Wooden Pile Case Study

AUTHOR
Dean Matthews

Timber Piling Council, 2006

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Timber Piles: A Brief Overview

Efficient pile foundations are based on good engineering practices — practices which include such important phases as adequate preliminary soil exploration at the site of the proposed structure, load tests on sample piles where these are deemed necessary to establish safe design loads, selection of contractors experienced in driving foundation piles, and thorough engineering supervision of driving operations.

Timber continues to be a major player in driven piles, along with steel and concrete. Timber piling is generally readily available and usually does not require long lead times for shipment. Because of its light weight timber piles are easier to handle and install than other types of driven pile materials. They are also easy to drive, cut to elevation and to dispose of the cut-offs. Timber is a natural renewable resource and

is replanted after harvesting. The South, where most Southern Pine trees are grown, is harvesting its fourth generation of forests, or more, since the Europeans settled this country. Timber piling is an engineered material with published sizes, allowable design stresses and engineering design data.

Ninety percent of the timber piling in North America is Southern Pine. Douglas Fir is most of the balance. Other species are occasionally used, but they comprise less than one percent. About 90 to 95 percent of Southern pine piling is pressure treated with Chromated Copper Arsenate (CCA), the rest is creosote. About 85 percent of Douglas fir piling is pressure treated with Ammoniacal Copper Zinc Arsenate (ACZA) with the remainder treated with creosote.

From the Rocky Mountains east there are many huge rivers which have dropped silt where the water flow slows as it approaches the ocean or passes through a relatively flat area such as the Mississippi River valley. Many cities grew up on those areas, and timber piles have and continue to be used in these areas. The same is true in the west, but the predominant use of timber piles is East of the Rocky Mountains.  On many jobs timber piling will save owners considerable money as they are usually less expensive. Dollars per ton of load carry capacity is a key comparison.

Timber Pile Performance

A home built on timber piles in Pearlington, Miss., is reported to be the only one along the Biloxi coast which survived flooding from Hurricane Katrina. Timber piles extend from below the ground to the second floor level. Water came within several inches of the girders, but never entered the house.

Similarly constructed beach homes, meeting coastal wind load standards, are prominent along the Eastern and Gulf coast from New Jersey to Louisiana. Timber piling resists attack from both alkaline and acidic soil, and corrosion protection is not required. They are also unaffected by electrolysis from stray electrical currents. Timber piling installs with standard, readily available equipment and takes advantage of a plentiful renewable resource.

Timber marine piling is well suited to small boat marinas and heavy marine construction as it resists battering by wind, wave, storms and tides as wood is a resilient material. This is because wood has high damping characteristics which provides built-in shock resistance against hurricanes and earthquakes.

Piling History

Credit for the concept of a pile has been given to a Neolithic tribe called the Swiss Lake Dwellers who lived in what is now Switzerland around 6,000 years ago. They built their homes on platforms supported by timber piles for protection from the wildlife. Evidence of these structures still exists today in Lake Lucerne.

Around 1620 B.C. the Romans built a timber bridge across the Tiber River in Rome which lasted over 1,000 years. Some of the Roman roads and aqueducts were supported on timber piles which were still in good condition 1,900 years later. The Romans also built the first bridge across the River Thames in London on timber piles in about 60 A.D. The Venicians built their homes on timber piles from about 100 B.C. to 400 A.D.

The Romans had several methods for driving piles. One was to build a tripod out of three large trees with a pulley up in the top. With slaves on one end of the rope and a rock on the other end, the slaves pulled the rock up and dropped it on the pile enough times to drive it in the ground. That must have been the first drop hammer!

The modern age of wood preserving began in England in 1832 when the concept of injecting chemicals into wood was developed. The first treating plant in North American was built in 1848 for treating railroad ties.

Today, 96 percent of railroad ties are still wood and most of them are creosote treated. Piling was the second major product to be pressure treated which began in 1865.

