Understanding the advantages of earth construction in urban housing in the United Kingdom

Earth building material is benecial in urban housing in the United Kingdom, particularly with regard to the promotion of environmental sustainability. Due to higher demand to achieve sustainable development around the world, researchers and innovators are increasingly turning their attention to ecient ways to address climate change and excessive CO 2 emissions. Contemporary earth construction certainly contributes to reducing current global CO 2 emissions and achieving environmental sustainability. Existing literature documents numerous benets to constructing in earth, yet a critical review of the literature shows that, in many cases, these benets are empirically unsubstantiated. Moreover, some benets found in literature seemingly conict and are often context and/or project specic. This paper aims to address some of these incongruities through an in-depth analysis of the advantages of earth construction in the development of UK urban housing. To achieve this aim, an interpretivist research philosophy was employed, including an up-to-date state of art literature review on the topic, and validation through the Delphi technique and in-depth interviews of construction professionals in the eld of earth construction. The results show that earth building is very safe for the environment, it saves energy, promote self-help construction and signicantly contribute achieving all aspects of environmental sustainability in the United Kingdom. round. The results show that the participants identied all but three of the benets mentioned in literature review. They did not identify any new benets that were not previously mentioned in literature. The benets are ranked according to the number of experts who mentioned them in the rst phase of the Delphi survey.


Introduction
Sun-dried earth blocks was used in construction approximately 8000 B.C and it is estimated that earth-based shelters house about 50% of the world's population [22]. Earth has been most widely used construction material in many developing countries and continues to be the major means of construction [18]. According to Schroeder & Lemke [25], earth building materials has less negative impact on the environment, they are inherently energy e cient compared to other building materials, and less risky on people health. Earth construction, for example, consumes less non-renewable energy than red masonry, concrete blockwork and heat/humidity buffering [4]. According to Reddy et al [23], in modern interpretations, earth as a construction material has negligible life-cycle impact, is completely recyclable, and it contributes almost zero-carbon footprint.
Earth is not only environmentally viable; it is also cheaper, available in large quantities and easy to work with especially forming into building components [1]. In other words, earth construction carries many economic bene ts and a primary reason for adopting contemporary earth construction is that it can address urban housing crises, especially in developing countries where it is readily embraced [29]. Interest in earth construction in developed countries, by contrast, is more often driven by potential environmental gains as opposed to its economic value. In order to achieve overarching purpose of this study of informing the diverse bene ts of earth material to construction practitioners, this study aims to identify and explore the wide range of bene ts associated with earth construction in UK urban housing. Considering the exploratory nature of these aims, the study begins with a critical review of relevant literature on the advantages of earth material as a primary construction material. Furthermore, the paper analyses and validates these bene ts through empirical research methodology including the Delphi technique and in-depth interviews -a process in which ndings in literature are compared and contrasted against the perspectives of professionals in the eld (i.e. construction professionals) through questionnaires and interviews.
According to Houben & Guillaud [11], through modern construction methods, earth is typically stabilised through three major processes: Physically by modifying the percentage of mixed soil particles and texture of the earth by varying soil ingredients.
Chemically by mixing chemicals (stabilisers, such as, cement, lime, gypsum, y ash, etc.) or other materials to modify the properties of the soil.
Mechanically by applying force directly onto the soil compressing or ramming. It alters the permeability, density, durability, strength, compressibility, and porosity of the earth. Therefore, earth stabilisation must not be understood as only mixing cement with it. However, the bene ts of using earth as building material are many, as summarised in Table 1, which presents an in-depth summary of the generic bene ts of earth material as identi ed in literature. Table 1 Advantages of earth as building material [30,31].
3. It promotes housing construction in the form of 'selfhelp'.
4. Earth can build extra secured and strong structures.
5. Earth material has low embodied energy; therefore, considered as energy saver.
6. Naturally balance interior temperature and humidity in an earth building.
7. It is re resistant.
8. It creates employment opportunity.
9. Earth is considered as sustainable building material.
10. Earth walls preserve organic materials, such as timber.
11. Pollutants are absorbed by earth wall.
12. Designing with earth is easy and can produce building with high aesthetic value.
13. Earth wall possess high insulation property, therefore, excellent in controlling noise.
14. Earth is local building material; therefore, it promotes heritage, tradition and cultural practice.
15. It is available worldwide in abundance.
As shown in Table 1, a major focus of research to date has been on economic bene ts associated with contemporary earth construction. However, researcher Sanya [24] stated that economic advantage of earth is not always be achieved.
Given con icting views on the economic viability of earth construction, it is pertinent to investigate this bene t in more detail. Environmental bene ts also dominate current literature sources, particularly with regard to certain contexts. In India, for example, studies have shown that one m 2 CSEB construction of masonry consumes ve times less energy than one m 2 of wire cut red brick masonry and fteen times less than country red bricks [17]. Maini [17] also states that CSEBs are eco-friendlier than red bricks and consume less energy and pollutes less air during manufacturing than red bricks. Further to this, Adam and Agib [1] emphasize how processing and handling soil requires low energy input; the author referred to a study (Desert Architecture Unit) and stated that 36 MJ (10 kwh) energy is consumed in the production of one m 3 of earth, whereas 3000 MJ (833 kwh) energy is consumed in the production one m 3 of concrete.
Schroeder and Lemke [25] systematically explain and illustrates how earth construction can become self-sustaining.
The soil is collected/extracted from the ground with its natural state; preparation such as classi cation, pulverisation, drying is done with its aim to be used in the construction. Thereafter, the soil is used to complete the building. The earth building last long time, however, in the case of demolition the earth building goes back to the ground, thereafter can be reused and recycled as earthen materials. This is how the life cycle of earth becomes self-sustaining. In this way, earth material is ecologically advantageous. As research emphasizes, contemporary earth construction is often more economically and environmentally sustainable than conventional building materials (i.e. red brick and concrete) and may play a fundamental role in cutting excessive CO 2 emissions in the urban housing projects. A critical review of existing literature, however, reveals that sparse research so far carried out that identi es, explores and analyses the advantage of earth material speci cally in the case of UK urban housing. Bene ts mentioned by various construction professionals and authors in literature are also, more often than not, anecdotal and based on individual perceptions; thus, empirical data is lacking and re ning of data via rigorous scienti c processes. Responding to the need for informing the bene ts of contemporary earth construction to UK professionals, therefore, this study aims to identify the bene ts in UK context.

