Comment on “Laboratory Measurement and Analysis of the Deteriorated Layer Permeability Coefficient of Soil–Cement Deteriorated in a Saline Environment”

The derivation of a formula to compute the permeability coefficient in the commented paper assumes that the mass flow is homogeneous in a homogeneous layer of a specimen. This assumption is not correct when there is also, at least, one heterogeneous layer. Moreover, a mathematical lapse was found on one equation, that would prevent the right computation of the permeability coefficient, even if the assumption was correct. Although this does not invalidate the major conclusions of the study and has not an outstanding effect on the presented results, for the sake of rigor and sound background for future studies in this field, corrections to the published formulas and model are proposed.

The investigation published in Materials, by Jin et al. [1], was found interesting and invited further analysis, that resulted in the present Comment. In fact, two issues were found.
The first issue consisted of a lapse in the development of the formula to compute the permeability coefficient, in a specimen with different media. The mistake occurred when the authors, after substituting Equation (6) in Equation (4), divided the resulting expression by H, to introduce the R H term. Besides the several ratios d/H, the ratio H m /H was amiss also taken as R H . Thus, Equation (7) of Reference [1] should be Consequently, the formulas to compute parameters A and B of Equation (8) in Reference [1], are not correct (assuming that Equation (9) is intended to provide the formula for A). The correct formulas are for Equation (9) of Reference [1], and for Equation (10) of Reference [1]. This mistake is made clear in the results for the soil-cement with a cement content of 10%, tested at 45 days. From Figure 10 of Reference [1], k 0 and k c have the same value (near 0.32 × 10 −8 cm/s), while k d has a different value (near 0.17 × 10 −8 cm/s, following Figure 12 of Reference [1]), that cannot be correct. If the permeability coefficients of the two different media of a specimen are different, then the permeability coefficient of the specimen shall lay between the permeability coefficients of the different media. If both media have the same permeability coefficient, then the permeability coefficient of the specimen will be equal to the permeability coefficient of the media. This will happen if Equations (2) and (3) are used (together with Equation (8) of Reference [1]).
Secondly, a more important issue, that justifies this Comment, was judged to exist in the background of the derivation of the formula to compute the permeability coefficient in a specimen with two different media.
In the following, two layers of different media crossed by a perpendicular flow are designated as serial association (SA), and two layers of different media being crossed by mass in a flow that is parallel to both are designated as parallel association (PA).
Both (SA and PA) exist in a non-completely deteriorated soil-cement specimen. The question is in that Jin et al. [1] adopt an approach considering that the mass flow in a homogeneous layer is homogeneous as well, regardless of eventual heterogeneities in other layers previously crossed by mass. Actually, in a layer with PA there will be different flows, and the continuity of the flow(s) imposes that in subsequent layers, even though they are homogeneous, there will be different flows as well. Figure 1 depicts the approach adopted in Reference [1] and the approach suggested in this Comment.
Materials 2020, 13, x FOR PEER REVIEW 2 of 5 the permeability coefficient of the specimen shall lay between the permeability coefficients of the different media. If both media have the same permeability coefficient, then the permeability coefficient of the specimen will be equal to the permeability coefficient of the media. This will happen if Equations (2) and (3) are used (together with Equation (8) of Reference [1]). Secondly, a more important issue, that justifies this Comment, was judged to exist in the background of the derivation of the formula to compute the permeability coefficient in a specimen with two different media.
In the following, two layers of different media crossed by a perpendicular flow are designated as serial association (SA), and two layers of different media being crossed by mass in a flow that is parallel to both are designated as parallel association (PA).
Both (SA and PA) exist in a non-completely deteriorated soil-cement specimen. The question is in that Jin et al. [1] adopt an approach considering that the mass flow in a homogeneous layer is homogeneous as well, regardless of eventual heterogeneities in other layers previously crossed by mass. Actually, in a layer with PA there will be different flows, and the continuity of the flow(s) imposes that in subsequent layers, even though they are homogeneous, there will be different flows as well. Figure 1 depicts the approach adopted in Reference [1] and the approach suggested in this Comment. Knowing that the overall permeability coefficient for permeation in SA, km,SA, is where ki is the permeability coefficient of material i and Li is the layer thickness of material i, as well as that the overall permeability coefficient for permeation in PA, Km,PA, is where ki is the permeability coefficient of material i and Li is the layer thickness of material i, it is easy to repeat the approach followed by Jin et al. [1], where the overall permeability of the specimen, kc, will be computed as where d is the thickness of the deteriorated layer, Hm is the thickness of the layer with PA, kd is the permeability coefficient of the deteriorated material and km,PA is the equivalent permeability coefficient of the materials in PA, calculated from