Undrained Pore Pressure Development on Cohesive Soil in Triaxial Cyclic Loading

Round 1

Reviewer 1 Report

Many of the sentences have grammatical errors and sometimes not very clear what they mean. For example:  “Results discussed show that the greater susceptibility of cohesive soil to cyclic loading is linked with their pore pressure accumulation response.”  “In this case, the post-construction settlements may make unable to use the construction.”   A few statements are not necessarily true. The paper discusses a few conducted tests and the observed behavior without any coherent conclusion. It seems like the conducted tests are done without any prior planning. No compression between the test results are done to show the effect of any parameter on any observed behavior.   It is very well-known that pore water pressure generation in sands and clays are mainly affected by the shear strain. The observed behavior is just another confirmation of this fact, eventhough it seems like the authors are not aware of this fact. The cyclic soil behavior topic has been the subject of numerous studies for the past 50-70 years, but it seems like the authors are not aware of that and the findings.   I believe the authors should carefully study what has been done before and then plan a test program to address the subjects not been studied before. I strongly recommend conducting strain-controlled cyclic testing instead of stress-controlled for any meaningful soil-behavior study.

Author Response

Thank You for the review of this manuscript. In the attached file we are sending to you the response to the comments. In the manuscript, we highlighted all changes with a green background.

Sincerely,

Andrzej, Emil, Alojzy and Wojciech.

Reviewer 2 Report

The paper presents experimental data obtained on cohesive soil investigated by means of cyclic triaxial tests. In particular, the study aims to capture the difference between cohesive soils consolidated in isotropic and anisotropic stress conditions.

I have some major comments, regarding the organization of the paper and the presentation of the results. Providing that the authors answer to these points and to some other minor comments, the paper can be published in the Applied Sciences after another round of review.

Major comments

1.      Abstract. The sentence “Therefore, the pore pressure accumulation in the first few cycles plays a key role in total accumulated plastic strains and excess pore pressure” (lines 20-22) seems a contradiction because pore pressures are related to themselves. Please, revise accordingly.

2.      Introduction. The literature section (lines 50-54) should be expanded with some more details about the main findings reported by several authors. The last part of the introduction (after line 70) is not easy to follow since the mentioned literature results are not supported by figures. I suggest that the Authors should improve this section by expanding the main concepts and supporting them with some literature figures. English should be also reviewed to present the concepts in a more clear and understandable manner.

3.      The site where the tested soil has been retrieved should be still roughly introduced at the beginning of section 2.1 and some geological features of this deposit should be added in the text. The sentence “This type of clay is common for the northern and central part of Poland” (line 105) should be moved in the previous section 2.1.

4.      Section titles are often inconsistent with the content of the paragraphs, e.g., the section entitled “sampling method and sample properties” does not report any information on the sampling method as well as no description of the test apparatus is reported in the section “Test apparatus and test procedure”. It is very important that the title section should be informative of the contents reported in it. Please, revise the section or the section title, accordingly.

5.      Please revise the sentence “The saturation step was terminated when the Skempton parameter B was equal to 0.95 which indicates full saturation conditions.” (line 116-117), underlining that full saturation conditions imply a Skempton parameter B equal to 1, but a value of 0.95 has been assumed as a saturation condition from a technical point of view.

6.      Results discussion. The shape of the excess pore water pressure vs. the number of cycles curve is not well-recognized because of the adopted scale of the horizontal axis. A good compromise would be to add a new plot where the excess pore water pressure is reported as a function of a smaller number of cycles (e.g., the number of cycles where the maximum value of Δu is attained). I suggest adding some comments on these plots, highlighting the differences with the typical curves observed in sands and natural silty sands, as reported in the literature (Park et al. 2015; Chiaradonna et al. 2016; Chiaradonna et al. 2018). The study can be further enriched with comparisons between tested soils and sand response during cyclic triaxial tests (Salvatore et al. 2018; 2019). 

Minor comments  

Some typing mistakes and other minor comments are reported in the following list.