Methodology
To understand the bene ts of using earth material in the UK, this study followed a four-stage, methodological framework as shown in Fig investigations, to date, on the topic of identi cation and understanding the bene ts of earth as building material, particularly in the UK is very few. After careful consideration of the nature of the research problem, an interpretivist philosophical stance was adopted to achieve the aim of the study.
Stage Two (Exploratory phase -the Delphi technique): This phase employed the Delphi method where an expert panel of participants responded to questionnaires in two rounds to achieve a consensus to nalise the advantages of using earth material in urban housing. Speci cally, this method was chosen as an initial research technique to accumulate scienti c data to support the advantages summarised from the literature review (Table 1). The data collected from the Delphi technique, combined with the comprehensive literature review, resulted in an extensive list of bene ts associated with earth construction and provided data from which a robust interview schedule was constructed for the validation phase (Stage Three), i.e. in-depth interviews with experienced experts working in UK earth construction.
Stage Three (Validation Phase of in-depth interviews): This phase employed a number of in-depth interviews with the objective of identifying and validating the advantages of earth as building material in UK urban housing. This validation phase played a central role in identifying distinctive, unique bene ts and hidden agendas (such as the condition upon which the earth construction is bene cial are not well explained) particularly within the context of the UK. The accumulation of this validated data allowed the researcher to make a strong foundation of understanding the diversi ed, known, and unknown and conditions of bene ts and then re ned the generic bene ts to understand them better in UK context.
Stage Four (Synthesis of the research process): The aim of the nal phase was to synthesize the research ndings; summarize and discuss the contribution of knowledge the results make to academia and the construction industry; recommend future research and acknowledge limitations. This concluding phase facilitates an overall holistic understanding of the advantages of earth material in UK urban housing. Considering the exploratory nature of this study, a methodological framework of four stages was employed. Each stage collected key data that assisted and contributed to the development of the subsequent stages.

