CE 417

Construction Engineering and Management

Spring Semester 2011

Term Project

Scraper Production Rate and Cost

Presented by:

Nelson Francisco Salgado

Instructor:

Dr. Waheed Uddin

May 6, 2011.

"I pledge myself to uphold the highest standards of honesty in my university life and I

will not tolerate dishonesty on the part of others.”

May 6, 2011.

Dr. Waheed Uddin,

Professor and Director, CAIT,

Department of Civil Engineering,

203 Carrier Hall, University of Mississippi,

Oxford, MS.

Dear Dr. Uddin,

By this mean, I am submitting to you the final term project for the course CE417

Construction Engineering and Management, due on May 6, 2011. This report has been

entitled Scraper Production Rate and Cost.

The main purpose of this document is to discuss the use of scrapers in earthwork,

presenting a case of study, which shows a scraper Caterpillar model 631E. Data and

calculations to find its production rate and cost has been presented and discussed

throughout the content of this document. Along these calculations, a section with proper

discussions about the results has been included, providing some recommendations related

to this project.

Your evaluation and comments on this report will be greatly appreciated.

Sincerely,

Nelson Francisco Salgado,Exchange Student,University of Mississippi,Oxford, Mississippi.nfsalgad@olemiss.edu

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TABLE OF CONTENTS

Page

1. Executive Summary ……………………………………………………………... 5

2. Introduction………………………………………………………………………. 6

3. Data and Design Information…………………………………………………….. 8

3.1 Data Description………………………………………………………... 8

3.2 Additional Information…………………………………………………. 11

4. Analysis and Design……………………………………………………………... 11

4.1 Weight………………………................................................................... 11

4.2 Rolling Resistance……………………………………………………… 12

4.3 Grade Resistance………………………………………………………... 12

4.4 Total Resistance, Travel Speed and Travel Time………………………. 12

4.5 Dump Time……………………………………………………………... 14

4.6 Turning Times…………………………………………………………... 14

4.7 Total Cycle Time……………………………………………………….. 14

4.8 Pusher Cycle Time……………………………………………………… 15

4.9 Balance Fleet……………………………………………………………. 15

4.10 Efficiency……………………………………………………………… 15

4.11 Production Rate………………………………………………………... 15

4.12 Cost……………………………………………………………………. 16

4.13 Number of Work Time To Accomplish a Task……………………….. 18

5. Evaluation of Design and Discussion of Results……………………………….... 19

5.1 Summary of Results…………………………………………………….. 19

5.2 Discussion of Results…………………………………………………… 19

6. Recommendations………………………………………………………………... 20

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Works Cited (References)…………………………………………………………... 22

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1. EXECUTIVE SUMMARY

Earthwork is a very important part of a project, involving a considerable percentage

of its total cost. The Transportation Research Board Executive Committee argues that

“good earthwork is most easily ensured by firm control operations early in the contract

when many seemingly more important operations also require attention” (Muri et.al, 26).

For this particular project involving earthwork, it has been considered to use a

scraper model Caterpillar 631E, with a single powered axle. The basic approach consists in

determining the scraper cycle and its production cost in a dollars-per-cubic-yard basis

through a systematic analysis. For the calculation of its cycle time, and therefore its

production rate, some other considerations have taken into account. A short distance with a

lowered speed fixed in five miles per hour, has been used to consider the effect of both

acceleration and deceleration experimented by the scraper “when coming out of the pit

(cut), approaching the dump area, leaving the dump area, and again when entering the pit”

(Peurifoy et.al, 233). Since the scraper will be loaded at its entire volumetric capacity, it has

been considered to provide assistance to the machine through the use of a push tractor in

order to minimize the loading time as much as possible. In the second part of the project, a

different type of soil with a different swell factor has been considered to be hauled and

dumped in an area of terrain with a fixed average-fill depth. In this case, it is asked to find

the number of days that the fleet of scrapers will take to accomplish the task of transporting

the material to fill this volume.

