A Complete Design Manual for Corrugate Plastic Drainage Pipe is Available
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Before beginning installation,
one should note that OSHA Files show that trench Cave-Ins kill more
construction workers per year than any other type of construction accident.
This includes any type of pipe installation, cable installation or
A Complete Design Manual for Corrugate Plastic Drainage
Pipe is Available if you visit the PPI’S Web Site by clicking here:
Corrugated HDPE pipes have been installed at
depths as shallow as twelve inches (12”) and as deep
as one hundred and four feet (104’) in H-20 traffic bearing live
load situations. This wide range of depths is facilitated by burying
the pipe in a good quality soil load bearing envelope
that eliminates soft voids around the pipe so the pipe will
not be prone to shift. When Live Loads are applied to the pipe, the
flexible design of the pipe can utilize the stable compacted side-wall
backfill for load bearing support. Good Quality soil backfilled and
compacted in layers to recommended engineering standards will provide
long term, Service Strength, Lateral Support against pipe deflection.
Soil Quality analysis procedures necessary for constructing a load
bearing embedment envelope around flexible plastic pipes to provide
long term load bearing Pipe-Soil interaction is principally referenced
in ASTM-D-2321, and for soil compaction in ASTM-D-698 or AASHTO-T-99.
Table 1 describes Soil Quality classification.
Minimum Cover Height
Based on Class III Backfill Compacted to 90% Standard Proctor
Density and AASHTO H-20 Load
Minimum Cover Height Table 2 and CANDE graph source: The Corrugated
Polyethylene Pipe Association Study titled “Minimum Cover Height
for HDPE Corrugated Plastic Pipe Under Vehicular Loading” by
Katona - 1988
Trenching & Bedding
Trenching Installation Sequence
An undisturbed, firm trench
wall is the unmovable base against which the compacted Soil Envelope
material that immediately surrounds the pipe rests. The undisturbed,
firm trench wall and the Compacted Envelope together actually carry
the load. In stable soils excavate as narrow a trench as needed to
install, and compact around the pipe. Narrow trenches will save time
and compaction costs. If trenching in unstable soils, a trench of
1.5 times the pipe diameter plus twelve inches is the minimum width
required. A soils engineer however may require additional excavation
in order to stabilize or bridge over soft soil pockets. Also when
trenching in unstable soil, the trench walls should be slanted on
a slope both for safety, and to assist the installer in preventing
the pipe from being misaligned by unstable soil cave-ins. As noted
in Table 1, Class IV and V soils require the evaluation
of a Geotechnical Engineer. Gravel packed envelopes enclosed
in a filter wrap are generally required per ASTM-D-2321 to provide
pipe support, and control water migration of cohesionless material.
ASTM-D-2321 also notes other soil stabilization option strategies
in addition to gravel packed filter wrapped pipe support cells.
Bedding Installation Sequence
Just as firm trench walls provide
solid side support and prevent side shifting in plastic pipe systems,
so too must the bed upon which the pipe lays be free of
soft spots to provide longitudinal support along the length
of the pipe. If the pipe is not uniformly supported along its length
by a firm, unmovable floor or bedding, then differential settlements
can cause pipe sagging to occur. Sagging can create water flow backups.
To provide good longitudinal bed support, construct the Bedding Zone
per the appropriate Soil Type as described in Table 3. The Bedding
Zone as shown in the Pipe Cover Sequence Diagram is the top of the
trench grade that the pipe lays on. This Bed should be Smooth and
Free of Large Rocks or other protrusions, which may cause point loading
on the pipe. Do not allow rocks of over 1 ½” to come
in contact with the pipe. Class I soils are smooth graded. Class
II and III
soils are smooth graded, and then compacted to desired grade.
Class IV and V soils must be over excavated by at least 6 inches
and filled with compacted material to grade per Table 3. A geotechnical
evaluation may require additional over excavation
and filter wrapped fill if a future high water table level is expected
in the pipe support zone.
Trench & Bedding
Construction Methods Per Soil Classification
Class II & III
Excavate to grade and begin installation.
