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A Decision Support System for
Excavation Equipment Selection
Francisco Eduardo Contente Calhau
Extended Abstract
Mestrado Integrado em Engenharia Civil
(Integrated Master in Civil Engineering)
Supervisor:
Prof. Dr. Fernando António Baptista Branco
April 2013
The reason for writing the dissertation "A Decision Support System for Excavation Equipment
Selection" arose from the need to associate the unit cost for an excavation operation with the
equipment involved in it.
This paper presents the modeling of a simple and straightforward method for calculating hourly rates
for hydraulic excavators, using Uni-Variable Exponential Regression (UVER) and Multi-Variable
Linear Regression (MVLR), as well as the software EXCselector designed to calculate hourly/unit
costs and productivity for excavation operations, which gathers information on the operating
conditions, the volume involved, the UVER and MLVR methods, and the equipment, materials and
costs database.
The objectives of this study are to define standardized criteria for the characterization and selection of
excavation equipment (rotary excavators); to analyze excavation materials; to define output
parameters; to apply a deterministic model for calculation of production and costs; and to collect
information about equipments provided by brands representatives, combining their features, prices
and services.
Therefore, this research provides a useful tool for decision making support for the selection of
excavation equipment, costs and productivity calculation, which will be capable of meeting the market
needs.
Abstract
1
The thesis developed is part of the curriculum of the Master Degree in Civil Engineering; post
Bologna, taught at Instituto Superior Técnico.
The reason for the dissertation "A Decision Support System for Excavation Equipment Selection "
arose after studying some subjects during the course, such as: planning, economics, organization and
management in the construction business and the difficulty of linking the unit costs for an excavation
operation with the equipment involved in it.
This paper has the following aims:
to define standard criteria for the characterization and selection of excavation equipment;
to apply a deterministic model for the calculation of production and costs;
to collect information about equipments on the market, combining their features, prices and
services provided by brands representatives and model a simple and straightforward method
for calculating hourly rates;
to concentrate production data, materials and costs in a software designed to calculate
excavation hourly/unit rates.
2.1. ISO 6165:2006
ISO 6165:2006 “Earth-moving machinery - Basic types - Identification and terms and definitions” gives
terms and definitions and an identification structure for classifying earth-moving machinery designed
to perform the following operations:
“excavation;
loading;
transportaion;
and drilling, spreading, compacting or trenching of earth and other materials, for example,
during work on roads and dams, and building sites”.
This standard divides the machines into groups according to their function and design configurations:
“dozer;
loader;
backhoe loader;
excavator;
trencher;
dumper;
1. INTRODUCTION
2. EXCAVATION EQUIPMENTS
2
scraper;
grader;
landfill compactor;
roller;
pipelayer;
rotating pipelayer;
and horizontal directional drill”.
This study examines the group of excavators as defined in section 4.4 of the standard as: “self-
propelled machine on crawlers, wheels or legs, having an upper structure capable of a 360º swing
with mounted equipment and which is primarily designed for excavating with a bucket, without
movement of the undercarriage during the work cycle”. This point adds two notes:
NOTE 1: “An excavator work cycle normally comprises excavating, elevating and discharging
of material ”;
NOTE 2: “An excavator can also be used for objects or material handing/transportation”.
2.2. Excavator: Terminology and commercial specifications
Commercial specifications, terminology and normative references established for hydraulic
excavators are defined in ISO 7135:2009 "Earth-moving machinery - Hydraulic excavators -
Terminology and commercial specifications." Sometimes the data provided by equipment suppliers
are limited, insufficient and may not be in complete agreement with the normative references. In
general, the best characterization of excavators is their operating mass, engine power and the bucket
capacity.
However, the catalogs contain transport dimensions, dimensions of reach, lifting capacities, and may
include: contact areas and pressures, traction, noise, tool forces, capacity of the hydraulic system,
among others.
Operating mass:
The operating mass of the equipment, accessories and components, and the methods for their
determination are defined in ISO 6016:2008 “Earth-moving machinery - Methods of measuring the
masses of whole machines, their equipment and components”.