The final goal of the project is to find the most economic and efficient alternative in

terms of the number of scrapers to be used per push tractor.

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2. INTRODUCTION

Scrapers are a very useful type of construction machinery in the development of

several different kinds of projects. For instance, for highway construction, some typical

applications of scraper units are “earthmoving operations, transporting, dumping, and

spreading earth material” (Wright, 481). In the sixth edition of his work called “Highway

Engineering”, Wright argues that in a large-scale operation involving borrow pits where the

borrow material is to be incorporated into embankment, scrapers show a very efficient

performance when the material is different than rock (487).

Push-loaded scrapers are “loaded by one, two, or rarely three crawler tractors. The

use of three tractors is justified when this third one is a tractor-ripper equipped with a

bulldozer. Otherwise “ripping is completed for the time being and the operator is simply

and efficiently keeping the machine busy” (Church, 13-38). This assumption has been taken

for the earthwork scenario shown in this paper, therefore, only one push tractor will be used

to match the number of scrapers.

Besides the number of tractors to match the number of scrapers, scraper loading

time depends of other variables, such as the size and characteristics of the scraper, grade

resistance, conditions of the loading area, if the operator is skilled enough to operate the

machine, push tractor horsepower, or nature of material (Church, 13.41). Table 1 shows

different loading times for wheels-tires scrapers according to the nature of the material.

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Table 1. Loading Times for Wheels-Tires Scrapers According to Nature of Rock-Earth.

Nature of Rock-earth

Loading time, min

Push-loaded scraper Self loaded, single-axle drive

scraperSingle-axle

driveAll-axle

driveGravel-sand alluvia 0.8 0.7 1.2Residuals, silts and clays 0.6 0.5 1.0Average-weathered rock, ripped 0.8 0.7 1.2

Rock well blasted 1.2 1.1Not

recommended

Source: Excavation Handbook. McGraw-Hill Inc., 1981, New York. H. Church,

13-43, Table 13-15.

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Another factor with significant influence on the loading time is the haul distance,

which is related to this in a directly proportional way, meaning that at greater haul

distances, loading time will increase (Peurifoy et al, 240). An average of 0.85 minutes has

been used for the calculations about to be presented.

The purpose of this paper is to show the entire process for computing both

production rate and number of scrapers to be used in earthwork, addressing the problem to

find the ideal amount of machines to afford the project to be as economical as possible. The

data and design information are described next.

3. DATA AND DESING INFORMATION

3.1 Data Description

The following information has been extracted from the seventh edition of the book

‘Construction Planning, Equipment, and Methods’ by Robert L. Peurifoy, Clifford J.

Schexnayder, and Aviad Shapira (which will be referred as ‘the course textbook’

henceforth in this document). This specific extract can be found on page 251, problem 8.9:

“Based on the scraper specifications in table 8.1 and the performance charts in

figures 8.9 and 8.10, and for haul conditions as stated here, analyze the probable scraper

production.

How many scrapers should be used and what will be the production in bank

cubic yard per hour (bcy per hr)?

The material to be hauled is cohesive.

It has a swell factor of 0.76 and a unit weight of 2,900 pounds per bank cubic

yard (lb/bcy).

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The expected rolling resistance for the well-maintained haul road is +3%.

Assume a 0.80 minutes (min) load time and that average load will be 90% of

heaped capacity.

To account both acceleration and deceleration use an average speed of 5 miles

per hour (mph) over a distance of 200 feet (ft).

Use a 50-min hour efficiency factor.

The total length of haul is 2,600 ft and has individual segments (see figure 1)

when moving from cut to fill”.

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Cut Fill

200ft 200ft400ft 1,800ft

+5% -2%-4%

Figure 1. Haul road profile showing the grades for each individual segment.

Source: Construction Planning Equipment and Methods, 7 ed., Peurifoy et al.,

251.

