Driven Piles...

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>>>Soil Stabilization

 

Driven piles are structural elements for the transfer of loads through weak soil layers to a suitable bearing layer of soil or rock. They are suitable foundations for all types of land-based and marine-based structures. Their choice over other deep foundation methods is a matter of economic and site-specific factors.

 

Types of Driven Piles and Their Capacities

Piles develop their capacity through a combination of side (skin) friction and end bearing. Piles driven to a very hard layer such as rock will develop their capacity primarily by end bearing, while piles driven soil will develop their capacity from both side friction and end bearing. Piles have been driven to capacities over 1,000 tons (US) in the offshore oil industry. However, most projects only require piles with less than 100-ton design loads

  • Steel (over 25 tons)
  • Concrete (up to 100 tons)
  • Wood (up to 25 tons)

 

Sheet Piles are a special type of pile used to form in situ walls for earth retention and excavation support. Steel sheet piles are commonly used today, while wood and concrete sheet piles can only be used in very limited conditions.

 

Driven Pile Applications

  • Deep foundations (most common)
  • Excavation support (Sheet and soldier piles)
  • Docks and wharfs

 

Important Driven Pile Parameters

  • Skin friction and end bearing values
  • Allowable stress for the pile material
  • Drivability of the pile hammer, cushion, capblock, and pile system

 

Driven Pile Design Steps

  1. Geotechnical site investigation
  2. Determine load to be supported
  3. Design pile-to-structure connection
  4. Design pile-to-soil or rock load transfer
  5. Develop a pile testing program
  6. Determine the driving resistance by the Wave Equation Analysis

 

Driven Pile Installation Methods

Piles are typically driven into the ground using impact hammers. These hammers are powered by:

  • Compressed air (or steam)
  • Diesel
  • Hydraulic fluid
  • Gravity (drop hammers)

drive head containing cushion material is used between the hammer and pile to prevent damage to the hammer and pile. The drive head also keeps the pile centered under the hammer. The hammer and pile are guided by set of leads that are supported by a crane. Typically, 50 to 150-ton capacity crawler cranes are used to drive piles. Vibratory hammers can also be used to drive piles, particularly low displacement piles such as sheet piles, but these do not penetrate hard soils or give an accurate indication of pile.

 

Vibration and Noise Considerations

  • Vibrations generally will not harm structures in good condition when using small to medium sized impact hammers.
  • Vibrations pose more risk to deteriorated structures, and can interfere with nearby sensitive electronic instruments.
  • Steady-state vibration produced by vibratory hammers is much more likely to cause damage to structures and cause settlement of loose soil. Predrilling greatly reduces the magnitude of the vibrations transmitted through the ground to nearby structures.
  • Noise from pile driving can be cause for concern on sensitive job sites such as hospitals. Timber and concrete piles driven with air or hydraulic hammers are much quieter than steel piles driven with diesel hammers.
  • Noise levels near the pile and hammer are high enough to require hearing protection for those on and near the job site.

 

Driven Pile Quality Assurance/Quality Control

  • Consider the use Wave Equation Analysis software to model the pile – hammer – soil interaction. This program estimates ultimate load. Typically, a factor of safety of 2 is used with this analysis. This program can be used to check the driving stress in the pile, and guide hammer selection. This relationship, once established for a pile project, offers a very reliable quality control tool.
  • Test selected piles to local code requirements (usually two times the static design load) ASTM standards D 1143, D 3689, and D 3966, provide standard static testing methods for compression, tension and lateral test, respectively. Also, the pile can be tested using ASTM D 4945 High-Strain Dynamic Testing of Piles. This test can be done in lieu of the static test, and is very economical for testing a large number of piles.
  • Determine, from the Wave Equation Analysis and load test results, the required driving resistance for the production piles.

 

Benefits of Driven Piles

  • High-performance
  • Appropriate for a wide range of ground conditions
  • Design loads up to 1,000 tons
  • Suitable for land-based and marine-based structures
  • Usually the fastest deep foundation

 

 

by: www.serranodee.com