Make sure no rock over 1 ½” in size makes contact
with the pipe.
Excavate to a point above grade & compact
loose material to the desired Bedding Compaction Density Percentage.*
Since this soil is not readily compactable, excavate at least
6” below grade. Then fill & compact a Bedding Layer
to grade. ** A geotechnical evaluation is required.
Similar to Class IV. However, may need to
excavate over 6” to below existing groundwater or future
predicted grade water level. **A geotechnical evaluation is required.
The above chart only gives a general overview of each soil type. For actual engineering design work, the expanded description necessary for understanding the obtainment of a satisfactory pipe embedment stiffness can be seen in ASTM-D-2321 on Tables 1 & 2.
* NOTE: If using a Class
I Stone envelope material in a trench of Class II soil, use a filter
wrap. If using a Class II sand envelope material in a trench of Class
III soil, use a filter wrap.
**NOTE: Must use a filter wrap or employ other soil
stabilization strategies in Class IV or V soils as noted in ASTM-D-2321
per the determination of a geotechnical engineer.
Pipe Cover Sequence
If a Class I stone material
is used for Bedding material in a Class II or lower soil type classification
trench, a Filter Wrap should be placed down prior to adding the Bedding
material. A Filter Wrap, as shown in Filter Diagrams A & B, will
allow water to pass, but prevent the trench Bed or Wall soils from
migrating with rain waters into the rock envelope. This will stabilize
both the Envelope Zone and the Support Soils that surround
it. If a Class II soil is used for Bedding material in a Class
III or lower soil type trench classification a Filter Wrap should
be used also. In addition to a filter wrap, an Anti-Seep
Collar maybe required along the pipe run to prevent the
water flow migration of soil fines along the length of
| Lay down filter fabric prior to placing loadbearing envelope
material and foundation bedding materials.
|| Water can pass easily through the gravel-pack or a sand envelope’s
exterior filter wrap, while waterborne fines are filtered out.
This construction technique will provide both a stable envelope
soil and a stable trench soil.
Haunching Backfill Sequence
The Haunching Zone as shown in the pipe cover
sequence diagram is between the Bedding Zone and the Lower Half of
the pipe below the Spring Line. If a Class I type of rock or stone
was used in the Bedding Sequence, it is mandatory that it be used
in the Haunching Backfill Sequence up to the Spring Line. This continued
use of Class I material will prevent the loss of Haunching’s
side support that would occur if a Class II or Class III soil were
used since these latter soil types would migrate into the voids of
a Class I stone type bedding base. If a Class II sand was used in
the Bedding Sequence of a Class III soil, the Class II should be
used up to the Spring Line for haunching backfill material also as
different soils in the haunching area may react differently to moisture
The continued use of the same material from Bedding
to Spring Line in conjunction with the use of a filter fabric will
stabilize the environment in which the pipe is buried. If the pipe
is being installed below existing or future predicted groundwater levels,
a Class I must be used to the Top of the pipe encased in a filter wrap.
CPP Water Tight or Mastic Wrap leak resistant couplers as described in the coupler
section must be used at expected high water sites to prevent the water
migration of cohesionless soils into pipe joints that could cause future
If a Class I stone is used as Haunching Envelope material,
simple dumping and leveling can achieve the required compaction level.
If a Class II sand is used, employing the water spray compaction method
to obtain the required compaction density is effective. Layout the
sand in 6 inch lifts and spray each lift, but avoid saturation and
puddling. As a rule of thumb, the dryer the soil, the stronger and
more stable it is. However, a slightly damp material will generally
result in maximum compaction with minimum effort. Do not use this method
in freezing weather though, as ice pocket voids maybe created.
