specific needs of the structure, site conditions and budget.
Driven piles easily adapt to variable site conditions to achieve uniform minimum capacity with high reliability, thus eliminating uncertainty due to site variability. Driven piles are usually installed to established criteria (e.g., minimum blow count per unit penetration, sometimes with a minimum penetration). Because they are normally driven to a blow count to assure the desired minimum capacity, pile lengths may vary when subsurface conditions are not uniform. Driven piles may either be cut-off to shorten their length or spliced to extend their length. Splice designs usually meet or exceed the strength of the pile itself. Pile shoes (or "points") can be added to assist penetration requirements and provide very reliable contact with rock. The optimum length is used for each pile which accommodates all site conditions.
Driven piles adapt well to unique site conditions and restrictions. They are ideally suited for marine and other near shore applications. There are no special casings required and there are no delays related to the curing of concrete. Piles driven through water can be used immediately, allowing construction to proceed in a timely manner. For bridges or piers, driven piles can be quickly incorporated into a bent structure allowing the bridge or pier itself to be used as the work platform for succeeding piles in top-down construction.
To minimize disturbance in wetlands or allow work over water, driven piles can be used to construct temporary trestles. Piles installed to meet any temporary construction need can be extracted when the need is ended.
In earthquake prone regions, large diameter driven piles are well suited to resist seismic forces. Non-displacement pile sections (e.g. H piles) can be utilized to minimize vibration effects on nearby existing structures. In corrosive environments, coatings and/or additives can be used to mitigate the effects of corrosion thereby lengthening the service life of a structure. Coatings can also be used to mitigate the effects of negative skin friction.
Reliable and Available
The equipment and installation methods for driven piles are time-tested and well proven. Advances in materials, equipment, methods, and testing continually combine to improve the efficiency of driven piles. Modern driven pile installation equipment is capable of providing consistent, known energy that can be easily measured during operation. Advancement of the pile into the earth can be monitored and recorded for future study and comparison to other tested and installed piles.
Recording the blow count versus depth during pile driving easily documents successful pile installation. Because driven piles are usually driven to a blow count criterion, they will have a measurable capacity providing assurance that they meet the project requirements. Piles can be easily driven through upper soft soil layers regardless of the soil type and groundwater conditions.
Driven piles have vastly superior structural strength. Driven piles almost never fail structurally during static testing or static loading. They have high lateral and bending resistance for their entire length making them ideal to resist wind, berthing and seismic loading conditions. Driven piles can tolerate moderate eccentricity in the application of superstructure loads due to their full-length strength. Piles can be driven either vertically or at various angles of inclination to increase support for lateral loads. In special cases, piles can even be driven horizontally.
Pile driving is relatively easy in many soils. Since the soil at the toe is in a compacted condition for displacement piles, end bearing can often carry a substantial load. There are no "soft bottom" soil conditions so large settlements for end bearing piles are eliminated.
Driven piles displace and compact the soil. Other deep foundation options can require the removal of soil and considerable subsidence, which can undermine the support of adjacent structures and cause excessive deformations, both of which can result in structural problems.
Drilling for cast-in-place piles relieves soil pressures and reduces unit shaft resistances. In groups of drilled piles the removal of soil generally loosens and weakens the soil structure, reducing the capacity of previously installed piles. Groups of driven production piles densify the soil, improving the capacity of previously driven piles. In groups, driven production piles usually have a higher capacity than the test pile while drilled production piles often have a lower capacity than the test pile. Thus, driven piles generally have higher capacities than other pile types of the same diameter and length.
Driven piles require no curing time and can be driven in natural sequence rather than skipping alternate piles, thus minimizing the moving of the equipment and speeding installation.
Driven pile installations usually produce no spoils for removal and therefore no exposure to, or costly disposal of, potentially hazardous or contaminated materials. The site is left clean and ready for the next construction activity.