A Numerical Investigation of Induced and Embedded Trench Installations for Large-Diameter Thermoplastic Pipes under High Fill Stresses

Round 1

Reviewer 1 Report

In this paper, the mechanical properties of the Induced Trench Installation (ITI) and Pre-Embedded Trench Installation (ETI) models for large diameter thermoplastic pipes under high filling stresses are investigated by numerical analysis in a pipe-EPS-backfill system. a number of points need clarifying and certain statements require further justification. There are given below:

1. Figure 1, Figure 2, Figure 3, Figure 5: These figures in your paper are a bit blurry. Please consider replacing them with clearer ones.

2. Page 6, Page 7, Page 10, Page 20: The figures, tables and formulas in the manuscript are recommended to be arranged more neatly.

3. Figure 4: The “Fill height” is recommended to be marked in the picture.

4. Figure 12: The title is a comparison between different tubes, while the picture is written with different stiffness, Make sure these are not contradictory.

5. Page 8, 2nd paragraph, 1st sentence: Since it is the verification of the numerical model, it is recommended to briefly analyze the reasons why both horizontal and vertical stresses are greater than the measured values.

6. Page 19, 1nd paragraph-5nd paragraph: These paragraph is not part of Pipe material (HDPE–PVC) effects and a separate heading is recommended.

Author Response

Comments and Suggestions for Authors

In this paper, the mechanical properties of the Induced Trench Installation (ITI) and Pre-Embedded Trench Installation (ETI) models for large diameter thermoplastic pipes under high filling stresses are investigated by numerical analysis in a pipe-EPS-backfill system. a number of points need clarifying and certain statements require further justification. There are given below:

1. Figure 1, Figure 2, Figure 3, Figure 5: These figures in your paper are a bit blurry. Please consider replacing them with clearer ones.

Response 1: The authors appreciate the reviewer’s comment. The specified figures are placed more clearly in the text of the manuscript.

2. Page 6, Page 7, Page 10, Page 20: The figures, tables and formulas in the manuscript are recommended to be arranged more neatly.

Response 2: The authors appreciate the reviewer’s comment. The figures, tables and formulas mentioned are arranged in the manuscript.

3. Figure 4: The “Fill height” is recommended to be marked in the picture.

Response 3: The authors appreciate the reviewer's comment. In Figure 4, the surcharge stress is added depending on the embankment fill height.

4. Figure 12: The title is a comparison between different tubes, while the picture is written with different stiffness, Make sure these are not contradictory.

Response 4: The correction indicated in Figure 12 has been made.

5. Page 8,2nd paragraph, 1st sentence: Since it is the verification of the numerical model, it is recommended to briefly analyze the reasons why both horizontal and vertical stresses are greater than the measured values.

Response 5: The authors appreciate the reviewer’s comment. The soil environment in which they are located affects the pressure cell used in field measurements. The response of a pressure cell as an inclusion within a soil medium depends on the interaction between the cell and soil, which is controlled by many factors such as soil type and grain size, cell type, cell-to-soil stiffness ratio, aspect ratio, membrane deflection, installation procedure, etc. (Talesnick, 2013; Weiler & Kulhawy,1982; Abbott, 1967; Monfore, 1950). Field measurement results show that the pipe vertically deflects under the applied stress, and the vertical stresses decrease by developing a positive arching mechanism. Depending on the vertical deflection of the pipe, the arching rate changes, and passive thrust is applied to the side wall of the pipe depending on the amount of stress transferred over the pipe to the side backfill and the rigidity of the backfill. Thus, the vertical and horizontal stresses acting on the pipe show a redistribution due to deflection and positive arching, depending on the loading amount, pipe stiffness, and backfill stiffness.

Corrugated thermoplastic pipes were used in the field experiment, the pipe cross-section was modelled as straight-walled in numerical analysis.

Therefore, instead of the values by the field measurement, the percentage changes with respect to the values calculated in the Reference installation were utilized in the discussion of calculated stresses.

