Construction site layout planning practices in inner-city building projects: space requirement variables, classification and relationship

ABSTRACT

An inner-city building construction site for vacant land development or redevelopment of existing property is a site surrounded by buildings, occupants, and busy street(s); its space is the most important but limited resource that needs wisely planned and professionally used. The study aimed to assess the construction site layout planning (CSLP) practice and the classification of space requirement variables for inner-city building sites. Thus, the study adopted a triangulated study approach and the data was collected through a sequential mixed method that includes case studies: semi-structured interviews, document evaluation, and site visits observation followed by a two-round Delphi survey. The study has identified the space requirement variables, classification, and relationship for CSLP of inner-city sites; classified under three categories: Micro-space (four), Macro-space (18), and Paths (five); and have significant to strong associations with each other. The current practice shows the CSLP is overlooked or fragmented which causes a congested site condition. So, the provided insight on the strong relationship between space requirements will be a basis for shifting from the temporary-facility-centered planning methods towards an integrated CSLP approach to overcome contractors’ problems in the continued redevelopment demand of the inner city.

KEYWORDS:

• Inner-city
• redevelopment
• building
• construction site layout planning
• space requirements

Introduction

Rapid and sustained urbanization as a major global concern is much more challenging in Africa today and in the future. According to the institute for security studies forecast, in 2020, approximately 488 million people lived in African cities; by 2030 this will reach 791 million, and 1.4 billion by 2050, which is 58% of the entire population (Bello-Schünemann & Aucoin, 2016). Thus, the number of African urban residents is projected to grow to 20.2 percent of the global share in 2050 from 11.3 percent scored in 2010; the Sub-Saharan region is known as the world’s fastest-urbanizing region (Saghir & Santoro, 2018). However, the rapid and sustained urbanization in Africa is a major challenge to governments’ capacity for the provision of infrastructure, creating massive backlogs in urban areas. Hence, urban centers across this region are characterized as informal settlements or slums which are growing fast and substantially exceeding the capacity of government investment in urban upgrading (Bello-Schünemann & Aucoin, 2016; HABITAT, 2014; Saghir & Santoro, 2018).

To properly manage urbanization, cities and municipalities have to first contain the redevelopment of their inner city. So, African capital cities like Lusaka are enforcing the governments and private sectors to invest in inner-city redevelopment construction projects. Besides, planners and governments are giving priority to the redevelopment of inner cities over new developments at the periphery to gain the advantages of the new urban agenda: building considerably higher densities for economic, environmental, and social values. Thus, urban redevelopment is now common agenda in Africa for the process of renewing cities or sub-cities (Chiwele et al., 2022; El-Anwar et al., 2014; Habitat, 2020).

Inner-city building construction

Urban redevelopment includes projects for developing pocket lands, clearing slums, and upgrading and revitalizing the sub-city with building construction projects and infrastructures. Therefore, building construction projects in inner-city sites are mainly the result of urban redevelopment plan projects and the general urban plan. These projects need to apply urban redevelopment techniques for design, construction, and operation by adapting ways to meet the many and changing needs of urban residents (Saghir & Santoro, 2018; Towers, 2005; Tsenkova, 2002). Hence, the new urban agenda promotes the implementation of a sustainable urban development program that positions ‘housing’ at the center of the planning approaches, and governments have reaffirmed their commitment to achieving ‘the right to adequate housing for citizens’ (Habitat, 2020).

Inner-city building construction projects like other construction projects may be categorized by their level of complexity, site condition, technology, and project delivery method used. They are characterized based on types of buildings, implementing bodies, and relationship to the city plan, but shared common features among each type of such building project. These features are described differently by different studies but commonly stated are: limited space, site access limitation, materials storage area availability limited, disorderly and extremely dense occupation, difficulties in logistics/traffic jams, the presence of physical obstacles, noise levels difficult to control, and parking shortage (Anwar & Aziz, 2011; El-Anwar et al., 2014; Lambeck & Eschemuller, 2009; Radziszewska-Zielina & Kania, 2017; Spillane et al., 2013). These features are the major problems for inner-city building construction caused by the unique urban environment challenges: the presence of populated workforce, high-rise buildings, narrow sidewalks and streets, mass transportation systems, public utility lines, local building regulation restrictions, neighboring occupants at public and residential buildings, and union requirements (Lambeck & Eschemuller, 2009).

Therefore, inner-city redevelopment building projects can be classified based on the calling purposes, implementing bodies, the relationship to city planning, building types, and others as shown in Table 1. The common features mentioned above are all site space-related challenges for the construction operation in such locations. Table 1 was used as a checklist for selecting the case study and data collection process for this study.

Table 1. Inner-city building construction projects types Vs category of urban redevelopment. After (El-Anwar et al., 2014; Habitat, 2020; Uzon, 2005; Özdemirli, 2014).