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3.2 Additional Information

Find the production cost using the results provided by the previous information

using the following cost data:

Scraper operating and ownership (O&O) cost = $89.00 per hour

Push tractor O&O cost = $105.00 per hour

Scrapper operator cost = $12.00 per hour

Push tractor operator cost = $20.00 per hour

Find how many 6-hour days will be required to complete a site development project

that requires an average 2-ft fill for 500 ft x 900 ft area? Assume 1.0 swell factor.

4. ANALYISIS AND DESIGN

4.1 Weight

The following specifications for a scraper model Caterpillar 631E has been

extracted from table 8.1 on page 232 of the course textbook:

Heaped capacity = 31 cubic yards (cy)

Operating weight = 96,880 lb

Empty Weight (EVW) 96,880 lb

Load Volume = 0.9 x 31cy = 27.9 (loose cubic yards) Lcy

Load volume bank measure 27.9 x 0.76 = 21.2 bcy

Weight of load = 21.2 bcy x 2,900 lb/bcy = 61,480 lb

Gross Weight (GVW) 158,360 lb

4.2 Rolling Resistance (RR)

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From data of the problem:

RR = +3%

4.3 Grade Resistance (GR)

From data of the problem:

600 ft +5%

1,800 ft -2%

200ft -4%

4.4 Total Resistance, Travel Speed, Travel Time

The calculation of total resistance, and travel time is shown on table 2 of this paper.

To compute the total resistance, it has been use the following equation:

TR=GR+RR

where TR: Total resistance

GR: Grade resistance

RR: Rolling resistance

To compute the travel time, it has been use the equation 8.2 on page 238 of the

course textbook, described as following:

Travel Time Segment (min .)= Segment Distance , ft88 ×travel speed , mph

The travel speed for the first and last segment has been fixed in 5mph in order to

account of the acceleration and deceleration. For the rest of haul segments, speed has been

extracted from figure 8.9 of the course textbook.

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Table 2. Calculation of Travel Time

Distance (ft)

RR (%)GR (%)

TR (Lb/ton)

TR (%)Speed (mph)

Travel Time (min)

Haul(164,450Lb)

200 3 5 160 8 5 0.45400 3 5 160 8 11 0.41

1800 3 -2 20 1 33.5 0.61200 3 -4 -20 -1 5 0.45

             

Return (96,880Lb)

200 3 4 140 7 5 0.451800 3 2 100 5 17 1.20400 3 -5 -40 -2 33.5 0.14200 3 -5 -40 -2 5 0.45

Total Travel Time 4.18

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4.5 Dump Time

Dump time for scrapers depends on the machine size and project conditions. The

machine considered in this case has a heaped capacity of 27cy. Its dump time has been

obtained from table 8.7 on page 341 of the course textbook:

Dump time = 0.37

4.6 Turning Times

It is argued on the course textbook by its authors that “turning time is not

significantly affected by either type or size of scraper” (Peurifoy et al, 242). For this

project, the Federal Highway Administration (FHWA) provisions have been used fixing the

turning time values as following:

Turn time fill = 0.21 min

Turn time cut = 0.30 min

4.7 Total Cycle Time

The total cycle time is the summation of the travel time, load time, dump time and

turn times of the scraper.

Travel Time 4.18 min

Load Time 0.80

Dum Time 0.37

Turn Time fill 0.21

Turn Time Cut 0.30

Total Cycle Time 5.86 min

4.8 Pusher Cycle Time

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For computing the pusher cycle time, it has been used equation 8.3 on page 243 of

the course textbook, described as follows:

Pusher cycle time=1.4 × Scraper load time+0.25

Pusher cycle time=1.4 × 0.80+0.25

Pusher cycle time=1.37 min

4.9 Balance Fleet

For computing the number of scrapers per push tractor to be used in the project, it

has been necessary to make the calculations with the equation 8.4 shown on page 245 of the

course textbook, described as follows:

Balanced number of scrapers= Scraper CycleTimePus h Tractor Cycle Time

Balanced number of scrapers=5.86 min1.37 min

=4.28 min≅ 4∨5 scrapers

4.10 Efficiency

The efficiency of the machine used in this project has been established on a value of

50 min-hr based on the provided data.