Operating mass: “mass of the base machine, with equipment and empty attachment in the most usual
configuration as specified by the manufacturer, and with the operator (75 kg), full tank and all fluid
system at the levels specified by the manufacturer”.
3
Engine:
The function of excavator motors is not to provide motive power directly to the equipment, but rather
to provide power to the hydraulic system. The diesel engine system designed to operate continuously
over long periods can be distinguished by the number of cylinders, displacement (cm3), and power
(Watt). Net power is expressed in kW and measured as specified in ISO 9249:2007 "Earth-moving
machinery - Engine test code - Net power".
Bucket capacity:
The bucket capacity or nominal capacity (QN) refers to the volume of material which may be contained
in a backhoe bucket. ISO 7451:2007 "Earth-moving machinery - Volumetric ratings for hoe-type and
grab-type buckets of hydraulic excavators and backhoe loaders" establishes a method for the
calculation of QN. The volume assessments are based on the internal dimensions of the bucket and
on the representative volumes at the top of it, heaped capacity of 1:1, regardless of the type of the
excavated material (see Figure 1).
Figure 1: Bucket capacity. (1)
This chapter analyzes briefly excavation materials. According to Ricardo and Catalani (2)
: “the need to
classify excavation materials, comes from the simple fact that the toughest, are more difficult to
disassemble, demand a greater number of hours of equipment or require a more intensive use ,
generating obviously higher cost of digging”.
Greco (3)
states that the factors influencing the excavation of a soil are the moisture content, voids and
size and shape of the particles, taking also into account properties such as specific gravity and swell
factor.
For this study it is necessary to recall the basic concepts of soil mechanics, testing, ratings and
expeditious methods of analysis, eg:
genesis and geomorphology;
size and shape of the particles;
3. CHARACTERIZATION AND CLASSIFICATION OF
EXCAVATION MATERIALS
4
Atterberg limits;
soil classification;
rocks;
seismic refraction;
swell factor.
The productivity of excavation (PE) sets the volume of land which an excavator moves on average,
within a certain time under certain conditions. It depends on the cycle time (tCiclo), the overall efficiency
of the work (EG), and the available bucket capacity (Qu). The productivity is usually expressed in m3/h
and it can be obtained as follows: (2) (4) (5)
(1)
(1)
tCiclo is expressed in seconds and Qu in m3. Considering the ratio "tCiclo /operating mass" it is possible
to make an extrapolation for machines with an unknown tCiclo to a rotation between 60 and 90º.
(2)
EG considers the hourly efficiency (EH), mechanical efficiency (EM) and operator efficiency (EO), and
may also include meteorological factors, slopes and other restrictive conditions:
(3)
QU is expressed in volume of loose excavated soil. This depends on the nominal capacity of the
bucket (QN) and the type of material, and it is described by the following expression:
(4)
FEB is the fill factor of the bucket according to SAE J296. This corresponds to the ratio of the actual
volume contained in the bucket and QN.
5. COSTS
Any equipment has operating fixed costs and variable costs. The former are associated with
equipment availability and can be accounted directly or indirectly in relation to work performance.
Direct costs are calculated for each work and by the time of actual use and it may be so designated
as property costs (CPRP).
Indirect costs are allocated to the work, construction site costs, regardless of the type and duration of
the work. Moreover, variable costs or operating costs (COP) just depend on the work done. (6)
4. PARAMETERS OF PRODUCTION
5
CPRP can be further divided into two types, direct (CdPRP) and indirect (C
iPRP). C
dPRP occurs by
immobilization of capital invested in equipment and is calculated according to its depreciation and
amortization. CiPRP expresses indirect charges, inherent to the acquisition of property and equipment,
such as interest rates, insurance and taxes.
COP results directly from the use of work equipment. This includes: cost of supplies, fuel and
lubricants, wear material, maintenance and repair costs.
The cost of the operator (CMan) is also included in the hourly rate of the excavation equipment
because the operator is often associated only with the task of running the equipment, not performing
other tasks at work.
The total hourly cost (CHT) of the excavations is:
(5)
5.1. Cost Estimates
This paper presents a simple and straightforward method for calculating hourly rates, according to
commercial specifications.