Basics	Types of urban redevelopment project types
Reason for redevelopment		Informal settlement	Slums	Rezoning/land-use change	None-worthing to the optimum value for owner/communities
Relationship with City plan		Regularization plan	Upgrading Plan	Included in the Regular city plan	Out-of-city plan requirements/based on optimum valuing
Implementing bodies	Local government agencies	Housing Programs	Housing Programs
	Landowners	Standardizing	standardizing	Changing the requirements	High-value building type
	Land lease right holders	>>	>>	>>	>>
	Real-estate developers	Development	Development	>>	>>
	Redevelopment Association	Standardizing	standardizing	>>	>>
Types of Building	Single-family house	✓	✓	✓	✓
	Apartment	✓	✓	✓	✓
	Mixed Use building	✓	✓	✓	✓
	Public services	✓	✓	✓	✓
	Hotels	✓	✓	✓	✓
	Malls/markets	✓	✓	✓	✓
	Neighborhood development	✓	✓	✓	✓

Urban redevelopment construction project (URCP) is one of the emerging potentials for development projects in the construction industry located in the inner city which has overcrowded road access and a congested area. Hence, besides the project performance fulfillment requirements, contractors for such locations are always demanded to manage the potential disputes with the surrounding community and fulfill the municipality requirements (Spillane et al., 2013). Consequently, if an efficient and effective construction site management practice is not implemented, it negatively affects both the surrounding community and the project performance; ‘efficiency is defined as the ability to accomplish tasks with the possible minimum waste (time, money, and effort)’; whereas, ‘effectiveness is the degree to which the work is successful’ in achieving the project objective, that both are important tools for cities’ development, and performance improvement for building construction projects (Kliem, 2014; Pooya & Bizhan, 2015).

Construction site layout planning practices

Space in urban construction sites is among the most important but limited resource that needs to be planned wisely and professionally used. However, it has been overlooked for so long and caused congested working conditions that result in challenges for inner-city construction (Osman et al., 2003). The inner-city building construction site is either vacant land ready for new development or redevelopment of an existing property that is located in an area neighbored on its sides by public and/or residential buildings with occupants and a high-traffic road(s). Hence, construction works at such sites have to apply an efficient and effective site management approach by developing a site management plan to get resources and execute the work with insignificant effect on the existing neighborhood, traffic flow, and public utilities by assuring safety within the construction site and surrounding (Lambeck & Eschemuller, 2009). Besides, even at an inner-city site having less to moderate built area-to-site area ratio, the location of temporary facilities within the site influences the productivity of workflow and site safety. Therefore, site layout planning for inner-city building projects is a complete class problem in which lack of control during construction and inferior planning in any condition of the site can cause material losses and extra costs, reduction of productivity, and safety hazards to workers at site and surrounding community’s (Singh et al., 2019). So, to have optimum usage of the limited available space, optimized site layout planning is always in demand for inner-city building construction projects (Elbeltagi & Hegazy, 2001).

Even though project safety requirement is not a point of discussion in construction contracts of developing countries (Mwanaumo et al., 2014), it affects the CSLP as the space utilization, supervision, accessibility, geometry, and topography of the site affect the Construction site layout planning. In many studies, the temporary facilities (TFs) are considered the major components of the CSLP including laydown areas, fabrication shops, maintenance shops, batch plants, and residence facilities; their requirement varies from project to project based on different factors such as project type, scope, design, location, and organization of construction work (Ghanim, 2020; Moradi & Shadrokh, 2019). So, according to Ning et al. (2011), the CSLP as a decision-making process needs to involve sub-processes: identifying problems and opportunities, developing solutions, and choosing and applying the best alternative. However, the current practice is far away from this, and most studies usually focus only on the three sub-processes: identifying and selecting TFs needed for the project, size and shape determining, and (optimally)allocating them within the available free space areas for a specified duration within the site boundaries. Some studies also tried to integrate with the construction project logistics or material distribution plan (Moradi & Shadrokh, 2019; Ning et al., 2011; Oral et al., 2018; Papadaki & Chassiakos, 2016; Salih et al., 2019; Thomas & Ellis, 2017).

Construction activities, and associated temporary facilities, require work and storage space on construction sites and inside constructed facilities (building). Moreover, multiple activities of a project occur at the same time demanding complex networks of material handling paths, storage spaces, and space to execute tasks. As a result, space becomes a scarce resource, especially in inner-city project sites. So, a construction project manager has to plan and allocate spaces for each construction trade (crew) to perform work efficiently and to meet effective project goals (Akinci et al., 2002; Riley, 1994). Accordingly, CSLP is an approach needed for efficient site space utilization based on the space requirements to ensure the construction workflow is efficient and ahead of schedule (Chavada et al., 2012; Ghanim, 2020; Misron et al., 2019; Sutt et al., 2013).

Inferior CSLP and lack of its control during construction can result in site congestion that leads to material losses and extra costs, reduction of productivity, and/or increased safety hazards to workers at the site and surrounding community (Singh et al., 2019). But optimized construction site layout plan enhances and ensures productivity that reduces costs, minimizes materials relocation at the site, and assures a secure and accessible construction site and safe work environment. Besides, high-rise building construction projects in inner-city sites need a dynamic and optimized construction site layout plan recommended to be developed as a staged plan for the different stages of construction (Elbeltagi & Hegazy, 2001; Sutt et al., 2013).

