DRAFT (1)
FACTORS AFFECTING COITSTHUCTIOW COST
OF
MUNICIPAL 3EWEIR PROJECTS
By
Robert L. Michel
August 1970
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ENVIRONMENTAL P^OT??'"'^ /GFNCY
-------
INTRODUCTION
The Construction Grants Program of the Federal Water Quality
Administration (FWQA) provides grant-in-aid funds for municipal waste-water
treatment plants and interceptor-outfall sewers. In the course of admini-
stration of these municipal projects, detailed "bid data is obtained showing
the contract price of many components including the installed cost of
interceptor sewers, the subject of this study. Although this cost analysis
is for the most part limited to "bid data received in nine months of 1968,
statistical relationships were derived for order -of -magnitude estimating
of current costs for installing vitrified clay, cast iron, and reinforced
concrete sewer pipe at a wide range in diameters and depths of cut.
f;
X EFFECT OF PIPE DIAMETER
Cost per linear foot increases Toy a constant percentage (or ratio)
with increased pipe diameter at a constant depth of cut. As shown in
Figure 1 (for vitrified clay pipe), Figure 2 (for cast iron pipe), and
Figure 3 (for reinforced concrete pipe) such a relationship plots as a
straight line on semi-logarithmic charts. On the top half of these same
figures the "handbook" weight per linear foot is likewike plotted against
diameter. Note that the divergency of slopes in both weight and cost
occurs near the same diameter a logical effect if you assume cost is
directly associated with product weight. Somewhat similar observations
-------
FIGURE 1. EFFECT OF DIAMETER OF VITRIFIED CLAY PIPE 0!I irEIGHT AND INSTAT,LFD COf.T
-------
FIGURE 2. EFFECT OF DIAMETER OF CART IRON PIPE ON WEIGHT AND INSTALLED COST
-------
FIGURE 3. EFFECT OF DIA-'STER OF EEIITFORCED CONCRETE PIPE
,_ r.__ ,.____TQ1L VZiGIIT. AlID JIISTALIZD CCST ^ .., ,
-"1
-------
2
are in a study of installed costs of outside piping for the chemical
process industries as related to small-diameter clay and cast iron pipe*- '.
The cost estimates developed from these statistical relationships includes:
(a) normal excavation, (b) cost of pipe, (c) placing and joining of pipe,
(d) "backfilling of the trench. Rock excavation, sheeting and shoring,
cradle or encasement of pipe, gravel foundation, and surface restoration
are excluded.
Tables I, II, and III give the cost trend equations (from regression
analysis) and some resulting point estimates. Equations (l) and (2) from
Table I can be converted to the following more "natural" form (free of
logarithms) by taking anti-logarithms of the original equation constants:
(1-A) Y = (1.&2) (1.101)X
(2-A) Y = (2.71*0 (1.082)X
where Y = Installed Cost (^/linear ft.) at 0-6 ft. cut of
vitrified clay pipe.
and X - Nominal pipe diameter (in.)
The above equations read that estimated pipe cost increases approximately
10.1 percent with each inch of diameter up to 12 inches; while at 15 to
36 in. for vitrified clay pipe, the rate of change reduces to 8.2 percent
per inch. Table IV summarizes the indicated rate of increase for each
added inch of diameter for all three types of pipe. The low rate of cost
increase for reinforced concrete pipe above 12 in. explains why this pipe
is more commonly used at larger diameters than vitrified clay pipe.
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TABLE I. INSTALLED COST OF VITRIFIED CLAY PIPE AT 0-6 FT. CUT
Poim: Estimates
Nominal
Pipe
Diameter (inches)
8
10
12
15
18
24
30
36
Installed Cost (March 19.68 Dollars).
Lower Licit
$ 3.22
3-90
4.73
7-72
8.92
15.73
25.26
40.58
Expected' Value
$ 3-97
4.82
5.84
8.87
11.25
18.08
29.03
46.64
UwDer Limit
$ 4.88
5.93
7*. 18
10.20
12.94
20.79
33.38
53.64
Regression Equations
Size
Range
Equation
Correlation
Coefficient
Sample
Size
(1) 8-12 in.
(2) 15-36 in.
Log y - o.o4i75x + 0.2653
Log Y - 0.03431X + 0.4336
0.65
0.90
31 Bids
7,
Where Y = Installed Cost ($/linear foot)
X = Nominal Pipe Diameter (in.)
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TABLE II. INSTALLED COST OF CAST IRON PIPE AT 0-6 FT. CUT
Point Estimates
7
Nominal
Pipe-
Diameter (inches)
6
8
10
12
-Lh
16
18
2k
Installed Cost (March 1968 Dollars)
Lover Limit
$ 4.02
5.18
6.68
8.62
9.28
10.90
12.81
20.73
Expected Value
$ 5-18
6.68
8.62
11.11
11.51
13.52
15.89
25-71
Upper Limit
$ 6.68
8.62
ll.li
1^.33
lU.27
16.76
19.70
31.88
Regression Equations
Size Range
(inches)
(3) 6-12
(10 lk-2k
Equation
Log Y - 0.05526X -
Log Y = 0.03^99X -,
r 0.3828
1- 0.5712
Correlation
Coefficient
0.72
0.80
Sample
Size
37 Bids
IT
Where Y = Installed Cost ($/linear foot)
X = Nominal Pipe Diameter (in.)
