is the minimum cover for the pipe?
The minimum cover for HDPE pipe is 1'-0" for H-20 traffic loads if
installed in accordance with AASHTO Section 30. This is based on
empirical calculation of load response (Iowa formula, Burns and Richards
formula, etc.), manufacturer's testing and field experience with the
pipe. AASHTO specifications Section 126.96.36.199 defines the minimum cover
as "ID/8 but not less than 12 inches." This cover is measured
from the pipe OD to the top of a rigid (concrete) pavement or the bottom
of a flexible (bituminous) pavement. Both AASHTO and ASTM, as well as
most manufacturers, require additional (temporary) cover for
construction loads greater than H-20. Generally, an additional 2' of
temporary cover, mounded over the pipe and removed for final grading and
paving, is sufficient for large construction vehicle loads.
2. Is crushed stone backfill always
No. Many native soils are well suited for backfill provided they are
compacted properly. These soils meeting the unified soil classifications
of a GW, GP, SW, SP (ASTM D-2321 Class II) and GM, GC, SM, SC (ASTM
D-2321 Class III)
3. What is the cost comparison to traditional
HDPE pipe is generally less expensive in initial material cost than
RCP and less expensive or the same cost as CMP, but the real savings are
realized in installed costs. Due to the light weight, longer lay lengths,
ease of handling, and push-together joints of ADS pipe, the installed
cost savings of HDPE ranges from 10 to 30% when compared to RCP and CMP.
4. What is its crush
When specifying pipe, the crush strength is a term used for pipes that
experience a brittle failure. In other words, the crush strength is the
load at which the pipe physically breaks. This is not applicable to a
visco-elastic material such as HDPE or an elastic material such as CMP
since the material will deflect under load to the point of failure. A
HDPE pipe profile can actually deflect until the material is flattened
due to reverse curvature. Therefore, an analogous test to crush strength
for HDPE pipes would be AASHTO's pipe flattening test, which requires
the pipe be deflected 20% with no signs of wall buckling, cracking,
splitting, or delamination.
5. How high a fill will the pipe
ADS recommends a maximum fill height of 60' for pipe sizes up to and
including 36" and 30'. For 42 and 48", ADS recommends a
maximum fill height of 30'. Greater fill heights for all pipes are
possible provided attention is given to backfill materials and
compaction. In fact, ADS has successfully installed both singlewall and
N-12 pipe in cover heights of 100 feet and greater for highway, landfill,
and mining applications. For fills exceeding ADS recommendations,
contact an ADS representative to review site specific data.
6. Do you use recycled
HDPE pipe manufactured to meet the specifications of AASHTO M-252 or
M-294 require virgin compounds be used for the manufacturing process.
However, the specifications do permit reclaimed material from the
manufacturing process to be utilized since this is virgin material that
was wasted during start-up or trimmed from the pipe. The properties of
the resin are therefore known and meet the requirements for the virgin
7. How do I connect dissimilar
There are several methods of connecting dissimilar materials either from
ADS or outside companies. The most common method is to butt the two
pipes together, wrap the joint with a non-woven geotextile material, and
pour a concrete collar around the connection. ADS also offers a spigot
adapter that consists of a HDPE cylinder that is welded to the pipe and
then slipped into the bell end of the existing pipe. Then, depending on
the joint location and performance requirements, a geotextile wrap and/or
concrete collar can be employed. Additionally, ADS offers a watertight
repair coupler that is wrapped around the joint and tightened with
a mechanical coupler. The watertight repair coupler is most effective
when used with pipes of similar outside diameters. For additional
details on field connections, contact your local ADS representative.
8. What is the expected shelf
HDPE meeting AASHTO M-252 or M-294 is manufactured with a carbon black
coloration that protects the material from UV degradation. Carbon black
has been proven to protect exposed HDPE pipe for over three years
with no change in tensile properties from UV effects. Once installed,
the earth cover protects the pipe from UV effects. Exposed ends of pipes
are located in areas of little or no tensile stress (due to no
overburden), and therefore not a consideration for UV effects.
9. Will sunlight affect the
pipe? Gasoline? Will the pipe burn? Float?
Let's address these questions individually.
Sunlight contains ultra-violet rays that reduce the tensile
properties of plastics with time. HDPE pipe installed in the ground is
primarily in compression due to the annular profile design of the pipe.
Slight tensile stresses in the pipe arising from trench anomalies and
residual stresses from the manufacturing process are overcome by the
large compressive stresses due to the soil overburden. Therefore, the
net stress in the pipe is compressive. Additionally, once the pipe is
backfilled it is protected from the effects of UV rays. The pipe's
exposed ends are in areas of little or no stress and therefore a
reduction in tensile strength due to UV does not affect the pipe's
performance. AASHTO and ASTM specifications include requirements of
coloration of HDPE pipes with carbon black to inhibit the effects of UV
of the material. Carbon black has been proven to protect HDPE pipe for
over three years with no change in tensile properties from UV effects.
Therefore, when the pipe is stored prior to installation, the carbon
black coloration inhibits the effects of UV rays and, once the pipe is
installed, it is protected from UV by the backfill materials and exposed
ends are located in an area of little or no stress. To illustrate the
fact that carbon black protects HDPE from UV degradation, consider the
insulation coating on overhead telephone and electric lines. These lines
are insulated with HDPE that is continuously exposed. However, since the
lines are under relatively low stress, the carbon black coloration
protects the lines from UV degradation.
The effects of gasoline and whether the pipe burn will be addressed
together since the concern with fuel spills is the potential for a
catastrophe which could damage the pipe. Gasoline does not adversely
affect HDPE. The material does not soften or lose strength when exposed
to gasoline, however, the material will burn if exposed to an outside
fuel source (gasoline). When examining the effects of a catastrophic
event, one needs to consider the performance of other materials as well
as the effects of the event itself. In the case of a fuel spill and a
fire in a closed storm drain system, the fire would extinguish itself
rather quickly due to an inadequate air supply. Studies have shown that
even in an open culvert the fire extinguishes itself within the first
few feet of the culvert. Repairs would be relatively simple and involve
excavating the ends of the pipe and cutting off the damaged section.
This could then be replaced with new pipe.
The Florida DOT conducted a study into the potential damage to HDPE
pipes from fire and concluded the risk of a fire was minimal and
headwalls would effectively eliminate damage. For a catastrophic event,
many commonly used materials would be damaged and require repairs. For
example, asphalt pavements would burn, metal pipes (especially asphalt
coated) would be damaged and concrete pipes would be weakened due to
cracks from expanding aggregate. Therefore, fire has the potential to
damage a number of common construction materials. When evaluated from
the risk potential of damage, HDPE pipe is no more susceptible to fire
damage than other pipe materials.
10. Is the pipe competitive to RCP and
HDPE offers a very competitive alternative to RCP and CMP. HDPE material
costs are generally equal to or less than comparative sizes of RCP and
CMP pipes. Due to the low mannings 'n' value of ADS N-12, HDPE pipes are
generally one diameter smaller than CMP pipes for equivalent flow
characteristics, making HDPE even more cost effective. The inert nature
of HDPE and its high abrasion-resistance allow HDPE pipe to be used in
environments when RCP and CMP require thicker walls or additional
coatings to obtain the required performance. HDPE is manufactured in
longer lengths (20') and is significantly lighter than RCP, which
equates to faster, safer installation. Finally, ADS N-12 offers a bell
and spigot coupler that allows faster installations than CMP. Therefore,
considering the competitive cost of ADS N-12 pipe and the savings due to
higher performance and faster, safer installations, ADS N-12 is logical
alternative to providing cost effective storm drain pipe applications.