Works of literature on the CSLP methods have over the years changed from experience (expert judgment) to modern scientific planning techniques leading to the current stage of an optimum plan. Hence, different studies tried to formulate an optimization of site operation-related objective function(s) (cost, productivity, safety, environment, and stakeholder relationship) and have developed CSLP methods and tools: a fuzzy-based multi-objective (Soltani & Fernando, 2004), BIM-based automated site layout planning (Kumar & Cheng, 2015; Le et al., 2019), dynamic CSLP using harmony search algorithm and integrating genetic algorithm (Farmakis & Chassiakos, 2017; Lee et al., 2016; RazaviAlavi & AbouRizk, 2017), and multi-stakeholders conflict minimization using dynamic hybrid bacterial and ant colony algorithm (Son, 2021) are some from the many.

However, previous studies in this area have little coverage on construction site space requirements, classification, and relationship in particular to inner-city sites. The current CSLP methods are mainly used to allocate the space requirements of TFs to optimize their objective function(s) by minimizing travel distance between TFs. Hence, the parties involved in a construction project, especially the contractors, do not seem to get the attention of CSLP methods and are not considered important to this matter. Contractors prefer to use the conventional methods for site layout which are based on expert judgment: by simply planning with initial planning knowledge and experience about site layout, and using two-dimensional drawings as mediums for project communication among stakeholders; but mostly used during the project mobilization and remained un-updated for the entire project life (Singh & Kumar, 2018).

Space requirements for inner-city building projects

The CSPL is getting attention from researchers and starting from the last few decades different studies have introduced and come up with solutions and few have been implemented in the developed world. However, there are a lot of problems unsolved especially in developing countries like Zambia CSLP is not addressed at all. According to Zolfagarian and Irizarry (2014), among the eight areas of current CSLP practice, the movement and the close relationship of resources and facilities respectively within the site, the use of few variables during the decision, and unavailability of predefined rules and regulations for CSLP are the space-related issues that lead to inefficient construction site layout that are causing site congestion and safety hazards (Zolfagarian & Irizarry, 2014).

In urban construction, site boundaries constraints, physical overlaps constraints between TFs locations, max/min distance constraints among TFs; zone constraints for TFs placement; and construction activities zone and other specific urban constraints are some of the space requirement variables (Abdel-Raheem & Khalafallah, 2012; Zolfagarian & Irizarry, 2014). However, the space planning for a construction site practice is fragmented into two areas of planning: space schedule which deals with activity execution workspace planning by generating, allocating, and conflict detection and resolution for workspaces, and site layout planning which mainly focuses on positioning TFs in the unoccupied areas on the specified duration within the site boundaries (Igwe et al., 2020; Papadaki & Chassiakos, 2016).

The space requirements of the construction site include space for the construction process such as areas for a prefab, unloading, storage, work(crew), debris, layout, tools/equipment, material and personnel paths, protected space, hazard area, and construction product space (building components) (Igwe et al., 2020). Similarly, Chavada et al. (2012) have narrowly categorized the space requirement of activities on construction sites as a working area for the crew, material storage, equipment area, and area for support infrastructures. Moreover, Akinci et al. (2002) formerly extracted and classified the 13 commonly known types of space requirements during construction operations into three categories as Macro-level for spaces required with large-scale spaces across the site such as storage, staging, layout, unloading, prefabrication areas; Micro-level for spaces required near to the permanent components of the building for the crew, equipment, hazard, protected area, and building component itself; and Paths required to be left clear for material, debris, and crew to transport called as the Material path, Personnel path, and debris path (Akinci et al., 2002; Chavada et al., 2012).

Salih et al. (2019) in their study about ‘performance evaluation of construction project in Erbil city’ which is typically an inner-city site have identified 12 types of space requirement elements: laydown area, storage, material handling (equipment and tools), utilities (water supply and sanitary), office, accommodation, security (entrance & fencing), display for safety rule and labor relations policy, Site accessibility (road and footpath), safety (fire prevention, first aid kit, personal protective equipment), warehouses, Material staging, Craft change-house, Batch plant, and workshop. Similarly, Kaveh et al. (2016) in their urban site case study have also identified the facilities which occupy the site space as the main gate, side gate, batching plant, steel storage yard, formwork, storage yard, bending yard, cement, sand, and aggregate storage yard, curing yard, refuse dumping area, casting yard, and lifting yard. Hence, the space requirements for an inner-city site are grouped into Jobsite access, Material handling, worker transportation, temporary facilities, and Jobsite security; the planning process mainly focuses on identifying the optimum size, type, and location at sites during different stages of building construction (Kaveh et al., 2016; Kumar & Cheng, 2015; Salih et al., 2019; Whitman, 2014).

The above reviews have shown that construction site space requirements are multi-dimensional including micro space, macro space, and paths-related requirements; the practice of construction site space planning is fragmented; the CSLP-related literature only focuses on the allocating of TFs in the provided site space. Thus, CSLP methods developed previously, try to optimize their objective function(s) by minimizing the travel distance between TFs. However, the actual travel distance of a crew in a construction site is not only from TFs to TFs; rather, it is a travel cycle from the working station to TFs.

Therefore, this study aimed to test if CSLP can be used for planning all the space requirements of inner-city building construction projects by identifying and classifying all space requirements and their relationship.

Research methodology

The choice of a methodology for this study is a triangulated study approach which is the combination of quantitative and qualitative approaches with a number of data collection techniques. After an extensive review of the literature, the literature on CSLP practices in inner-city building sites showed a gap. Hence, this research adopted sequential methods of data collection to contextualize, include, and verify the preliminary variables extracted from the literature review by practitioners of inner-city building construction. These data collection processes are summarized in the following sub-headings.

