AGENDA:
STORM DRAIN DESIGN SEMINAR: PART II
AGENDA: CONTINUED
GENERAL STEPS FOR STORM DRAIN DESIGN
IF YOUR STORM DRAIN CONSTRUCTION IS PART OF A TRACT IMPROVEMENT, SOME
OF THESE STEPS MAY NOT APPLY BECAUSE THE TRACT MAP APPROVAL PROCESS
WILL USUALLY TAKE CARE OF STEPS ONE THROUGH FIVE.
- ENVIRONMENTAL DOCUMENTATION - EIR, ND, CE, EIS, EA, FONSI
- ENVIRONMENTAL & LOCAL AGENCY PERMITS
- GENERAL PLAN CONFORMITY
- COOPERATIVE AGREEMENTS
- RIGHT OF WAY REQUIREMENTS
- UTILITIES (HIGH IMPACT ON STORM DRAIN DESIGN)
GENERAL STEPS FOR STORM DRAIN DESIGN
- MATERIALS SOILS/GEOTECHNICAL REPORTS
- SURVEYS
- PRELIMINARY ALIGNMENT(S)
- HYDROLOGY
- HYDRAULIC DESIGN (FOCUS OF TODAY'S PRESENTATION)
- STRUCTURAL DESIGN
- PLANS, SPECIFICATIONS, AND ESTIMATES
- PUBLIC MEETINGS/SEMINARS
- CONTRACT BID AND AWARD
GENERAL STEPS FOR STORM DRAIN DESIGN
- CONSTRUCTION SUPPORT/INSPECTION
- RECORD DRAWINGS/AS-BUILT PLANS
- CLOSE OUT DESIGN FILE
- CHECK TO ENSURE THAT ERRORS AND OMISSIONS INSURANCE IS UP TO DATE.
- LAW SUITS AND DEPOSITIONS
NOTE: THERE ARE ONLY THREE THINGS THAT HAVE NO STATUTE OF LIMITATIONS
IN THE STATE OF CALIFORNIA:
- MURDER
- CHILD MOLESTATION (2003)
- AND YOUR ENGINEERING LICENSE
IN FACT, LAWYERS HAVE GONE SO FAR AS TO SUCCESFULLY SUE AN ENGINEER'S
WIDOW FOR DAMAGES.
GENERAL STEPS FOR STORM DRAIN DESIGN
THE FOLLOWING WAS FROM A CELSOC DRAINAGE LAW COURSE:
CIVIL ENGINEERS CAN BE SUED FOR INCOMPETENCE AND/OR NEGLIGENCE. SINCE
NEGLIGENCE CAN BE HARD TO PROVE, MANY LAW SUITS FOCUS ON TAKING OF
PROPERTY.
MOST PUBLIC AGENCIES ARE SUED FOR:
- NEGLIGENT DESIGN, CONSTRUCTION, AND MAINTENANCE
- INTENTIONAL OR UNINTENTIONAL DAMAGE OR TAKING OF PROPERTY OR PROPERTIES.
- CREATION OR MAINTENANCE OF A DANGEROUS OR DEFECTIVE CONDITION
GENERAL STEPS FOR STORM DRAIN DESIGN
PRIVATE PROPERTY OWNERS MAY HAVE TO DEFEND AGAINST ALLEGATIONS OF:
- VIOLATION OF STATUATORY REQUIREMENTS.
- NEGLIGENTLY DIVERTING SURFACE, STREAM, OR FLOOD WATER.
ONCE SUED, MANY PUBLIC AGENCIES AND PRIVATE PROPERTY OWNERS HAVE A
TENDENCY TO "SHARE THE PAIN" WITH THEIR HIRED ENGINEERING CONSULTANTS.
CELSOC DEFINES NEGLIGENCE AS THE FAILURE TO PERFORM AS WOULD A REASONABLE
ENGINEER OPERATING UNDER LIKE CONDITIONS.
OR
NEGLIGENCE CAN BE THOUGHT OF AS FAILURE TO PERFORM IN ACCORDANCE WITH
THE ACCEPTED STANDARDS OF THE PROFESSION.
GENERAL STEPS FOR STORM DRAIN DESIGN
THE FOLLOWING PRESENTATION WILL REFERENCE THE ORANGE COUNTY LOCAL
DRAINAGE MANUAL AND ORANGE COUNTY PUBLIC FACILITIES AND RESOURCES DEPARTMENT
STANDARD PLANS AS THE:
STATUTORY REQUIREMENTS
AND ACCEPTED ENGINEERING STANDARDS
OF WHICH A STORM DRAIN MAY BE DESIGNED IN ORANGE COUNTY.
CITIES MAY ALSO HAVE THEIR OWN STANDARDS AND STATUATORY REQUIREMENTS
REALISTICLY, THE ONLY TIME AN ENGINEER MUST BE CONCERNED WITH ADHERING
TO COUNTY/DISTRICT STANDARDS AND REQUIREMENTS IS WHEN YOU ENCROACH,
CROSS, OR CONNECT TO COUNTY/DISTRICT ROW, OR WHEN YOU WANT US TO ACCEPT
YOUR FACILITY FOR OWNERSHIP, MAINTENANCE, AND OPERATION.
STORM DRAIN DESIGN
- DETERMINE A ROUGH/PRELIMINARY ALIGNMENT FROM BEGINNING (D/S) TO
END (U/S). (or vice versa)
EXAMPLE: THE ORCHID STREET STORM DRAIN
[Picture showing overview of area]
OUR TASK IS TO DESIGN A STORM DRAIN TO PICK UP ALL THE DRAINAGE ALONG
ORCHID STREET
STORM DRAIN DESIGN
- DETERMINE A ROUGH/PRELIMINARY ALIGNMENT FROM BEGINNING (D/S) TO
END (U/S).
- ASSISTS IN ACCURATELY ORDERING SURVEYS, GEOTECHNICAL STUDIES,
R/W, ETC.
- ASSISTS IN REQUESTING PERTINENT UTILITY INFORMATION.
- EVERY HOUSE OR BUSINESS USUALLY HAS AT LEAST 3 UTILITY
CONNECTIONS WITHIN THE STREET R/W: GAS, WATER, AND SEWER.
IN AREAS WITH UNDERGROUND UTILITY DISTRICTS THERE MAY
BE AS MANY AS SIX (6): INCLUDING ELECTRICAL POWER, TELEPHONE,
AND CABLE TV. (MANY LAY UNDER THE SIDEWALK)
ON ORCHID STREET WE HAVE WATER, GAS, AND SEWER LINES TO RELOCATE FOR
EVERY HOUSE ON THE WEST SIDE OF THE STREET
[diagram of street]
FOR THIS STREET: ELECTRICAL, TELEPHONE, AND CABLE LINES ARE LOCATED
ON UTILITY POLES BEHIND THE HOUSES.
STORM DRAIN DESIGN
- ASSISTS IN REQUESTING PERTINENT UTILITY INFORMATION.
NOTE: IT IS COMMON FOR SANITARY SEWER LINES TO OCCUPY THE STREET CENTERLINE,
AND MANY TIMES IT CAN BE DIFFICULT TO RELOCATE SEWER LINES & LATERALS
BECAUSE THEY ALSO FLOW BY GRAVITY.
