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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