Indian Geotechnical Journal

Improvement of the mechanical properties of Tunis Soft Soil using plastic waste

Manuscript Number: IGTJ-D-24-00730

Full Title:
Improvement of the mechanical properties of Tunis Soft Soil using plastic waste

Abstract: Tunis soft clay is a type of soil known for its poor mechanical properties. Plastic pollution is
a major environmental challenge, but the beneficial properties of plastic waste can improve
soil stability. This research marks an unprecedented effort to enhance the compressibility and
shear strength of Tunis soft soil using plastic waste of grains. Reconstituted soil samples
mixed with 2%, 5%, and 7% waste will be pre-consolidated under pressures of up to 80 kPa
and then discharged. Experimental results show that the addition of plastic waste significantly
reduces consolidation time, void ratio while increasing undrained cohesion. Moreover, the
optimum percentage of plastic waste identified in this study is 5%, exceeding the 4% reported
in several previous studies. For geotechnical engineers, the mix is suitable for foundation soil
and backfill, though further studies into placement techniques, such as deep mixing or
drainage columns, are needed by researchers.

The authors sincerely thank the Chief Editor for the opportunity to revise this paper in response to the reviewers' comments and hope
that the revised version meets the standards for publication in the journal.
The authors also extend their gratitude to the reviewers for their valuable evaluations, comments and insights, which have significantly
improved the quality of the revised paper.
The revised version of the paper has been carefully reviewed and updated in response to the reviewers' suggestions.
An initial draft version was prepared to better illustrate the changes to the reviewers:
- All changes made in this initial draft version are indicated by text crossed out in black and replaced with modifications highlighted in
yellow for greater clarity.
- Sections crossed out in red are permanently deleted from the revised paper.
- Changes highlighted in turquoise represent new additions made in response to the reviewers' requests.
In the responses, the lines marked in bold refer to the revised paper, those in parentheses refer to the initial draft, and those highlighted
in turquoise indicate lines added for the new section.
Reviewer #3

Questions Response

1-The manuscript contains many spelling, We sincerely appreciate the reviewer’s feedback regarding the spelling, grammatical, and
grammatical and editorial errors and should be editorial errors, as well as the incorrect referencing of certain articles. We have thoroughly
reviewed. In addition, the manuscript's text reviewed the manuscript to address these issues. Specifically:
contains incorrect references to certain articles. Spelling, Grammar, and Editorial Errors: The entire manuscript has been carefully proofread
This must be reviewed, particularly for articles and revised to correct all identified errors.
with more than two authors
We have reworded certain sections of the revised document, highlighted in yellow, to correct
grammatical and spelling errors.
All references have been thoroughly reviewed and corrected to ensure accuracy, with particular
attention given to citations involving works with more than two authors. This includes adhering
to referencing guidelines for entries such as
Line 051 (Line 073): Geyer, Jambeck, &Law, 2017;
Line 080 (Line139): Giri, Panda, & Sahoo, 2020;
Line 176 (Line 241): Ben Khalifa, Ben Salem, & Frikha, 2023;
Line 178 (Line 243): Jebali, Frikha, & Bouassida, 2017;
We revised the citation details of other references without DOIs and corrected them in the
references section, including examples such as:
 Line 1588 (Line 1899): Mirzababaei, M., Arulrajah, A., Haque, A., Nimbalkar, S., Mohajerani,
A. (2018). Effect of fiber reinforcement on shear strength and void ratio of soft clay.
Geosynthetics International, 25 (4), 471-480,
https://doi.org/https://doi.org/10.1680/jgein.18.00023 Retrieved from
https://www.sciencedirect.com/science/article/pii/S175176131800056X
We believe these revisions have significantly improved the quality and accuracy of the
manuscript. Thank you for bringing these issues to our attention.
Line 1611(Line 1918): Okoro, C., Vogtman, J., Yousif, A., Agnaou, M., Khoury, N. (2011).
Consolidation characteristics of soils stabilized with lime, coal combustion product, and plastic
waste. Geo-frontiers 2011: Advances in geotechnical engineering (pp.1202–1209). Retrieved
from https://doi.org/10.1061/41165(397)123
2- Despite the importance of using plastic The authors wish to express their gratitude for the pertinent comments regarding the need to
waste in improving soil, especially weak soil, clearly articulate the innovation of the manuscript. In response, we have revised the abstract to
as a solution to get rid of this waste and the fact more clearly highlight the innovative aspects of our work, particularly regarding the
that this method is not new in improving these reinforcement of Tunis soft soil with plastic waste and the optimal utilization of plastic waste.
soils, the innovation of the manuscript needs to These aspects are emphasized in new sentences founded the following lines
be further stated and more clearly stated in the
-From line 030 to Line 031 (From line 037 to line 038):” This research marks an unprecedented
abstract.
effort to enhance the compressibility and shear strength of Tunis soft soil using plastic waste of
grains”.