Case study data collection

To identify the practices and space requirements of the construction site layout planning on

inner-city building construction site, a case study means of data collection that includes expert semi-structured interviews, evaluation of related documents, and site visit observation on selected sites were considered and implemented. The data collection was primarily based on the variables extracted from the literature review. To acquire on-site information from construction practitioners and site observation, building construction site data were collected from the registered projects database in Lusaka city. After careful site visits and consultation with industry experts, the number of case study sites selected from existing building projects registered at the Nation council for construction (NCC) and Lusaka city council (LCC) was narrowed down to two. A selection process has been established that permitted comparisons and deducing differences between the projects chosen concerning the scope of the study.

Two sites were selected that are located in Lusaka’s central business district: planned for urban redevelopment and densification, and both are high-rise buildings of reinforced concrete but have different site-space conditions. Project one is a medium scale with G + 4 and one basement floor of a mixed-use building. It is located on the side of one of the busiest roads in the city that has no parking permission. The local road on the other side of the project is narrow and the only parking and logistic access to the site. The other two sides of project one share a line of property with neighboring flat houses with residents. Project two is a large-scale high-rise building with 20-plus floors located on the side of one of the busiest roads in the city. Unlike project one, the construction site is not directly connected with neighboring houses and streets and has a large space but with numerous facilities for the project.

Therefore, the case studies involved two possible inner-city building construction sites that were selected based on their location in the city master plan for redevelopment and/or densification area (JICA, 2009), available space, scope, and the overall complexity of the project. For this study, the two sites have been identified as exhibiting the construction site layout planning scenarios for the shortage of space due to the high rate of the built area-to-site ratio and project scope requirements for large-scale facilities, or both.

The interviewees for the semi-structured interview (SSI) were selected based on the criteria established: practitioners who currently or previously work on inner city building construction projects like real estate, redevelopment, rezoning, and refurbishment projects, and some sector scholars. Site engineers, a health and safety officer, project managers, resident engineers, and project engineer from the two sites, a chief engineer, building inspector, and quantity surveyor from LCC as a regulatory body, and from NCC: senior Engineer and Architect as a regulatory body and the construction school representatives have participated. Therefore, 12 industry practitioners with working experience ranging from 7 to 35 years had been included in the SSI; having accumulated work in inner-city building projects and others working with the regulatory bodies for urban construction. The accumulation of data from experienced personnel firsthand was achieved through in-person interviews with each participant; it also provided the interviewer to extract and understand the perspectives of knowledgeable individuals about the construction site layout. Interviewees were nominated by both selecting and convincing individuals and companies to ensure that the individuals have knowledge and experience and are willing to be interviewed and share their opinions.

To complement the literature survey variables by including, contextualizing, and verifying the classification and lists of space requirements of construction site layout planning to inclusively represent the practices for building construction projects in the inner-city, 12 expert SSIs were undertaken. Thus, it explored and added new space requirement variables not identified, or miss classified during the literature review. This helped to understand the current CSLP practices and level of awareness among professionals.

Besides, the semi-structured interview, construction documents for each site in the case study: schedule, site establishment plan, resource, and logistic plan, and method statement were evaluated and used as a guide for the site visits observation. New variables have been extracted regarding the practical space requirements for the building construction sites by observing and recording the available space and current way of utilization. The variables for space requirements extracted from the site observation and documents were included and prepared for comment by the experts. Thus, the SSIs responses are summarized by adding, contextualizing, and removing variables that are rejected by most of the interviewees and clustered into three groups which will be tested and developed by the next stage of the sequential method of data collection using the Delphi survey. Table 2 shows the summarized space requirement variables and classifications from the case study.

Table 2. Space requirement variables and classification for inner-city building construction projects.

S/No.	Lists of space requirements in building construction sites	S/No.	Lists of space requirements in building construction sites
A	Micro-level spaces (spaces required close to the building)	B	Macro level space (area for temporary facilities placement)
1	Construction activities working area for crew	5	Temporary office
2	Layout areas (bending yards, prefabrication areas)	6	Storage facilities for scaffolding and form works
3	Protected space	7	Storage facilities for Cement warehouse
4	safety zone and hazard area	8	Rebar storage yard
C	Paths (space required to be left clear for transporting)	9	Storage facilities for MEP
26	Worker personnel path	10	Tile storages
27	Material and debris path	11	storage for panels
28	Site accessibility (road and footpath)	12	Tools storage
29	Traffic control	13	stockpile (sand and aggregate storage yard)
30	Weighbridge	14	Tower crane/hoist area
31	Main gate and side gate	15	Batching plant area/Concrete mixer
		16	Utility facilities area (sanitary)
		17	Utility area (water tank)
		18	Utility area for generator
		19	Change room (Lockers)
		20	Jobsite security house
		21	Debris area (waste stockpile)
		22	Unloading area
		23	Temporary material staging area (for placing from a crane)
		24	Material Distribution area next to a workstation
		25	Equipment area

Delphi questionnaire survey

The catalog of the variable identified for space requirements reviewed by the case study is included in a Delphi questionnaire survey. By assuring participants are well versed and experienced and providing sufficient diversity with the Minimum number (10 required responses), and the willingness of the experts, the Delphi survey members were selected from the target population that encompasses: Professionals having inner-city building construction projects exposure, representing the major stakeholders (Contractors, Consultants, Regulators, and higher education institutions), and having Construction professions (Project managers, Civil Engineers, Quantity surveyors, and Architects). Therefore, to gain the agreed perspective of practitioners of the industry in the study area working in both on-site management and office works/planning with major stakeholders, a panel of experts having 15 members with 15 years of average industry duties was established; it included many of the practitioners who participated in the interview and other experts from the sector including university lecturers.

