THE OFFICIAL PUBLICATION OF THE PILE DRIVING CONTRACTORS ASSOCIATION ISSUE 3 2018 | VOLUME 15, NO. 3

“A Driven Pile … Is a Tested Pile!”

DRIVING EXTRA LONG STEEL SHEET PILESChesterfi eld Associates replaces piling at a New York pier

The Design and Installation of Cost-Effi cient Piles Conference

Savings from Testing the Foundation of a High-Rise Building

GROUP Deep Foundations’ Short Service Worker Safety Program

TIMBER

Timber piles have been widely used the world over for thousands of years. For most of that time, there were no rules or standards for their proper preparation and use. Over time, techniques were developed to protect and preserve wood, and stan-

dards were developed to provide a consistent level of quality, consistent terminology and to protect the market from substandard products.

Timber piles are a unique deep foundation product in that they are naturally grown. They are produced from trees subject to variations in size, straightness and strength. The two predominant species used for timber piles in the United States and Canada are Southern Pine and Douglas fir.

The American Society for Testing and Materials (ASTM) developed the first stan-dard for timber piles – ASTM D25, Standard Specification for Round Timber Piles – in 1915. It has been updated numerous times over the last century. This standard defines the physical characteristics of timber piles and establishes minimum criteria to ensure each timber pile will perform as intended. ASTM D25 focuses primarily on the quality, straightness and size of a pile.

To ensure reliable design values, ASTM D25 specifies that piles shall be of any spe-cies of wood for which strength values are provided by ASTM D2555, Standard Practice for Establishing Clear Wood Strength Values. ASTM D25 ensures the quality of each pile by requiring that each pile be of sound wood and establishing a minimum number of growth rings per inch and a minimum percentage of summerwood in the outer 50 percent of a pile tip.

The standard also provides limits on the straightness of each pile, spiral grain and the size and number of knots to ensure consistency while accommodating minor variations in this naturally grown product.

In addition, ASTM D25 also addresses the size of timber piles by providing mini-mum tip and butt dimensions for each designated length. These minimum dimensions ensure a predictable taper for each pile so designers can calculate the diameter at any loca-tion along its length. Tables are provided for both Southern Pine and Douglas fir that allow a designer to specify a tip circumference or butt circumference. For an end bear-ing pile, a tip circumference is specified. The table provides a corresponding minimum pile butt circumference, measured three feet from the large end of the pile, based on the designated length of pile. Conversely, for piles that rely primarily on skin friction, a butt circumference is specified. The table provides a corresponding minimum tip circumfer-ence, based on a designated length of pile. There is also a table that specifies minimum tip

Which Wood?The proper specification of timber pilesBy Van Hogan, Timber Piling Council

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The two predominant species used for timber piles in the United States and Canada are Southern Pine and Douglas fir.

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sizes of Class A (14-inch butt) and Class B (12-inch butt) piles for designers that prefer these designations.

Material strength values for the vari-ous species of timber piles are published in the National Design Specification for Wood Construction by the American Wood Council. The published design val-ues apply only to timber piles conforming to ASTM D25. These strength values are used in determining the bearing capacity, lateral capacity and allowable driving stress of a pile.

With the development of the wood preservation industry, the need arose for standards to specify the preservatives and treatment processes to be used. There are a number of preservatives available to protect wood from biological attack. The American Wood Protection Association (AWPA) establishes the standards neces-sary for consistent applications and pro-cesses. AWPA standards provide the proper preservative to use with each species of timber and the retention level for various exposure conditions.

AWPA has established Use Category System Standards. These standards deter-mine the appropriate treatment based on how timber will be used during its ser-vice life. Use Category System Standard U1 is used by specifiers to determine the appropriate treatment for a particular service condition.

Standard U1 contains five major Use Categories that describe the service con-ditions to which wood will be exposed. Within the various Use Categories there are Commodity Specifications that provide spe-cific information, including retention values for each species/preservative combination and any applicable special requirements.

Timber piles are found in two Use Categories. Foundation piles and piles used on land or in fresh water are list-ed in UC4C, Commodity Specification E: Round Timber Piling. Piling used in saltwater environments are listed in UC5, Commodity Specification G: Marine Applications. Timber piles used in marine applications are further divided into sub-categories UC5A, UC5B and UC5C. These

subcategories are based on geographical location and define the amount of preserva-tive needed to provide optimum protection.

The following is a chart showing the various Use Category subdivisions for timber piles in marine applications fol-lowed by a list of the prescribed treat-ments and retention levels for the com-

TIMBER

Sample Specification:Round Timber Piles: Piles shall be Southern Pine and shall conform to ASTM D25, unused, clean peeled, uniformly tapered, one piece from butt to tip. [Specify butt or tip diameters from Tables X1.1 through X1.5. The pile length should be specified as well.]

Preservative: Pressure treatment shall be in accordance with the following AWPA Use Category Standard: UC4C, UC5A, UC5B or UC5C.

Handling, storage and field fabrication, including the treatment of cut ends or penetrations, shall be in accordance with AWPA M4.

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mon preservatives used with Southern Pine (Chromated Copper Arsenate (CCA)) and Douglas fir (Ammoniacal Copper Zinc Arsenate (ACZA)).

To properly treat timber piles, mois-ture must first be removed from the wood so that it may receive the preservative. Once the timber pile is sufficiently dry, it is placed into a pressurized cylinder. The cylinder is flooded with liquid preservative, which is then pressurized, forcing preservative into the empty wood cells. The concentration of the preservative solution determines the amount of preservative retained in the pile.

Southern Pine

Douglas Fir

AWPA Reference

Type CCA Retention (PCF)

ACZA Retention (PCF)

UC4C Foundation 0.80 1.0

UC4C Land & Freshwater

0.80 1.0

UC5A Marine – North 1.5 1.5

UC5B & UC5C Marine – South 2.5 2.5

TIMBER

Once the piles are removed from the cylinder, testing is conducted to ensure the preservative has penetrated the wood and the proper retention levels have been met.

Note that the preservative treatment does not penetrate the entire pile. It cre-ates a protective envelope around the pile. It is important to avoid puncturing or damaging the protective envelope prior to or during installation. Should dam-age occur, or if the pile is cut or drilled after installation, the protective envelope will need to be repaired. AWPA M4

provides the procedures for the proper handling, storage and field treatment of timber piles.

The proper specification of timber piles to meet ASTM D25 material stan-dards and AWPA treatment, handling and repair standards ensures that each pile will provide the best service possible for its intended use. t

The Timber Piling Council serves as a resource for the proper design and use of treated timber piles.

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