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TABLE III. INSTALLED COST OF REINFORCED CONCRETE PIPE AT 0-8 FT. CUT
Point Estimates
Nominal Pipe
Installed Cost (March 1968 Dollars)
Diameter (.incnes;
12
15
18
21
24
27
30
33
36
42
48
54
60
Lower Limit
$ 6.25
7-52
9.03
10.9
15.2
16.3
17-5
18.9
20.3
23.4
27.0
31.1
35-8
Expected Value
$ 6.82
8.20
9-85
11.9
17.2
18.4
19.8
21.3
22.9
26.4
30.5
35-1
40.5
Upper Limit
$ 7-43
8.94
10.7
13-0
19.4
20.8
22.4
24.1
25.9
29.8
34.5
39.7
45.8
Regression Equations
Size Range
(inches)
(5) 12-21
(6) 24-60
Equation
Log Y - 0.02668X.
Log Y = 0.01036X
+ 0.5134
+ 0.9863
Correlation
Coefficient
0.92
0.96
Sample
Size
8 bids
"12
where y = Installed Cost (^/linear foot)
X - Nominal Pipe Dianeter (in.)
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TABLE IV. INDICATED RATE OF INCREASE IN INSTALLED FIFE COST
BECAUSE OF DIAMETER
Type of Pipe
Estimated Percent Cost
Increase for Each
Additional Inch
Diameter
Range (inches)
Reinforced Concrete
Vitrified Clay
Cast Iron
Reinforced Concrete
Vitrified Clay
Cast Iron
6.4
10.1
13.5
2.1
8.2
8.4
12-21
8-12
6-12
24-60
15-36
12-24
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A study^ ' in 1964 of sever costs on a basis of cost per diameter
inch per foot reported that there was a constant difference in price
between pipes of varying diameteran arithmetic progression. To test
this relationship, unit-length costs from a selling price quotation
sheet (dated October 1968) for undelivered reinforced concrete pipe
(ASTM C-76 Class 2) vere converted to dollars per diameter inch and
plotted against nominal pipe diameter in feet. Figure k, the resulting
linear plot, shows this relationship to be still valid.
The other main quantitative factors in the interceptor cost
picture are slope and depth. These variables are inter-dependent with
diameter as they affect the capacity and velocity required for a given
project flow. Consequently, depth-of-cut is the next subject for
statistical analysis.
-------
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COMBINED EFFECT OF PIPE DIAMETER AND DEPTH OF CUT
Previous discussion revealed the effect of diameter, per se, on
cost of sever pipe. However, this gives only a fraction of installed
costs with excavation and "backfill contributing., is a minimum, over
half of most pipe-in-trench costs. A simple linear regression analysis
of cost vs. depth of given pipe diameters provides a measure of the
effect of these variables. Thus, given a fixed diameter of pipe and
proposed depth of cut, a quick non-definitive estimate of installed
costs can "be made for many combinations of diameter and depth for
vitrified clay and reinforced concrete sever pipe. In addition,
adjustments can "be made for regional costs and price escalation.
Table V gives the estimating equations for "average United States"
costs, -while Tables VI and VTI gives the resulting point estimates.
Tables VIII and IX gives the cost index -values required for escalation-
location corrections. Figures 5 and 6 illustrate the depth of cut
effect on installed cost of vitrified clay and reinforced concrete clay
pipe.
-------
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-------
TABLE VI. ESTIMATES OF INSTALLED COST OF VITRIFIED CLAY FIFE
AT DIFFERENT DEPTHS AND DIAMETERS^/
Expected Cost at
Depth of Cut
(feet)
5
11
'l5
19
Nominal
8 Inch 10 Inch
$ 3.5 $ 5-2
6.6 9.5
10.2 14.3
15-7 21.3
Diameter
2k- Inch
$18.4
25-3
31.2
35-8
30 Inch
$28.1
3^-3
39-0
U.5
I/ U.S. Average Cost in March 1968 Dollars.
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TABLE VII. ESTIMATES OF INSTALLED COST OF REINFORCED CONCRETE PIPE
AT DIFFERENT DEPTHS AND DIAMETERSl/
-
Depth of Cut
(feet)
5
11
15
19
23
15 Inch
$ 8.2
10.2
11.8
13-7
15.9
Expected Cost at
21 Inch 27 Inch
$12.5 $15.2
14. 7 17-5
16.4 19.1
18.3 21.0
20. 4 23.0
Nominal
33 Inch
$21.5
23.6
25.1
26.9
28.5
Diameter
48 Inch 60 Inch
$31.7 $4l.6
3^.6 44.6
36.6 46.8
38.8 49.0
4l.2 51.4
I
I/ U.S. Average Cost in March 1968 Dollars.
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TABLE VIII
SEWAGE TREATMENT PLANT
AND SEWER
CONSTRUCTION COST
INDEX
(1957-1959 = 100)
City
No. Cities
01
02
03
Oh
05
06
07
08
09
10
Atlanta
Baltimore
Birmingham
Boston
Chicago
Cincinnati
Cleveland
Dal3.as
Denver
Detroit
WPC-STP
Values
110.06
115-64
107 . 34
121.95
125.31
118.63
128.01
106.42
111.94
129 . 67
WPC-S
Values
109.44
123.51
101.30
126.42
123.47
121.37
128.11
102.03
117-75
137 A6
March 1968
City
No.
11
12
13
14
15
16
17
18
19
20
Cities
Kansas City
Los Angeles
Minneapolis
New Orleans
Nev York
Philadelphia
Pittsburgh
St. Louis
San Francisco
Seattle
WPC-STP
Values
112.93
128.59
125 . 04
110.25
139.77
119.36
123.07
125.95
133.52
130.66
WPC-S
Values
118.52
137.37
137-77
115.30
159.38
130.91
133-59
128.41
144.25
144.40
National Index Values 121.21 127.04
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TABLE IX
SEWAGE TREATMENT PLANT and SEWER CONSTRUCTION COST INDEX
(1957-1959 - 100)
April 1970
City
No.. Cities
01
02
03.