The Delphi technique applied a questionnaire survey as an instrument for data collection. This study has run two rounds of the Delphi survey, for a period of three months: starting from July to September 2022. All the questionnaires were distributed in person and online using Google Forms. A letter of introduction from the school of engineering and a letter of invitation to the experts were attached with the survey questions in round one. Similarly, the Delphi survey objective, procedure, and instruction on how to complete the survey were briefly included in both rounds. Each expert was given a code name (i.e. A, B, C … .) during the second round by trussing the first-round in-person response codes (PM1= Project manager site one, SE1=Site Engineer for site one, etc.) respectively with the help of the requested and answered email address during the two round surveys, to track the individual’s feedback at every round and to make data analysis easier (Akins et al., 2005; Okoli & Pawlowski, 2004; Sekayi & Kennedy, 2017). The detailed process for each round is presented below.

Delphi round one survey

The Delphi survey is designed, to gain comprehensive insight from the team of experts about the space requirement classification and variable lists by putting their agreement on the probability of occurrence and the level of impact of each variable on construction site layout planning; to add any variable assumed to be in the classification. Thus, the survey was circulated to each expert manually with each variable ranked using five ranks: Very low impact, low impact, moderate impact, high impact, and very high impact; never, rarely, sometimes, frequently, and always to their level of impact and probability of occurrence of the space requirements respectively. 12 expert responses were collected and analyzed using python’s pandas library for descriptive statistics: the measures of central tendency (means, median, and mode) and level of dispersion (standard deviation and inter-quartile range); these are used to summarize and send information about the collective judgments of respondents to the next round by rephrasing and including the variables with less consensus rate and new recommended variables by experts respectively. The summarized information of the collective judgments of respondents and their recommendations were organized for the next round.

Delphi round two survey

The questionnaire was circulated electronically using a Google form to 20 experts including all previous Delphi members and some other experts who work in the industry for about 15 and above years such as university lecturers. The Google form is used to improve return rates and to reach experts who are difficult to reach in person. A question was asked to rate using the Likert Scale of five from 1(Very low impact) to 5 (very high impact), and 1 (Never) to 5 (always) to their level of impact and probability of occurrence of each space requirement respectively. Hence, accumulated 13 responses from the team of experts were recorded; documenting the return rate per survey the response rate was low, but the requirement for data analysis for the Delphi survey was satisfied. Thus, the data is prepared using python for further inquiry.

Before, finding the answer to the research question, it is a requirement to develop a stopping criterion for the Delphi rounds using the concept of stability and consensus tests in the Delphi method of data collection. The stability test helped to check if the consistency of responses between the two rounds is achieved by testing the obtained responses in rounds one and two have shown statistically insignificant from each other no matter the level of consensus among the respondents to each variable they have. Following the stability test, the respondents’ consensus on the level of impact and probability of occurrence of each variable was justified for the second round of Delphi survey data.

Data analysis and results

The summarized and clustered space requirement variables from the case study developed into the Delphi questionnaire survey to a total of 31 space requirement variables classified under three categories: Macro space (21), Micro space (04), and path space (06). The questionnaires were circulated to the panel of experts using the Delphi survey in two rounds to obtain experts’ perspectives on variables classification of space requirements for inner-city building construction sites based on their consensus on the variables’ impact level, and probability of occurrence in the building construction projects in inner-city sites.

Therefore, to test the stopping criterion, stability of response is achieved by a nonparametric chi-square (X²) test by developing hypotheses. Thus, using the resulting statistic’s degrees of freedom magnitude to the product of the number of rounds(i) minus one and the number of response intervals (j) minus one and the level of significance, the critical values of x² based on level of significance(0.05), and degree of freedom= (i-1)*(j-1), in this case (2–1) (3–1) = 2, has been selected from the standard statistical table which is 5.991 and is compared with the obtained value for each variable (Akins et al., 2005; Dajani et al., 1979).

The responses in round one and round two with common respondents were collected and the Likert scale given in two rounds was pooled into three groups as low impact, moderate impact, and high impact for the impact level of each space requirement and rarely, sometimes, and frequently for the probability of occurrence in inner-city building projects. The X² test then proceeds by stating a null hypothesis (H₀) and an alternative hypothesis (H₁) statement as shown below.

A null hypothesis (H₀): The Delphi round one and two are independent of the responses obtained in them

An alternative hypothesis(H₁): The Delphi round one and two are not independently obtained in them

The observed frequencies with the resulting expected frequencies and the statistical value of X² are given in Table 3.

Table 3. Stability test for Delphi round one and two: Pooled response on the level of impact of the variables on CSLP practices for inner-city building projects.