Justi cation and process of execution of the Delphi technique
As indicated, two major methods were used to collect data. Namely, the Delphi technique and in-depth interviews were employed due to their ability to explore the bene ts of earth material. If there is a problem of getting information about any issue/phenomena and there is incomplete knowledge about it, the Delphi method can be used to acquire information [2,7]. Through this technique information can be generated from the subjective judgments of experts on a collective basis and this method is suitable investigating problems that do not lend themselves precise analytical techniques [2]. Therefore, Delphi technique helped understanding the advantages of earth material, explored areas about the advantages of earth building material that may have been overlooked, areas where there are con icting ideas and that do not have su cient data to generalize.
As mentioned, the literature review concluded that sparse research produced evidence validated via structured scienti c processes with regard to the bene ts of earth construction. The Delphi technique, therefore, was selected as an initial study method because it offered a more rigorous, informed and grounded way to explore the potential bene ts of contemporary earth construction. The Delphi technique was able to capture a more in-depth analysis of the bene ts of earth construction from a carefully selected group of construction professionals. The data produced by the Delphi technique provided several clari cations and substantially contributed to knowledge in the eld with regard to the type of conditions that may need to be met in order for earth construction to be bene cial.
The Delphi method consisted of two rounds of questionnaires, where the second questionnaire was built from the questions and experts contributions summarised from the rst round of questionnaires. The aim of the questionnaire in the rst round was to extract advantages of earth in urban housing. In the second round of the Delphi, the bene ts identi ed from the rst round were listed and ranked according to the number of experts who identi ed each bene t. These results (list of bene ts) were then presented back to the experts for their second thought, reconsideration and coming up with nal list.
Of the few of professionals specialised on earth material all over the world, the most renowned of this population were selected as participants for the Delphi method. In total, thirty-four international experts who were considered to have the up to date knowledge and exposure for the study were communicated. Fourteen international experts, resulting in a 41% response rate, responded and agreed to participate. Of the fourteen participants, all of them were practitioners but also researchers. The panellists required only two rounds of questionnaire to reach a consensus. Figure 2 illustrates the methodological process adopted to execute the Delphi technique in this study.

Justi cation and process of execution of the in-depth interviews
To formulate a semi-structured questionnaire for in-depth interviews conducted in phase three the researcher used the results of the Delphi technique. The data acquired from the in-depth interviews with UK construction professionals were used to validate the consolidated or generic list and generate a list of the advantages of earth material in UK. In depth interviews are appropriate qualitative method for encouraging participants to talk about their personal feelings, opinions, and experiences [16]. Interview method help to gather hidden information or data from sensitive topics that people might be reluctant to discuss in a group setting. Therefore, in depth interviews helped exploring areas about the bene ts that may have been overlooked, areas where there are con icting ideas and that do not have su cient data to validate the advantages of earth material in UK.The methodological processes used (and the results) of the Delphi technique and interviews are discussed in turn.
The data collected from the Delphi technique combined with the literature review resulted in an extensive list of bene ts and helped direct the construction of the interview schedule for the validation stage (Phase Three). Speci cally, the third phase incorporated in-depth interviews with experts experienced in earth construction in the United Kingdom. This phase played an important role in highlighting any distinctive factors and hidden agendas (such as certain local conditions upon which the bene t is depending upon) in the UK. The accumulation of these data also allowed the author to build a grounded foundation of knowledge from which to explore the bene ts of earth material in urban housing of UK. According to Loosemore [15], the researcher must make sure that the research techniques used in any interviews should be able to communicate most effectively between themselves and the expert participant. Therefore, in a research, feedback is essential convergence. It is the knowledge transferring to the researcher from the participant. Such knowledge can be obtained from verbal and written formats, facial expressions and body languages. Interviews are excellent methods in theory building, not only because interviewers they can elicit perceptions of key concepts from experts, (i.e. industrial practitioners in the case of this study), but also because the data is contextualized through the stories and experiences of those interviewed [6].
All the interviewees were an average of over thirty-ve years experienced in earth construction. They ful lled the following set of criteria used to recruit participants with su cient knowledge and experience in the eld: British natives and locally trained construction professionals with knowledge in earth construction who may shed light on local factors affecting bene ts of earth construction in the UK.
Professionals who are active members of various UK based international associations related to earth construction.
Migrated construction professionals trained in the UK and have international experience, with the ability to clarify local factors affecting earth construction considering their global exposure.
In the third stage (interviewing), after establishing rapport, the experts were asked to discuss what they considered to be the advantages of earth material in UK, if any. In this stage all the experts gave their own list of bene ts; they were then shown the list of bene ts established from the results of the Delphi phase and asked to validate by comparing between their own list. However, they were further asked to discuss and explain if there is any mismatch between the two lists.