-From Line 034 to Line 036 (From line 059 to line 060): “Moreover, the optimum percentage of
plastic waste identified in this study is 5%, exceeding the 4% reported in several previous
studies.”

3-In Table No. 2, the year of the standard used We are grateful to the reviewer for theses valuable comments.
must be indicated next to its number. The soil's
-Regarding the first point, we have updated Table No. 2, Table No. 3 to include the year of the
dry unit weight has two possibilities: either its standard used next to its number, as requested.
units are t/m3 or there is a typographical error
Following the reviewer’s comment, the year of the standards [ASTM D2435 and ASTM D652]
in its value. used in Line 483 (603) has also been added.
The standards used are also properly cited in the references from Line 1228 to 1289
- Regarding the second point, we have reviewed the soil’s dry unit weight and identified a
typographical error in its value. The corrected value, 17.2 kN/m³, has been updated in Table No.
2 in Line 328 (Line 419).
4-What do the authors mean by the upper and We appreciate the reviewer’s inquiry regarding the upper and lower limits in Figure 3. These
limits represent the specified particle size distribution range for Tunis Soft Soil. For greater
lower limits in fig.3, please clarify them
clarity, we have revised the legend of Figure No .3 and added a brief explanation in the text,
supported by an appropriate reference as mentioned in Line 357 (Line 450):” The upper and lower
limits define the specified particle size distribution range for TSS (Frikha, Tounekti, Kaffel, &
Bouassida, 2015).”

5- What is the benefit of linear regression? It We value the reviewer’s insight regarding the benefit of linear regression. The authors
acknowledge that this relationship is limited to the range of plastic waste percentages used in the
finds an equation that links one of the
experiments and does not account for other consolidation parameters. Its primary utility in this
properties to the percentage of plastic waste study is to derive an equation that links the void ratio or compressibility coefficient to the
which is limited to the percentages of PW used percentage of plastic waste.
only. In fact, the authors agree with the reviewer’s remark, particularly regarding the use of linear
regression to predict the compression index 𝐶𝑐 as a function of only the initial void ratio 𝑒0. In
this case, the coefficient of determination 𝑅² was approximately 0.8, indicating a relatively weak
correlation between the variables.
However, in response to the reviewer’s request, and considering the influence of various factors
on the consolidation behavior of soft soil as well as the complexity of the composite Tunis soft
soil-plastic waste mixture, multivariate regression models have been employed in the revised
paper, replacing the linear model. In fact, in our case, where the independent variables are highly
correlated, linear regression can become unstable, leading to unreliable coefficient estimates. To
address the issue of multicollinearity, three separate multiple regression models are employed in
 the next response.

6-It will be good contribution for this We sincerely appreciate the insightful feedback to incorporate multiple regression models using
additional variables such as the initial void ratio, the initial dry unit weight, the initial water
manuscript, multiple regression can be used,
content ratio, and the percentages of plastic waste added. In response, we have revised the
using more than one variable for each property, manuscript to include multivariate regression models that consider these factors, as they provide
such as the initial void ratio, the initial dry unit a more comprehensive understanding of the relationship between the properties and the inclusion
of plastic waste.
weight, the initial water content ratio, and the
percentages of PW added. To estimate this relationship, multiple regression models have been used in the revised version,
specifically LASSO, Ridge, and Elastic Net. The Leave-One-Out Cross-Validation (LOOCV) was
used to select the best-fit model, with the LASSO model producing the highest R² value. LASSO
can address multicollinearity by shrinking less important variables’ coefficients to zero, effectively
selecting the most relevant predictors for the model.
Lasso model is used in this work to predict:
-the dry unit weight as function of plastic wastes and initial water content (Line 573)
𝛾𝑑 = 0.2 − 0.03 𝑃𝑊 + 0.05 𝜔𝑖 (1)
-the void ratio as function of plastic wastes and dry unit weight (Line 790)
𝑒0 = 0.906 − 0.038 𝑃𝑊 + 0.0071 𝛾𝑑
-the compressibility index as function of plastic wastes and the void ratio (Line 853)
𝐶𝑐 = 0.4471 − 0.0141 𝑃𝑊 + 0.0033 𝑒0
We believe these additions strengthen the analysis and enhance the overall contribution of the
study, particularly as the R² values for these multiple correlations exceed 0.9, indicating a strong
fit between the observed data and the LASSO model's predictions.