4.11 Production Rate

In this case, it is possible to choose among two alternatives, 4 or 5 scrapers. If 4

scrapers are selected, the production rate should be obtained through equation 8.5 on page

246 of the course textbook, because 4 is less than the balanced number of scrapers 4.28.

∏ . Rate= efficiency ,min /hrscraper cycle time , min

× number of scrapers × volume per load

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∏ . Rate=50 min/hr5.86 min

×4 scrapers × 21.2bcy=723.5 bcy /hr

If 5 scrapers are selected, the production rate is controlled by the pusher tractor

because 5 is greater than the balanced number of scrapers 4.32. The production rate for this

scenario can be obtained through equation 8.6 on page 246 of the course textbook,

described as follows.

∏ . Rate= efficiency ,min /hrpusher cycle time , min

× volume per load

∏ . Rate=50 min/hr1.37 min

×21.2 bcy=773.7 bcy /hr

4.12 Cost

Table 3 of this document shows de calculation for the unit cost in dollar-per-hour

basis for the different two alternatives: using four or five scrapers. Unit cost per item has

been extracted from the additional data.

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Table 3. Cost Calculation

4 Scrapers 5 Scrapers

Unit Cost ($/hr)

Quan-tity

Total Cost ($/hr)

Quan-tity

Total Cost ($/hr)

ScrapersO&O cost 89.00 4 356.00 5 445.00Operator cost 12.00 4 48.00 5 60.00

Push Tractor

O&O cost 105.00 1 105.00 1 105.00Operator cost 20 1 20.00 1 20.00

529.00 630.00

Note: To compute the unit cost in dollars-bank-cubic-yard units, the following equation has

been used:

Unit Cost ( $ per bcy )= TotalCost , $ per hourProduction Rate ,bcy per hour

Unit Cost ( 4 scrapers )= $ 529.00 per hour723.5 bcy per hour

=$ 0.731 per bcy

Unit Cost (5 scrapers )= $630.00 per hour773.7 bcy per hour

=$ 0.814 per bcy

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4.13 : Number of Work Days to Accomplish a Task

In this part of the project, it is asked to find the number of six-hour days that the

machine will take to fill a volume with a material presenting a 1.0 swell factor. This means

that no conversion is needed in the previously estimated production rate.

Having discussed this, the volume-to-be-filled calculation is presented as following:

Volume , cy=Average fill depth, ft × Area , ft2×1cy

27 ft3

Volume=2 ft × (500 ft × 900 ft ) × 1 cy

27 ft3=33,333.3 ccy of fill

Given that the swell factor is now to be assumed 1.0, this fill volume presents the

same value in bank cubic yards. The rest of calculations are shown on table 5 of this paper.

Table 4. Number of Six-hour Days Calculation

5 Scrapers 4 ScrapersQuantity Unit Quantity Unit

Load Bank Measure 21.2 bcy 21.2 bcyPusher Cycle Time 1.37 min 1.37 minScraper Cycle Time 5.86 min 5.86 minProduction Rate 773.7 bcy/hr 723.5 bcy/hrVolume to be filled 33333.3 bcy 33333.3 bcyNumber of 6-hour days 7.18 days 7.68 daysRounded number of 6-hour days 8 days 8 days

Note: To compute the production rate, equation 8.5 when using 4 scrapers, and equation 8.6

when using 5 scrapers. Both equations are shown on page 246 of the course textbook and

have been retyped in this paper in section step 14 from the data and analysis. To compute

the number of days, the following equation has been utilized.

Number of 6 hour days=Volume¿ be filled , bcy ¿Production Rate , bcy per hour

×1 day

6 hours

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EVALUATION OF DESIGN AND DISCUSSION OF RESSULTS

5.1 Summary of Results

Based on the previous data and analysis, table 6 has been constructed in order to

show a summary of the obtained results. It has been taken into account that the balanced

number of scrapers resulted to be 4.32, opening the possibility of choosing either four or

five machines.