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
Atlanta
Baltimore
Birmingham
Boston
Chicago
Cincinnati
Cleveland
Dallas
Denver
Detroit
Kansas City
Los Angeles
Minneapolis
New Orleans
New York
Philadelphia
Pittsburgh
St. Louis
San Francisco
Seattle »
WPC-STP
Values
129.29
124.
116.
142.
143.
l4o.
146.
124.
128.
152.
128.
137.
141.
126.
160.
136.
149.
l4l.
150.
147.
66
00
86
84
87
86
10
70
08
09
92
44
91
87
99
21
44
43
19
5« Change
April 1969
+ 9-
+ 3-
+ 3-
+ 8.
+ 8.
+ 7-
+ 5-
+ 6.
+ 7-
+ 8.
+ 5-
+ 1.
+ 5-
+ 5-
+ 8.
+ 5-
+12.
+ 7-
+ 5-
+ 4.
2
6
0
8
8
4
2
5
6
6
6
6
4
8
3
6
1
0
8
5
WPC-S
Values
132
128
119
150
146
155
157
119
127
156
135
142
150
137
170
153
154
147
168
162
.05-
.09
99
.16
.75
.45
.78
.06
.23
-95
.26
.83
.26
.16
.33
.04
55
.12
34
.33
% Change
April 1969
+ 8
+ l
+ 1
+ 9
+10
+ 6
+ 6
+ 6
+ 4
+11
+ 5
+ 1
+ 5
+ 5
+ 1
+15
+ 7
+ 7
+11
+ 4
7
3
.8
5
.8
.6
.1
.6
3
.2
.4
.1
.8
.8
.8
.8
9
.8
.1
.8
National Index Values
138.49
+ 6.5
145.74
+ 6.7
-------
Figures 5 an(i 6 and the corresponding.tabulated data show different
constant relative increases of cost with depth "depending on diameters, i.e.,
as diameter increases the relative amount of cost increase per foot of
depth decreases. This is due to the fact that at longer pipe sizes,
diameter accounts for a greater portion of the overall installed cost than
does depth, per se.
-------
Figure 5. Effect of Cut Depth
On Installed Cost of Vitrified
Clay Pipe (March 1968 $)
-------
: Figure 6. Effect of Cut Depth
1 On Installed Cost of Reinforced
; Concrete Pipe (March 1968 $)
._m____4-- ,__, J*, _ . i_^ _i __.
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-------
COST OF ANCILLARY WORKS vs. FLAM? COST and PLANT CAPACITY
In most previous studies few, if any, attempts have been made to
perform regression analysis of costs for appurtenances that are ancillary
to the central treatment plant. By using total eligible cost and plant
construction cost data from plant-interceptor projects for previously
unsewered communities, a significant association was found between costs of
ancillary works and size of plant. This relationship was found by (l)
subtracting plant construction cost from Total Eligible Cost, (2) denoting
the resultant value as "Y".or ""Ancillary Costs", and (3) comparing this with
plant size "X". Both costs were converted to 1957-1959 dollars prior to
analysis by using the Plant Cost Index for X and Sewer Index for Y. The
Y variable is assumed to be predominately the cost of ancillary works
(pumping stations, force mains, interceptors and outfalls) as required for
a given project although there is a fraction of this cost earmarked for
legal, administrative and engineering costs of the entire project.
By using this rationale for computing costs, the ancillary costs
were compared with plant design capacity (population equivalent) from projects
wherein a complete treatment system was installed.
Linear regression and scatterdiagram (Figure T) analysis showed a
log-linear relationship "between these two variables. The results of the
regression analysis are described in Tables X through XIII for waste stabiliza-
tion ponds, primary, trickling filter and activated sludge plants. The lower
ancillary cost for ponds is probably due to the greater number of gravity-type
interceptors in communities using pond treatment.
-------
" s
s
Quite frequently cost for interceptor piping and other ancillary
equipmemt is expressed as a flat percentage of total plant cost. A
modification of this approach is the comparing of construction costs for
ancillary vorks (interceptors, force mains, outfall, and lift stations)
vith construction cost of various size treatment plants, as is done in
Figure 8. The relatively low cost of interceptor sewers serving small
plants is probably due to the lower cost-expansion exponent (slope of
line on log-log plots) at the top of the range in small diameters and
the shorter runs of interceptors. In general, a cost of two miles at
a given depth and diameter is about twice the cost of one mile leaving
little economy-of-scale in construction cost because of distance alone.
Therefore, interceptor costs apparently reach a point (at approximately
30,000 EE) as shown in Figure 8 where they rise above plant costs as
greater diameters and distances are required to intercept the greater
volumes of sewage. Table XIV data taken from other contract cost
summaries also shows a similar change for ratio of interceptor cost to
plant cost because of population served size groups. Of course, addition
of collection sewer cost to the above-noted interceptor costs would exceed
plant costs in all cases.
