ISO 6165:2006 “Earth-moving machinery - Basic types - Identification and terms
and ... excavators are defined in ISO 7135:2009 "Earth-moving machinery ...
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
Abstract 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.
1. INTRODUCTION 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. EXCAVATION EQUIPMENTS 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
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: “selfpropelled 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”.
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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 3
over long periods can be distinguished by the number of cylinders, displacement (cm ), 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.
3. CHARACTERIZATION
AND
(1)
CLASSIFICATION
OF
EXCAVATION MATERIALS 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
Atterberg limits;
soil classification;
rocks;
seismic refraction;
swell factor.
4. PARAMETERS OF PRODUCTION 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 3
of the work (EG), and the available bucket capacity (Qu). The productivity is usually expressed in m /h and it can be obtained as follows:
(2) (4) (5) (1)
(1) 3
tCiclo is expressed in seconds and Qu in m . 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.
4
(6)
CPRP can be further divided into two types, direct (C
d PRP)
i
and indirect (C PRP). C
d PRP
occurs by
immobilization of capital invested in equipment and is calculated according to its depreciation and i
amortization. C PRP 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.
Máx.
Operating mass kg 38686
Engine power kW 236,0
Bucket capacity 3 m 1,49
CPRP €/h 27,54
COP €/h 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), 3
engine power (kW) and bucket capacity (m ). 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.
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Table 2: Correlation values.
kg kW 3 m CPRP COP
Operating mass kg
Engine power kW
Bucket capacity 3 m
CPRP €/h
COP €/h
100%
94% 100%
93% 89% 100%
84% 74% 66% 100%
97% 97% 87% 86% 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)
3
Bucket capacity: UVER (m ) (10)
6
(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
0,84626113
0,980917422
Standard error
2,081590998
1,163456322
Observations
24
24
R
2
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.
Interception
Coefficients -2,840995989
Standard error 1,874992858
Student's t -1,515203633
P-value 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
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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. 3
UVER (kg)
UVER (kW)
UVER (m )
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.
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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 9
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.
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