Table 6. Summary of Results

4 Scrapers 5 Scrapers

Problem 8.5 data

Production Rate 723.5bcy/hr 773.7 bcy/hr

Production Cost $529 /hr $630 /hrUnit Production Cost $0.731 /bcy $0.814 /bcy

Additional dataVolume to be filled 33,333.30 cy 33,333.30 cyNumber of 6-hour days 7.18 days 7.68 daysRounded number of 6-hour days 8 days 8 days

5.2 Discussion of Results

It is appreciable that the balanced number of scrapers is closer to 4 than 5. In the

first part of this project, using 4 scrapers will generate savings up to $101/hr with the cost

of reducing the production rate in 6.49% in comparison with the 773.7 bcy/hr production

rate developed with 5 machines. These savings represent 10.20% less than the alternative of

working with 5 pieces of equipment, and around $0.083 every cubic yard of bank volume.

In the second part of the project, the additional data does not provoke significant

variation on the balanced number of scrapers, meaning that the same criteria of choosing

between either 4 or 5 machines also applies for this case.

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Now, time is presented as the critical variable of selection in a 6-hour daytime basis.

Although both alternatives provide similar numbers resulting both in a rounded number of

8 days, working with 5 machines will provide 0.82 days (around 5 hours from a total of 6

hours in that day) of float time in the last day. Other parameters that will be considered for

the final recommendation are the size of this work, which is relatively small-scale, and

production rate.

5. RECOMMENDATIONS

The first case, without a fixed volume to fill, does leave the cost estimator to

consider several scenarios. A large-scale project may justify the use of 5 scrapers, meaning

to increase costs in $101 per hour. With a higher production rate, the employment of these

5 machines should reduce the time to accomplish the task resulting in more savings. Other

aspects to consider are the type of material and the type of machine to be used. In this case

the material was categorized to be a cohesive soil and a push loaded scraper. In the case of

a different, harder soil, another type of scraper may be considered. In his work, Church

argues that “soft to medium formations favor the self-loading scraper and medium to hard

rock-earth favors the push-loaded machine”. When several locations require excavations at

the same time “as in cuts for confined home sites, the self sufficient self-loading scraper is

more economical” (13-51).

Other circumstance for large-scale projects can be to improve haul roads. Grades in

the haul roads determine the travel speed of the selected piece of equipment, therefore it is

important to study its effect on the production cost. Church agrees on “the more moderate

the grades, both unfavorable, the faster the average travel speed”, adding that maximum

values for the haul-road grade ranging in “±8% as being conductive to economical all-

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weather operation” (Church 15-5). The improvement of haul road conditions can also make

the rolling resistance to decrease (Peurifoy, 315). “While close attention to haul roads is

characteristic of large works” it tends to be neglected in “planning for small rock-earth

excavations” (Church, 15-20).

In the second part of this project, a relatively small volume was considered to be

filled using a fleet of scrapers and a push tractor. In this case, the difference between

the two alternatives (either four or five scrapers) becomes more noticeable due to

the scale of this work. Though with both number of scrapers the number of days to

accomplish the task turn out to be the same, it is demonstrated than using 5

machines, we are likely to have a 0.82-day float time. Using 4 scrapers, the balanced

number of days is almost exactly 8 days, without leaving much float time. Since no

more tasks are mentioned to be completed by the fleet of scrapers, it is more

recommendable to use only 4 scrapers. Selecting this alternative would generate

savings up to one hundred dollars, and only for this second case, the cost related to

improve haul roads may not be justified based on the size of the earthwork.

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Works Cited

Church, Horace. Excavation Handbook. New York: McGraw-Hill, 1981. Print.

Muri, Wayne et al. Guide to Earthwork Construction. Washington: Transportation

Research Board National Research Council, 1990. Print.

Peurifoy, Robert et al. Construction Planning, Equipment and Methods. 7th. ed. New

York: McGraw-Hill, 2006. Print

Wright, Paul. Highway Engineering. 6th. ed. New York: John Wiley & Sons, 1996.

Print.

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