WE MUST ALSO BE AWARE OF STANDARD/CODE CLEARANCES FOR UTILITIES.
AND INTERESTINGLY ENOUGH, THE AS-BUILT PLANS OF THE UTILITIES MOST LIKELY
TO INJURE OR KILL YOUR CONSTRUCTION WORKMEN SHOW THE LEAST AMOUNT OF DETAIL
IN TERMS OF LOCATION AND DEPTH.
- MAJOR AND PRIMARY ARTERIAL HIGHWAYS USUALLY CONTAIN MAJOR
TRANSMISSION AND DISTRIBUTION UTILITY LINES IN ADDITION TO
THE ABOVE.
CROSS SECTION OF SOUTH BRISTOL STREET SHOWING NINE (9) UTILITIES WITHIN
THE STREET RIGHT OF WAY. FIVE (5) OF THE NINE (9) UTILITIES ARE TRANSMISSION
LINES.
[diagram of cross section of Bristol Street]
MINIMUM UTILITY CLEARANCES FOR THE CMSD
[diagram of CMSD]
STORM DRAIN DESIGN
- DETERMINE A ROUGH/PRELIMINARY ALIGNMENT FROM BEGINNING (D/S) TO
END (U/S).
- ASSISTS IN DELINEATING THE TRIBUTARY AREA OF YOUR STORM
DRAIN FOR YOUR HYDROLOGY STUDY, IF REQUIRED.
- HYDROLOGY STUDY
- USUALLY ACCOMPLISHED U/S TO D/S.
- MAY ASSIST YOU IN PRELIMINARY LOCATION OF CATCH BASINS
- MAY ASSIST YOU IN PRELIMINARY SIZING OF STORM DRAIN MAINLINE
- WILL PROVIDE YOUR DESIGN INLET DISCHARGES
HYDROLOGY STUDY SUBAREAS
[map]
HYDROLOGY STUDY PRELIMINARY STORM DRAIN ALIGNMENT
[map of Possible Storm Drain Alignment]
HYDROLOGY STUDY: POSSIBLE CATCH BASIN LOCATIONS
LOCATE BASINS BASED ON HYDROLOGY STUDY CONVEX ROUTING
INITIAL AREA
STREET FLOW ROUTING
PIPE FLOW ROUTING
WE USUALLY TRANSITION FROM STREET FLOW TO PIPE FLOW WHEN FLOW DEPTH
IN GUTTER EXCEEDS THE TOP OF CURB ELEVATION
STORM DRAIN DESIGN
- HYDRAULIC DESIGN
- USING YOUR HYDROLOGY STUDY AS A GUIDE, PICK PRELIMINARY LOCATIONS
OF EACH CATCH BASIN.
- CALCULATE STREET FLOW AND DEPTH
HYDROLOGY STUDY CALCULATIONS ARE PRELIMINARY, SO WE MUST USE STREET
FLOW TABLES TO ACCURATELY COMPUTE STREET FLOW DATA.
AT 0.43 FEET THE STREET CROWN HAS BEEN EXCEEDED, SO ASSUME THAT WATER
FLOWS EVENLY ON BOTH SIDES OF STREET.
11.38/2 = 5.69
COUNTY STANDARD TYPE A2-6 & A2-8 CURBS
COUNTY STANDARD ROLLED CURBS
STREET ½ WIDTH IS APPROX. 17 FEET
FOR OUR EXAMPLE THE STREET SLOPE ( S) IS EQUAL TO: 56.62 - 55.85 =
0.77 FT DIVIDED BY THE LENGTH OF SUBAREA 3,4: L = 238.99 FT
S = 0.77 / 238.99 = 0.00322
Q/S½ = (5.69)/(0.00322)½ = 100.3
COMPUTED DEPTH OF FLOW IS APPROXIMATELY AT HEIGHT OF STREET CROWN
OR 0.43 FEET
LIMITATIONS OF THE STREET FLOW TABLES:
TABLES ARE ONLY PROVIDED FOR PFRD STANDARD PLAN A2-6, A2-8, AND COUNTY
STANDARD ROLLED CURBS AS PREVIOUSLY SHOWN
TABLES ASSUME COUNTY STANDARD PLAN TYPE STREET CROSS SECTIONS AS PREVIOUSLY
SHOWN
TABLES ASSUME THE FOLLOWING MANNING'S N ROUGHNESS COEFFICIENTS:
CURB TO CURB: N = 0.015
CURB TO R/W: N = 0.030
FOR A2-6 & A2-8 CURBS, THE WETTED PERIMETER IS CALCULATED ASSUMING
VERTICAL DEPTH AT THE CURB FACE AND HORIZONTAL DISTANCE AT THE GUTTER,
PAVEMENT, AND PARKWAY.
FOR ROLLED CURBS, THE WETTED PERIMETER IS CALCULATED ASSUMING A HORIZONTAL
DISTANCE FOR THE PARKWAY, GUTTER, AND PAVEMENT. ACTUAL SLOPE DISTANCE
IS USED FOR CURB SECTION.
LIMITATIONS OF THE STREET FLOW TABLES: (CONTINUED)
A COMPOSITE MANNING'S COEFFICIENT IS USED WHEN FLOW ENCROACHES INTO
PARKWAY.
ROADWAY CROSS SLOPE/CROSS FALL IS ASSUMED TO BE SX = 0.017
VALUES ASSUME TRIANGULAR FLOW.
VALUES SHOWN ARE FOR ONLY HALF THE STREET UNTIL THE STREET CROWN HAS
BEEN EXCEEDED THEN FULL STREET CROSS SECTION IS UTILIZED.
DEPTH TIMES VELOCITY MAY NOT EXCEED SIX:
V x y < 6
FLOW DEPTHS LESS THAN 0.20 FEET ARE NOT GIVEN.
STREET HALF WIDTHS ARE MEASURED FROM CENTERLINE TO GUTTER FLOW LINE.
STORM DRAIN DESIGN
- HYDRAULIC DESIGN
- USING YOUR HYDROLOGY STUDY AS A GUIDE, PICK PRELIMINARY LOCATIONS
OF EACH CATCH BASIN.
- CALCULATE STREET FLOW AND DEPTH
- DETERMINE REQUIRED LENGTH AND TYPE OF CATCH BASIN.
FOR A PFRD STANDARD PLAN 1301 (TYPE I) AND 1302 (TYPE II) INLET
IN CONJUNCTION WITH A STD. PLAN 1308 LOCAL DEPRESSION (TYPE A),
h = CURB FACE + 4"
FOR A ROLLED CURB, WE MUST TRANSITION INTO A STD. 1308 LOCAL DEPRESSION.
CATCH BASIN FACTS:
OCLDM RECOMMENDED LOCATIONS:
CATCH BASIN FACTS: CONTINUED
PHIL'S RECOMMENDED LOCATIONS:
-
PRIOR TO CROSS GUTTERS AT INTERSECTIONS (IF POSSIBLE) TO INTERCEPT
PARTIAL FLOW, SUCH THAT DEEP FLOWS WITHIN CROSS GUTTERS WILL NOT
CAUSE TRAFFIC PROBLEMS (TRAFFIC SLOW DOWNS, STALLED CARS, LOSS
OF STEERING CONTROL, ETC.)