REFERENCES

Talesnick, M. L. (2013). Measuring soil pressure within a soil mass, Can. Geotech. J., 50 (7):716–722. dx.doi.org/10.1139/cgj-2012-0347.

Weiler, W. A. & Kulhawy, F. H. (1982). Factors affecting stress cell measurements in soil. J. of the Geotech. Engrg. Div., Proc. ASCE, 1529-1548.

Abbott, P. A. (1967). Arching for vertically buried prismatic structures. J. of the Soil Mec. And Foun. Div., ASCE, 93(SM): 233–255.

Monfore, G. E. (1950). An analysis of the stress distribution in and near stress gauges embedded in elastic soils. Structural Report No. SP 26, U.S. Bureau of Reclamation, Denver, CO.

6. Page 19,1nd paragraph-5nd paragraph: This paragraph is not part of Pipe material (HDPE–PVC) effects and a separate heading is recommended.

Response 6: The authors appreciate the suggestion by the reviewer. The specified sections are presented under the following heading.

4.6. Effect of using EPS together with thermoplastic pipe

The stress values determined around the reference pipe were less than the 30 m fill stress (30 × 20 = 600 kPa) with a unit volume weight of 20 kN/m³. This shows that some positive arching develops due to the vertical deflection of the thermoplastic pipe under the applied stress. However, when EPS Geofoam is used in the ITI and ETI models, the stresses determined at the pipe crown are smaller than the reference. It can be said that the use of EPS Geofoam material together with the pipe increases positive soil arching regardless of pipe deflection and has a positive effect on pipe behaviour.

The uniform placement of the backfill material around the pipe affects the local and general stability of the pipe. While using mechanical tools to place and compact the backfill, care was taken not to damage the pipes. The backfilling process was controlled to prevent excessive and asymmetrical deformation. Non-uniform stresses around the pipe can cause non-uniform deformation, local bending and damage by exceeding the performance limits. Brackman et al. (2008) emphasized that pipe deflection and strain vary according to the type of backfill material and the compaction equipment’s efficiency, especially the uniform placement of the backfill around the pipe. Buried pipe with EPS Geofoam in the ETI model will not require very controlled backfill placement, and more uniform stress distribution will be created around the pipe than in the ITI model.

According to Ma et al. (2019) as stated in the flexible pipe at the low embankment thickness will deform elastically and form the soil arching itself without the necessity of any soft inclusions. However, under high embankment fills, the use of EPS at the top of flexible pipes can help with load reduction and safe design.

In Santos et al. (2020), it is stated that the ITI method provides a better reduction in stress and deflection around the corrugated steel pipe crown, while the ETI method provides a better reduction in soil stress around the pipe spring line.

Author Response File: Author Response.pdf

Reviewer 2 Report

In this study, induced trench installation (ITI) and embedded trench installation (ETI) models for large diameter thermoplastic pipes were investigated by using numerical method. It's a good academic paper.  The only question is that more experimental data should be added to prove the numerical results.

Author Response

Comments and Suggestions for Authors

In this study, induced trench installation (ITI) and embedded trench installation (ETI) models for large-diameter thermoplastic pipes were investigated by using numerical method. It's a good academic paper. 

The only question is that more experimental data should be added to prove the numerical results.

Response 1: The authors appreciate the suggestion by the reviewer. Other papers by the author are available based on Sargand et al 2002 field experiments. Numerical analyzes were carried out at different times. Related publications are listed below.

Akınay E (2010). Analysis on Behavior of Buried Flexible Pipes, MSc. Thesis, Yildiz Technical Univ., Science and Technology Institute, Istanbul, Turkey (in Turkish).

Bozkurt, S. (2021). Reduction of Stresses Acting on Buried Flexible Pipe by Using Compressible Material, Master’s thesis, Yildiz Technical University Science and Technology Institute, Istanbul, Turkey (in Turkish).

Kılıç, H. and Akınay, E. (2019). Investigation of Buried HDPE Pipe Deflection Behavior. Teknik Dergi, 30(5), 9373-9398. doi: doi.org/10.18400/tekderg.397254.