Timber piles support many notable structures. The Temple of Diana in Western Turkey, 600 B. C.; Campanile Tower in Venice, 900 A.D.; Pont Notre-Dame bridge in Paris, 1507; Royal Palace of Amsterdam, 1600; and the ferry terminal at the foot of Market Street in San Francisco, 1906. All are still standing on timber piles.

Timber Foundation Piling System

Testing Program

In 1999 and 2000 the timber piling industry sponsored full scale tests of both Southern Pine and Douglas Fir piling. These tests were conducted by EDM International, an independent laboratory in Ft. Collins, CO. Historically, stresses were based on tests of small clear samples and the results adjusted for wood characteristics by the procedure outlined in ASTM D2899 for timber piling.

Not only were these full scale tests run, but an additional step was added to the program which is not required by testing standards, nor a usual step in laboratory testing programs. Timber piling producers were surveyed to determine where they obtained the logs used for piling. The Southern Pine timber was from Virginia, Georgia and Florida. Douglas fir timber was from Oregon, Washington and British Columbia. Samples for the tests were selected in those growing regions in proportion to the amount used for piling.

EDM first conducted bending tests to failure and these tests showed the bending stresses could be up to 53 percent higher than currently allowed for bending strength design. While bending is not often a factor in foundation piles, it can be important with marine piles where the piles must resist the wind forces from the sides of ships, barges or small boats. These wind forces are resisted in bending by the marine piling.

Following the bending tests, an approximately three foot piece was cut from both the butt and tip ends and these were tested in compression. These tests showed that compressive stresses could be increased 12 percent more than currently allowed, thus allowing more load carrying capacity.

The testing program demonstrated that allowable stresses currently published in the National Design Specification (NDS) are conservative. A new ASTM standard under development will provide the protocol for developing stresses from full scale tests. Following ASTM approval and peer reviews by wood product testing laboratories, and academia, the resulting allowable working stresses will be published in the American Forest & Paper Association’s “National Design Specification.” This document is included by reference in the International Building Code.

Many field load tests have been conducted over the years on timber piling. These tests showed that loads up to 75 tons could be safely supported on timber piles with a factor of safety of at least two.

Durability of Timber Piles

Venice Grand Canal today. Many Venetian buildings were constructed on timber piles.

The durability record of timber piles is an often asked question. The conclusions of the Federal Highway Administration (FHWA) are:

• Foundation piles submerged in ground water will last indefinitely.
• Fully embedded, treated, concrete capped foundation piles partially above the groundwater will last 100 years or longer.

Source: FHWA HI 97-103, “Design and Construction of Driven Pile Foundations.”

It should be noted that the design life of steel is 100 years, also.

Timber Piling Standards and Treatments

Before treatment, piling is inspected and classified by size and conformance to ASTM D25 Standards. Quality control inspectors at the plant monitor treating processes, sample, and inspect piling throughout the manufacturing process. After treatment, the piling is checked for penetration and retention according to AWPA Standards. Every charge of piling in the cylinder is sampled.

ASTM D25, the oldest ASTM wood standard still in use, provides the requirements for wood characteristics in timber piling, defines the required straightness and provides size tables from which to obtain size information to design the timber piling foundation system. Timber piles are required in PDCA specifications to be pressure treated in accordance with the following American Wood Preservers Association (AWPA) Use Category Standards:

• AWPA Use Category for Timber Piles
a. Foundation, land and fresh water piling UC4C (formerly C3).
b. Highway construction UC4A and UC4B (formerly C14).
c. Marine construction (saltwater) UC5A, UC5B, UC5C (formerly C18).

The AWPA Standards were originally written in the 1920s, based on the performance of creosote, Any preservative allowed in Standards after that had to be equal to or better in performance than creosote. CCA came into the AWPA Standards in the 1930s. ACZA, which is used to treat Douglas Fir, was developed about the same time. It is not generally possible to treat Douglas Fir with CCA and consistently meet AWPA Standards.