7-The manuscript did not clearly explain how We are grateful for the constructive comment. To address the reviewer’s comment, we have
amended the manuscript to provide a clearer explanation of how the new method of preparing
the new method of preparing the samples and
the samples and recycling plastic waste affected the results.
recycling the plastic waste affected the results,
The advantage of the pre-consolidation technique is highlighted by the optimum plastic waste
which were confirmed by other researchers content (5%) identified in this study, as mentioned in the introduction of the revised paper. This
who used the traditional method of preparing section outlines several benefits of using the pre-consolidation process to prepare TSS-PW
the samples. mixtures.
From Line 090 to 117: “This methodology differs from conventional techniques, which primarily
rely on compaction to alter soil properties. The process of pre-consolidation followed by
unloading enhances the structural performance of the mixture, increasing its resistance to
deformation under load and simulating the long-term effects of settlement. This technique has
remarkably led to the use of a higher optimal percentage of plastic waste (up to 5%) in this study,
compared to traditional compaction methods, which typically use up to 4% plastic waste.”
To better understand the effect of pre-consolidation on the increase in optimal plastic waste
content to 5%—compared to approximately 4% achieved using compaction—the authors have
included a detailed comparison between the results of the new method and those obtained using
traditional sample preparation techniques. This comparison is discussed in the revised manuscript
at the end of the Oedometer Curve section and the Compression and Swelling Indices section.
From Line 799 to 807 “Using an optimum of 1% plastic waste in raw soils and the compaction
method, their study achieved a 9% reduction in the void ratio, while the current study, with
RTSS2% (pre-consolidated) and not at the identified optimum, achieved a 12% reduction. This
demonstrates that pre-consolidation enhances mixture resistance under loading and facilitates
achieving an optimum of 5% plastic waste in this study.”

From Line 831 to 839:” With RTSS5%, compressibility decreases significantly by 20%,
surpassing the 11% reduction in Cc values reported by Soltani-Jigheh (2016) for clay soil with an
optimum of 3% plastic waste using compaction. This highlights the effectiveness of the pre-
consolidation technique, which stiffens the mixture under loading and allows for increasing the
plastic waste content to 5% while improving consolidation parameters.”

8-The authors had not shown whether adding We thank the reviewer for emphasizing this important aspect, as environmental concerns are a
pressing global issue today. In response, we acknowledge that evaluating the environmental
PW has any impact on the environment
impacts of incorporating plastic waste into building materials requires thorough testing. This
includes Toxicity and Leaching Tests to assess potential risks of harmful substance release, as
well as studies on CO2 Emissions to evaluate the carbon footprint associated with the production
and use of plastic-infused soils. While this research primarily focuses on improving the
mechanical properties of Tunis soft soil using plastic waste, it also aims to explore
environmentally friendly construction materials. As such, we have included a brief discussion of
the environmental considerations in the revised introduction, drawing on previous work that
investigated the use of plastic waste in reinforcing concrete performance.
From Line 126 to Line 132:” However, long-term degradation of plastics in soil can present
significant environmental risks, including plasticizers flame retardants, antioxidants and
microplastic leakage, especially for waste smaller than 5 millimeters in size (Minde, Kulkarni,
Patil, et al., 2024).”
We once again appreciate the reviewer's concern regarding the environmental aspect, which could
serve as a perspective for future research to build upon the current work.
 Reviewer #4