1.      “Isotopically” should be “isotropically” see lines 57, 84, 262, 297, 304, 333, 391

2.      I suggest "soil response" instead of "soil responds" see lines 47, 266, 267, 276, 368, 396

3.      “freqiancy” should be “frequency” see line 447

4.      “responds” should be “respond” see line 11

5.      “cyclicloading” should be “cyclic loading” see line 475

6.      “cohesive soils… responds” should be “cohesive soils… respond” see line 11

Finally, the paper is poorly written and English should be reviewed by a native English speaker to improve the text all along with the article.

 

Suggested references

Chiaradonna A., Tropeano G., d’Onofrio A., Silvestri F. (2018). Development of a simplified model for pore water pressure build-up induced by cyclic loading. Bulletin of Earthquake Engineering, 16(9):3627–3652. https://doi.org/10.1007/s10518-018-0354-4.

Chiaradonna A., Tropeano G., d’Onofrio A., Silvestri F. (2016). A simplified method for pore pressure buildup prediction: from laboratory cyclic tests to the 1D soil response analysis in effective stress conditions. VI Italian Conference of Researchers in Geotechnical Engineering, Procedia Engineering, CNRIG2016, 158: 302-307. DOI:10.1016/j.proeng.2016.08.446

Park T., Park D., Ahn J.K. (2015). Pore pressure model based on accumulated stress. Bull Earthq Eng 13(7):1913–1926

Salvatore E., Ando E., Proia R., Modoni G., Viggiani G. (2018). Effect of strain localization on the response of granular materials subjected to monotonic and cyclic triaxial tests. Italian Geotechnical Journal, 52(2), 30-43.

Salvatore E., Spacagna R.L., Andò E. and Ochmanski M. (2019). Geostatistical analysis of strain localization in triaxial tests on sand. Géotechnique Letters 00, 1–6, https://doi.org/10.1680/jgele.18.00228

 

 

Author Response

Thank You for the review of this manuscript. In the attached file we are sending to you the response to the comments. In the manuscript, we highlighted all changes with a green background.

Sincerely,

Andrzej, Emil, Alojzy and Wojciech.

Author Response File: Author Response.doc

Reviewer 3 Report

The presented article deals with an interesting topic regarding the influence of the consolidation stage in the dynamic behavior before cyclical loading, however the investigation has important defects.

The experimental part and the register of tests is very useful for future research but it is clearly scarce; In addition, the work does not have an adequate
experimental strategy and a full discussion with more general conclusions.

Here are some aspects that I believe can help the authors improve their research:

-         A dynamic study of a soil should start from showing the static geomechanical characteristics.

-         It is necessary for the record to be adequate to indicate the geological origin of the samples, their exact location where they were obtained and the depth of the samples. It is also convenient to classify the samples according to the universal classification system.

-         Since it is mainly an experimental work, it is also necessary to comment on the preparation of the samples for testing and indicate more about the conditions of the test.

-         The choice of the confinement values indicated for the 5 samples (45, 90 and 135 kPa on one side and 65 and 125 kPa on the other) and the values of the deviator load should be indicated. Generally the value of the deviator is given in
relation to the one that produces the failure with static load, which allows us
to compare its effect with respect to the maximum deviator possible.

-         There is no comment on the limitations of the test with respect to the maximum strains in the equipment. Has not the strain been limited to any value?

-         It is also commented that a number of cycles from 10000 to 50000 are performed and nevertheless the 5 dynamic tests shown reach only 20000.

-         The study of the dynamic behavior with respect to the generation of interstitial pressure generated is usually normalized with respect to the initial pressure of confinement. In the work, this generated ratio is not commented, which is important to compare what happens with regard to the liquefaction of non-cohesive soils.

-         It is commented that the behavior of the sample of fig.1 follows 3 stages and that the first stage corresponds to the first cycle. But what causes the increase in interstitial pressure
and deformation in this first cycle? If we had the registered static test, we could see if
it is due to the speed of the load or if the stage of discharge and recharge of
the sinusoidal load also has an influence.

-         It is stated that the deformation and interstitial pressure are independent in the 2nd stage. What is the reason for this?. It is not understandable that their dependence is total in the 1st and 3rd stage and however independent in the 2nd; rather, one should think of a more complex dependence
and that it is necessary to investigate.

-         In the 3rd stage of the test of the sample of fig.1, the interstitial pressure and the deformation are stabilized, does this mean (as commented in the introduction) that the critical state of the soil has been reached?

