concrete pipe has been one of the most economical and durable construction
materials for underground drainage and sewerage. . The "Pipe of the 21st
Century" may well be Non-reinforced Concrete Pipe slip-formed directly in an open cut
trench. With thirty-five years of successful history in Western United States, CIPCP has
been established as a simple, practical, economical and durable method of producing high
quality concrete pipes from 24 inches to 120 inches in diameter rapidly and at sharply
reduced cost in comparison to alternatives. More than 3,500 miles of this advanced pipe
system are in daily service in Western irrigation, storm drainage and waste water systems.
CIPCP eliminates the customary precast manufacturing plant making large diameter
concrete pipe economical and available wherever and whenever needed. Environmentally
trucking and stringing of large pipe sections along an open ditch are important advantages
of CIPCP. Heavy cranes and the usual heavy construction equipment are not needed. The
installation process proceeds rapidly and with minimal disturbance in residential streets.
An industry developed National American Concrete Institute Specification, (ACI 346-90)
and Recommendations for Cast-in-Place Non-reinforced Concrete Pipe (ACI 346R-90) are the
result of 20 years of industry effort. While the CIPCP process is simple in concept, and
the referenced specifications are fully responsive to the product there remains a
requirement for the presence of a third-party professional engineer/inspector to assure
field quality control over each construction phase from trenching, concrete placement and
backfill to acceptance of the finished product. 2/,3/
The continuous and jointless monolithic casting process practically eliminates leakage
providing a distinct advantage over jointed precast pipe where mortared joints are
required for each precast pipe length. Even with rubber gasketed joints, the precast
product does not surpass the monolithic CIPCP conduit's performance. There are no joints
in the conduit other than construction joints at the start and finish of each pour.
Hairline transverse shrinkage cracks occur at about 30-40 ft. intervals, but these close
when wetted. If larger width cracks occur they are sealed.with cement mortar paste, epoxy
injection or an elastomeric sealant.
The art of slip forming took a giant leap forward on January 24, l956 when the United
States Patent Office issued Patent No. 2,731,698 for an APPARATUS FOR FORMING CONCRETE
PIPE IN-SITU to Elmer Le Roy Tunsen assigned to NO-JOINT (TM) CONCRETE PIPE COMPANY, Butte
Co.,California. Quoting in part from the Patent document, one of the objectives of the
patent is...."to obviate the need for prefabricated pipe sections and pipe joints by
casting cementitious pipe in a continuous manner directly in the ground in which the pipe
is to lay." The Inventor chose diameters ranging upward from 24 inches, I.D.. in six
(6) inch increments. Wall thicknesses are quite similar to those of Reinforced Concrete
Culvert Pipe, ASTM C-76 Wall "B". Monolithic Cast-In-Place Pipe superseded the
two-stage systems that were commonly used in irrigation systems and which were not
generally acceptable of Public Works projects..2/
Not until 1961 and 1962 were other monolithic cast-in-place pipe systems patented and
entered into competition with No-Joint (TM) Pipe. In the five years between 1961 and 1966
the Trademarked name "No-Joint" actually became the generic name for CIPCP. .4/5/6/
States, Cities Highway Departments, Flood Control Districts and other Public Agencies
in the Columbia Basin of Washington, the Great Central Valley of California and the Salt
River Valley of Arizona were leaders in specifying and purchasing slip-formed concrete
pipe for Public Works Projects. Today CIPCP has has achieved more than 35 years of highly
successful historical performance and the simplicity, economy and durability of CIPCP are
filling the demand for cost-effective storm drainage and waste water systems throughout
the Western United States. It is an excellent product for environmental control systems
where suitable soils exist and sound concrete is near at hand.