While Class III soils are allowed as a compaction envelope material,
more care and compaction time must be taken to achieve the required
proctor density compaction level. Class III soils should be laid out
in 3 inch lifts. Even though water-spray compaction is allowed in
Class III soils, Hand-Held tamping is generally recommended because
this can be faster than waiting for the sun and air to dry out each
lift. As a result of the extra time spent on compaction, Class III
soils can be more expensive to use than imported Class I or Class
II soils even if the Class III soil is provided free.
pipe contractors economize both labor cost and imported soil envelope
cost at Class III sites by importing a Class II sand, and mixing
this 50/50 with the site native Class III soil. The native Class III
soil generally provides enough dampness when mixed with the Class
II sand to provide a quick maximum compaction job with little effort.
The final compacted soil’s
moisture content should be 9.5% or less. If a higher post-construction,
soil moisture content is expected, a filter wrapped soil support
cell should be considered.
To make sure that no movement occurs
in the pipe during Haunching, Special Care should be taken to adequately
fill and shape the Haunching material to the Bottom Half Curvature
of the pipe equally all on each side. Failure to adequately fill and
compact under the Spring Line bottom half Curvature can create loose
voids that will later allow movement during a live load. To avoid
this, place the Haunching fill material in 3-inch or 6-inch layers,
and compact prior to laying down another lift. If using the water
spray compaction method, do not puddle, and allow each layer to solidify.
Be sure to compact each layer out to the undisturbed trench side wall
for assured side wall lateral support.
Equal compaction on each side in layers
will prevent pipe from shifting during the construction phase. By
extending the compaction fully to the firm and undisturbed sidewall,
the lateral stiffness needed to prevent pipe side – shifting
under traffic loads is assured.
Initial Backfill Sequence
|| Compact bedding and the envelope zone in layers to eliminate
soft spots that can cause shifting.
This sequence covers the space between the
pipe’s Springline to a point 6” above the top of the
pipe; it takes place in two steps: (1) Step one is from the Springline
to the Top of the pipe. (2) Step two is from the top of the pipe
to a point 6” above the Top of the pipe. Place each lift,
and compact to the required standard proctor density. If using the
water spray compaction method, allow time for each damp layer to
solidify until it will support the weight of a man before adding
the next lift. In compacting this initial backfill sequence, do
not use heavy mechanical compaction equipment directly above the
top of the pipe until ample backfill has been added to prevent the
compaction event from causing the pipe to deflect or bend out-of-round.
Light hand tamping or water spray compaction, which will solidify
soil and eliminate voids without deflecting the pipe, is recommended
here for Class II or Class III soils; Class I soils will compact
upon dumping with little extra effort other than smooth grading.
In this compaction phase, compact the sidewall portion of the backfill
fully to the sidewall, as sidewall lateral support is what is essential
for the load carrying. Again, only use light hand tamping or water
spray to compact away loose material over the top. Provide at least
48 inches of cover Directly Over The Top before the utilization
of a hydrohammer for compaction. If using a front-end loader to
deposit Class I or Class II material in the trench, Featherdump
in 6” layers so the required compaction can be achieved quickly.
If using a Class III material, dump in 3” layers. Once step
two has been completed 6” above the top of the pipe in this
phase, close off the filter fabric wrap by overlapping if filter has
| 6” Bedding Zone Required Under Pipe
|| Water spray compaction is effective, and labor cost efficient
in Class II sand soil types. Class III soils however can be compacted
over a wide range of densities due to moisture variability potential.
To achieve good compaction in Class III soils, the soil’s
moisture content should be 9.5% or less. In these soils, compaction
installation economies can be achieved by mixing imported sand
50/50 with the native Class III clays.
Final Backfill Sequence
When the Initial Backfill Sequence has been
installed to a point 6” above the top of the pipe, and the
filter has been overlapped to the closed position, only an additional
6” of Compacted material is needed in this, the Final Backfill
Sequence, if Live Load Traffic Bearing is expected. Otherwise, Compacted
Class I, II or III material is not needed in the Final Backfill
zone except as specified by an engineer in a Class IV or V soil
or high water table situation. Also at depths of more than 20
feet deep, additional compacted material maybe needed in the Final
Backfill zone depending on the soil type profile at each site as
determined by an engineer.
||Class I Rock & Marl cover provides H-20 live loadbearing
service with 12” of cover. H-20 loadbearing includes cars,
vans and tractor-trailers as shown elsewhere in this booklet.