the following equation Knowing that the overall permeability coefficient for permeation in SA, k m,SA , is where k i is the permeability coefficient of material i and L i is the layer thickness of material i, as well as that the overall permeability coefficient for permeation in PA, K m,PA , is where k i is the permeability coefficient of material i and L i is the layer thickness of material i, it is easy to repeat the approach followed by Jin et al. [1], where the overall permeability of the specimen, k c , will be computed as where d is the thickness of the deteriorated layer, H m is the thickness of the layer with PA, k d is the permeability coefficient of the deteriorated material and k m,PA is the equivalent permeability coefficient of the materials in PA, calculated from the following equation where k 0 is the permeability coefficient of the sound material, and A d and A 0 are the areas of deteriorated and sound material, respectively, in the cross-section of the specimen, within H m . Introducing Equation (7) in Equation (6) and making A 0 /(A d + A 0 ) = R a , an equation equivalent to Equation (1) will be achieved.
However, considering the approach suggested in this Comment, a different k c is obtained. In this case where k m,SA is the permeability coefficient of the materials in SA, calculated as Finally, introducing Equation (9) in Equation (8) and transforming variables in order to consider the same parameters as in Equation (1), results in Although Equation (10) is quadratic in k d , considering the available computation means, its numerical solution to find k d is advised, for the sake of reliability. In Figure 2, a comparison between the results presented in Reference [1] and those obtained with Equation (10)  where k0 is the permeability coefficient of the sound material, and Ad and A0 are the areas of deteriorated and sound material, respectively, in the cross-section of the specimen, within Hm. Introducing Equation (7) in Equation (6) and making A0/(Ad + A0) = Ra, an equation equivalent to Equation (1) will be achieved.
However, considering the approach suggested in this Comment, a different kc is obtained. In this case where km,SA is the permeability coefficient of the materials in SA, calculated as Finally, introducing Equation (9) in Equation (8) and transforming variables in order to consider the same parameters as in Equation (1), results in Although Equation (10) is quadratic in kd, considering the available computation means, its numerical solution to find kd is advised, for the sake of reliability. In Figure 2, a comparison between the results presented in Reference [1] and those obtained with Equation (10) is shown.
A mean relative error of 35% was observed. Although it was not a considerable error, having presented the range of permeability coefficients in soil-cement, the scientific soundness requires the application of Equation (10) instead of Equation (1).
At last, an update of the function to model the evolution of the permeability coefficient of a deteriorated part of soil-cement is also recommended. Therefore, fitting Equation (12) of Reference [1], copied below, to the kd values computed from experimental results, through Equation (10), the parameters presented in Table 1  A mean relative error of 35% was observed. Although it was not a considerable error, having presented the range of permeability coefficients in soil-cement, the scientific soundness requires the application of Equation (10) instead of Equation (1).
At last, an update of the function to model the evolution of the permeability coefficient of a deteriorated part of soil-cement is also recommended. Therefore, fitting Equation (12) of Reference [1], copied below, to the k d values computed from experimental results, through Equation (10), the parameters presented in Table 1 were obtained. The corresponding functions of permeability coefficient evolution in time are shown in Figure 3.  It shall be mentioned that the permeability coefficient considered for the soil-cement from Qingdao Port was 14.06 × 10 −8 cm/s, corresponding to the mean value of 13.96 × 10 −8 cm/s, 13.70 × 10 −8 cm/s and 14.52 × 10 −8 cm/s, instead of 14.41 × 10 −8 cm/s. Furthermore, it is worth noticing that ku = 9.74 × 10 −8 cm/s for soil-cement with a cement content of 15%, as found in Table 1 of Reference [1], cannot be correct, once there are two intermediate kd values (at t = 90 d and t = 7300 d) that are greater than it.
Funding: This research received no external funding.

Conflicts of Interest:
The author declares no conflict of interest.

A0
Non-deteriorated area in the cross-section of the specimen Ad Deteriorated area in the cross-section of the specimen d Deterioration depth H Total height of the specimen Hm Height of the non-contaminated portion of the specimen k0 Permeability coefficient of non-deteriorated soil-cement kc Overall permeability coefficient of soil-cement specimen kd Permeability coefficient of deteriorated soil-cement ki Lower asymptote of logistic function km,PA Equivalent permeability coefficient for parallel association of materials km,SA Equivalent permeability coefficient for serial association of materials ku Upper asymptote of logistic function p Shape parameter of logistic function PA Parallel association of materials  It shall be mentioned that the permeability coefficient considered for the soil-cement from Qingdao Port was 14.06 × 10 −8 cm/s, corresponding to the mean value of 13.96 × 10 −8 cm/s, 13.70 × 10 −8 cm/s and 14.52 × 10 −8 cm/s, instead of 14.41 × 10 −8 cm/s. Furthermore, it is worth noticing that k u = 9.74 × 10 −8 cm/s for soil-cement with a cement content of 15%, as found in Table 1 of Reference [1], cannot be correct, once there are two intermediate k d values (at t = 90 d and t = 7300 d) that are greater than it.