There are two principal construction methods for CIPCP. Both feature a semi-circular
bottomed trench as the outer concrete form; one method uses internal metal forms while the
other method features an inflated tube inner form. The latter developed many problems when
diameters exceeded 48 inches.and is no longer an available construction option.. 4/5/6/
Today the metal form processes continue in use in diameters ranging from 24 inches to
120 inches The initial proprietary Patents and subordinate franchises have expired. There
are many independent Contractor/Operators employing a variety of machines with
modifications from the original Patents. Specifications are often written by individual
contractors to exclude competitors and to protect their own interests, particularly as to
workmanship and quality control. There is no CIPCP Industry Trade Association to
promulgate uniform industry standards. Purchasers rely heavily on the individual
Contractor/Operator to furnish design, construction and inspection know how. Our
Engineering/Inspection Firm, The Moote Group has recognized and filled this void for more
than 15 years, earning National recognition for leadership in CIPCP Engineering and Field
Quality Control. Our services include feasibility studies, review and critique of design
plans and furnishing expert technical supervision over field operations.
Installation of CIPCP begins as soon as a backhoe opens a trench. Daily production
ranges from 800 feet/day for 24-inch to 350 feet/day for 120-inch pipe. Even larger
diameters are feasible, if and when there is a need. Approximately four miles of 14-ft.
I.D. CIPCP were laid and tested in Arizona in 1977 as an alternative storage system for
the U.S. Air Force's MX-Missle.. The Moote Group was among the special consultants
selected by Ralph M. Parsons to assist in this undertaking.
CIPCP is now firmly established among an increasing number of Western Professionals.
The informed engineer knows that CIPCP has exceeded every load test, has an "n"
factor (friction coefficient in the Manning equation) equal to that of reinforced concrete
pipe alternates, is readily available wherever needed at 20 to 40 percent less cost than
RCP. By eliminating the factory and manufacturing lead time CIPCP could well be discovered
as "The Pipe for the 21st Century". There is a challenge and an opportunity for
innovative minds to modernize and improve the CIPCP process. The original machinery
designs and concept remain virtually unchanged. The process, although presently cost
effective, is relatively labor intensive requiring an crew of 7 to 12 men.. Modernization
of mechanical features and use of automation for concrete control would obviate the
present need for third-party professional quality control in the field. 3/21/
Sacramento County, California was one of the earlier Public Works users of CIPCP
starting in the year 1956. In the next 14 years the County installed 156,000 lineal feet
of CIPCP in diameters from 24-inches to 84-inches at a savings of $2,000,000 to taxpayers.
Sacramento County realized 30-40 percent cent installed cost savings in over conventional
pipe. 7/
CIPCP trenches are excavated with a special semi-circular bucket for each pipe size.
The typical CIPCP trench has a semi-circular bottom and must stand vertically to one-half
of the pipe diameter above the crown of the pipe without sloughing Above that elevation
the sidewalls may be sloped back to lessen the possibility of caving and to minimize the
need for shoring. The trench is from 1-1/2 to 3 inches wider than the outside diameter of
the slip-form (or "boat"). Prescribed line and grade are established by means of
a laser set up in the trench and targeted on the digging bucket. Spoil is cast along one
side of the trench so the opposite bank is clear for internal form stringing and
accessible to transit mixed concrete delivery trucks. PHOTO
When ground water is encountered, a french drain is constructed along the trench invert
center line. The slip form passes over the drain, filled with gravel, and water flows
harmlessly beneath the pipe.
In locations where unsuitable soils are encountered the trench may be filled with
suitable imported soil, compacted and re-excavated. Marston Negative Projecting Conduit
Condition is the generally accepted method for Installing CIPCP. In this method the pipe
is laid in shallow trench before the final embankment is placed to finished grade. (This
method is well known to design engineers.).
The No-Joint (TM) slip form comprises two parts. The outer hull, or "boat"
contains the diesel or electric power unit and cable winch and an after section or
"mandrel" that unitizes the concrete hopper, electric vibrators and the upper
and lower skirts which shape the extruded concrete. The mandrel is vertically hinge
mounted to provide articulation in negotiating curves. Curve radii are limited to 45 feet
for 24-inch and 160 feet for 120-inch ID CIPCP. Precast RCP pipe manufactured to the
specified radius are inserted in curves having radii too short to be negotiated by CIPCP.