-
REMEMBER THE FOLLOWING PROTECTION LEVELS:
-
FOR ARTERIAL HWYS: ONE TRAVEL LANE MUST BE FREE FROM INUNDATION
IN EACH DIRECTION IN A 25-YEAR STORM (MEDIANS AND LEFT TURN POCKETS
ARE NOT TRAVEL LANES)
-
FLOODING WIDTH FROM MEDIAN CURBS IN SUPERELEVATED SECTIONS SHALL
NOT EXCEED TWO FEET.
CATCH BASIN FACTS: CONTINUED
OCLDM UNDESIRABLE LOCATIONS:
PHIL'S ADDITIONAL UNDESIRABLE LOCATIONS:
BASIC CATCH BASIN FACTS:
THREE (3) CATCH BASIN CLASSIFICATIONS:
- CONTINUOUS GRADE
- LOW POINT
- SUMP (100% CLOG RULE)
THREE (3) STANDARD INLET TYPES:
- CURB OPENING
- GRATED
- COMBINATION
BASIC CATCH BASIN FACTS: (CONTINUED)
FOR CONTINUOUS CLASS CATCH BASINS, THE INLET WILL BASICALLY ACT AS
A SIDEWEIR WITH A ZERO HEIGHT SPILLWAY.
FOR A LOW POINT OR SUMP CATCH BASIN:
THE INLET WILL ACT AS A STANDARD WEIR WITH THE FLOW PASSING THROUGH
CRITICAL DEPTH AT THE ENTRANCE UNTIL THE OPENING BECOMES SUBMERGED.
Q = 3.087 LH3/2
WHEN THE DEPTH OF WATER EXCEEDS ABOUT TWICE THE HEIGHT OF THE INLET
ENTRANCE/OPENING, THE INLET WILL ACT AS AN ORIFICE.
Q = 5.62h3/2 (H'/h)1/2 L, H' = H - (h/2)
H = DEPTH OF FLOW, h = HT. OF INLET OPENING, L = LENGTH OF INLET
BETWEEN THESE TWO DEPTHS, THE INLET WILL OPERATE SOMEWHERE BETWEEN
A WEIR AND AN ORIFICE. A TRANSITION IS USED AS THE OPERATION OF THE
INLET IS UNDEFINED.
From Page 5-35 of the OCLDM:
The inlet hydraulic tables contained herein are applicable to both
cases of face-plates as long as the depth of water (y) does not reach
the face-plate (pressure flow).
From Page 5-38 of the OCLDM: Inlet design procedure
- Determine the best design of inlet to use, checking that depth
of depression at curb inlet plus depth of flow in approach gutter
(a+y) is less than the height of the curb opening per Table 5-4.
This means that Figures 5-10a & 5-10b on Page 5-39 will not give
accurate results unless the depth of flow in the street gutter is less
than 3.5" (0.3') for a 6" curb face.
Since 0.3 feet depth in the street gutter is not very high, many streets
may require catch basin inlets at unreasonably short intervals.
So, what can we do to improve this situation?
If we start a transition from an A2-6 to A2-8 curb about ten feet
before our catch basin inlet, we can increase our street slope by 0.01667
and increase our hydraulic opening (h) from 7.5" to 9.3". Therefore,
we increase our maximum gutter depth from 3.5" to 5.3".
For our example: If we increase our street slope from 0.00322 to 0.019887.
Q/S½ = 5.69/(0.019887)½ = 40.35
From our street flow table: Flow depth = 0.37 feet (4.4 inches)
Since 4.4 inches is less than 5.3 inches, Figures 5-10a&b are
applicable.
However, you must be very careful that you don't create a sump condition
with your transitions. Bypass flow conditions may require much greater
distances between inlets if you try to transition back to an A2-6 curb
to allow the hydraulics to perform effectively. Standard minimum distance
between basins is 12 feet. This distance may need to be increased.
Depth of flow is changed gradually over distance as governed by gradually
varied flow computations. A ten foot long transition is fairly short
to change the depth of flow in the street and gutter especially if
the water is moving fast. The longer your transition or distance to
the inlet opening, the more confidence you may have that the depth
of flow in the gutter is reflective of the street capacity tables.
GRAPH EQUATION: Q = 0.7L(a + y)3/2
a = depth of catch basin's local depression
L = length of clear opening, y = depth of flow in gutter
Standard Weir Equation
Q = 3.087 LH3/2
(y from street capacity tables)
Q/L = 0.7(a + y)3/2
So, Q/L = 0.37 We know that Q = 5.69 cfs, therefore,
L = 5.69/0.41 = 13.88 feet.
[FIGURE 5-10a OF OCLDM]
The Los Angeles County Flood Control District Hydraulic Design Manual,
1982, has four (4) charts/graphs for the design of "Curb Opening Catch
Basin Capacities."
The LACFCD Manual does not indicate any gutter flow depth limitations
for their catch basin inlets as they may relate to a hydraulic opening.
LACFCD Manual graphs do, however, indicate that for gutter flow depths
in excess of 0.67 feet the curves on the graphs are extrapolated and
not validated by laboratory experiments.
Another limitation of the LACFCD graphs is that there are only four
(4) sheets corresponding to only four(4) selected street slopes: 0.005,
0.01, 0.03, and 0.05. I believe that any street slopes less than, greater
than, or between the selected street slopes must be interpolated and/or
extrapolated.
Gutter flow depth = 0.67
Each curve represents a standard inlet length.
These curves are all for a local depression of 4 inches.
Curves for 1" and 2" local (gutter) depressions are also available.
S = 0.005
S = 0.01
S = 0.03
STORM DRAIN DESIGN
- HYDRAULIC DESIGN
- USING YOUR HYDROLOGY STUDY AS A GUIDE, PICK PRELIMINARY LOCATIONS
OF EACH CATCH BASIN.
- CALCULATE STREET FLOW AND DEPTH.
- DETERMINE REQUIRED LENGTH AND TYPE OF CATCH BASIN.
- DETERMINE ACTUAL LENGTH OF CATCH BASIN AND ANY BYPASS
FLOW, IF REQUIRED.
NOTE: BE MINDFUL OF BYPASS FLOW ON STEEP GRADES AND ON CURVES.
A MAX. BYPASS OF 15% IS ALLOWED.
IF WE ARE TRYING TO DESIGN AN INLET OR INLETS ON A STEEP STREET WHERE
SOME BYPASS Q MAY BE LIKELY, WE CAN USE FIGURE 5-10b TO CALCULATE THE
BYPASS.
IF WE HAVE CALCULATED L (REQUIRED INLET LENGTH TO INTERCEPT 100% OF
GUTTER FLOW) AND OUR ACTUAL INLET LENGTH (LP) IS LESS, CALCULATE
LP/L AND a/y, ENTER FIG. 5-10b AND DETERMINE QP/Q.
PARTIAL FLOW INTERCEPTED QP, IS THE RATIO QP/Q
TIMES THE TOTAL GUTTER FLOW.
THE FLOW CARRIED OVER TO THE NEXT INLET QC = Q-QP.
