Pál Vásárhelyi Doctoral School of Civil Engineering and Earth Sciences 

Department of Engineering Geology and Geotechnics

Supervisor: Dr. Nagy László

Investigation of the Properties of Dispersive Clays

Introducing the research area

The stability of flood protection structures in Hungary is a top priority, which is justified by the nearly 4200 km long embankments in the country. When constructing dams, a rule of thumb is that leakage related problems can be prevented by using cohesive soil in the embankment so that stability can be ensured. The structure of cohesive soils is similar in many cases to a dispersed system where the size, the quantity and the distribution of particles and the forces and impacts between each particle determine the properties and changes of the system.

During my PhD research, properties of dispersive soils were investigated on soil samples collected from various locations in Hungary to obtain as accurate results as possible.

Brief introduction of the research place

Most of my research is conducted in the Soil Mechanics Laboratory of the Department of Engineering Geology and Geotechnics of the Budapest University of Technology and Economics, the Faculty of Civil Engineering Material Testing Laboratory and the Institute for Soil Sciences and Agricultural Chemistry of the Hungarian Academy of Sciences. The supervisor guiding my doctoral research is Dr. László Nagy, author of a number of publications related to national and international flood control.

History and context of the research

Damages of flood protection structures can vary and can be linked to a number of factors. Different failure mechanisms and related soil mechanics can be well matched, piping, slope failures, drying cracks are all well distinguishable phenomena and the identification of the associated geotechnical problems is important for proper interventions. This seems to be an exception to the failure mechanism of dispersive clays , because  the method of degradation, the tunnel erosion (Figure 1) can be easily discovered on the spot, however, the type of soil in which the steady erosion can occur is difficult to "grasp".

 

Figure 1.Tunnel erosion in dispersive embankment based on [H7]

The studies on dispersive soils started in the United States, analyzing the damages to conventionally stable embankments. They discovered a property of cohesive soils (which were only connected to granular soils) that they erode when interacting with water. In the 1970’s, the research focus broadened, when additionally to treating the damages, the causes of the problem were also investigated [H4]. Nowadays, the main object of the researches is to choose the most suitable tool for soil treatment.

The research goal, open questions

The aim of my research is to reveal the behavior of dispersive soils and the properties of soils susceptible to erosion. During my research, the emphasis is on the property that is responsible for the tendency to erode in dispersive soils. The reasons for the "unfavorable" soil structure (Figure 2) are studied by phase analytical methods (X-Ray diffraction and DTA analysis) and to find an identification method of dispersive properties in the composition of soils [S4].

 

Figure 2. Connections between soil grains based on [H2]

The first step in the process of damaging the dispersive clay embankments is the formation of a secondary gap, a leakage channel in the soil structure. In most cases, this is a crack caused by temperature fluctuations, through which the water can reach the soils and then erode the layers, thus damaging the structure.

I examine the variability properties of soils, including the Galli-type void ratio [S2], and I apply measurement methods such as the double hydrometer test, which being a modified version of a routine soil mechanics test can be more easily implemented in practical use.

An important research line is the way and preconditions of applying dispersive clays in earth structures [S5] after soil treatment was used. This requires including tests of various kind of soil treatment methods and materials in my research schedule.

Methods

The standards background

Studies of dispersive soils cover relatively rarely applied methods in geotechnics. During my doctoral research, I thoroughly examined the methods used for the tests of dispersive soils proposed by the current standard MSZ-EN 1997-2-2008 (Eurocode 7).

 

Reference test

It is important to emphasize that both the reliability and the applicability of the pinhole test (Figure 3), which examines the question on the hydraulic principle, that is, what hydraulic condition is necessary for the cohesive soil to erode and the particles to break apart. In this case, in the studied soil sample, in addition to different hydraulic gradient values ​​(at different pressure levels), the degree of expansion of the artificially created path (pinhole) of the sample is investigated, and also the degree of water movement that damages the soil structure.

 

Figure 3. Pinhole test device

Phase analytical studies

The X-ray diffraction (XRD, X-Ray Diffraction) and DTA (Differential Thermal Analysis) measurements can be used to determine the mineral composition of soils and the proportion of dominant minerals in the composition [H1]. The X-Ray diffractogram of the samples provides the mineral constituents, while the derivatograph shows the ratio of the determined components (Figure 4). They can be evaluated together with the degree of dispersion of the soils, so it provides more useful information on the reason of “unfavorable” soil properties.

 

 

Figure 4. X-Ray diffractograms (left) and DTA analysis results (right) of dispersive clays

Geotechnical investigations

Double hydrometer test (SCS dispersion test) was developed by the American Soil Conservation Service. The test involves two hydrometer tests. One is the "traditional" hydrometer test where a dispersing agent is added to the soil slurry so that the soil particles do not bond together and the measurement gives the proportion of the individual fractions within the sample according to the Stokes Law. In parallel, another sedimentation has to be carried out, but without the addition of a dispersing agent, the grains, if they are able to link together, should also be determined and the resulting grain size distribution of the composition should be determined. Based on the two measurements, the (weight) percentages of particles with a diameter of 0.005 mm are compared to determine the degree of dispersion.

Results

During the exploration of the properties of dispersive soils, I have identified several research directions, ranging from the detection of the properties of dispersive soils to the treatment of proven dispersive soils, and to the exploration of the relationship between soil sciences and geotechnical engineering.

