MECHANICAL AND PHYSICAL PROPERTIES OF THE MONTMORILLONITIC AND ALLOPHANIC CLAYS IN THE

NEAR-SURFACE SEDIMENTS OF CHALCO VALLEY, MEXICO:

ANALYSIS OF CONTRIBUTIG FACTORS TO LAND SUBSIDENCE

Martín HERNANDEZ-MARIN Department of Geosciences, Virginia Tech, 3051 Derring Hall,

Blacksburg, VA 24061, USA. Tel.: +1-540-2312404. Fax: +1-540-231-3386 mhmarin@vt.edu

Dora Celia CARREON-FREYRE. Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Apartado Postal 1-742, 76001 Querétaro, Qro., México. Tel.: +52-442-2381104 ext. 112

freyre@geociencias.unam.mx Mariano CERCA

Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 México, D. F., México. Tel.: +52-55-56224288 ext. 118

marianoc@geol-sun.igeolcu.unam.mx

Abstract Land subsidence in the Chalco Valley, near Mexico City, is caused by the consolidation of the upper aquitards, which consist mainly of saturated silty-clayey sediments interbedded with sandy lenses of volcanic ash. Detailed characterization of the physical and mechanical properties of the top 15 m of the sequence was performed. Laboratory analyses included gravimetric water content, density, grain size, consolidation tests, and X-Ray diffraction. Five distinctive silty-clayey layers were observed in this sequence with allophanes and montmorillonites as the predominant mineralogy. Results indicate a maximum clay content of 28%, minimum density of 1.61 g/cm3, high porosity of 86 %, and a high gravimetric water content of about 350 %. A maximum high compressibility index of 3.11 was also estimated for the sequence. Our results confirm a progressive increment of fracturing in the upper aquitard. The combined presence of fractures and sandy lenses contribute to the progressive increase of hydraulic conductivity, which accelerates consolidation. We emphasize the relation between clay mineralogy, physical, and mechanical properties of uppermost aquitard as contributing factors to land subsidence in the Chalco Valley. Keywords: subsidence; clay mineralogy; allophane; montmorillonite; Chalco.

1. INTRODUCTION

Chalco Valley is an 1100 km2 sub-basin located to the southeast of the Mexico Basin, near to Mexico City. The basin is bounded by volcanoes including Popocatepetl and Iztaccihuatl stratovolcanoes. Chalco Valley is one of several ancient lakes located on the lowest part of the Mexico Basin (figure 1), in which saturated silty-clayey sediments represent the main grain size of the near surface sedimentary sequence. Furthermore, several layers of volcanic ash and lapilli erupted by adjacent volcanoes during the Quaternary time can also be observed intercalated in the sedimentary sequence. During the last two decades, groundwater extraction from the aquifers in Chalco has been an economically important source of water for Mexico City. Groundwater extraction from deep aquifers began in 1980 (Ortega et al., 1993), and since then subsidence has been documented by several researchers not only in Chalco Valley but also in the entire Mexico Basin (Ortega et al., 1993; Rudolph and Frind, 1991; Ortega, 1996; Ortega et al, 1999; Rivera et al., 1994). The main problems resulting from subsidence in Chalco is flooding of large areas adjacent to artificial water channels, and the damage to urban constructions.

Figure 1. Location of sampling site in the Chalco sub-basin near Mexico City. The zone is largely occupied by channels and surrounded by volcanoes in highlands.