-------
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FIGURE T. SCATTERDIAGRAM, AIvCILLAHY COST P5B PLAOT POPULATION EQUIVALENTACTIVATED SLUK3
-------
TABLE X
CORRELATION OF PLA1W CAPACITY AND ANCILLARY COST
FOR
COMPLETE TREATMENT SYSTEM WITH ACTIVATED SLUDGE PLANTS
"A"
Point Estimates of Ancillary Costs
Design
Population
Equivalent
1,000
2,500
5,000
10,000
25,000
50,000
75,000
Total Ancillary Costs (1957-1959 Dollars)
Lover Limit
$ 17,000
39,000
73,400
136,800
315,000
598,500
865,400
Expected Value
$ 37,700
86,700
163,000
304,000
700,000
1,330,000
1,923,000
Upper Limit
$ 83,300
191,600
360, 200
450,800
1,547,000
2,939,000
4,249,800
"B"
Regression Equation
Log Y = 0.911 LogX + 1.843
Where Y «= Ancillary Cost (Difference "between Plant Cost
and Total Eligible Cost)
X «= Design Population Equivalent of Plant
Valid Size Range - 1,000 - 75,000
Sample Size = 89
Lover Limit Ratio = 0.45
Upper Limit Ratio = 2.21
Correlation Coefficient =0.84
-------
TABLE XI
COKREIATION OP PLANT CAPACITY" AHD ANCILLARY COST
FOR
COMPLETE TREATMENT SYSTEM WITH PRIMARY PLANTS
Point Estimates of Ancillary Costs
Design
Population
Equivalent
1,000
2,500
5,000
10,000
25,000
50,000
75,000
Total Ancillary
Costs (1957-1959 Dollars)
Lover Limit Expected Value
$ 29,300 $
57,900
97,000
162,500
321,700
539,200
729,300 l
53,200
105,200
176,300
295,500
584,900
980,400
,326,000
Upper Limit
$ 96,800
191,500
320,900
537,800
1,064,500
1,784,300
2,413,000
"B"
Regression Equation
Log Y = 0.7450 Log X + 2.4906
Where Y = Ancillary Cost (Difference between Plant Cost
and Total Eligible Cost)
X - Design Population Equivalent of Plant
Valid Size Range = 1,000 - 75,000
Sample Size = 78
Lover Limit Ratio = 0.55
Upper Limit Ratio = 1.82
Correlation Coefficient =0.89
-------
TABLE XII
CORRELATION OF PLANT CAPACITY AND ANCILLARY COST
FOR
, COMPLETE TREATMENT SYSTEM WITH TRICKLING FILTER PLANTS
Point Estimates of Ancillary Costs
Design
Population
Equivalent
1,000
2,500
5,000
10,000
25,000
50,000
75,000
Total Ancillary Costs (1957-1959 Dollars)
Lower Limit
$ 25,300
51,800
89,200
153,400
31^,500
5^1,100
7^3,600
Expected Value
$ ^4-7,700
97,700
168,300
289,500
593,^00
1,021,100
1,^03,000
Upper Limit
$ 90,200
18^,600
318,100
5Vf,200
1,121,500
1,929,700
2,652,000
"B"
Regression Equation
Log Y = 0.7832 Log X + 2.3288
VThere Y - Ancillary Cost (Difference between Plant Cost
and Total Eligible Cost)
X = Design Population Equivalent of Plant
Valid Size Range = J.,000 - 75,000
Sample Size = 97
Lover Limit Ratio = 0.53
Upper Limit Ratio - 1.89
Correlation Coefficient =0.82
-------
TABLE XIII
CORRELATION OF PLANT CAPACITY AND ANCILLARY COST
FOR
COMPLETE TREATMENT SYSTEM WITH ₯ASTE STABILIZATION PONDS
Point Estimates of Ancillary Costs
Design
Population
Equivalent
1,000
2,500
5,000
10,000
25,000
Total Ancillary Costs (1957-1959 Dollars)
Lover Limit
$10,100
20,600
35,300
60,400
123,700
Expected Value
$ 32,500
66,400
114,000
195,000
399,000
Upper Limit
$ 77,000
157,300
270, 200
462,200
9^6,000
s-1
"B"
Regression Equation
Log Y ' * '0.779 Log X 4 2.175
Where Y = Ancillary Cost (Difference "bet-ween Plant Cost and
Total Eligible Cost)
X = Design Population Equivalent of Plant
Valid Size Range = 1,000 - 30,000
Sample Size = 317
Lover Limit Ratio = 0.31
Upper Limit Ratio = 2.37
Correlation Coefficient = 0.63
-------
FIGURE 8. RELATIONSHIP BETWEEN ANCILLARY VJORKS AND PLANT CONSTRUCTION COST
* ^£nclTI<
f '
-^- Trea-i
ary'vlcrks* "
;ment- Plant ---.
\ --
. ._ T ! \ , - - , I
. . - - < ) - . '
" ; | ' ,
. . . - | -i
T.OOOO
foDooo
3 A 5
Design Population Equivalent of Plant
Includes interceptors, force mains, liftLstations and their engineering costs but
ludes collection system.
-------
TABLE XTV. INTERCEPTOR COST AS A RATIO TO NEW PLANT COST
Population Served
Group
500
500 - 999
1,000 - 2,499
2,500- 4,999
5,000 - 9,999
10,000 - 24,999
25,000 - 49,999
50,000 - 99,999
100,000 - 249,999
250, ooo
Overall
Ratio to Plant Cost at Each Location
1
532
.548
.710
1.030
1.127
1.512
2.120
2.186
2.514
3-527
1.800
2
.294
.302
.392
.568
.622
.834
1.170
1.207
1.388
1.946
.99^
3
373
.384
.498
.722
.790
1.060
1.487
1.533
1.763
2.473
1.256
4
'.249
.438
.438
.607
.607
.718
1.079
1.079
1.712
1.712
.882
5
.549
.565
733
1.063
1.163
1.560
2.189
2.496
2.595
3.640
1.858
6
.450
.450
.450
.527
.527
.806
1.496
1.496
1.496
1.496
923
National
.373
.384
.498
.722
.790
1.060
1.487
1-533
1.763
2.473
1.262
1 = California, Idaho, Nevada and Oregon.
2 - Iowa, Minnesota, Missouri, Montana, Nebraska, N.Dakota, S.Dakota & Wyoming.