Table 1: Sample data.
Operating mass kg
Engine power kW
Bucket capacity m
3
CPRP
€/h COP €/h
Máx. 38686 236,0 1,49 27,54 53,79
Mín. 16500 86,0 0,52 9,14 21,54
Med. 24992 129,8 0,97 15,59 31,73
Desv. P. 6169 37,7 0,27 4,95 8,85
Sayadi et al (7)
presents two models to estimate costs: “these models estimate the capital and
operating cost using uni-variable exponential regression (UVER) as well as multi-variable linear
regression (MVLR)”. This approach considers as independent variables the operating mass (kg),
engine power (kW) and bucket capacity (m3).
The present study analyzes a sample of 24 machines (see Table 1) from 7 different manufacturers
and sold in Portugal. To this end, the current values of CPRP and COP are calculated. At some points,
cost data were provided directly by representatives of brands.
In Table 2 we can see the correlation values between the variables and schedule costs.
6
Table 2: Correlation values.
Operating mass kg
Engine power kW
Bucket capacity m
3
CPRP
€/h COP €/h
kg 100% 94% 93% 84% 97%
kW 100% 89% 74% 97%
m3 100% 66% 87%
CPRP 100% 86%
COP 100%
UVER:
In Table 2 the variable that shows a significant correlation over time in relation to costs, is the variable
operating mass. This result can also be observed in the graph bellow (Figure 2) and in the equations
resulting from UVER:
Operating mass: UVER (kg)
(6)
(7)
Figure 2: Operating mass Vs Hourly costs.
Engine power: UVER (kW)
(8)
(9)
Bucket capacity: UVER (m3)
(10)
7
(11)
MVLR:
Table 3 summarizes the coefficients of determination for the MVLR model, applied to the calculation
of the hourly costs.
Table 3: Coefficients of determination MVLR.
Estatística de regressão CPRP COP
R2
0,84626113 0,980917422
Standard error 2,081590998 1,163456322
Observations 24 24
In Tables 4 and 5, MVLR summaries can be observed, relating to CPRP and COP. For values of
"Student's t", it is possible to evaluate the significance of the regression coefficients, concluding that
the variable "operating mass" is the most effective in calculating costs. This result is consistent with
the remarks made by the UVER analysis method.
With the results obtained by MVLR an estimate of CPRP and POC can be made, using equations (12)
and (13).
(12)
(13)
Table 4: Regression summary MVLR for CPRP.
Coefficients Standard error Student's t P-value
Interception -2,840995989 1,874992858 -1,515203633 0,145366468
(kg) 0,001651744 0,000261976 6,304943448 3,73065E-06
(kW) -0,053925594 0,034175905 -1,577883395 0,130278905
(m3) -16,37156722 4,454229778 -3,675510254 0,00150037
Table 5: Regression summary MVLR for COP.
Coefficients Standard error Student's t P-value
Interception 1,956654109 1,047983152 1,86706638 0,076620961
(kg) 0,000956996 0,000146425 6,535727038 2,27559E-06
(kW) 0,109530002 0,019101818 5,734009162 1,30414E-05
(m3) -8,633463695 2,489586956 -3,467829744 0,002429395
8
Analysis of results:
According to Sayadi et al (7)
the performance of the models, the Mean Absolute Error Rates (MAER)
of different functions are calculated as follows:
(14)
MAER values obtained from the UVER and MVLR models are shown in Table 6.
Table 6: The MAER obtained from the UVER and MVLR.
UVER (kg) UVER (kW) UVER (m3) MVLR
CPRP 12,73% 17,25% 18,44% 9,23%
COP 7,68% 5,60% 8,37% 2,65%
As shown in Table 6, the results of MAER are smaller in the MVLR method, in relation to COP and
CPRP. These results confirm the MVLR method as the one that best applies to the estimation of hourly
costs.