The steel "boat" serves as a sturdy safety shield to protect CIPCP workmen.. Photo
Fig --
Aluminum alloy forms, equipped with hook and eye linkage bars are lapped and fed
through the mandrel beneath the concrete hopper. Metal spreaders (semi circular trusses)
are placed at the 6-inch form laps by workmen within the mandrel. The weight of the fresh
concrete sets the metal forms in position. The 90 degree exposed pipe invert is smoothly
troweled as the slip form advances. A variable speed electric powered winch driving a
cable anchored to a dead man within the trench advances the slip form. Larger slip forms
have a half-track in the "boat" to decrease towing friction.
When the concrete takes its initial set (in 4-6 hours), spreaders are removed, the
linked forms are dropped onto the invert, pulled out of the pipe cleaned and oiled for the
next day's pour. The conduit is quickly inspected for repairable deficiencies. Openings to
the conduit are closed and the heat of hydration of the cement raises the ambient
temperature as much as 30-40 degrees providing an ideal warm moist curing environment. The
exposed outer upper quadrant of the pipe is immediately covered with a plastic sheet and
"shaded" with 6 inches of loose native material. The concrete reaches 80 per
cent of design strength in 48 hours, more or less, and full design strength in 5-7 days.
Photo Fig. --
It is important that the initial 24-inches of backfill be compacted to the relative
density of the trench soil. The soil envelope surrounding the CIPCP must be a homogeneous
and of uniform density to assure uniform 180 degree load distribution over the pipe crown.
Backfill above the initial 24 inches is placed and compacted as directed by the Soils
Engineer to satisfy overlying structural requirements (such as roads streets, etc.).
Backfilling commences when concrete test cylinders reach 80 to 100 per cent of design
strength and continues thereafter without interruption.
The Soils Engineer. furnishes a Soils Report, renders his judgment as to the
suitability of the soil for CIPCP, and provides soils information to the design Engineer
required for structural calculations. Data includes unit compacted soil weight and the At
Rest lateral soil pressure. Field soils technicians inspect and direct excavation,
backfilling and testing. The Project Engineer depends upon the Soils Engineer for for all
of the soils data that affect the CIPCP product and uses the data when evaluating the load
carrying capability of the CIPCP..8/
The At Rest lateral pressure Coefficient (Ko=1-Sin phi) is a conservative design
criterion., as numerous load tests have corroborated. Casagrande says " (In the case
of buried concrete pipes)........ "horizontal earth pressures often surpass at-rest
pressures and approach the passive pressure range.".9/
Quoting from Project No. 15-3 Final Report on "Rational Structural Analysis and
Design of Pipe Culverts" by the Department of Civil Engineering, Northwestern
University, page 39. "..On the basis of 0.003 (in/in) strain limit for concrete, and
other assumptions, Lum found that the allowable conduit deformation is equal to the square
of the pipe diameter (in inches) divided by 1200 times the wall thickness (in
inches)".. 11/. As an example, a 60 inch CIPCP with a 6 inch wall could theoretically
deflect 0.5 inch before failure. In no In Situ Load Bearing Test of CIPCP has a measured
vertical and/or horizontal deflection ever been measured in excess of 7 per cent of the
theoretical ultimate.13/14/
As loads are imposed on the pipe,the elastic CIPCP ring tends to deflect and presses
against the restraining trench The unusually high test loads and small deflections bear
this out. In the case of 96-inch CIPCP, the maximum recorded deflection was 0.034 inch
under a static load of 52,250 lbs. Applying the above deflection formula the 96 inch CIPCP
could deflect 0.833 inch before failure. The actual deflection is only 4 (four) per cent
of the failure mode.10/
Northwestern University Department of Civil Engineering developed a formula for
computing Moments from measured deflections in concrete rings. The result Moment = 3.6 x
EI X Delta R/Delta R squared., where R = measured deflection of radius. 11/.