Akinay E. and Kilic H. (2010) “Use of Empirical Approaches and Numerical Analyses in Design of Buried Flexible Pipes”  Scientific Research and Essays, Vol. 5 (24), pp. 3972-3986, ISSN:1992-2248.

Stress and deflection curves of some other pipes from Bozkurt, S (2021) master thesis are presented. Other details can be found in the thesis.

Author Response File: Author Response.pdf

Reviewer 3 Report

This study investigated the induced trench installation (ITI) and embedded trench installation (ETI) of large-diameter thermoplastic pipes subjected to high fill stresses. The study used numerical analysis to evaluate the effects of different backfills, pipe diameters, stiffnesses, and thermoplastic materials on the stresses and pipe deflections. The results showed that the ETI model reduced the stresses acting on the pipe and caused a more uniform stress distribution around the pipe. The diameter of the HDPE pipes and the stiffness of the PVC pipes were found to affect the pipe stresses. The HDPE pipes were found to have lower stresses compared to PVC pipes due to its lower stiffness. The use of EPS in the installation of thermoplastic pipes greatly affected the stress values, leading to a significant reduction in the horizontal stresses that act on the pipe wall. The study provides valuable information for the design and installation of thermoplastic pipes under high fill stresses. A minor language correction may be required for full clarity and precision.

The Authors didn,t write what is a scientific novelty in the presented paper.

I suggest replacing "Findings" with "Results".

The paper includes many editorial errors, for example: "Key words", drawing captions, bold words.

Author Response

Comments and Suggestions for Authors

This study investigated the induced trench installation (ITI) and embedded trench installation (ETI) of large-diameter thermoplastic pipes subjected to high fill stresses. The study used numerical analysis to evaluate the effects of different backfills, pipe diameters, stiffnesses, and thermoplastic materials on the stresses and pipe deflections. The results showed that the ETI model reduced the stresses acting on the pipe and caused a more uniform stress distribution around the pipe. The diameter of the HDPE pipes and the stiffness of the PVC pipes were found to affect the pipe stresses. The HDPE pipes were found to have lower stresses compared to PVC pipes due to its lower stiffness. The use of EPS in the installation of thermoplastic pipes greatly affected the stress values, leading to a significant reduction in the horizontal stresses that act on the pipe wall. The study provides valuable information for the design and installation of thermoplastic pipes under high fill stresses. A minor language correction may be required for full clarity and precision.

The Authors didn,t write what is a scientific novelty in the presented paper.

I suggest replacing "Findings" with "Results".

The paper includes many editorial errors, for example: "Key words", drawing captions, bold words.

Responses: The authors appreciate the suggestion by the reviewer. The manuscript has been reviewed in terms of typographical errors and scientific novelty. Many corrections have been made to them.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments for author File: Comments.pdf

Author Response

This paper evaluates the stresses and deflections in large-diameter pipes by
considering the pipe diameter, stiffness, and backfill performance. The results can provide a
reference for large-diameter thermoplastic pipes under high filling stress conditions.
Therefore, I think this paper can be considered for publication after revision. Please see the
comments below.

1. The pictures, tables, and equations in the article are not aligned, making the overall
   layout of the article somewhat cluttered.

Response 1: The authors appreciate the reviewer’s comment. Manuscript corrections have been made regarding the table, figures and format.

2. 
   The resolution of the pictures in the article is not very clear, and it is recommended
   to insert vector graphics.

Response 2: The authors appreciate the reviewer’s comment. Figures have been revised. Vector graphics could not be added because the model was too large.

3. Some English phrases in the article are not used properly. For example, in line 12 of
   the abstract, "decrease by 65 to 45%" should be "drop from 65 to 45%"; in line 9 of the
   third paragraph of 3.4, "as a result of" is to indicate what the cause is, "as a result" indicates
   the result, should be changed to "as a result".