List of space requirements	a. Observed Frequencies				b. Expected Frequencies
	Round	Less impact	Moderate impact	High impact	Less impact	Moderate impact	High impact	X^2
Construction activities working area for crew	1	0	0	12	0	0.5	11.5	0.47
	2	0	1	11	0	0.5	11.5
layout areas (bending yards, prefabrication areas)	1	0	4	8	0	3	9	0.505
	2	0	2	10	0	3	9
protected space	1	2	6	4	1	4	7	0.193
	2	0	2	10	1	4	7
Safety zone and hazard area	1	2	3	7	1	2	9	0.378
	2	0	1	11	1	2	9
Temporary office	1	0	2	10	0	1.5	10.5	0.663
	2	0	1	11	0	1.5	10.5
Storage facilities for scaffolding and formworks	1	0	3	9	0	3	9	1
	2	0	3	9	0	3	9
Storage facilities for Cement warehouse	1	0	3	9	0	2	10	0.439
	2	0	1	11	0	2	10	0.439
Rebar storage yard	1	2	4	6	1	2.5	8.5	0.268
	2	0	1	11	1	2.5	8.5
Storage facilities for MEP	1	3	6	3	1.5	4	6.5	0.112
	2	0	2	10	1.5	4	6.5
Tile storages	1	2	3	7	1	3	8	0.57
	2	0	3	9	1	3	8
Storage for panels	1	2	6	4	1.5	4.5	6	0.513
	2	1	3	8	1.5	4.5	6
Tools storage	1	0	3	9	0.5	2	9.5	0.599
	2	1	1	10	0.5	2	9.5
Stockpile (sand and aggregate storage yard)	1	0	2	10	0.5	2	9.5	0.769
	2	1	2	9	0.5	2	9.5
Tower crane/hoist area	1	0	0	12	0.5	1	10.5	0.424
	2	1	2	9	0.5	1	10.5
Batching plant area/Concrete mixer	1	1	3	8	1	2	9	0.737
	2	1	1	10	1	2	9
Utility facilities area (sanitary & water tank)	1	0	4	8	0.5	3.5	8	0.752
	2	1	3	8	0.5	3.5	8
Utility facilities area for power(generator)	1	2	3	7	1	2.5	8.5	0.505
	2	0	2	10	1	2.5	8.5
Change room (Lockers)	1	5	3	3	2.5	5	4	0.170
	2	0	7	5	2.5	5	4
Jobsite security house	1	5	4	3	2.5	5	4.5	0.202
	2	0	6	6	2.5	5	4.5
debris area (waste stockpile)	1	2	4	6	1	5	6	0.549
	2	0	6	6	1	5	6
Unloading area	1	0	3	9	0	3	9	1
	2	0	3	9	0	3	9
Temporary material staging area (for placing from a crane)	1	0	1	11	0	1.5	10.5	0.663
	2	0	2	10	0	1.5	10.5
Material Distribution area next to a workstation	1	3	3	6	1.5	4	6.5	0.409
	2	0	5	7	1.5	4	6.5
Equipment area	1	2	3	5	1	2.5	7.5	0.380
	2	0	2	10	1	2.5	7.5
Worker personnel path	1	2	3	7	1	2.5	8.5	0.505
	2	0	2	10	1	2.5	8.5
Materials and debris path	1	1	3	8	1	2.5	8.5	0.937
	2	1	2	9	1	2.5	8.5
Site accessibility (road and footpath)	1	0	3	9	0	2.5	9.5	0.722
	2	0	2	10	0	2.5	9.5
Traffic control	1	7	3	2	3.5	2.5	6	0.044
	2	0	2	10	3.5	2.5	6
Weighbridge	1	8	2	2	4	4	4	0.050
	2	0	6	6	4	4	4
Main gate, side gate	1	1	2	9	0.5	3	8.5	0.650
	2	0	4	8	0.5	3	8.5

Therefore, to accept the null hypothesis as true, and conclude the responses obtained in the two rounds are stable, the critical value must be greater than the test value for each variable. The calculated values of X² for each variable as shown in Table 3 are less than the critical value drawn from the chi-square standard table (X² = 5.991). Thus, the null hypothesis is true to each variable; there is no dependency on the rounds of the Delphi to the response of each question. So, the response of respondents is stable on the two consecutive rounds (one and two) irrespective of respondents’ agreement on each variable.

After approving the stability test of Delphi rounds its stopping criterion is satisfied, and the second stopping criterion: the agreement or convergence of the responses at the second round has been evaluated using quartile deviation (QD) and median of the responses.

The quartile deviation is calculated by:$QD=\frac{Q3-Q1}{2},whereQ3isthirdlevelquartile\text{break}\left(75\mathrm{\%}\right),Q1isfirstlevelquartile\left(25\mathrm{\%}\right)$ (Ab Latif et al., 2017; Ansah et al., 2021).