3 = Arkansas, Arizona, Colorado, Kansas, N.Mexico, Oklahoma, Texas & Utah.
4 = Alabama, Florida, Georgia, Kentucky, Louisiana, Mississippi,
North Carolina, South Carolina, Tennessee, Virginia.
5 = Delaware, Illinois, Indiana, Maryland, Michigan, Ohio, W.Virginia &
Wisconsin.
6 = Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York,
Pennsylvania, Rhode Island, and Vermont.
-------
COST OF MUNICIPAL COLLECTION SEWERS
Sanitary sever systems vhich collect vastevater from individual
dwellings comprise several components starting with the house sever.
This house drainage plumbing empties into the street or lateral sever.
Branch severs receive sewage from a few laterals in a small area and empty
into a sub-main sever vhich, in turn, empties into a trunk severthese
sewers in conjunction vith manholes make up the so-called collection sewer
system vhich is exclusive of interceptor and outfall severs. For the
purpose of this study, force mains and pumping stations are considered as
part of the interceptor system.
Isard and Coughlin^J developed a collection sever cost model
based on population density (persons per acre). Their model shoved
exponential decrease in unit cost ($ per capita) vith increased population
density (persons/acre). These two variables, consequently, plot as a
straight line on logarithmic paper.
Although analysis of land use parameters in terms of population
density and average daily flov per acre is the "best way to accurately
determine size and subsequent cost of sewers, a correlation between population
served and installed cost in 100 communities yields order-of-magnitude cost
estimates of collection sewer systems on an overall population basis.
Tables XV and XVI and Figure 9 give the results of the correlation of
logarithm of the total cost with the reciprocal of the logarithm of the
population served. As throughout this report common logarithms (base 10)
are used.
-------
9\
Following is a rough comparison of given per capita cost levels as
related to population density (Coughlin-Isard study) and population served
(from the at>ove Tatiles):
Per Capita Cost Population Population Density
(1968 $) Served (people/acre)
50 25,000 36
100 T,500 15
200 800 6 . '
-------
->
3
"
TABLE XV
CONSTRUCTION COSTS OF COLLECTION SEWERS IN SMALL COMMUNITIES
"A"
Point Estimates for Cost of Collection Severs
Population
Served
250
500
750
1,000
1,250
Lower Limit
49,900
77,300
101,000
120, 000
135,000
Construction
Expected Values
69,100
119,000
155,000
184,000
207,000
Cost (Dollars)
Upper Limit
106,000
183,000
239,000
283,000
319,000
Expected
Per Capita Cost
276
238
206
184
166
'B"
Regression Equation
Log Y
Where Y
X
Valid Size Range
Sample Size
Lower Limit Ratio
Upper Limit Ratio
Correlation Coefficient
6.9529 - 5.0685
logX
Total Installed Cost (1968 $)
Population Served
100 - 1,250
57 projects
0.65
1-54
- 0.85
-------
TABLE
CONSTRUCTION COSTS OF COLLECTION SEWERS IN INTERMEDIATE SIZE COMMUNITIES
"A"
Point Estimates for Cost of Collection Sewers
Construction Costs (Dollars)
Population
Served
2,500
5,000
7,500
" 10,000
25,000
Lover Limit
236,000
356,000
^,000
^95,000
712,000
Expected Values
kikfOOO
62^,000
762,000
868,000
1,250,000
Upper Limit
724,000
1,090,000
l,3lK>,000
1,520,000
2,190,000
Expected
Per Capita Cost
166
125
102
89
50
"B"
Regression Equation
Log Y = 7-6979 - 7.0381
LogX
Where Y = Total Installed Cost (1968 $)
X = Population Served
Valid Size Range = 1,500 - 25,000
Sample Size = 9^- projects
Lower Limit Ratio =0.57
Upper Limit Ratio =1.75
Correlation Coefficient = - 0.77
-------
U£1T»
FIGURE 9. Correlation of Collection Sever
Cost with Population Served
567 SS
i"6oo
' 100,boo
Population Served
-------
The population densities for the above ranges in costs varies from
that categorized for lov density residential to high density residential.
Both procedures, although on a different basis, produce cost estimates
of the same order of magnitude.
INSTALLED COST PER MILE
A gross approach to a non-definitive estimate of interceptor cost
is the analysis of cumulative cost per mile vs. the largest diameter pipe
used in a particular project (most interceptor projects use 3-^- different
size pipes). Moreover, with other costs in the data, there is a more
integrated cost picture as shown in Figure 10, including Table XVII. For
examples of the number manholes used with interceptors are plotted in
Figure 11. In spite of this different approach, the resulting cost
curves are similar to the refined unit cost curves discussed earlier in
the report--both showing the geometric progression effect of diameter on
pipe cost per unit length installed. Converting the largest diameter in
an interceptor project to a flow-capacity value and comparing this to
cost-per-mile forms another statistical relationship as shown in Figure 12.
This is termed "log-linear" or exponential, a rather common type of
association between cost and capacity.
-------
TABLE ;X"f II. COST PER MILE OF INTERCEPTORS
Point Estimates
Largest Pipe
Diameter (inches)
8
10
12
15
18
2k
30
36
42
48
54
60
72
90
Installed. Cost (1967 Dollars)
Lover Limit
$ 26,800
29,600
32,800
38,100
44,400
60,200
81,300
123,000
141,000
162,000
185,000
207,000
279,000
417,000
Expected Value
$ 41,900
46,300
51,300
59,600
69,400
94,000
127,000
176,000
202, 000
231,000
264,000
296,000
398,000
596,000
Upper Limit
$ 65,800
72,700
80,500
93,600
109,000
148,000
199,000
252,000
289,000
330,000
378,ooo
423,000
569,000
852,000
Regression Equations
Size Range
(inches)
8-30
36 - 90
Equation
Log Y = 0.0219X +
Log Y = 0.0098X +
4.447
4.893
Correlation
Coefficient
0.75
0.73
Sample Size
(Projects-*)
21
14
Where Y = Installed Cost ($/mile) of Entire Project
X = Nominal Pipe Diameter (in.) of largest pipe laid.