Questions Response
Thank you for your comment regarding the manuscript length. We understand the importance of
1-the manuscript is somehow long. The authors concise presentation and have worked diligently to shorten the paper while maintaining its
should remove unnecessary parts of the coherence and clarity. Specifically:
manuscript and try to present it in almost half
✓ The manuscript has been reduced from 31 pages (1398 lines) to 26 pages (1193 lines),
number of pages than existing version
excluding references. This represents a significant reduction achieved through the
following efforts:
✓ Streamlining Discussions:
• we rewrote the abstract as per the reviewers' requests and shortened it from 14 to 12
lines.
• In the materials identification section, we removed unnecessary details (highlighted in
red strikethrough) and focused only on essential values presented in Tables 2 and 3.
• For the standards used, we omitted detailed descriptions, restricting references to
citations only.
• In the conclusion, we removed explanatory details, keeping only key findings,
reducing its length by 20 lines.
✓ In the results and discussion section:
• We removed primary equations commonly used in civil engineering (e.g., the void
ratio formula).
• Long sentences were rewritten for brevity (black strikethrough replaced with shorter
yellow-highlighted text).
• Background and less critical details were summarized for conciseness.
✓ Removing Redundancies:
• Figures 9, 13, and 14 from the original version were replaced with multiple regression
analyses, with corresponding data retained in Table 6, reducing the length of this
section.
• Figure 17 and Table 8 were removed, as Figure 18 summarizes the same information.
• Figure 19 was also removed, retaining Table 9, which conveys the same data.
To highlight for the reviewer, the current revised paper includes 7 tables and 14 figures, whereas
the original version contained 9 tables and 18 figures.
• Condensing Data Presentation:
✓ We carefully revised figures, tables, and regressions to avoid overlap and present
information more efficiently.
Despite these efforts, reducing the manuscript to half its current length may compromise its
coherence and the depth necessary for readers to fully understand the methodology and
implications. We have worked to balance clarity and thoroughness to ensure the research is both
accessible and scientifically rigorous.
We welcome your suggestions on any specific sections that could be further condensed or
refined without affecting the flow and comprehensiveness of the paper.
Thank you for your understanding and constructive comments.

2- Who will be benefited from the research and We thank the reviewer for this valuable suggestion. In response, we have revised the abstract,
how it should be beneficial in future research, introduction, and conclusion to clearly highlight the beneficiaries of this research and its potential
the necessity of performing the research contributions to future studies.
should be mentioned in the abstract,
Specifically, we emphasize in the abstract that this work can benefit researchers and engineers as
introduction, and conclusion clearly
mentioned in:

From Line 036 to Line 038: “For geotechnical engineers, the mix is suitable for foundation soil
and backfill, though further studies into placement techniques, such as deep mixing or drainage
columns, are needed by researchers.”

We have highlighted in the revised introduction that policymakers working in the fields of soil
stabilization, waste management, and sustainable construction can benefit from this research as
follow:

From Line 076 to 079 (135-193): “The integration of recycled plastic wastes in the field of
geotechnical engineering presents a viable and friendly solution in this case (Giri, Panda, &
Sahoo, 2020).”
 From Line 119 to Line 120: “The necessity of this research lies in the increasing demand for
sustainable building materials and the growing challenge of managing plastic waste”

From Line 086 to Line 087: “This study is innovative, as it marks the first use of plastic waste to
improve the properties of Tunis soft soils (TSS).”

From Line 120 to Line 125: “For geotechnical engineers, the mix demonstrates significant
potential as both foundation soil and backfill material, especially with 5% reinforcement, offering
an environmentally sustainable and cost-effective alternative to traditional soil stabilization
methods.

Line 132 to Line 140: “Therefore, to fully optimize the mix’s performance and ensure its
effectiveness in different geotechnical conditions, further research is needed to explore advanced
placement techniques, such as deep mixing or the use of drainage columns.”

The advantages of the results of this study have been elaborated upon to better explain to
geotechnical engineers and researchers how they can benefit from them in the revised conclusion
as mentioned in:

From Line 1170 to Line 1173 (1468-1471):” It was deduced that the speed of the consolidation
process accelerates the consolidation time, thereby shortening construction time

From Line 1183 to Line 1188: To quickly predict Cc values, the LASSO model was selected,
achieving an R² of 0.91 and providing a relationship between Cc, e0 and PW. This improvement
is particularly significant in the design and construction of foundations and slope stability.

From Line 1190 to Line 1193: The results also reveal that the permeability of TSS decreases as
the substitution of PW increases, which can reduce the risk of erosion and TSS weakening under
foundations.

From Line 1207 to Line 1209 (1521-1524): This result is very useful for improving the load-
bearing capacity of the TSS, which obviously contributes to improving the load-bearing capacity
of the foundations.