To assess consensus, the interquartile range (≥1.25 on a scale from 1–5) is an indicator for consensus measure. So, the consensus of the experts was tested based on the median, inter-quartile range, and quartile deviation of the data collected at round two Delphi survey. So, variables with quartile deviation less or equal to 0.5 (QD≤0.5) are considered respondents reached a high consensus; for QD values more than 0.5 and less than or equal to one (0.5 ≤ QD≤1.0) have a moderate consensus; QD values with more than one (QD > 1.0) is concluded as no consensus. Similarly, the impact level and probability of occurrence are high and frequent respectively if the median values are four and above, moderate and sometimes for median≥3.5, and low if the median values are less than 3.5 (Ab Latif et al., 2017; Ansah et al., 2021; Beiderbeck et al., 2021). Mode frequency and a visual inspection of histograms were utilized to infer information about the consensus range as shown in Table 4.

Table 4. Consensus on impact level and probability of occurrence of the space requirement variables on the CSLP practice for inner-city building projects.

Category	Code	Space requirement lists	Impact Level			Probability of occurrence
			Median	Mean	QD	Median	mean	QD
Micro Space	V1	Construction activities working area for crew	5	4.31	0.5	4	4.08	0.5
	V2	layout areas (bending yards, prefabrication areas)	4	4.23	0.5	4	4.15	0.5
	V3	protected space	4	4	0.5	4	4.08	1
	V4	Safety zone and hazard area	4	4.23	0.5	4	3.85	1
Macro Space	V5	Storage facilities for Cement warehouse	5	4.54	0.5	5	4.23	1
	V6	Rebar storage yard	5	4.38	0.5	4	3.77	0
	V7	Tools storage	5	4.23	0.5	4	4	0.25
	V8	Tower crane/hoist area	5	4.23	0.5	4	4.08	0.25
	V9	stockpile (sand and aggregate storage yard)	5	4.15	1	4	4.08	0.25
	V10	Utility facilities area for power(generator)	4	4	0	4	3.85	0.25
	V11	Temporary material staging area	4	4	0	4	3.92	0.25
	V12	Temporary office	4	4.23	0.5	4	4	0.25
	V13	Storage facilities for MEP	4	4.08	0.5	4	4.23	0.5
	V14	Tile storages	4	3.92	0.5	4	4.08	0.5
	V15	Batching plant area/Concrete mixer	4	4.08	0.5	4	3.54	0.5
	V16	Utility facilities area (sanitary)	4	3.77	0.5	4	3.62	0.5
	V17	Equipment area	4	4.15	0.5	4	4	0.5
	V18	Utility facilities area (water tank)	4	3.83	0.63	4	4.31	0.5
	V19	storage facilities for scaffolding and form works	4	4.08	1	4	3.77	0.5
	V20	storage for panels	4	3.85	1	4	3.69	0.5
	V21	Unloading area	4	4	1	4	3.85	0.75
	V22	Material Distribution area next to work station	4	3.92	1	4	3.77	0.75
Paths	V23	Traffic control	4	4	0	5	4.38	0.5
	V24	Worker personnel path	4	4.15	0.5	4	3.92	0
	V25	Site accessibility (road and footpath)	4	4.23	0.5	4	4.08	0.5
	V26	Main & side gates	4	3.77	0.5	4	4.23	0.5
	V27	Materials and debris path	4	4	1	4	4	1
	V28	Change room (Lockers)	3	3.46	0.5	4	3.85	0.75
	V29	Jobsite security house	3	3.62	0.5	4	4	1
	V30	debris area (waste stockpile)	3	3.69	0.5	3	3.23	0.75
	V31	Weigh-bridge	3	3.46	0.5	3	3.15	0.5

Therefore, after identifying the median value, inter-quartile range, and quartile deviations, the Delphi round decided to stop its round because the stability test shows statistical values are much less than the critical values of the chi-square as shown in Table 4, and the convergence of the responses of respondents to most variables score a high level of consensus (with QD = 0.5), and few variables have a moderate level (QD = 1) were satisfied. So, classifying items according to the consensus level on impact level, and probability of occurrence of each variable, and analyzing the coefficient of association to know the level and direction of relationship among variables is the subsequent analysis technique. As a result, the median of each variable score shows all have a high level of impact and frequent occurrence with a score greater than or equal to four (median≥4.0), except four variables scored a median of three (median = 3) for the impact level which means they have a low impact in the CSLP practice for inner-city building projects. Table 4 shows the level of respondents’ consensus on each variable statement.

Thus, each of the variables in each category is ranked based on the experts’ perceptions of each question. So, the panel of experts has agreed on the importance of most of the variables listed in the three categories of the space requirements to CSLP based on their level of impact and the probability of occurrence. Besides, to test the association of variables, a correlation analysis using the Spearman coefficient of relation was studied for the level of impact and probability of occurrence values. Accordingly, both values showed a significant correlation. Especially, the values for impact level showed a clear relationship among variables as shown in Figure 1. From the topmost prominent space requirement variables for CSLP: Construction activities working area for the crew, have a spearman coefficient greater or equal, rho (ρ) = 0.5 with most of the macro space category elements that even reach rho (ρ) = 0.8 with stockpile (sand and aggregate storage yard). This supports the consensus of the experts on the elements and classification of space requirements for construction site layout and shows their relationship among the categories as shown in Figure 1.

Figure 1. Spearman correlations among space requirements for inner-city building projects.

