Environmental Science & Engineering - www.esemag.com - November 2005
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Concrete pipeline designers transitioning to Standard Installations

by Paul Smeltzer, P.Eng.

There are two options for designers of concrete pipe drainage systems to ensure that the pipe functions as a conduit and a structure. The Indirect Design method determines the required strength of the pipe and then a class of pipe is selected that meets that load requirement based on a given bedding design. Using this method, the pipe manufacturer selects the reinforcing steel required in the pipe. Direct Design, otherwise referred to as Standard Installations Direct Design (SIDD), is the design of pipe in the installed condition.

The magnitude and distribution of the loads are determined and the physical properties necessary to support those loads are calculated. In some extreme cases the particular class of pipe determined by the Indirect Design method may not be suitable. For example, the pipe may be required to meet strengths above the highest standard (i.e. 140D) classification listed in tables, or a more exacting design may be appropriate. In such situations, designers can use the Direct Design method to consider flexure, shear, radial tension and crack control, thereby directly determining the reinforcement required to meet the specific project criteria.

In the Indirect Design method, the earth pressures and their distribution around the pipe and the resulting moments, thrusts, and shears in the pipe are not calculated. Instead, procedures developed by Anson Marston and Merlin G. Spangler in the 1910s to 1930s are used to calculate bedding Installation of test pipe in Ottawa. Installation of test pipe in O ttawa. factors for the pipe, which relate the in situ load to the pipe to the load applied in a three-edge bearing test. Many of the design practices currently in use are based on the pioneering research completed by these two men.

The Indirect Design method was the only standard industry practice through the 1900s, until research by Dr. Frank J. Heger and Dr. Timothy J. McGrath in the 1970s and early 1980s led to improvements in understanding the structural behaviour of buried pipe in its installed condition. The finite analysis known as soil pipe interaction design and analysis (SPIDA), identified conservatism in the Marston Spangler work.

Trial installations carried out in Ohio from 1985 to 1990 led to development of the four distinct Standard Installations of SIDD. In 1993, Standard Installations were adopted by the American Society for Civil Engineers (ASCE) as Specification 15-93-Standard Practice for Direct Design of Buried Precast Concrete Pipe Using Standard Installations. It was adopted later in the 1996 (16th) Edition of the American Association of State Highway and Transportation Officials (AASHTO) Standard Specification for Highway Bridges, Section 17, Soil-Reinforced Concrete Structure Interaction Systems. In 1997, Standard Installations were incorporated into the new Canadian National Bridge Design Code CAN/CSA S06- 00. Now, the standards are being accepted by several Canadian municipalities.

In 2000 the Ontario Concrete Pipe Association and its partners, the National Research Council of Canada, the Ministry of Transportation and the City of Ottawa, commenced a research project on the design and installation of concrete pipe using ASCE 15-93. This research included a literature review of the work to date and a demonstration site in the City of Ottawa, which is a 1350 mm circular culvert installation. The site instrumentation was monitored until fall 2003. The NRC has now distributed a draft final report to the project partners for review and comment, which is expected to be made public in 2005.

Marston and Spangler’s development of the Indirect Method
With help from Professor A.N. Talbot of the University of Illinois and a theory for pressures in grain bins published by Janssen, Marston proposed the following equation for calculating the earth load on a pipe in a narrow trench.
Where We represents earth load on pipe, (lbs/ft.), Cd the coefficient for calculating earth load in trenches, w the unit weight of earth (lbs/cu. ft.), Bd the width of trench at top of pipe (ft.). The value of Cd is dependent on K, the ratio of lateral to vertical earth pressure as determined by Rankine’s equation, and by the coefficient of internal soil friction.
This formula means that the earth load on the pipe in narrow trenches is independent of the size of the pipe, depending only on unit weight of earth backfill over the pipe, w, the width of the trench at the top of the pipe, Bd, the ratio of the height of fill over the pipe to the width of the trench H/Bd and the coefficients of friction of the fill soil, µ, and between the fill soil and the side of the trench, µ1. After publishing his classic work, The Theory of Loads on Pipes in Ditches and Tests of Cement and Clay Drain Tile and Sewer Pipe, Marston and his co-workers focused on supporting strength of pipe in trenches as it is affected by bedding conditions, and investigated methods for determining supportive strengths using laboratory tests.