6. SELECTION CRITERIA
To ensure that an excavator meets the expectations of a particular application, one must know its
productive capacity, costs and necessary requirement to perform the task. Choosing a machine only
on the criteria of productivity and costs does not guarantee that it is the best option. For example,
certain demanding operating conditions may lead to a high wear and fatigue, increasing the likelihood
of failure and consequent higher costs. If so, a machine with different specifications, a superior
operating mass class and more suitable characteristics can be the right choice. Another option is to
adopt more appropriate and less demanding excavation techniques, which although less productive,
ensures a better long-term performance. (8)
Besides the identified criteria, such as those that best describe an excavator: operating mass, engine
power and bucket capacity; it must also be taken into account the specifications of the digging tool,
combining the boom, arm and bucket and the application of these criteria to the design teams
(excavator(s) and equipment(s) of transport).
7. SOFTWARE EXCselector
One of the aims of this work is to concentrate production, materials and costs data in software
designed to calculate hourly/unit costs excavation. This pursuit of a computerized process that
facilitates the calculation of excavation costs and assists in the selection and comparison of
excavators, responds to the present reality in Civil Engineering.
9
Microsft ™ Excel ™ provided a grid interface for the development of software EXCselector and the
treatment of data: equipment (brand operating mass, engine power and bucket capacity); excavation
materials (soil class, material, condition, density, blistering and fill factor) and costs (COP, CPRP e CHT).
Phyton™ programming language was also used for the development of a software tool able to
operate in Windows™ environment (64 bits). This software gathers information on the operating
conditions, the volumes involved, the UVER and MLVR methods, and equipment, materials and costs
databases. Thus it is possible to calculate and provide the user with information on operating
conditions, productivity, costs, tCiclo, efficiency, volumes and times (see Figure 3).
Figure 3: Fluxograma do EXCselector.
8. CONCLUSION
Throughout the development of this work it became clear that the establishment of a Decision Support
System for Excavation Equipment Selection has to go through future monitoring and adjustment of
the adopted model. The constant changes and evolution of equipment, markets and regulations
confer any static model an immediate and ephemeral character.
Another question that can be raised is the choice of a deterministic model. This type of model does
not allow evaluating the variability of times, efficiencies, productivity and costs throughout the
excavation process. The optimization approaches suggest the best configuration of the modeled
10
system, but do not necessarily provide the optimal solution. However, the results are essentially
informative and will be useful as a resource to the decision making.
The results of this research provide a useful tool for decision making support for the selection of
excavation equipment, costing and productivity, able to meet the needs of a market consultation.
In the future it may be useful to develop this methodology, characterizing not only one type of
equipment (excavators), but also the different equipments that may be involved in tasks of
earthmoving, developing EXCselector as a computer joint application tool to various types of
equipment.
1. Caterpillar. Manual de Produção Caterpillar, Edição 37. Peoria, E.U.A. : Caterpillar INC, 2007.
2. Ricardo, Hélio De Souza e Catalani, Guilherme. MANUAL PRÁTICO DE ESCAVAÇÃO -
TERRAPLENAGEM E ESCAVAÇÃO DE ROCHA. 3ª Edição. São Paulo : PINI, 2007.
3. Greco, Jisele Aparecida Santanna. Terraplanagem (Notas de aulas). Belo Horizonte, Minas
Gerais : Departamento de Engenharia de Transportes e Geotecnia, UFMG, 2012.
4. Caterpillar. CATERPILLAR PERFORMANCE HANDBOOK 42. Peoria, Illinois, U.S.A. : Caterpillar
Inc., 2012.
5. Komatsu. SPECIFICATIONS & APPLICATION HANDBOOK, Edition 30. Japan : s.n., 2009.
6. Faria, José Amorim. Gestão de obras e segurança - 5 - Equipamentos de construção civil -
Versão 8. Porto : FEUP, Março de 2008.
7. ESTIMATING CAPITAL AND OPERATIONAL COSTS OF BACKHOE SHOVELS. Sayadi, Ahmad
Reza, et al., et al. s.l. : Taylor & Francis, 2012, JOURNAL OF CIVIL ENGINEERING AND
MANAGEMENT, Vols. 18(3): 378–385. 1822-3605.
8. Volvo Construction Equipment. Volvo Excavator, Performance Manual. Konz : s.n., 2008.
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