As a footnote to Trench
and Cover installation requirements, the paving material of concrete
and asphalt should be noted. In most construction cycles, pipe
installation comes at the beginning of a job, and paving at the
End. The minimum cover height shown in Table 2 is based on the Unpaved Cover
height needed to substain H-20 Construction Load Traffic, but not
the heavier H-25/H-30 construction loads like brick, ready-mix,
loaded motor graders, etc. Both asphalt and concrete greatly reduce
the deflection loading on shallow buried plastic pipe, and 6” of
pavement material can be substituted for the last 6” of the
backfill phase. Local highway department subdivision rules however
may very. Thus area road officials should be consulted prior to
installation to identify site-specific construction needs.
cases where the pre-paving depth is less than three feet, and
the paving cannot be rescheduled towards the beginning of the construction
cycle, Mounding should be employed to protect the pipe during construction
in a way similarly recommended for concrete, clay or metal pipes.
The mounding of three feet of compacted fill over the pipes should
be used at Construction Cross-Over Sites. Non cross over areas should
be red flagged to prevent accidental crossings. The Mounding Ramp
should extend to each side of the pipe for a distance of 1.5 times
the diameter plus twelve inches. During regrading of the Mound for
paving preparation, care should be taken not to deflect the pipe out-of-round.
Care should also be taken by the paving equipment operator to avoid
pipe damage or setting deflection in the pipe. If installed correctly,
concrete and asphalt paving allow for the use of CPP pipe systems
at any depth.
A Complete Design Manual for Corrugated Plastic Drainage Pipe is Available
if you visit the PPI’S Web Site by clicking here:
Because paving does allow for traffic bearing
over CPP’S durable, lightweight pipe systems at shallow or
deep depths, CPP pipes fill a multitude of cost saving needs on
large paving jobs like shopping mall parking lots and airport runaway
stormwater control systems. Both asphalt and concrete can be easily
formed to make catch basins as well as attractive, useful headwalls.
Preformed or cast-in-place headwalls and flared end sections will
allow for the mowing of roadside vegetation and other weed control
programs, like Fall burnings, without damage to plastic pipes as
is also done with bituminous coated metal or concrete pipes.
Other Installation Tips
As with concrete, metal and clay pipes, the
installer should take precautions at the usual listing of sites
that historically have been a problem for any pipe, no matter its
raw material base. These sites include: Manhole tie-ins; Catch basin
tie-ins; Drop box tie-ins; Foundation or retaining wall penetrations;
Service lateral connections and crossing over an existing pipeline. Dams and all sites with soils that are mushy prone due to rising and falling seasonal water tables require the services of a certified engineer. Also note that any job disturbing 1.0 acres or more of wetland requires a construction permit. At these points, extra care should be taken to insure that uniform gravel pack support is provided around the entire pipe or pipes,
and that the firm bedding of previously installed pipes is not changed.
Also, do not excavate more trench length than can receive pipe in
a day. Should multiple day – extra excavation occur, all native
soils to be used as backfill should be tarp covered to prevent extra
soil moisture related softness in the fill.
Soil Remediation / Stabilization
Sites such as Detention ponds always have wet, mushy
soils that must be improved for loadbearing with or without traffic
loading in order to eliminate differential settlement that occurs
with changing water levels.
A concrete pad is poured to provide
firm bedding support around a CPP pipe fitting. Such extra
installation measures will reduce the possibility of future construction
related damage to any pipe system, no matter its raw material
At sites where water tables vary with annual rainfall peaks, In Place Shoring
in combination with filter wrapped gravel-packs can be required to assure long
term pipe-soil support per ASTM-D-2321.
Such measures are needed at these sites
whether traffic is expected or not. In addition, sites where soil
types vary greatly, either naturally or from previous fill construction
practices, are likely to provide the conditions that require extra
soil stabilization efforts.