BASIC CATCH BASIN FACTS:
OUR COMPUTED L = 13.88 FEET
STANDARD CATCH BASIN LENGTHS ARE:
3.5, 7, 14, AND 21 FEET
EXAMPLE OF STEEP STREETS WITH CURVES
[aerial photo of streets]
INDUS STREET
REDLANDS
REDLANDS
INDUS
CATCH BASINS LOCATED TO FIX DRAINAGE PROBLEM
DESIGN OF A SUMP CATCH BASIN
APPROXIMATE STORM DRAIN ALIGNMENT
SUMP CATCH BASIN
Actual Plan View of Our Sump Catch Basin Inlet
FINISHED PRODUCT
OKAY, SO HOW DO WE DESIGN THIS BEAST?
DESIGN OF SUMP CATCH BASIN
- DETERMINE Q TO INLET.
FROM OUR HYDROLOGY REPORT Q OF SUBAREA 6,7 = 5.47 CFS
- ESTIMATE A CATCH BASIN LENGTH "L"
- DETERMINE THE HEIGHT OF THE CATCH BASIN OPENING "h" OR HYDRAULIC
OPENING.
HYDRAULIC OPENING CAN BE OBTAINED FROM OCLDM TABLE 5-4
FOR A 6" CURB HEIGHT, 4" LOCAL DEPRESSION, AND CURVED CATCH BASIN FACE
PLATE ( WHICH IS TYPICAL FOR PFRD STANDARD PLAN INLET STRUCTURES) h = 7.5" OR
0.625'
- ENTER NOMOGRAPH OF OCLDM FIGURE 5-13 WITH Q/L AND h TO DETERMINE
H/h.
H = 0.64h = 0.625x0.64 = 0.40' = 4.8"
SINCE 4.8" IS LOWER THAN OUR 6" CURB HEIGHT, HYDRAULIC OPENING AND
INLET LENGTH ARE SUFFICIENT.
GRATED TYPE INLETS
THE USE OF GRATED TYPE INLETS IN STREETS WITH SUMP CONDITIONS IS NOT
PERMITTED.
GRATES GENERALLY ACT AS STRAINERS CATCHING DEBRIS WHICH TENDS TO PLUG
THE GRATE OPENINGS.
THE HYDRAULIC EFFICIENCY OF GRATED OPENINGS IS INCREASED WHEN STORM
RUNOFF IS ALLOWED TO FLOW PAST THE INLET.
THIS IS DUE TO THE INCREASED HEAD/DEPTH IN THE CROSS SECTION OF FLOW
OVER THE GRATING.
GRATES NEED TO BE DESIGNED SUCH THAT BICYCLE TIRES WILL NOT GET STUCK.
GENERALLY, ONLY 50% OF THE TOTAL GRATE AREA IS FLOW OPENINGS.
MINIMUM CLEAR SPACE BETWEEN LONGITUDINAL BARS IS ONE INCH (1") AND
CROSS BARS ARE TO BE PROVED AT A MINIMUM SPACING OF NINE INCHES (9").
WITH GRATES THERE IS NO LOCAL DEPRESSION
THE DESIGN PROCEDURE FOR GRATES ON A CONTINUOUS STREET GRADE IS FAIRLY
SIMPLE:
- DETERMINE THE DEPTH OF FLOW IN THE STREET AS PREVIOUSLY SHOWN BY
USING THE STREET CAPACITY TABLES. DETERMINE STREET SLOPE (S).
- DETERMINE THE CAPACITY OF THE GRATE BY USING OCLDM FIGURES 5-15,
5-16, AND 5-17 DEPENDING ON GRATE LENGTH.
THESE GRAPHS ARE BASED UPON THE FORMULA:
LMIN = 0.675v(y+t)
LMIN = MIN. LENGTH OF SLOT
v = MEAN VELOCITY OF FLOW IN THE APPROACH GUTTER
y = DEPTH OF WATER IN APPROACH GUTTER
t = THICKNESS OR DEPTH OF GRATE
WE HAVE FIGURES FOR THREE STANDARD GRATE LENGTHS.
GRATE INLETS FOR A SUMP CONDITION
REMEMBER: GRATE INLETS ARE NOT ALLOWED WITHIN STREETS THAT HAVE A
SUMP CONDITION. HOWEVER, GRATE INLETS IN A SUMP CONDITION CAN BE UTILIZED
IN STREET ALLEY WAYS AND PARKING LOTS.
GRATES TEND TO ACT AS A WEIR FOR DEPTHS (OR HEADS) OVER THE GRATE
UP TO 0.4 FEET.
GRATES TEND TO ACT AS AN ORIFICE FOR HEADS GREATER THAN 1.4 FEET.
FOR HEADS BETWEEN 0.4 AND 1.4 FEET THE OPERATION IS NOT DEFINED DUE
TO VORTICES AND EDDIES THAT GENERALLY OCCUR OVER THE GRATE.
WHEN PROPOSING A GRATE INLET IN A SUMP CONDITION, THE DESIGNER MUST
ALSO ASSUME 100% CLOGGING, 100-YEAR FREEBOARD CRITERIA, AND OVERFLOW
TO AN EMERGENCY OUTLET SYSTEM.
GRATE INLETS FOR A SUMP CONDITION
DESIGN PROCEDURE:
- DETERMINE Q, GRATE CONFIGURATION (ADJACENT TO CURB OR IN OPEN AREA).
- ASSUME GRATE DIMENSIONS
- COMPUTE THE PERIMETER: P=2W+L (W/CURB), P=2(W+L) (W/O CURB), THEN
DIVIDE THE RESULT BY 2. THIS ACCOUNTS FOR CLOGGING.
- COMPUTE THE TOTAL CLEAR OPENING, EXCLUDE AREA TAKEN UP BY BARS,
(A) AND DIVIDE BY 2 TO ACCOUNT FOR CLOGGING.
- ENTER FIGURE 5-18 USING THE DESIGN DISCHARGE Q.
- IF Q INTERSECTS APPROPRIATE P (COMPUTED IN STEP 3) CURVE READ CORRESPONDING
DEPTH d.
- IF Q DOES NOT INTERSECT P CURVES, USE A CURVES (WITH AREA COMPUTED
IN STEP 4) AND READ CORRESPONDING DEPTH d.
FORMULAS FOR CALCULATING THE DEPTH OF WATER OVER GRATE INLETS IN ALLEYS:
OCLDM FIGURE 5-21
COMBINATION INLETS: CURB INLETS + GRATE INLETS
TYPICAL LOCATIONS: TURN LANES ADJACENT TO LANDSCAPED MEDIANS, CU-DE-SACS,
AND AREAS WHERE FLOW-BY CONDITIONS NEED TO BE ELIMINATED OR MINIMIZED.
TWO STANDARD COUNTY SIZES: 7 FOOT AND 10 FOOT.
DESIGN PROCEDURE:
- DETERMINE STREET SLOPE (SO), Q, AND DEPTH OF FLOW IN
THE GUTTER (y).
- ENTER FIGURE 5-23 OR 5-24 WITH SO AND y TO DETERMINE
DISCHARGE THAT WILL BE CAPTURED BY INLET.
- COMPARE Q FROM FIGURE TO ACTUAL GUTTER FLOW TO DETERMINE INLET
EFFECTIVENESS.
STORM DRAIN DESIGN
- CALCULATE INLET DEPTHS (Id):
- (1) CHECK FOR 30" MIN. COVER CRITERION PER COUNTY CODIFIED
ORDINANCE (CCO) 6-3-69.
- (2) TRY TO DETERMINE ANY UTILITY CONFLICTS BETWEEN YOUR INLET
DEPTH AND PROPOSED MAINLINE ALIGNMENT.