* Projects requiring force mains and pumping stations excluded from
estimate preparation.
21
-------
7
Q , .,
g , _ , ,
j._.FI.GraE..10._INSTAHED..COST .PER MILE .BASED ON LARGEST DIAMETER OF- INTERCEPTOR PIPE !
-(X
. .. , . _ .J
J
i
i I
i. X. _:
..." .-4: -".
_j
;
,
^ . .._.
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58 72 95 12D
Nominal Diameter of Largest Pipe (in.)
-------
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-------
RELATIONSHIP BETWEEN COST AND CAPACITY
The study so far primarily correlates cost of a unit length of pipe
with single dimensions, i.e., the depth of trench and the diameter of pipe.
By converting the diameter dimension to a volume-capacity equivalent, an
exponential relationship evolves as indicated previously in Figure 12.
This relationship was tested again by converting to "equivalent carrying
capacities" -with 12" = 100 "by using handbook^ ' values, and calculating
unit length costs at an installed depth of 0-6 feet for the various
diameters. The resulting data, plotted in Figure 13 gives the log-linear
exponential relationship "between installed cost per foot and relative
carrying capacity. A characteristic of this 'log-linear" relationship is
that each doubling of capacity increases cost by a uniform percentage.
The approximate percentage cost increases taken from this plot are as
follows for each doubling of capacity:
Reinforced
Clay Cast Iron Concrete
Smaller Diameters 30$ 25$ 23$
Larger Diameters 90$ 55$ 2U$
With these nondefinitive estimates, the trend, per se, for increase in
cost with capacity is less in the smaller diameter ranges with a doubling
in capacity costing only 30$ more for clay pipe up to 18-inch diameter,
while above this pipe diameter, a 90$ increase in cost buys a double
capacity. Engineers generally concede that vitrified clay pipe is
-------
relatively expensive in larger diameters. The curves in Figure 12
show this graphically. Conversely, the economy-of-scale is high and
the cost lov for this type of pipe in smaller diameters attesting
to the need for careful economic analysis of each construction situation
to select the optimum diameter and type of pipe. The uniform
"cost-expansion" factor -within a fairly "broad range of diamerters for a
given type of pipe should provide a useful tool for sewer system cost
analysis.
-------
J
P4
(U
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H
o
D
8
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p
--- FIGURE 13. ' Cost-capacity Relationships
Different Diameters and Types of Plants
T
Vitrified Clay
'Reinforced
Concrete
10
12
_»;
--15"
18
:| Ul_:_l.j
21). : 30 - : - 36 - Inch diameter
67891
100
3/15
7S81
1,000
Relative Carrying Capacity
-------
LOCATION FACTORS
There are other determinative factors that affect interceptor cost
other than depth of cut and diameter of pipe. Soil type at the construction
sites can vary from plastic clay to almost pure sand. Some areas vith high
water tatles require extensive veil-pointing. Rock excavation is very
expensive compared to normal soil excavation. Tahle XVIII, accordingly,
illustrates the order-of-magnitude effect of special installation require-
ments, "which increase unit costs in some cases "by a factor of ten.
A study published in 196^ of sever installation costs stated that
pipe-in-trench accounts for only k6.62$ of the overall cost vith only 25-5$
of this fraction going for the pipe itself. ^J rp0 illustrate, three recent
large interceptor projects vere noted to have the folloving percentages of
pipe cost:
Cost of Pipe
Project Cost (percent)
$ 426,000 25
61,332,000 9
'2,13^,000 5
-------
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-------
THE COST OF PUMPING STATIONS
Pumping stations are installations -which pump or lift sewage from
a lover level in a sewer system into a higher sewer or a receiving chamber
for transportation to a treatment plant or point of discharge. Most
municipalities find it necessary or economical to pump at least part of
their sanitary sewage. Pumping stations are cheaper than laying sewers
at increasingly greater depths. The topography of the area of a sewer
»
system may require the system to include several small pumping stations
at various points and one or more large stations.
A sewage pumping station has a wet well, which receives the sewage,
and a dry well, in which are placed the pumps, motors, and switches. A
house is usually placed over the dry well, although some small stations
are placed entirely underground. Most pumping stations have a number of
pumps of various capacities. The array of pumps are necessary to permit
flexibility of operation to meet various rates of sewage flow with reserve
pump capacity to allow for breakdowns. The stations are usually lighted
and ventilated; heating' is also required in more severe climates. Standby
power generators and remote alarm and flowmeter systems are other auxiliary
equipment that can affect the cost of pumping station installations.
This section of the interceptor study provides cost information
which can be used in preparing preliminary estimates of investment required
for a pumping station (connected to a municipal interceptor-force main sewer
system) in terms of .-peak sewage flow, installed pump capacity, number of
pumps and actual construction contract cost. A simple linear regression
-------
- 2 -
analysis was utilized to assess the effect on cost of total available
pump capacity and peak flov on installed cost of pumping stations. The
resulting cost estimates includes the complete installed construction
cost for the station but excludes the cost of acquiring the station site
or right-of-vay.
Data for the analysis vere extracted from FWQA project contract .
files. Most contract statements were for builders to furnish and install
complete, factory-built automatic stations with all necessary equipment
factory-installed and factory-tested. The principle items of equipment
usually include:pumps and motors or compressors, valves, internal piping,
central control panel with circuit breakers, motor stations and automatic
level controllers, lighting, sump pump, motor driven ventilation,
dehumidifieij and all internal wiring.