Response 3: The authors appreciate the reviewer’s comment. The specified corrections were made, the manuscript was reviewed and similar errors were corrected.

4. 
   In the first paragraph of 3.4, why did you select only the results of Pipeline 4 and
   Pipeline 14?

Response 4: The authors appreciate the reviewer’s comment. There is 6.1m of embankment on Pipe 4 and 12.2m of embankment on Pipe 14. Pipe4 and Pipe14 were chosen to represent the two fill levels. Numerical analysis results for other pipes are presented in Bozkurt’s (2021) master thesis. Stress and deflection curves of some other pipes from Bozkurt, S (2021) master thesis are presented. Other details can be found in the thesis.

Bozkurt, S. (2021). Reduction of Stresses Acting on Buried Flexible Pipe by Using Compressible Material, Master’s thesis, Yildiz Technical University Science and Technology Institute, Istanbul, Turkey (in Turkish).

5. In the second paragraph of 3.5, why are stresses and deflections evaluated but not
   deflections, when it was mentioned earlier that pipe performance is judged based on
   deflections and stresses?

Response 5: The authors appreciate the reviewer’s comment. Within the scope of this study, horizontal and vertical deflections are presented under the title of “4.5 Vertical and horizontal deflections”.

6. In the first paragraph of 3.6, why is the horizontal coordinate of the intersection of
   the first and second sections of EPS15 determined to be 1.45% of the axial strain, and what
   is the rationale for the 1.45%?

Response 6: The authors appreciate the reviewer’s comment. In numerical analysis, the values of the modulus of elasticity determined experimentally according to the strain level for EPS15 material were used. E₁ = 3560 kPa in the range of 0 – 1%, E₂ = 470 kPa in the range of 2 – 6% and E₃ = 170 kPa in the range of 6 – 30%. The 1.45% strain value indicates the yield limit of EPS15 material. The experiments carried out within the scope of the study were carried out according to the standards stated below.

ASTM D1621 Standard describes the procedure for determining the compressive properties of rigid cellular polystyrene (RCPS; i. e., EPS and XPS), while ASTM D6817 Standard specifies the minimum compressive requirements (Table 1, row 2) for RCPS products of different densities. The compressive resistances presented in Table 1 in ASTM D6817 are the values that should be achieved when test procedure (i. e., the sample dimensions and shearing rate) is in accordance with ASTM D1621 Standard.

According to ASTM D1621, the test specimen shall be square or circular in cross-section with a minimum of 25.8 cm² (4 in²) and maximum of 232 cm² (36 in²) in area. The minimum height shall be 25.4 mm (1 in) and the maximum height shall be no greater than the width or diameter of the specimen. The shear rate should be equal to 10 % of the sample thickness per minute ±25 mm (±01 in.)/minute. Summarily, ASTM D1621 and ASTM D6817 Standards are used to evaluate the quality control of RCPS products.

In this study (this manuscript) using cylindrical samples with a diameter and a height of 50 mm have been adopted. As the shearing rate, based on the studies mentioned in the literature, selecting the value of 1% strain/minute has been found proper by the authors for drained unconfined compression tests (Akınay 2017).

REFERENCES:
ASTM D1621. Standard test method for compressive properties of rigid cellular plastics.
American Society for Testing and Materials, West Conshohocken, Pennsylvania,
USA.
ASTM D6817/D6817M. Standard specification for rigid cellular polystyrene geofoam.
American Society for Testing and Materials, West Conshohocken, Pennsylvania,
USA.

Akınay, E. (2017). Investigating the effect of using compressible bedding material on the
behavior of buried flexible pipes. Ph.D. thesis, Science and Technology Institute, Yildiz
Technical Univ., Turkey, 356 p. (in Turkish).

7. In the first paragraph of 4.1, only the pipe diameters of 1.524m and 0.762m were
   considered in the test, but the results of other tests conducted from 0.762m to 1.524m were
   described earlier.