Marston defined four beddings from the lowest to the highest quality:
  1. Impermissible (Class D). Hard flat bottom assumed.
  2. Ordinary (Class C). Bottom support over a shaped arc of 60 to 90 degrees is assumed with soil placed with ordinary care to give the equivalent of 90 degrees of bottom support.
  3. First Class (Class B). Bottom support over a shaped arc of at least 90 degrees with the pipe surrounded by thoroughly compacted soil to at least 15 degrees above the springline.
  4. Concrete Cradle (Class A) Concrete placed around the lower part of the pipe. Various designs are used for extent of concrete.
Marston then developed the following formula for loads on embankment conduits:
The coefficient Cc is a function of the soil friction, the Rankine lateral soil pressure coefficient, and the ratio of fill height to pipe outside diameter. It is also a function of the projection ratio, and the settlement ratio. The projection ratio is the ratio of the vertical height of the top of the conduit above the embankment subgrade to the conduit outside pipe diameter, Bc . When a conduit is placed in a sub-trench with its top below the general embankment grade, it is said to have a negative projection ratio. For positive projecting installations, the settlement ratio is the difference between the settlement of the subgrade and projection height of the adjacent soil prism and the settlement of the crown itself (change in vertical diameter plus pipe settlement) to the compression of the adjacent soil in the projection height.

In the late 1920s and 1930s, pipe research at the Iowa Experimental Station concentrated on developing a method for determining the supporting strength of buried rigid culverts in embankment installations, termed projecting culverts. The results of the research were given in a comprehensive paper in 1933 by Merlin G. Spangler entitled, The Supporting Strength of Rigid Pipe Culverts. Spangler presented three bedding configurations and the concept of a bedding factor to relate the supporting strength of the buried pipe to the strength obtained in a three-edge bearing test.

Spangler’s theory postulated that the bedding factor for a particular pipeline and, consequently, the supporting strength of the buried pipe, is dependent on two installation characteristics: For the embankment condition, Spangler developed a general equation for the bedding factor, which partially included the effects of lateral pressure. For the trench condition, he established conservative fixed bedding factors, which neglected the effects of lateral pressure, for each of B, C and D beddings.

The objective of Spangler’s research was to determine the supporting strength of buried rigid pipe when subjected to the earth load predicted by Marston’s theories of earth loads on projecting culverts. Supporting strength was defined as the load that caused cracking, or later, as the load that caused a specified width of crack such as 0.01 inch. Based on rational assumptions about pressure distribution and tests of pipe installations constructed with “ordinary bedding” and pipe subjected to three-edge bearing loads, he determined the ratio of the field earth load that cracks the pipe, to the three-edge bearing load that cracks the pipe. He termed the ratio of the in situ load with a particular bedding and projection condition to the three-edge bearing load that produces the same invert moment, the “Load Factor” for that field condition. In current practice, this ratio is called the bedding factor, Bf .

Spangler based his development of load factors for relating the strength of a projecting conduit to the pipe’s threeedge bearing strength by making rational assumptions about the distribution of the earth pressure around the circumference of a buried pipe with various bedding and projection conditions, and determining the moments at invert, crown, and springline by an elastic analysis of the pipe, as a ring with uniform stiffness.

Spangler’s elastic analysis of a pipe ring resulted in the following equations for bedding factor, Bf where bedding factor is defined as the ratio of the field load to the three-edge bearing load that produces equal bending moments at the invert of the pipe.
In this equation, N varies with the distribution of vertical reaction (type of bedding), x varies with the projection ratio, and q varies with the Rankine pressure coefficient and the total vertical load on the culvert.