The early attempts to assign D-Loads to CIPCP proved fallacious. D-Load is determined
in a 3-edge bearing test. and is the load on one foot of conduit that produces a 12-inch
long crack .01 inch in width. The ratio of the test load/foot divided by the pipe diameter
in feet., is termed "D-Load (.01)" Upon pipe installation a Load Factor
corresponding to one of four classes of bedding enhances the the D-Load (.01) to the
design D-Load. The Load Factor of 3.0 is usually assigned to a CONCRETE cradle. CIPCP
qualifies for a load factor of at least 3.0, but the D-Load approach, which neglects
modern soil mechanics, is grossly inappropriate for CIPCP..
The Working Stress Design Method is the more appropriate method. The conduit is
subjected to earth and live loads which are ameliorated by At Rest lateral pressures.
Structural calculations using Paris Coefficients (16/) to determine the working stress are
based on applying uniform 180 degree load distribution and reaction of earth and live
loads and five loading conditions . These are (1) Vertical earth load (2) Live Load (10
feet of cover maximum), (3) Triangular Lateral Support, (4) Uniform Lateral Support, (5)
Weight of the conduit. (Water load is neglected in storm drainage.) A safety factor of 2
is applied based on the Modulus of Rupture in Flexure of the particular concrete design.
(Usually 650 psi for concrete f'c = 4,000 psi. permitting a working stress of 350 psi.)
Manning's equation (V=1.486/"n"R2/3S1/2), is used for calculating open
channel flow in CIPCP. Manning's "n" of 0.013 is the proper coefficient for both
CIPCP and RCP alternates. .Comprehensive tests performed by the U.S. Bureau of Reclamation
and The Salt River Project, jointly, on an operating irrigation system over a 3-year
period on CIPCP lines 24 to 54 inches in diameter, and ranging in lengths from 856 feet to
4,267 feet, clearly establish this as the correct "n" value to apply to CIPCP.
No equally comprehensive "n" factor test has ever been performed on reinforced
concrete pipe alternates. All of the test lines included in-line structures, bends,
detritis and silt accumulation. The scope of these tests is unprecedented and conclusive.
21/
Cast-In-Place Non-reinforced Concrete Pipe has earned the title "Pipe for the 21st
Century" in 35 years of meeting and exceeding every challenge. While it is a
utilitarian conduit and not to be touted as the "pipe for all seasons" it will
satisfy the requirements for gravity and low head (under 25 ft) storm, irrigation and even
in certain waste water conduit systems (where H2SO4 is not generated) in the future.
Environmental and pollution control problems recognized 20 years ago by Fischer (Retired
Editor of Harper's) ( 17/) and H.U.D. Official Jones (18/ ) remain unsolved and far
greater problems loom on the horizon. CIPCP is a "sleeping giant" awaiting
discovery;. This product, at one time used in India, South Africa, Mexico and in some
South American countries under franchises of NO JOINT (TM) patent owners has never been
promoted beyond a few "Western States. CIPCP is so versatile, simple to install and
cost effective that it certainly will have applications beyond the arid lands of the
West... It provides the means of building large diameter conduits at any place and at any
time without the usual supporting concrete pipe factory.
CIPCP has inherent advantages.: (1) The supporting strength of a Tunnel, (2) Proven
economy and speed of installation, (3) Almost "bottle tight" leakage
performance, (4) Simplicity of Design, (5) Independence from Factory Manufacture, (6)
Availability at any suitable location without lead time. (6) Availability at any suitable
location without a factory and attendant manufacturing lead time, and (7), CIPCP is a
prototype for other slip-formed concrete product configurations such as horseshoes and
arches.
The CIPCP. industry lacks a professional Trade Association to promote its products. The
basic machines have undergone only minor improvements. This opens an opportunity of
unprecedented magnitude.for developing the "Sleeping Giant". For the adventurous
entrepreneur, modern technology, computer chips, and mechanical improvements supported by
well directed research and engineering, portends opening exciting new horizons.
The needs in America, Europe, Russia, Asia, and Third World Countries are certainly
fertile ground.
Who will build