-         Regarding the test plan, the investigation is not well considered. It gives the sensation rather that tests that were available have been taken and afterwards the article has been written as an investigation. The investigation should try to isolate the change variables keeping the remainder constant to study its effect, but in the presented article this is not possible because the 5 tests have all the different variables (eg they have different relative density, confining pressures and deviator load ...) Only the frequency of the tests has been maintained.

-         A dynamic investigation with only 5 tests, with 3 isotropic and 2 anisotropic is clearly insufficient.

-         In the test of the sample of fig. 2 does not seem to distinguish the 3 stages Why in the
first case of Figure 1 there are three stages and not in this one?

-         In the test of figure 2 and figure 3 it does not seem to stabilize the generation of interstitial pressure with the number of cycles Why has it stopped at 20000?

-         Having different confinement values the isotropic tests with respect to the anisotropically consolidated ones, it is inappropriate to check and see their influence, especially being only 5 tests. This is precisely the objective of the article.

-         In the test of figure 5 of anisotropic consolidation, the generation of interstitial pressure with the number of cycles does not seem to be stabilized either.

-         The article reaches conclusions that are already known as the generation of interstitial pressure is fundamentally due to the state of initial density or that the deformation depends mainly on the state of confinement.

-         The different inclination of the graphs of figure 6 and 7 is also commented. The obtained is evident since obviously in the anisotropically consolidated case one starts from a static tangential tension that produces a lower cyclic deformation in each load cycle.

-         It is necessary to differentiate in the article the cyclic deformation of the cumulative permanent, since the evolution of both is completely different. In the anisotropic case the cyclic deformation is much smaller but nevertheless due to the initial static tangential tension, a permanent deformation much greater than in the isotropic case should be produced.

Author Response

Thank You for the review of this manuscript. In the attached file we are sending to you the response to the comments. In the manuscript, we highlighted all changes with a green background.

Sincerely,

Andrzej, Emil, Alojzy and Wojciech.

Author Response File: Author Response.doc

Reviewer 4 Report

This is the discussion on the cyclic test results on clay samples.

This kind of tests were often conducted in the field of earthquake enginerring, from the vie point of liquefaction susceptibiliy.

However, the style of the content presentation in this paper is different from the reearches in these field, the reviewer cannot understand the content clearly.

First of all, the strain level of clay specimen in these tests was not clear.

There are researches on cyclic threshold strain for pore water pressure generation (and some similar threshold strain) , by such as Dobry, Vucetic, and otehr researchers. 

So, the discussion with there threshold strain level shall be incorpolated, referring these related researches.

Second, the effect on the cyclic loading on specimen often indicated by CSR (Cyclic stress ratio). However, it is quite difficut to clarify the CSR level in these tests.

Especially, in K0 conditions.

Maybe, I am not sure the meaning of the indication of Table 3.

I want to know clear idear of sigma_v (sigma_1), sigma_h (sigma_h), and how it varies.

...what is the deviator stress median? why it is not equal to effective confining pressure?

(If it is isotorpically consolidated, it should be equal...I suppose...)

Third, can you show the test results such as Figure 4 and figure 5, in the scalte compatible with Figure 9?

I cannot identify the soil behavior around 300 cycles, which is quite unique.

Also, why Mr, and Mdu were used in Figure 9?

Qualification of the variation of dq, eps_r, du, (the terms in (1) and (2))

will give us beter understanding on the soil behavior.

...I belive dq is constant, if the test is stress controlled. Am I right?

Fourth, please show how the point moved in Figure (4)(e) and Figure (5)(e).

Why there is the line close to origin?

From  Figure (4)(a) and Figure (5)(a), I cannot understand why the effective stress goes close to zero. Excess pore water pressure inclease is limited in these cases.

Also,why the lines in Figure  5(a) are not connected?

Fifth, what is the meaning of the hysteresis in Figure 6 and Figure 7.

Is this excess pore water pressure related to dilatancy? (plastic volumetiic strain?)

For me, it looks just the phase difference (delay of the response of the transducers), 

since the loading is not so slow. 

(I guess, 1Hz loading is sometimes quite severe for machine control, depending on strain level).