Response 7: The authors appreciate the reviewer’s comment. In numerical analysis, to examine the effect of pipe diameter on stresses, HDPE pipes of 1.524 m in diameter –specified as F type – and 0.762 m in diameter – specified as C type –were considered and are shown in Tables 1 and 3. The pipes specified in the Table 1 below were used in the comparisons made in this manuscript.

Table 1. Field test pipe properties and installation conditions.

⁽¹⁾Sargand et al. (2002) for details, ⁽²⁾Initial values, Cr. S. – Crushed Stone, RC- Relative compaction.

Table 3. Parameters of pipe types.

8. 
   In the last paragraph of 4.1, the experimental data is described according to the pipe
   diameter, but at the end, the type of backfill is added to affect the stress on the pipe, so how
   is it affected? Why is this not described?

Response 8: The authors appreciate the reviewer’s comment. The following information has been added to section “4.1 Pipe diameter effect”. In addition, information is given in other parts of the article.

The type of backfill soil around the pipe also affected the stresses. As can be seen from Figs. 7 and 8, high stresses occurred in clay, sand and crushed stone backfills respectively. Similar stress distributions in thermoplastic pipes have been obtained in experimental and numerical analyses examining the pipe-soil interaction (Kang et al. 2007a; Brachman et al. 2008; Kang et al. 2009; Fang et al. 2019 and 2020).

In Fig.13, for the models using EPS with 0.762 m diameter PVC pipe, VAF values were very close to crushed stone and sand backfills. In the ITI and ETI models, VAF decreased to 0.3 and 0.2, respectively, and to 0.4 and 0.34 in clay backfill. HAF decreased up to 0.3 and 0.25 in the ITI and ETI model for crushed stone and sand backfills and up to 0.36 and 0.30 in clay backfill, respectively. In Fig.14, the VAF values for crushed stone and sand backfills were very close to each other in the 1.524 m diameter HDPE pipe. In the ITI and ETI models, VAF decreased to 0.3 and 0.25, respectively, and to 0.4 and 0.34 for clay backfill. HAF decreased up to 0.3 and 0.25 in the ITI and ETI models for crushed stone and sand backfills and up to 0.45 and 0.36 in clay backfills, respectively.

4.5. Vertical and horizontal deflections

In Figs.15 and 16, vertical and horizontal deflections are presented for 0.762 m diameter PVC and 1.524 m diameter HDPE pipes, respectively. In the reference, the highest deflection was obtained with clay, sand and crushed stone backfill, respectively. In Fig.15, the lowest deflections were obtained with the 0.762m diameter PVC pipe in the ITI model compared to the reference. Horizontal deflection decreased from 3.9% to 1.4%, from 5.7% to 1.5%, and from 7.5% to 0.2%, respectively, for crushed stone, sand and clay backfills. The vertical deflections decreased from −4.5% to −1.4%, from −6.2% to −1.5% and from −8.1% to −0.3%. The deflections obtained in the ETI model were greater than in the ITI model. In Fig.16, the lowest deflections were obtained with the 1.524 m diameter HDPE pipe with the ITI model compared to the reference. Horizontal deflections decreased from 2.62% to 0.70%, from 4.40% to 0.92% and from 6.85% to 1.00%, respectively, for crushed stone, sand and clay backfills. The vertical deflection values decreased from −4.77% to −1.7%, from −6.7% to −1.88% and from −9.32% to −0.25%. The deflections obtained in the ETI model were greater than in the ITI model.

Author Response File: Author Response.pdf

Reviewer 5 Report

The research content of this paper is very interesting. The large diameter Thermoplastic pipe as the research object, and it gives meaningful results. It is recommended to publish.

“EPS Geofoam”-This should be given its full name.

Author Response

Comments and Suggestions for Authors

The research content of this paper is very interesting. The large-diameter Thermoplastic pipe is the research object, and it gives meaningful results. It is recommended to publish.

“EPS Geofoam”-This should be given its full name.

Response : “EPS Geofoam” it is defined as “Expanded Polystyrene Foam (EPS) Geofoam”.

Author Response File: Author Response.pdf