The unit cost data secured from project records was converted
from a regional basis to a national basis and then updated from construction
time to April 1970 dollars by using the Sewer Construction Cost index.
A considerable variation in project-to-project costs of pumping
stations was observed with no real clue to the explanation of the variances.
However, there exists some statistically significant relationships between
total costs and total installed pump capacityi/ that allows crude estimation
of the investment required for station costs.
If Total installed pump-capacity is the sum of the capacity of individual
pumps in the station. Capacities less than 500 gpm- -had 1-2 pumps.
-------
1
Tables XIX and XX contain the results of the log-linear regression
analysis; while the corresponding Figure 1^ shows the costs for pumping
stations up to 500 gpm and over 500 gpm, respectively. This graphic
analysis (Figure 14) indicates that cost is an exponential function of
the variable "total capacity." These figures indicate a trend without.
a vide scatter. Not shown were several scatterdiagrams of cost versus
the highest single pump capacity, and cost versus average sewage design
flow. In both cases the data points were too widely scattered to justify
further analysis.
-------
TABLE XIX
RESULTS OF REGRESSION ANALYSIS FOR CONSTRUCTION COSTS
OF PUMPING STATIONS
Point Estimates for Cost of
Pumping Stations Up To 500 GPM Capacity
Design
Flow
(GPM)
100
200
300
koo
500
Unit Construction Costs (April 70 Dollars)
Lower Limit
11,326
1*1,752
19,628
22,658
25,331
Expected Values Upper
15,V70 21,
20,150 27,
26,810 36,
30,950 k2,
3k ,600 k7,
"B"
Limit
132
525
622
278
26k
Regression Equation
Log Y =
Where Y =
Valid Size Range =
Sample Size =
Lower Limit Ratio =
Upper Limit Ratio =
Correlation Coefficient =
.5000 (log x) + 3.1896
Cost per Pumping Station
Installed capacity GPM
50 - 500 GPM
13
.7321
1.366
.78
-------
0?
TABLE XX
RESULTS OF REGRESSION ANALYSIS FOR CONSTRUCTION COSTS OF PUMPING STATIONS
"A"
Point Estimates for Cost of
Pumping Stations of more than 500 gpm Capacity
Design Flow
700
800
1,000
1,200
1,500
3,000
5,000
10,000
12,000
Construction Costs (AT>r.
lover Limit
27,807
31,121
37,501
^3,689
52,653
94,096
144,424
257,959
300,237
Expected Values
^6,633
52,190
62,890
73,267
88,300
157,800
242, 200
432,600
503,500
70 Dollars)
Upper Limit
78, 204
87,523
" 105,^67
122,869
148,079
264,631
406,169
725,470
844,370
"B"
Regression Equation
Log Y = .8373(logX) + 2.2868
Where Y = Cost per Pumping Station
X = Installed Capacity (gpm)
Valid Size Range = 700 - 12,000 gpm
Sample Size = 14
Lover Limit Ratio = .5963
Upper Limit Ratio = 1.677
Correlation Coefficient = .87
-------
Figure ~ik
Regression Line of Unit Cost vs. Total Pumping Capacity
(Under 500 gpm and over 500 gpm)
XX)
3 -a t 6 7 a e i
5,000 10,
55o6 7.8f,6oo
Total Capacity - Gallons per Minute
3 -4 5 6 7
-------
!/
*
Peak Flow Regression Analysis
A log-linear regression analysis was also performed using peak
sewage flow, which the stations are designed to transport, as the
independent variable.
Figure 15 is a graphic representation of the sample, the regression
line derived, and the upper and lower limits. Table XXI shows the results
in detail including a very high correlation coefficient (0.96).
Table XXII is a comparison of cost estimates generated by the
regression equations developed. The estimates are for various gpm peak
flows and pump capacities. The first column of estimates is based on
formulae in a report of Alan M. Voorhees & Associates, Inc. (AMV).' '
The AMV formulae was converted from a cfs to gpm basis.
A fair comparison can be made between the AMV and F₯QA peak flow
estimates since they seldom vary more than 20$ of each other, as shown by
Figure 6. One differing aspect of the AMV estimates is that they
reportedly include land cost which should, in reality, cause the AMV costs
to be higher than the-FWQA estimates.
Further observation of the FWQA cost estimating analyses indicated
that for the peak flow regression; as the flow is doubled, costs increased
by approximately Jl$. For the installed capacity regression (up to 500 gpm)
a doubling of the capacity increases costs 4l$ and approximately QQffo for .
the 500 gpm and over regression. All three of these expansion factors
indicate economy-of-scale in pumping station installations.
-------
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3.303
- uofC).T3q.s
-------
TABLE XXI
RESULTS OF REGRESSION ANALYSIS FOR CONSTRUCTION COSTS
OF PUMPING STATIONS
Point Estimates for Cost of
Stations vs. Peak Flow
Peak
Flow
(GPM)
100
300
500
800
1000
1200
1500
3000
5000
12,000
Construction
Lower Limit
10,911
25,516
37,853
5M28
6l*,696
7^93
88,100
151,098
22k, 6kQ
Costs (Apr 70) Dollars
Expected
Values Upper Limit
15,035 20,718
35,160 ^8,i*50
52,160 71,876
75,000 103,350
89,150 122,81*9
102,650 ll*l,l*52
121,1*00 167,289
208,210 286,913
309,560 U26, 5 7k
608,280 838,210
"B"
Regression Equation
Log Y = .7730 (log
x) + 2.6311
Where Y = Cost per Pumping Station
X = Peak Flow
Valid Size Range = 0 - 12,000
Sample Size = 18
Lower Limit Ratio - .7257
Upper Limit Ratio = 1.378
(GPM)
GPM
Correlation Coefficient
.96
-------
TABLE XXII
COMPARISON - PEAK FLOW AND INSTALLED
5. ALAN M.
GPM
100
300
500
800
1,000
1,200
1,500
3,000
5,000
12,000
VOORHEES AND
. AMV*
Peak Flow
19,399
45,244
53,829
50,406
62,651
74,750
92,633
177,198
277,407
515A99
ASSOCIATES COST
FWQA .