The research undertaken by Marston and Spangler and their coworkers, and subsequent theories have formed the basis for American and Canadian standards and specifications for the selection of bedding, backfill and pipe strengths for over seven decades.

The evolution of Standard Installations Direct Design (SIDD)
Although developed for the Direct Design method, the Standard Installations simplify the Indirect Design method. The Standard Installations are easier to construct and provide more realistic designs than Marston’s B, C and D beddings. Development of bedding factors for Standard Installations follows the concepts of reinforced concrete design theories. The basic definition of bedding factor is the ratio of maximum moment to the three-edge bearing test to the maximum moment in the buried condition, when the vertical loads under each condition are equal.

where:
Bf = bedding factor
MTest = maximum moment in pipe wall under three-edge bearing test load, inch-pounds.
MField = maximum moment in pipe wall under field loads, inch-pounds.

To evaluate the proper bedding factor relationship, the vertical load on the pipe for each condition must be equal, which occurs when the springline axial thrusts for both conditions are equal. After considering the formulae for and , the formula for SIDD bedding factors is
where:
NFS = axial thrust at the springline under a three-edge bearing test load, Newtons per metre (pounds per foot).
D = internal pipe diameter, mm (inches)
t = pipe wall thickness, mm (inches)
MFl= moment at the invert under field loading, (Newtonmm)/ m (inch-pounds) /ft.
NFi = axial thrust at the invert under field loads, Newtons per metre (pounds per foot)
c = thickness of concrete cover over the inner reinforcement, mm (inches)

Using SIDD, bedding factors were determined for a range of pipe diameters and depths of burial. These calculations are based on one-inch cover over the reinforcement, a moment arm of 0.875d between the resultant tensile and compressive forces, and a reinforcement diameter of 0.075t. Evaluations indicated that for A, B, and C pipe wall thicknesses, there was negligible variation in the bedding factor due to pipe wall thickness or the concrete cover, c, over the reinforcement.

The six-step Indirect Design procedure for the selection of pipe strength is still appropriate.
  1. Determine earth load.
  2. Determine live load.
  3. Select Standard Installation.
  4. Determine bedding factor.
  5. Apply factor of safety.
  6. Select pipe strength.
The four Standard Installations that evolved from Marston and Spangler’s research (commonly referred to as Spangler And Marston Method - SAMM) provide an optimum range of soil-pipe interaction characteristics. For the high quality materials and high compaction of a Type 1 installation, a low strength pipe is required. Conversely, a Type 4 installation requires a higher strength pipe because it was developed for a condition of little or no control over materials or compaction. Following is a description of the four Standard Installations:

Type 1: Use of high quality backfill materials and high compaction values.
Type 2: Similar materials as Type 1 with lower compaction values.
Type 3: Lower grade backfill material with lower compaction values.
Type 4: No bedding with little or no compaction of native backfill required (not recommended).

The fundamental differences between SAMM and SIDD are: In general, SAMM minimized the inherent strength of concrete pipe, while SIDD maximizes the consideration of concrete pipe strength classes in the selection of bedding types.

The advantages of Standard Installations are: minimized need for imported granular material, therefore a more economical installation; the inherent strength of the pipe is recognized for design of the system; less supervision of the installation is required; and failure of a concrete pipeline is highly unlikely. Standard Installations allow engineers to design concrete pipe installations by determining the most cost effective installation design, increasing the use of native materials, and using appropriate construction and on-site quality control processes. The Direct Design method provides flexibility in design and cost savings on projects. Although many pipeline designers are still using the Indirect Design method, a faster transition to the Direct Design method is desirable and specifications in many jurisdictions need to be changes to accommodate SIDD.

Paul Smeltzer, P.Eng is Executive Director, Ontario Concrete Pipe Association
Contact: paul.smeltzer@ocpa.com

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