- DETERMINE MAINLINE TAIL WATER (TW) DEPTH OR CONTROL WATER SURFACE
ELEVATION (WSEL).
- ASSUME STORM DRAIN MAINLINE SIZES AND PERFORM MAINLINE HYDRAULICS
TO DETERMINE HGL.
- CHECK HGL AT EACH JUNCTION STRUCTURE TO ENSURE THAT THE
TOP OF CURB (TOC) CATCH BASIN ELEVATION IS HIGHER THAN HGL+0.5'+CF,
WHERE CF=HEIGHT OF CURB FACE PLUS DEPTH OF THE LOCAL DEPRESSION.
The calculation procedure for Inlet Depth (Id) is almost
identical for both the LACFCD Hydraulic Design Manual and the OC Local
Drainage Manual.
STORM DRAIN DESIGN
- IF TOC<HGL+0.5'+CF, RESIZE STORM DRAIN MAINLINE OR MAINLINE
GRADES ACCORDINGLY AND RECALCULATE HGL UNTIL ALL CATCH BASINS HAVE
PROPER FREEBOARD.
- FINALIZE MAINLINE ALIGNMENT.
- RECALC MAINLINE HGL FOR ANY SIGNIFICANT CHANGES THAT MAY
AFFECT HYDRAULICS
- SELECT ACTUAL CATCH BASIN LOCATIONS AND FINALIZE LATERAL LINE ALIGNMENTS.
- RECALC INLET DEPTHS IF ANY SIGNIFICANT CHANGES HAVE OCCURRED
SINCE ORIGINAL Id CALCULATION.
NOTE: MINIMUM PERMISSIBLE VELOCITY OF ALL STORM DRAINS (AND UNDERGROUND
SYSTEMS) IS THREE FEET PER SECOND (3.0fps) & ENGINEER SHALL VERIFY
THAT THE SYSTEM IS SELF CLEANING UNDER LOW FLOW CONDITIONS.
STORM DRAIN DESIGN
NOTE: NEVER PLACE A CATCH BASIN WITHIN SOMEONE'S DRIVE APPROACH.
NEVER PLACE A CATCH BASIN AT A CURB RETURN OR HANDICAP RAMP.
HINT: TRY NOT TO LOCATE CATCH BASINS AT THE CENTER MEDIAN OF MAJOR
ARTERIAL HIGHWAYS. INLETS WITH GRATES NOT RECOMMENDED AND LOCAL DEPRESSIONS
ARE NOT ALLOWED, INLETS COULD CREATE HAZARDOUS PONDING CONDITIONS (ESPECIALLY
WHERE DRIVERS LEAST EXPECT IT).
HINT: ALTHOUGH NOT CONDONED OR ENDORSED BY THE COUNTY OR YOUR PRESENTERS,
YOU MIGHT TRY TO LOCATE CATCH BASIN INLETS IN FRONT OF VACANT LOTS,
DISTRESSED, DEPRESSED, AND/OR OVERGROWN PROPERTIES.
THOSE PROPERTY OWNERS TEND NOT TO COMPLAIN AS MUCH OR AS ADAMANTLY
AS THOSE WITH THE MANICURED LAWNS AND GARDENS.
Determination of Minimum Inlet Depth, Id
Id = CF + 0.5 + 1.2(V2/2g) + [D/cos(SD)],
Tan-1(pipe slope) = Angle in Degrees (SD)
Id = CF + 0.5 + 1.2(Q2/2gA2) + [D/cos(SD)]
For a PFRD Standard Plan Type A Local depression and Type A2-6 curb
or 6" rolled curb: CF = 10" or 0.833'
Determine minimum Id for Orchid Street inlet no. 4.
From our hydrology study: total runoff of mainline storm drain to
this point is 23.17 cfs and the previous subarea was 11.38 cfs. So,
the discharge our inlets must accept is
23.17-11.38 = 11.79 cfs.
We shall assume that there is no bypass Q from upstream subarea, and
that this subarea is symmetrical such that storm runoff is evenly distributed
on both sides of street. If we needed to we could use the street capacity
tables to check this. However, if flow depth exceeds street crown elevation,
assume equal flow on both sides of street.
HYDROLOGY STUDY PRELIMINARY STORM DRAIN ALIGNMENT
POSSIBLE STORM DRAIN ALIGNMENT
Approximate location of inlets no.3 & no.4
[diagram]
Determination of Minimum Inlet Depth, Id
Id = CF + 0.5 + 1.2(V2/2g) + [D/cos(SD)],
Tan-1(pipe slope) = Angle in Degrees (SD)
Id = CF + 0.5 + 1.2(Q2/2gA2) + [D/cos(SD)]
For a PFRD Standard Plan Type A Local depression and Type A2-6 curb or 6" rolled
curb: CF = 10" or 0.833'
Determine minimum Id for Orchid Street inlet no. 4.
From hydrology study: Q= 23.17-11.23=11.79cfs for subarea (subarea includes
both sides of the street, catch basin will be on only one side) so, ½Qsubarea to
inlet no.4, or Qinlet4 = 5.90cfs
SD = (45.15-44.05)/38.75 = 0.028387, Tan-1(0.028387)=1.626°
Id = 0.833 + 0.5 + [(1.2)(5.90)2]/[¶(1.5)2/4]2(2)(32.174)
+ 1.5/cos(1.626°)
Id = 0.833 + 0.5 + 0.208 + 1.501 = 3.042 feet
Actual inlet depth = 53.80' (TOC) - 45.15' = 8.65'
Since 8.65' (actual) > 3.042 (calc), inlet depth okay.
ROADWAY SURFACE
TOP OF STORM DRAIN LATERAL OR CONNECTOR PIPE
(Phil's) PRACTICAL MIN. Id = PIPE O.D. + 30" + CF - 4"
CALCULATING AVAILABLE HEAD
From a WSPG run of the Orchid Street Storm Drain mainline:
At junction structure no.2 where inlets no.3 & no.4 confluence with
the mainline, u/s mainline d = 7.533' and d/s mainline d = 7.010', so HGL
= 7.27' + (WSEL = 51.27')
L = 38.75 ft
Q = 5.90 cfs, D = 18"
Ha (min.) = 0.30 ft +
Ha (actual) = [TOC-(FB+CF)]-HGL
Ha (actual) = [53.8-(0.833+0.5)]-51.27
Ha (actual) = 1.2 ft > 0.3 ?
Q = CaÖ2gDh (KING'S
HANDBOOK p.4-9)
ANOTHER WAY WE CAN USE THIS NOMOGRAPH IS TO DETERMINE THE MINIMUM SIZE OF
A CONNECTOR PIPE.
ASSUME: Q = 6cfs, L = 38.75 ft
Ha = [TOC - (FB + CF)] - HGL
Ha = [53.8-(0.833+0.5)]-51.54
THIS TIME WE'LL USE THE HIGHER U/S HGL INSTEAD OF THE AVERAGE.
Ha = 0.93ft
SO, FROM THE NOMOGRAPH (aka Figure 5-30) THE MINIMUM CONNECTOR PIPE REQUIRED
GIVEN THE AVAILABLE HEAD WOULD BE A 15" RCP. HOWEVER, COUNTY/DISTRICT MINIMUM
PIPE SIZE IN THE PUBLIC R/W IS 18".