Peak Flow
15,035
35,160
52,160
75,000
89,150
102,650
121,400
208, 210
309,560
608,280
PUMP CAPACITY
ESTIMATING RELATION
FWQA
Total Installed
Capacity
15,^70
26,810
34,600
- 52,190
62, 890
73,267
88, 300
157,800
242, 200
503,500
* Includes cost of land.
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Figure 16
Comparison of FWQA and Alan M. Voorhees & Assocs., Inc.*
Peak Flov Estimating Relationships
5(ib 6 7 I'o'oo
7 eib;ooo
3 A 5 6 7 J "> 1
Peak Flov (gpm)
includes Cost of Land
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FUTURE PRICE ESCALATION
Most of the cost equations developed in this study used the sewer
construction cost index of FWQA for conversion to a common "basis. The
exception is the"doliars-per-mile" relationship -which used raw cost data
without adjustment to a "base date. Some fairly recent cost index values
are in Table IX. However, for most cost estimates even current cost
indexes are not sufficient because many months usually elapse "before the
first "bid quotations are received. For this reason, Figure 1J shows past
trends in the sewer cost index and attempts from these to extrapolate
into the near-term. Although actual future price escalation may or may
not exhibit a constant ratio of increase with time as did the previous
two indicated trends, the predicted values should suffice through 1970.
A regression analysis of trend lines similar to Figure 17 gave the
following equations indicating the varying rates of change in escalation:
Period Equation
Sept. 1963 - Dec. 1965 Y = 113 (l.OOl6)X
Jan. 1966 - Dec. 1967 Y = 118 (l.0028)X
Sept. 1968 - Jan. 1970 Y = 131 (l.0055)X
Sept. 1969 - June 1970 Y = 1^0 (l.0055)X
Where Y = Sewer Construction Cost Index X 100
X = Months in period
The above equations show a monthly escalation of the National sewer cost
index ranging from 0.16$ in 1964 to 0.55$ a month at present.
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1966 196?
Calendar Year
1970
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CONCLUSIONS
In spite of "being exploratory and limited in time (and corresponding
amount of data) this study indicates that interceptor costs follow certain
statistical patterns. These different relationships for installed
interceptor cost per unit length were observed:
1. Installed pipe cost per unit length increases at a constant
ratio with pipe diameter at a constant depth of cut and
within a given range of diameters.
2. After a certain diameter is reached, the ratio of increase
in unit length cost with diameter changes. This is related
to a change in the rate of increase in weight per unit length
with diameter.
3. Install led pipe cost per unit length increases exponentially
with pipe capacity at a constant depth of cut. This means that
each doubling of capacity within a given diameter range
increases pipe cost by a uniform percentage. This exponential
relationship depends on a range in pipe sizes for after a
certain pipe diameter is reached,'there is a higher exponential
rate of cost increase with capacity.
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4. These cost-size associations characterize economy-of-scale
because of pipe diameter. For example, analysis of projects
vith the largest interceptor diameter equivalent to 1.0 mgd
capacity indicate a cost of approximately $50/000/mile/mgd
while projects with 100 mgd lines going to the treatment
plant average ^}^OQ/w.±e/mg,d, a considerably lover unit-
capacity cost of installation. There is, however, little
evidence of any economy-of-scale for interceptor construction
cost because of total length of lines.
5. The ratio of total -waste-water treatment plant cost to
interceptor cost (including other ancillary -works) varies
according to the design population of the plant or the
population of the community served. In low population
areas (e.g., 1,000) the ancillary works are about one-third
the cost of a new plant, at 25,000 population the costs are
approximately equal while at 100,000 the ancillary works are
approximately 1.5 times the plant cost.
6. Collection sewer unit costs (per capita basis) are very
sensitive to population (total population served and popula-
tion density). Very small areas (250 population) have over
five times the per capita cost than large communities
(25,000 population).
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7. Actual pipe cost is rarely over 10-15 percent of total cost
for large municipal interceptor projects. In fact, the base
pipe-in-trench cost itself is usually only about one-third
to one-Half the total cost of interceptor projects. Special
installation requirements such as crossing roads, railroads
or river beds increase cost per unit length almost ten fold
over pipe-in-trench requiring common excavation and normal
back fill.
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Jtegiou V, i-v.-
230 £:,v,:.:) :.
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REFERENCES
1. Bos-worth, C.A., "Installed Costs of Outside Piping."
Chemical Engineering, (March 25, 1968).
2. "Sever and Sewage Treatment Plant Construction Cost Index."
. CWT-1, page 52, Federal Water Quality Administration,
(December 1967).
3- Isard, Walter and Coughlin, R.E., "Municipal Costs and
Revenues Resulting from Community Growth." Changler-Davis
Publishing Company, 1957-
4. National Clay Pipe Institute, "Clay Pipe Engineering Manual."
5. Federal Water Quality Administration, op. cit., p.lSff.
6. Alan M. Voorhees & Associates, Inc., "Sewer System Cost
Estimation Model." (April 1967).
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230 £'?v.-.
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