Results from Delphi Technique
The experts were asked to identify the bene ts (if any) of using earth material in urban housing and all fourteen participants replied accordingly. Eleven experts (79%) unconditionally agreed that earth material is advantageous in urban housing. One expert (7%) indicated that earth may possibly be advantageous, and two experts (14%) posited that earth construction is bene cial, but dependent upon the following conditions: The type of urban housing (terraced, semidetached, ats, etc.). For example, multi-storeyed (high-rise) ats might not be economically or structurally possible. Single storied houses are more economically sustainable than double storied terraced or semidetached houses.
The suitability and availability of local soil. Earth that is sourced from a great distance will result in increased expenses due to transportation costs.
The nature of the climate in the town/city. Although contemporary stabilized earth structures have a high success rate globally with regard to resisting natural disasters, un-stabilized earth structure naturally vulnerable to ood and earthquake zones.
The rationale and/or philosophical goal of the designer, client and/or occupant. For instance, earth construction may be chosen for urban housing for many reasons, such as environmental and/or economic sustainability, or the promotion of local technology and culture, for example. Therefore, the bene t of using earth construction will depend on the designer or client's intentions or the goal of the project.
Type of stabiliser. For example, un-stabilised earth may prove to be environmentally more bene cial than cement stabilised earth. In addition, many experts are opposed to cement stabilised earth construction because it lacks environmental and economic sustainability.
The above summary provides an important list of conditions that need to be considered when assessing the advantages of any earth construction project in urban housing. These conditions were not identi ed in the aforementioned review of literature. Table 2 presents 15 of the bene ts of earth construction identi ed from literature (see Table 1) and twelve bene ts mentioned by the expert panel in the rst round. The results show that the participants identi ed all but three of the bene ts mentioned in literature review. They did not identify any new bene ts that were not previously mentioned in literature. The bene ts are ranked according to the number of experts who mentioned them in the rst phase of the Delphi survey.