IF POINT IS BETWEEN TWO CURVES, CHOOSE THE HIGHER DIAMETER.
Analysis of Junction Structures
Thompson DY Equation (utilized by WSPG for junction
analysis)
(OCLDM) DY = (Q2V2 - Q1V1 -
Q3V3cosq3)/(Aavg)g
Where Aavg = (1/6)(A1+4Am+A2),
or
Aavg = (1/2)(A1+A2) for practical use
Thompson DY does not account for length of transition.
Analysis of Junction Structure No.2
(equation from Orange County Local Drainage Manual)
PLAN VIEW
[diagram of Typical Section of Junction Structure / Catch Basin]
Analysis of Junction Structure No. 2
A1 = [¶(2)2]/4 = 3.141 ft2, A2 =
[¶(2.25)2]/4 = 3.976 ft2, A3,4 = [¶(1.5)2]/4
= 1.767 ft2
Q1 = 11 cfs, Q2 = 22 cfs, Q3,4 = 5.5 cfs
V1 = (Q1/A1) = 3.502 fps, V2 =
5.533 fps, V3,4 = 3.113 fps
DY = ((22)(5.533) - (11)(3.502) - [(5.5)(3.113)cos(50°)
+ (5.5)(3.113)cos(45°)]) / (32.174)(1/6)[3.141
+ (4)(3.558) + 3.976] = 0.525 ft
ANALYSIS OF JUNCTION STRUCTURES: (continued)
DY = 0.525 ft from OCLDM Thompson formula
WSPG Thompson DY formula:
DY = [(Q2V2) - (Q1V1)
- (Q3V3cosq3)(1/g)(1/Aave)]
+ DL Sfav
Last term of formula accounts for associated loss for length of transition.
Where Aave = [(A1 + A2)/2]
And DY = D1 + DH
- D2
Sf = n2V2 / 2.22R4/3
R= A/P = ¼[1 - (sinq/q)]D
q = cos-1{1 - 8[y/D-(y/D)2]}, q< 180°,
where q> 180°,
360° - q
D = diameter of pipe, y = water depth in pipe
From a WSPG run of the storm drain system, Junction Structure No. 2 --- DY
= 7.533-7.010 = 0.523 ft which is 0.002 ft lower although we would have expected
it to be higher.
TRANSITION FROM LARGE TO SMALL CONDUITS:
AS A GENERAL RULE STORM DRAINS INCREASE IN SIZE FROM U/S TO D/S. HOWEVER,
A STORM DRAIN MAY DECREASE IN SIZE WITH THE FOLLOWING LIMITATIONS:
S<0.0025 & D<48": DECREASES ARE NOT ALLOWED!
S>0.0025, D>48": MAY BE DECREASED WITH AGENCY APPROVAL AND EACH REDUCTION
IS LIMITED TO 3" IN DIAMETER FOR D=48" AND 6" FOR D>48". THE MINIMUM LENGTH
BETWEEN MULTIPLE REDUCTIONS IS 40 FEET.
DECREASES IN PIPE SIZE SHALL BE BASED UPON THE U/S PIPE SIZE
TRANSITIONS REQUIRED FOR REDUCTIONS WILL REQUIRE ADDITIONAL CLEAN-OUT MANHOLES.
ENGINEER MUST DETERMINE THE MINIMUM LENGTH FROM THE STORM DRAIN GRADE BREAK
TO THE PIPE REDUCTION FROM THE FOLLOWING FORMULA/PROCEDURE.
Sf = n2V2 / 2.22R4/3
V = Q/A
DETERMINATION OF CHANGE IN HGL FOR A SIMPLE STRAIGHT SECTION OF PIPE:
DY = DHGL = DEGL
= hf = SfL = 4.66n2LQ2/(d16/3)
EXAMPLE FOR ORCHID STREET S.D. FROM STA. 12+59 TO STA. 10+28:
Q = 22 cfs, L = 231 ft, n = 0.013, d = 2.25
Hf = 4.66 n2LQ2/D16/3 = 4.66(0.013)2(231)(22)2/(2.25)16/3 =
1.165 ft
From WSPG run for Orchid: DHGL = 51.000 - 49.834
= 1.166 ft
SOLVE FOR X
OBVIOUSLY, IF YOUR TAIL WATER DEPTH (TW) LESS THAN YOUR PIPE DIAMETER, YOU
HAVE OPEN CHANNEL FLOW.
TOOLS FOR CIRCULAR CONDUITS
q = cos-1{1 - 8[y/D-(y/D)2]},
where q< 180° or
y < D/2
where q> 180° or
y > D/2, 360° - q = q
AREA: A = 1/8(q - sin?)D2
WETTED PERIMETER: P = ½ qD
HYDRAULIC RADIUS: R = ¼ (1 - sinq/q)D,
R = A/P
TOP WIDTH: T = Dsin½ q, or T = 2Ö y(D-y)
HYDRAULIC DEPTH: DH = 1/8(q - sinq)/(sin½q)D,
or DH = A/T
MAXIMUM DISCHARGE: QMAX ~ 1.07 QFULL
TOOLS FOR CIRCULAR CONDUITS: (continued)
DISCHARGE WHEN PIPE FLOWING FULL
Q = (0.46319 D8/3 S1/2) / n , where q =
360° = 2¶
FORMULA FOR THE CALCULATION OF NORMAL DEPTH:
(0.07372/n)(1/q)2/3 (q -
sinq)5/3 D8/3 S1/2 -
Q = 0
FORMULA FOR CALCULATION OF CRITICAL DEPTH:
{g[ 1/8 (q - sinq)D2]3 /
D(sin½q)} - Q2 = 0
Where, q = cos-1{1 - 8 [ y/D-(y/D)2 ]
},
Solve for y by trial and error.
MATERIAL SELECTION FOR STORM DRAIN PIPE:
The basis for structural design shall be a design life of 100-years for
all permanent drainage structures within the County.
The County tends to classify pipe structures as two types: Rigid and Flexible
Rigid Pipes are those generally made from mortar products, such as: RCP,
ACP, CIPCP, and VCP.
Flexible pipes are those generally made of metal and plastic materials,
such as: CMP, CSP, SRP, ABS, PVC, and HDPE.
From the Corrugated Polyethylene Pipe Association:
All pipe, whether flexible or rigid, relies on the backfill structure to
transfer loads into the bedding.
The American Concrete Pipe Association:
For flexible pipe, the structure must be built in the trench. This often
means replacing the excavated materials with higher strength granular materials,
and placing the backfill with greater precision and compaction effort.
[diagram of trench bedding]
Source CalTrans
[conduit designation list]
DESIGN LIFE ELEMENTS
ELEMENTS ESSENTIAL TO A PIPELINE'S DESIGN LIFE:
-
Pipe/soil structure to maintain the pipeline design cross-sectional
flow area and corresponding flow capacity.
-
Pipe/soil structural stability maintaining the pipeline design slope
and corresponding flow capacity.
-
Integrity of the soil structure adjacent to and over the pipe under
road beds and other engineered structures.
-
Pipe and joint integrity maintaining required seal against water infiltration
and exfiltration exceeding design limits.
-
Corrosion resistance maintaining pipe and joint integrity in the corrosive
environment projected in system design.