3-It seems you can explain more about the gap We appreciate this valuable suggestion to further clarify the gaps in existing research and to
and introduce the importance of your research emphasize the importance of this study and its contributions. In the revised version, we have
elaborated in more detail on the research gaps, specifically the poor mechanical properties of
and contributions at the end of the introduction Tunis soft Soil and the growing issue of plastic waste pollution as mentioned in:
part
From Line 073 to line 076 (126-127): Accordingly, Tunisia needs to invest in advancing its
recycling technologies to ensure that plastic waste is processed efficiently and sustainably.
From Line 081 to line 086: Soft soils, which typically have low bearing capacity and high
compressibility are a common challenge in many regions of Tunisia. Construction on such soils
is difficult and often necessitates the use of deep foundations or improvement techniques to ensure
the stability of structures making costs more expensive
Additionally, we have highlighted the significance of the study's contributions at the end of the
introduction, in order to provide readers with a clearer and more focused context for the research
as follow:
From Line 119 to line 120: The necessity of this research lies in the increasing demand for
sustainable building materials and the growing challenge of managing plastic waste
From Line 120 to Line 125: For geotechnical engineers, the mix demonstrates significant potential
as both foundation soil and backfill material, especially with 5% reinforcement, offering an
environmentally sustainable and cost-effective alternative to traditional soil stabilization methods.
4- Please try rewriting abstract. Start with Thank you for this constructive feedback on the abstract. We appreciate this suggestion to provide
explaining the goals of this research, a clearer structure, starting with the goals of the research, followed by the methodology,
contributions, a summary of the results, and the significance of these findings. In the revised
methodology, contributions, and a summary of abstract, we have reorganized the content to better align with these recommendations.
results and the reasons for the importance of Specifically, we have emphasized the main objective, which is the improvement of Tunis soft
the results. Please explain more about the soils using plastic waste. We have also highlighted the key methodology, which is the pre-
consolidation process, and the contributions of the study, including the increase in the optimum
results and benefits of the manuscript in the
reinforcement of plastic waste in soil to 5%, as well as the offer of a more stable soil foundation.
abstract and try to shorten the length of the Additionally, we have included a more concise summary of the results and their importance, such
abstract as necessary. as reductions in consolidation time and void ratio, along with an increase in undrained cohesion.
We have also reduced the length of the abstract from 14 lines (173 words) in the original
submission to 12 lines (146 words) in the revised version, as suggested, while ensuring that all
essential information is effectively conveyed.
We thank the reviewer once again for these valuable suggestions.
5-Please explain more about the results and We appreciate the helpful suggestion to elaborate further on the results and benefits of the
manuscript. In the revised version, we have provided a more detailed explanation of the key
benefits of the manuscript in the abstract and
findings, including the reduction in consolidation time, the decrease in void ratio, and the increase
try to shorten the length of the abstract as in undrained cohesion. Additionally, we have emphasized the optimal 5% reinforcement of plastic
necessary waste in the soil as mentioned below:
From Line 033 to Line 036: “Experimental results show that the addition of plastic waste
significantly reduces consolidation time, void ratio while increasing undrained cohesion.
Moreover, the optimum percentage of plastic waste identified in this study is 5%, exceeding the
4% reported in several previous studies
We also discuss the broader implications of these results, including their potential benefits for
improving soil stability and promoting sustainable construction practices through the use of
plastic waste. Additionally, we have paved the way for future research into effective techniques
for implementing this reinforced soil in practical applications as follow:
From Line 036 to Line 038: For geotechnical engineers, the mix is suitable for foundation soil
and backfill, though further studies into placement techniques, such as deep mixing or drainage
columns, are needed by researchers.

6-Please check for if any grammar, Thank you for the thorough review and for suggesting a check for grammar, punctuation, and
typographical errors. In the revised manuscript, we have carefully reviewed the entire text and
punctuation, and typo errors throughout the
made corrections as needed. All changes have been highlighted in yellow for your convenience.
manuscript, and rewrite the sentences. please We have also rewritten certain sentences to improve clarity and readability. we've also rewritten
double-check the manuscript for any typo error some sentences to make them clearer and more readable, instead of the black strikethrough
sentences in the initial draft. We believe these revisions enhance the overall quality of the
throughout the manuscript.
manuscript.
Thank you again for this valuable feedback.