Discussion

The case study on the construction site layout planning practices in an urban area and the assessment of professionals’ level of awareness on the possible space requirements in the study area has shown it is overlooked; if not it only focuses on the TFs-related space requirements. The semi-structured site experts’ interviews supported by site visit observations identified most of the experts agreed that the planning practice is simply based on individual judgment to allocate project temporary facilities; as a result, they always face space shortage-related challenges. Thus, the continuous engagements with experts to extract possible space requirements for building projects with close site visit observation on the two case study sites, an inner-city site about 31 types of space requirements have been classified into three categories namely Macro-space (space needed for large space requirements mainly for temporary facilities), Micro space (space left for working area related space requirements), and Paths (space needed for access and movement at the site).

Furthermore, from the two rounds Delphi survey, the space requirement variables were analyzed to classify and rank items according to the consensus level, impact level, and probability of occurrence. Thus, the variables are ranked based on their median score shows all have a high level of impact and frequent occurrence with a score of median greater than or equal to four (median≥4.0), except four variables scored a median of three (median = 3) for the impact level which means they have a low impact in the CSLP practice for inner-city building projects. From the site visit observation, the variables with low median scores were evaluated for triangulation. So, the weighbridge as a temporary facility was available at site two but it was placed along with the site access road; this showed that it shares the same space with road access. Similarly, the change room and Jobsite security house space requirements were available in both sites and the Delphi result also shows they have low impacts but frequent occurrences, but the space utilized for them at the two sites was within the built area of the buildings. Thus, the Delphi results are also supported by the case study except for the debris area (waste stockpile) result that shows a minimum score in impact and probability of occurrence contradicts the actual site observations; this may be due to the uninformed on-site waste or there is a good waste disposal strategy from the site (Chikezirim & Mwanaumo, 2013). However, even though it needs further study, the site visit observations at the case study sites support the uninformed on-site waste; because in both sites there were spaces occupied by waste such as rebar, block, and formworks.

Generally, each of the classifications and the variable is ranked based on the experts’ perception of each question. The panel of experts agree on the importance of the three categories of space requirements for CSLP based on their level of impact and the probability of occurrence. So, the variables that score high impact and frequent occurrence represent the three categories: Macro space (18), Micro space (04), and Paths (05).

After ranking the space requirement variables under each category, the two rounds Delphi survey studied the coefficient of association to know the level and direction of the relationship among variables. Thus, based on the Spearman correlations among space requirements for inner-city building projects, variables from all categories showed a significant association with each other. Especially, from the top space requirement of the micro space category, the construction activities working area has shown a strong association of rho (ρ) = 0.8, 0.62, and 0.51 with stockpile (sand and aggregate storage yard), batching plant area/concrete mixer, and Site accessibility (road and footpath) respectively. This shows that construction site layout planning should not only focus on how to (optimally)allocate the TFs within the site; rather, it should focus on integrating and allocating site space to the micro space, macro space, and path-related requirements.

Conclusion and recommendations

A confined site condition of building construction in inner-city sites is a major challenge that frequently happens due to a shortage of space and improper planning and use. This study identified that the current CSLP practice is overlooked or doesn’t include the micro space and paths space requirements, which are the main elements of other construction plans such as in schedule and cost estimation. However, the panel of experts agreed on the importance of the three categories of space requirements for CSLP and classified them based on their level of impact and the probability of occurrence: Macro space (18), Micro space (04), and Paths (05). Besides, the spearman correlation analysis among the space requirements has shown, variables from all categories have a significant association with each other.

This study has identified a strong relationship between the working area spaces and space requirements for TFs. However, most of the developed optimization methods for CSLP only focus on how to minimize the travel distance between the TFs to formulate and solve their objective function(s). Minimizing travel distance between TFs will indeed reduce travel costs, but it will not create optimum value if it doesn’t keep minimizing the travel distance from working areas to the TFs too. So, this study will insight researchers to formulate their objective function(s) based on minimizing both the travel distances between TFs and the working area spaces to develop an optimization method for CSLP.

Therefore, the provided insight about the space requirement variables, classification, and relationship for the inner-city building projects will be a basis for shifting towards an integrated CSLP approach from the current (temporary-facility-centered) planning methods by considering all space requirements in a construction site. So, the integrated CSLP will assist in efficiently utilizing the limited inner-city site space and allocate it to all space requirements for optimized and safe site operations.

The literature on construction site space requirements in inner-city projects is limited; in this study, the variables were extracted from general building construction and contextualized by case study. So, the classification is global but the ranking made based on the probability of occurrence and impact level of the space requirements may vary and is limited to the study area, the inner-city sites.

Finally, this study recommends future studies on how to create awareness among professionals on the characteristics and impacts of the three categories of space requirements in construction operations and developing a CSLP method that integrates these requirements for inner-city building construction. Besides, this study also initiated a question for future study: what is the effect of CSLP on the other construction plans for inner-city building projects such as cost estimation, scheduling, and logistic plans?

Ethical approval

All procedures performed in this study involving human participants followed the ethical standards of the University of Zambia Directorate of Research and Graduate Studies and received approval for the study with Ref No.NASREC-2022-APPRIL-008.

Informed consent

The research involved human participants who were informed about the purpose of the study and that their participation is voluntary.

Acknowledgments

We would like to express our gratitude to the institutions and study participants for their response and collaboration in the study.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This research is done under the scholarship of SPREE at the university of Zambia School of Engineering.