The design life for a storm drain has ended when:
-
The storm drain can no longer convey the rated flow capacity established
in the sewer system design.
-
The storm drain permits infiltration of excessive groundwater and surface
water bearing fines into the drain thereby undermining the embedment
for the drain and other structures above the storm drain.
-
The storm drain has become structurally unsound to the extent that it
creates a real hazard to the reliable function of the drain in conveying
water or that it creates a hazard to surface structures, human endeavor,
human safety, or the environment. (Health - Safety - and Welfare)
-
The maintenance required to assure proper design operation of the drain
exceeds the reasonable and affordable required maintenance projected
or assumed in the storm drain design.
-
The cost of operation of the drain exceeds the available funds projected
as required in the storm drain system.
Advantages of RCP (County/District's Preferred Pipe Material)
-
Offered in a wide range of nominal diameters.
-
Offered in a wide range of structural strengths.
-
Offered in a wide range of laying lengths.
-
Offered with gasketed joints providing a required seal even when exposed
to high groundwater heads.
-
Offers proven structural stability under severe loads.
-
Physical and mechanical characteristics minimize the need for special
installation procedures, conditions, and materials to assure required
long term structural strength.
-
RCP is usually not subject to shear or beam breaking.
-
RCP can be easily used in pipe jacking operations.
Disadvantages of RCP:
-
Weight per linear foot is generally greater than the weight of many
competing pipe products, especially plastics.
-
RCP is subject to chemical corrosion where acids, chlorides, and sulfates,
are present. May require special design, formulation, or corrosion barrier
in severely corrosive environments.
-
Proper joint must be specified and provided for projected internal and
external pressure at the joint.
-
Initial cost of RCP and its installation may be much more than competing
products.
Flexible Pipe Materials
Advantages of ABS (Acrylonitrile-Butadiene-Styrene)
-
Offered in long laying lengths which may offer advantage if installation
conditions do not restrict working and handling space.
-
Offered with relatively light pipe weight per linear foot.
-
The product can be cut and tapped in the field with relative ease.
-
Offered with relatively high pipe stiffness (200 lbs/in/in) for plastic
pipe.
-
Tolerates low ring deflection (less than 7½ %) without structural failure.
Vertical deflection is usually limited to 7.5% of the base inside diameter;
the base inside diameter is the nominal diameter less manufacturing and
out-of-roundness tolerances inherent to the manufacturing process.
Flexible Pipe Materials
Disadvantages of ABS
Some plastic pipe (ABS) is offered in limited range of available diameter
sizes.
Subject to environmental stress cracking.
Subject to excessive ring deflection and/or distortion when installed without
adequate bedding and haunching soil stiffness.
Vulnerable to point-load distortion.
Vulnerable to attack by certain organic chemicals (detergents, oils, solvents,
gasoline, etc.).
Vulnerable to ultraviolet light degradation.
Vulnerable to excessive heat or combustion.
Subject to shear and beam breaking in the field.
Relatively low impact strength.
Vulnerable to floatation in high groundwater tables.
Vulnerable to distortion under LL at shallow burial depths.
Flexible Pipe Materials
Advantages of HDPE (High density Polyethylene)
-
Offered in long laying lengths.
-
Offered with relatively light weight per linear foot.
-
Provides high impact strength.
-
Product can be cut and tapped in the field with ease.
-
Butt fused joints, when properly assembled, can provide a very good
water tight seal.
-
Not subject to shear or beam breakage.
-
Provides relatively high resistance to attack by dilute sulfuric acid.
-
Recyclable material.
Flexible Pipe Materials
Disadvantages of Corrugated HDPE
Butt fusion joint assembly, when used, requires special equipment with trained
operators
Vulnerable to excessive heat or combustion.
Subject to erosion due to abrasion.
Rib-profile reinforced pipe cannot be readily connected to manholes with
satisfactory waterstop devices.
Vulnerable to floatation when installed at shallow depths with high watertables
and live loads at shallow depths, as well.
Offered with relatively low tensile strength and pipe stiffness.
Subject to environmental stress cracking.
Subject to excessive ring deflection and/or distortion when installed without
adequate bedding and haunching soil stiffness.
Vulnerable to point load distortion.
Flexible Pipe Materials
Disadvantages of Corrugated HDPE (continued)
Vulnerable to attack by certain organic chemicals (detergents, oils, solvents,
etc.)
Vulnerable to ultraviolet degradation when exposed to sunlight.
Vulnerable to distortion under live load at shallow burial depths.
Only available in diameter sizes up to 60 inches, however CPPA design manual
tables only go up to 48" diameters.
Flexible Pipe Materials
Advantages Metal Pipe Materials: CMP, DIP, Steel Pipe, etc.
CMP and smooth wall steel pipe (SWSP) is offered with relatively light pipe
weight per linear foot.
Offered in long laying lengths.
DIP is offered with high pressure and load bearing capacities.
DIP and SWSP provides high impact, shear and beam strengths.
DIP is offered with gasketed joints providing required seal even when exposed
to high groundwater heads or internal pressures.
Flexible Pipe Materials
Disadvantages Metal Pipe Materials: DIP
Subject to chemical corrosion where acids are present. Will require use
of a corrosion barrier or cathodic protection.
Subject to electrochemical or galvanic attack when exposed to an electrolyte,
such as: water or soil. The lower the resistivity of the soil or water the
more likely it is the metal will corrode.
Weight of DIP is generally greater than other pipe materials per foot.
Generally not available in short laying lengths required for installations
with restricted room available for pipe handling during installation.
Flexible Pipe Materials
Disadvantages Metal Pipe Materials: CMP and SWSP (not mentioned with DIP)
Coating materials (zinc) vulnerable to attack by certain organic chemicals
(oils, solvents, etc.)
Not easily tapped for lateral connections.
Relatively high flow-friction in conveyance of water.
Subject to excessive ring deflection and/or distortion when installed without
adequate bedding and haunching soil stiffness.
Joints provide a poor seal against leakage (water exfiltration and infiltration)
Subject to erosion due to abrasion.
Generally requires strutting to limit ring deflection during installation
in larger diameters.
Flexible Pipe Materials
Disadvantages Metal Pipe Materials: CMP (not mentioned with DIP) continued:
Protective coatings vulnerable to excessive heat or combustion.
Vulnerable to point-load distortion.
Vulnerable to distortion under live load at shallow burial depths.
Vulnerable to flotation when installed a shallow depths with high watertables.
Flexible Pipe Materials Design Tips
A flexible pipe must be allowed to flex.
Each type of flexible pipe will have a different allowable deflection.
A flexible pipe will have ring deflection. This occurs when load is transferred
from the pipe walls to the side soil. The pipe will deflect and form an ellipse.
Allowable deflection will be exceeded when the ellipse changes to a double
ring deflection.
Concrete Pipe Failure Modes:
Flexible Pipe Failure Modes:
-
Excessive Deflection
-
Wall Buckling under hydrostatic or trench load conditions
-
Point-Load Buckling
-
Wall Crushing under compressive load
-
Joint Leakage
-
Bending (stain and stress) and Seam Separation
Usually, to design a flexible plastic pipe you should have a Hydrostatic
Design Basis (HDB). That is, testing of plastic pipe specimens in accordance
with ASTM D 2837 which establishes long-term tensile strength of specific
plastic material. The HDB is similar to long-term compressive strength
for concrete.