Results from In-depth interviews
As noted, during interviewing phase the experts were rst asked to share their views on what they believed to be the bene ts (if any) of earth construction. In general, all the interviewees agree on a number of conditions upon which the bene ts are dependent upon but also had differing views on the economic bene t of earth construction in the UK.
Expert 'A' explained that in order to consider the bene ts of earth construction, it is rst important to recognize that there are several, hundred, different types of soil available in the UK, and some soil will work with stabilizer, some without stabilizer and in some cases excessive stabilization may be required. Soil in the UK varies acre to acre or hector to hector; and, in order to reap the bene ts of earth construction, much will depend on the type of soil available. Expert 'A' further explained that there were two major reasons why clients chose an earth building, i.e. either they 'want a green project for the sake of environmental sustainability, or simply for aesthetic reasons'. In either case, the expert suggested that the decision of whether to stabilize the soil or not would be made during the design process. For instance, if clients chose earth because they wanted a green project, then the earth would not be stabilized, (especially not with cementation material). If the client, however, wanted to use earth material for aesthetic reasons, then that was a "completely different situation". The client would need to see how adding stabilizer would change the appearance of the building wall elevation before the actual construction starts. The practitioner would need to create a test wall before actual construction to ful l client's aesthetic reasons. During one particular class assignment aimed at designing earth buildings, the expert 'A' shared how she sensed a lack of interest in stabilized earth among students. Many, she explained, felt that it lacked sustainability, which "is a big issue for the UK".
Expert 'C' similarly suggested that un-stabilized earth construction provides many bene ts whereas stabilized earth construction brings very few, if any. A major issue with urban housing procurement and construction, he explained, is funding; "therefore, it all comes down to the cost of industrial input". Expert 'B' also felt that earth construction was bene cial in the UK but not universally so. He suggested that each project needed to be weighed up against alternatives with respect to: the current cost and availability of materials, the labour market, and the level of familiarity the design team has with earth building and their skill set. Overall, he summarized his thoughts by saying, "The broad answer is 'yes' it is bene cial but it is project speci c. Potential bene ts have to be weighed against what the alternatives are".
Overall, the experts unanimously agreed that earth material is advantageous in urban housing, but dependent on certain conditions. Three of the experts also suggested that un-stabilised earth is more bene cial than stabilised earth in the UK, and, again, dependent on certain conditions. The conditions, as mentioned by the experts, are summarized as follows: 1. There are different soil types in the UK. One has to characterise different types of soil to understand which of them will work with stabiliser, which will not and which requires excessive stabilisation.
2. Projects involving earth construction must be weighed up against alternatives in terms of the current cost and availability of materials. Therefore, the bene t of using earth construction is project speci c.
3. Un-stabilised earth construction provides many bene ts whereas stabilised earth construction brings very few, if any. A major issue with urban housing procurement and construction is funding; therefore, cost is important.
4. The decision to use stabilizer will depend on the goal of the client, i.e. whether they are focused on creating a green project for the sake of environmental sustainability, or whether they are building for aesthetic purposes.
5. Soil needs to be excavated locally and brought to the construction site easily.
6. Skilled labour needs to be locally available.
7. The design team needs to be skilled in earth construction.
The experts were then asked to discuss what they considered to be the advantages of earth material in UK urban housing, if any. All the experts gave their own list of bene ts. The experts were then shown the consolidated list of bene ts (Table 2) summarised from the literature review and the Delphi technique and were asked to compare with their respective lists and validate whether those advantages of earth building were applicable to urban housing in the UK context. Table 3 presents the result of the validation. Table 3 Advantages of using earth material in UK urban housing validated by experts.
Summarised advantages of earth building in UK 1. Earth building use simple tools and labour with less skill.
3. Earth material has low embodied energy; therefore, considered as energy saver.
4. It characteristically controls humidity and heat by reducing peaks and drops.
6. Earth construction creates job opportunity in the UK.
7. Earth material is easy to recycle and degrade; therefore, it is environmentally sustainable.
8. Designing with earth is easy and can produce building with high aesthetic value. 9. It produces structures that have sound insulation characteristics. 10. It is local building material; therefore, it promotes heritage, tradition and cultural practice.
11. It encourages self-help construction.
12. It is available in most regions in abundance and ready for use in UK. 13. It contributes money to the local economy