Test methodology

In addition to the standard specification, many other test methods for dispersive soils are available, so in my research I have included a uniform framework for identifying dispersive soils so that "simpler" methods can be used as a filter condition (Figure 5). Using the methodology, a large number of sample sizes can be rationalized, so that the samples can be evaluated more efficiently.

  

 

 

Figure 5. Recommended test program

Soil treatment

After experimentally detecting the presence of dispersive soils:

1. When choosing fill material, it is possible to decide whether or not the soil can be used or its treatment can be designed in case of unfavorable behavior;

2. When examining the existing fill material, interventions can be taken to prevent and treat the erosion problem caused by the dispersive behavior. For a large number of samples, it may be useful to use the described test program, which by means of "simpler" preliminary tests reduces the number of soil samples that are to be considered from a dispersive behavior.

In the presence of dispersive soil detected in existing embankments, the most important thing is that the leakage between the upstream and the downstream face, as well as the water originated from geometry and drainage conditions, should not get in touch with the dispersive layer / layers, as this may cause leaching of the fill material. Accordingly, interventions are needed that either remove the critical layers or alter the dispersive properties of the soil, [H3], or it is required to incorporate structures or materials that lead leakage water away from or prevent contact with the dispersive soil layers.

Although calcareous soil treatment procedure is known in Hungary for improving the strength properties of soils [H6], no domestic experimental results are available for the improvement of dispersive soils. I studied the lime treatment of several dispersive (D1, D2) soil sample groups in Hungary with measurements of the dispersion of the various cohesive soils by calcareous soil treatment (Figure. 6) [S1], [S3].

 

 

 

Figure 6. Effect of soil treatment on the plasticity of the soil

 

 

Expected impact and further research

During my PhD research I learned about the problem of dispersive soils with several directions of investigation, which would be useful to address further. Among these issues, the emphasis should be on double hydrometer analysis, clarification of the usability of phase analytical methods for dispersed soil, and the use of geophysical test methods to locate the soils with unfavorable properties on the site.

My aim is to draw up principles that can be used in the practice and can be the basis for further research on dispersal soils.

Publications, references

Related own publications

[S1] Nagy G. (2017): Effect of soil treatment and mixing on the dispersive behavior of soils, In: Proceedings of the 6th International Young Geotechnical Enineers’ Conference (iYGEC6), Szöul, Korea (accepted)

[S2] Nagy G., Nagy L. (2015b): A Galli-féle mértékadó hézagtényező használata kötött talajok jellemzésére (in Hungarian). In: Török Á., Görög P., Vásárhelyi B. (szerk.): Mérnökgeológia-Kőzetmechanika 2015, pp. 355-362

[S3] Nagy G., Nagy L. (2015c): Identification and Treatment of Erodible Clays in Dikes. In: Schweckendiek, T., van Tol, A. F., Pereboom, D., van Staveren M. Th., Cools, P. M. C. B. F. (eds.): Geotechnical Safety and Risk V. ISBN: 978-1-61499-1. Rotterdam, The Netherlands, pp. 530-534.

[S4] Nagy G., Nagy L., Kopecskó K. (2016): Examination of the physico-chemical composition of dispersive soils, Periodica Polytechnica-Civil Engineering, Vol. 60.(2), pp. 269-279.

[S5] Nagy G., Nagy L. (2016): Meszes kezelés hatása diszperzív talajok vizsgálata (in Hungarian), XXXIV. Hidrológiai Vándorgyűlés, 2016.06.06-08, Debrecen, Hungary

 

Reference list.

 

[H1] Kopecskó K. (2002): Derivatográfia és Rtg-diffrakció mérnöki feladatok megoldásában (in Hungarian). In: Proceedings of the 10th International Conference on Civil Engineering and Architecture, pp. 134-141. ISBN 973 85809 00.

 

[H2] Mitchell, J. K. (1974): Fundamentals of Soil Behavior. Wiley Publications, New York, NY. 592 p. ISBN 978 0 471 46302 3.

 

[H3] Savas, H. (2015): Consolidation and swell characteristics of dispersive soils stabilized with lime and natural zeolite. In: Science and Engineering of Composite Materials. Vol. 23.(6), pp. 589–598, ISSN (Online) 2191-0359, ISSN (Print) 0792-1233.

 

[H4] Sherard, J. L., Decker, R. S., Ryker, N. L. (1972): Piping in earth dams of dispersive clays. In: Proceedings of the ASCE Specialty Conference on the Performance of Earth Structures, pp.589-626.

 

[H5] Sherard J. L., Dunnigan L. P., Decker R. S. (1976): Pinhole Test for Identifying Dispersive Soils. In: Geotechnical Engineering Division, ASCE, Vol. 102, No. GT 1, pp. 69-85.

 

[H6] Szendefy J. (2013): Impact of the soil stabilization with lime. In: Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering: Challanges and Innovations in Geotechnics. Paris, France. pp. 2601-2604. ISBN: 978 2 85978 478 2.

 

[H7] Szepessy J. (1983): Szemcsés és kötött talajok járatos eróziója, illetve megfolyósodása árvízvédelmi gátakba. A veszély mértéke, csökkentése (in Hungarian), Hidrológiai Közlöny, 1983. vol. I. pp. 11-20.