Long term deflection of the pipe should not be over-looked in a design
especially in large diameter pipe.
How do we check or design HDPE pipe and other flexible pipes?
-
We first calculate the prism load on the pipe:
WC = H ?S OD/144
WC = prism load, lb/linear inch of pipe
H = burial depth to top of pipe, feet
?S = soil density, pcf
OD = outside diameter of pipe, inches
-
Second, calculate deflection: 7½ % is our allowable.
Dy = K(DLWC+WL)
/ (0.149PS+0.061E')
Dy = deflection, inches
K = bedding constant
DL = deflection lag factor = 1.0 when prism load is used
WC = prism load, lb/Linear inch of pipe
WL = live load, lbs/l inch of pipe
OD = outside diameter of pipe, inches
PS = pipe stiffness, from manufacturer's tables
E' = modulus of soil reaction, psi (usually chosen from Tables)
- Third, we look at buckling. We first determine the critical
buckling pressure (PCR) and then the actual buckling pressure
(PV). PCR > PV or our pipe may fail
in buckling. PCR has a safety factor in the equation which is
suggested at 2.0. This may be increased for pipes of questionable or short
history.
PCR = (0.772/SF) [E' PS / (1 - v2)]½
PCR = critical buckling pressure, psi
E' = modulus of soil reaction, psi
PS = pipe stiffness, psi
V = Poisson ratio, 0.4 for polyethylene
SF = factor of safety, usually 2.0
PV = (RWH?S/144) + (?WHW/144)
+ (WL/OD)
PV = actual buckling pressure, psi
RW = water buoyancy factor = 1 - 0.33(HW/H)
H = burial depth to top of pipe, feet
?S , ?W = unit weight of soil and water respectively
HW = height of groundwater above top of pipe, feet
WL = live load, lb/linear inch of pipe
OD = outside diameter of pipe, inches
- Fourth, determine the bending stress and strain. Each material
will have its own allowable bending stress and strain.
sB = 2Df EDy
y0 SF / DM2 Bending stress
sB = bending stress, psi
Df = shape factor from manufacturer
E = modulus of elasticity, psi, depends on material
Dy = deflection, inches
y0 = distance from centroid of pipe wall to furthest surface
of pipe, inches
Dm = mean pipe diameter, inches =ID + 2c, c = dist. to inside surface
from neutral axis
SF = safety factor
eB = 2Df Dy
y0 SF / DM2 Bending Strain
eB = bending strain, in/in
All other variables as described above
Fifth, check for wall crushing. For plastic pipe it is very difficult to
calculate the structural design required to prevent wall crushing without
a legitimate long-term design value for wall compressive strength.
It is imperative that the structural design of plastic pipes be based upon
established long-term strengths. Plastics will typically exhibit short-term
(minutes, hours, days, months) strengths much greater than long-term strengths
(50-years or more).
For flexible pipe: the max. allow. Wall crushing load < prism load, WC
Pc = 288 Sct / FS Do, for solid wall flexible
pipe
Pc = 288 ScA / FS Do, for profile wall
flexible pipe
Pc = maximum allowable wall crushing load, psf
t = pipe minimum wall thickness, inches
A = average wall profile area, in2/in
Sc = material design compressive strength, psi
FS = factor of safety (usually 2.0 min.)
Do = pipe outside diameter, inches
The compressive strength of plastics is time and temperature dependent.
Compressive strength should be based upon long-term stress data (10,000 hours
min. @ 73.4°F) per ASTM D 2837.
SIXTH, CHECK FOR WALL THRUST: FOLLOWING CALCULATION FROM HANCOR TECHNOLOGY
FOR HDPE PIPE.
Wall thrust: Tcr equal to or greater than T
Tcr = (Fy)(A)(fp)
Where:
Tcr=critical wall thrust, lb/linear inch of pipe
Fy=tensile strength of polyethylene, psi
=3000 psi for short term conditions
=900 psi for long term conditions
A=wall area, in2/inch of pipe (Table 2-1 or 2-2)
fp=capacity modification factor
for pipe, 1.0
T = 1.3(1.5WA+1.67P1C1+Pw)(OD/2)
= calculated wall thrust, lbs/in
WA = soil arch load, psi
P1 = LL transferred to pipe, psi
C1 = LL distribution coefficient = lesser of Lw/OD
or 1.0
Lw = LL distribution width at the crown
OD = outside diameter, in.
Pw = hydrostatic pressure at spring line of pipe, psi
-
Lastly, check for other allowables such as: minimum cover, impact loading,
environmental limitations, code limitations, etc.
COUNTY/DISTRICT POLICY ON RCP ALTERNATIVE STRUCTURES
Orange County Local Drainage Manual page 6-7
The use of products other than RCP must provide for 100-year life expectancy
or the proponent must provide an annuity to fund the replacement costs.
Privately Funded Projects by Developers for Dedication to County
Reinforced concrete pipe (RCP) will be the standard for local storm drain
design. Alternative pipes may be considered when the developer creates a
donation to the County based on an approved life-cycle analysis determined
by Figure 6-1.
OCLDM Corrugated Steel Pipe: A maximum life of 50-years shall be used for
CSP.
OCLDM Spiral Ribbed Pipe: Spiral Rib Pipe as set forth in this manual is
an alternative pipe to RCP with a life span equal to CSP (50-years).
Minimum size of SRP shall be 48 inches in diameter.
[diagram of Reinforced Concrete Pipe - Bedding Detail]
The structure of flexible pipes, such as SRP, is taken from its trench rather
than from the pipe walls as is the case with RCP.
Ring deflection failure is caused by excessive compaction or deflection
of the side walls of the trench.
One way to increase the design life of SRP, CMP, and other similar structures
(especially from abrasion) is to pave the pipe invert with concrete as shown
here.
Aluminum SRP is a popular material for construction of large storm drains,
however aluminum has many design challenges of its own.
Zinc is used to galvanize steel pipe and especially CMP
Galvanic corrosion will be an issue with aluminum pipe if you have dis-similar
metal connectors and connections.
You should be aware that aluminum corrodes as do many other metals. And
may corrode faster if the aluminum-oxide barrier is continually removed as
with water and debris.
Per the OCLDM, Corrugated Aluminum Pipe (as shown Below) may be specified
as an alternative having a 25-year life. CAP is considered more sensitive
to soil pH and resistivity.
CAP is not recommended for flow velocities greater than five (5) feet per
second.
OCLDM limitations on plastic pipes:
-
Maximum cover shall be 20 feet.
-
Minimum diameter shall be 4 inches.
-
Maximum diameter shall be 36 inches.
-
No plastic pipe in arterial highways (landscaped medians and subdrains
are okay).
-
Plastic pipes shall use a slurry backfill.
Hold downs are so the pipe will not float during installation.
OCLDM required backfill for plastic pipe materials.
Recommended trench backfill from the Corrugated Polyethylene Pipe Association
Table 1-2 of the CPPA design manual specifies the Class IA, IB, II, and
III backfill material composition.
NOTE: OCLDM is much stricter.
Class IA - Angular crushed stone or rock
Class IB - Angular crushed stone or rock with sand
Class II - Gravel and Sand
Class III - Silty gravels/sands
Questions?
Thank you for coming. It's lunch time!!!