Discussion
As shown in the earlier review of literature, much research, to date, has focused on the monetary advantages of earth material (dependent on certain conditions); whereas the results from the Delphi technique and panel of experts show that environmental bene ts dominate as the major advantage of earth construction. The results also emphasize that the bene ts of earth construction are likely dependent upon certain conditions and circumstances.
There were also many con icting perspectives in the literature review, and between the literature review and ndings from the Delphi survey, which may suggest that current literature on the advantages of earth may be lacking in empirical evidence. For example, according to the literature review, earth masonry units may be manufactured investing less capital following the same manufacturing process except ring or burning phase. However, literature also shows that the process of manufacturing CSEB is distinctive. Similarly, literature states that the thermal conductivity of earth is nearly zero (0), but technical guidelines show that the thermal conductivity of dry earth is 1.5 K-(W/ mK) [26]. Experts in this study agreed that the process of manufacturing CSEB is distinctive and that the thermal conductivity of earth is not zero. Moreover, according to the literature review, a bene t of earth walls (loam) is that it preserves organic materials such as timber and pollutants are absorbed by earth wall. In the Delphi technique, none of the experts mentioned these as bene ts of earth construction. However, this bene t of earth walls absorbing pollutant is not certainly overlooked by the experts of Delphi panel but in fact; there is lack of scienti c evidence in the literature review of proving that earth walls can absorb pollutants.
The major contribution of this study is that it validates and extends upon the bene ts of earth construction as previously identi ed in literature. Initially, this study identi ed fteen bene ts of earth construction from literature, and thirteen of those bene ts were corroborated through the empirical research employed in this study. Speci cally, the Delphi technique identi ed twelve bene ts similar to that found in research. In-depth interviews also validated twelve bene ts and further isolated one bene t not recognized in research (i.e. that earth construction contributes money to the local economy). Table 4 presents a comparison of the advantages of using earth material in urban housing (in the UK context) as extracted from the literature review, through the Delphi technique and in-depth interviews. Of note, this study may suggest a shift in attitude with regard to the leading factor as to why earth is advantageous in urban housing. Namely, where literature in the past isolated cost to be the dominant advantage, results from the Delphi technique suggest that energy saving may play a greater role in promoting the bene ts of earth construction. According to the experts in this study, environmental bene ts are the leading bene t to earth construction in the UK.
However, the type of conditions that may in uence the bene ts of earth construction, as identi ed in literature, have, to date, focused predominantly on issues of cost. An in-depth analysis of the results from this study (including ndings from the Delphi technique and interviews with experts in the eld of earth construction) reveal a much more comprehensive inventory of the type of conditions necessary upon which earth construction may be bene cial in urban housing in the UK context. Table 5 presents a summary of the conditions and circumstances upon which contemporary earth construction is bene cial as identi ed from literature and from the methodology employed in this study. Taken together, these conditions and circumstances provide an extensive list of key factors that need to be considered when assessing the bene t of earth construction in UK urban housing.  It is important to note here that experts in earth construction are relatively few and only small numbers of contemporary British earth building practitioners have worked in the past or are working now in the UK. Nonetheless, four British practitioners were face to face interviewed and the data was generated from the transcriptions of the interviews.

Conclusions
The UK experts in this study agreed that earth is extremely bene cial to UK urban housing development under certain conditions although the use of earth as construction material in the UK is not yet widespread. The environmental bene ts of earth construction, in particular, are a major advantage and can address excessive CO2 emissions in the development of UK urban housing. Thirteen major bene ts of earth construction in total were identi ed, validated and summarized from the feedback of experts in this study that, overall, corroborate all but two of the fteen major bene ts of earth construction previously mentioned and extracted from literature.
With innovative approaches, the utilization of earth will greatly bene t urban housing in the UK. The utilization of earth material as alternative to the conventional ones is extremely valuable in that it is extremely helpful in the achievement of sustainable environment and development (i.e. reduced CO 2 emissions) and promotion of self-help construction.
However, bene t number 10 and 11 in Table 4 were not stated by any of the earth construction professionals of this research. However, these bene ts were taken from the existing literature. Therefore, for future research it is pertinent to investigate and nd out through scienti c study on the capabilities of earth walls of absorbing pollutants, preserving timber and other organic material used in construction. Figure 1 Methodological framework: A graphic representation of four interconnected stages and three techniques used in the study.

Declarations
Page 21/23 Figure 2 The process of the Delphi technique adopted in this study.

Figure 3
In-depth interview process adopted in this study.

Figure 4
Factors that may in uence the bene ts of earth construction in UK. Author, 2020.