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Structure Backfill Design Aid

This design aid defines the three types of backfill, recommends their use, and illustrates how to calculate structure backfill for culvert designs. Use the Revised Standard Plan M-206-1 (instead of the 1992 Standard Plan M-206-1) and the Standard Special Provision Revision of Section 206, Structure Excavation and Backfill, whenever computing pipe quantities with this Design Aid.

TYPES OF STRUCTURE BACKFILL

Structure Backfill (Flow-fill)

Description: A lean concrete mixture. Defined in the Standard Special Provision, "206 Structure Backfill (Flow-fill).

Advantages:

  • Quick set-up time.
  • No compaction testing.
  • Reduces settlement problems.
  • Trench width may be reduced (see 206-Structure Backfill (FlowFill) dated June 26, 1997).

Disadvantages:

  • Availability limited.
  • High cost.
  • Culverts have a tendency to "float" when flow-fill is used. A plan detail showing a method for anchoring culvert is helpful
  • Must be placed in lifts to avoid distortion and movement of culvert.

Recommended Uses: Flow-fill is recommended for steel and plastic culverts in areas where settlement is a concern (curb and gutter or areas with high concentrations of utility lines or conduits.

Cost per Cubic Yard: $57 (1995).

 

Structure Backfill (Class 1)

Description: Graded material defined in Subsection 703.08 (a).

  • 100% passing the 2 inch sieve.
  • 30-100% passing the #4 sieve.
  • 10- 60% passing the #50 sieve.
  • 5- 20% passing the #200 sieve.
  • Maximum Liquid Limit of 35.
  • Maximum Plasticity Index of 6.

Advantages:

  • Readily available material.
  • Good backfill material.
  • Less expensive than Flow-fill.

Disadvantages: Requires compaction and testing.

Cost per Cubic Yard: $13 (1994).

Recommended Use: Class 1 is recommended for backfilling metal and concrete culverts. Also, plastic pipe when flow-fill is not available.

 

Structure Backfill (Class 2)

Description: Material developed on project. Defined in Subsection 703.08 (b) as, "Suitable materials developed on the project..... free of frozen lumps, wood, or other organic material." - See Section 703.08.

Advantages:

Least expensive type of backfill.

Available at all locations.

Disadvantages:

The wide variability creates a potential for backfill failure.

Cost per Cubic Yard: $7.20 (1994).

Recommended Use: Class 2 backfill should only be specified when native material can meet the requirements of Class 1 Backfill. The use of unsuitable materials for Class 2 has resulted in pipe failures and settlement problems due the use of unsuitable material as structure backfill.

 

Recommended Backfill for Culverts. 

Metal: Use either flow-fill or Class 1**.

Plastic: Use flow-fill or Class 1.

Concrete: Use Class 1** with 3 inches of bedding material. 

** Backfill Class 2 may be specified only if material generated on the project meets the requirements for Class 1. 

For all pipe trenches in rock, a 12" layer of Backfill Class 1 is required beneath the pipe.

TABLE 2

Backfill Rates for Flexible (Plastic and Metal) and Rigid (Concrete) Culverts
(Cu. Yds.)

 

Inside Diameter (In.)

I
FLEXIBLE
Rate per Lin. Ft. (Cu. Yds.)

II
FLEXIBLE
With 12" of Loose Bedding (Trench in Rock)

III
RIGID
With Mandatory 3 inch Loose Bedding

IV
RIGID
With 12" Loose Bedding (Trench in Rock)

15"

0.31

0.47

0.38

0.50

18"

0.36

0.52

0.42

0.55

24"

0.45

0.63

0.51

0.65

30"

0.53

0.74

0.61

0.76

36"

0.63

0.85

0.70

0.89

42"

0.73

0.97

0.80

0.98

48"

0.84

1.10

0.91

1.11

54"

0.94

1.22

1.01

1.22

60"

1.05

1.35

1.10

1.32

66"

1.17

1.48

1.21

1.45

72"

1.29

1.62

1.31

1.56

78"

1.41

1.76

1.42

1.68

84"

1.54

1.91

1.53

1.81

96"

1.81

2.22

1.75

2.06

The method used to calculate these rates is as follows:

Column I: Width (Ba + 3 feet) X Height (Ba + 1 foot minus the pipe volume)

Column II: Width (Ba + 3 feet) X Height (Ba + 2 feet minus pipe volume)

Column III: Width (Ba + 3 feet) X Height (Bc + 1'-3" minus pipe volume)

(Note that Bc is used to match with D206-1 Project Detail)

Column IV: Width (Ba + 3 feet) X Height (Bc + 2' minus pipe volume)

Where Ba = inside diameter and Bc = outside diameter

To determine structure backfill, follow these steps:

1. Determine pay length of pipe

2. Determine type of pipe needed. If using the 617 item, use column III to backfill. This will provide adequate quantity of backfill for any type of culvert.

3. Determine the type of backfill required.

4. Multiply the factor in the table times the pay length of pipe. This is the quantity for structure backfill and bedding (where required). This quantity is for information only and will be included in the cost of the culvert.

tga


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