Note, 

line 88, meat > mean?

line 263, fires > first?

Author Response

Thank You for the review of this manuscript. In the attached file we are sending to you the response to the comments. In the manuscript, we highlighted all changes with a green background.

Sincerely,

Andrzej, Emil, Alojzy and Wojciech.

Author Response File: Author Response.doc

Round 2

Reviewer 2 Report

The paper presents experimental data obtained on cohesive soil investigated by means of cyclic triaxial tests. In particular, the study aims to capture the difference between cohesive soils consolidated in isotropic and anisotropic stress conditions.

I really appreciated the effort of the Authors to improve the contents of the manuscript and also the Authors’ reply is satisfactory from a technical point of view. Nevertheless, I find that the English language is still under the standard of the journal and, consequently, the results of the study are difficult to convey to the readers. As an example, a list of the problems detected in the abstract is reported in the following.

1.      Line 14: “cyclic loading also will be different” should be better reported in a technical style as “cyclic loading is also different”. The same consideration can be done in lines 23 and 25.

2.      Line 21: the sentence “the pore pressure generated in the first few cycles plays a key role in total accumulated plastic strains and constitutes a major amount of excess pore water pressure” can be rewritten as “the pore pressure generated in the first few cycles plays a key role in the accumulation of plastic strains and constitutes the major amount of excess pore water pressure”

3.      Line 24-26: The sentence can be clearly rewritten as “The results of tests on isotropically and anisotropically consolidated soil samples are discussed in this paper in order to point out the main features of the cohesive soil behavior.”

Grammar mistakes (e.g., line 53: “tie” instead of “the”) and missing words (e.g., line 81: “continuous accumulation of the foundation” likely instead of “continuous accumulation of the foundation settlement”?) need to be corrected and the style need to be revised by a native English speaker in order to improve the readability of the paper.

Author Response

Dear Reviewer,

We wish to thank you for the second round of the revision. We corrected addressed problems with the language and we did check of the manuscript in order to find the misspellings. In attachment we are sending the respond to addressed problems. 

Thank you for your remarks to this manuscript.

Sincerely,

Andrzej, Emil, Alojzy, and Wojciech.

Author Response File: Author Response.doc

Reviewer 4 Report

 

Thank you for the coorection and answer to review comments.

 

Now the reviewer understand the content more, and

I respect to the large volume of cyclic tests.

 

Followings are some comments for more improvement.

1) (related to former review comments #4)

For Figure 2(e), 3(e), 4(e), 5(e), 6(e), 7(e), 8(e), 9(e)

it is better to point out "End of saturation phase",

"Endo of consolidation phase","End of the cyclic...",

as same as the figure in the answer to the reviewer comment.

Otherwise, reader cannot understant the path.

I think, in usual case, the path start fromt the end of the consolidation phase, since the cyclic phase is the main focus.

That's the reason why I could not understand the figure in initial draft, I suppose.

 

2)  (related to former review comments #5)

Still the meaning of the hysteresis is not clear for me.

Figure 10 is de(strain)-du(PWP) relationship. So, it is not equal to

usual hysteresis of shear stress-shear strain relationship.

So, the area is not equal to the disspated energy.

 

The point is, why du=0 at the end of unloading at de=0 (or dq=0).

If it is plastic, residual value shall be remained.

That's the reason why I daughted "delay of the response of the transducuers " in the initial review comment.

 

Note, the explanation of "The range up to 100 points per second" in the answer is not enough to exclude the delay of response.

The issue what I concern is not the sampling frequency, but the machanical behavior of sensors/actuators, and quality of servo for actuator control.

The best way is compare 1 hz reslut and 0.1 hz result, I suppose. But I am not requesting additional test.

Anyway, what I concern is, why both de and du are almost 0 at the end of unloading?

Also, how about the state of de is minus (negative value)? This is cyclicl loading, and de goes both positive value and negative value. Am I right?

3) New equation (3)

dumax is with " ' " as shown in equation, or without " ' ", as shown in line 465? 

Author Response

Dear Reviewer,

We wish to thank You for second round of the revision. In attached file we send You the respond to your remarks. 

Sincerely,

Andrzej, Emil, Alojzy and Wojciech.