According to both principles of Effective Stress developed by Terzaghi (Das, 1997), and Leaky Aquifers presented by Jacob (1946), vertical flow is induced from the aquitard to the aquifer during pumping, reducing pore pressure and increasing effective stress. Subsidence in Chalco Valley is thought to be caused by consolidation of the silty-clayey aquitards during pumping and drawdown of groundwater levels underlying the aquitards (Ortega, 1996). The high compressibility of Chalco uppermost aquitard has been reported in Chalco Valley (Hernandez-Marin, 2003) and in this aquitard the predominant clay minerals include montmorillonite and allophane. These minerals usually have high porosity and elevated gravimetric water content resulting in highly compressible materials (Mesri et al., 1975; Wesley, 2001). In addition, fractures in this valley were first reported by Vargas (2001) during slug tests on aquitards and later observed by Hernandez-Marin (2003) and Carreon-Freyre et al., (2003) during field and laboratory work. Fracturing results in abnormally high values of hydraulic conductivity in the uppermost aquitards and the current tendency is for more of these discontinuities to appear (Vargas, 2001). Both high compressibility and increasing fracturing are relevant in subsidence because with more rapid draining of pore water an aquitard undergoes a more rapid consolidation and possibly a larger total deformation. This paper explores the intrinsic properties, behavior and response of the uppermost silt-clayey lacustrine aquitard in Chalco Valley during subsidence, as well as the effects of sediment heterogeneities during consolidation. 2. METHODOLOGY

Samples were extracted from a 15 m depth core drilled in the central part of Chalco Valley (shown in figure 1), within individual Shelby tubes of one meter long. Sampling in the first two meters was detailed by excavating a trench. Immediately after sample extraction, gravimetric water content was estimated in each observed layer. Laboratory techniques including grain size determinations, density, consistency, and consolidation were performed in 20 cm intervals through each layer; although, analyses in layers < 20 cm thick were not performed. Some samples were preserved unaltered in order to apply consolidation, density, and consistency tests under natural moisture conditions. Remaining samples were air dried for grain size, density, consistency, and X-Ray diffraction techniques. 2.1 Determination of physical and mechanical properties The following physical properties were evaluated from collected samples: a) gravimetric water content, which can be defined as the ratio of the pore water weight to the mass of the solid phase. This property was obtained in

agreement with ASTM protocols (American Society for Testing and Materials) (ASTM D2216-92, 1998a), b) grain size, which was determined by sieving and hydrometer analysis (ASTM D422-63, 1998b), c) density or particle specific gravity, which was determined by the pycnometer method (ASTM D854-92, 1998c), d) porosity, which was estimated from undisturbed samples used in consolidation tests. Void ratio (e) is related to porosity (n) by the following expression: n=e/(1+e). Void ratio was used in the compressibility plots. e) Consistency or Atterberg limits. Liquid and plastic limits were considered in this paper and were determined according to ASTM guidelines (ASTM D4318-95, 1998e). The Casagrande coup method was applied for the determination of the liquid limit (LL) and the Terzaghi method was used for the plastic limit (PL). In addition to prepared samples according to ASTM standards, wet-natural samples were tested, because air-free drying effects in allophanitic and montmorillonitic clays can have an effect in the physical and mechanical properties of fine grained sediments according to Hernandez-Marin and Carreon-Freyre (2004). Naturally wet samples were mainly considered for limits properties determination. f) One-dimensional consolidation tests were performed using a hydro-pneumatic consolidometer, following the specifications of ASTM (ASTM 2435-96, 1998d). Additionally, the compressibility index was estimated from the plastic portion of the compressibility curve and computed from the slope of void ratio and the base-10 logarithm of pressure (Lambe and Whitman, 1969). 2.2 Consolidation Tests Consolidation tests were performed to estimate the compressibility index (Cc). Two different consolidation tests were applied on unaltered samples: i) the incremental test, in which, several incremental stresses are applied to saturated samples during a 24 hours period. During each stress increment, pore pressure, back pressure, vertical deformation were recorded and analyzed. ii) the gradient-controlled test proposed by Lowe et al. (1969), in which different stresses are applied to samples to maintain a constant hydraulic gradient (hydraulic gradient in this test is the difference between pore pressure and back pressure). In both consolidation tests, the compressibility index is computed from the plastic portion of compressibility curve (unit deformation or ratio void vs logarithm of effective stress). 2.3 X-Ray diffraction X-Ray diffraction was performed on samples to obtain the predominant mineralogy of the clayey fraction. Organic matter was eliminated from each sample by using 30 % oxygenated water and low heating (less than 60 ºC). Carbonates were also removed from each sample reacting with a solution of

10% HCl. In addition, one of three washings was performed using a solution of 10 % acetic acid under low heating. After the organic matter and carbonates had been eliminated, an aliquot of the fine fraction was analyzed in the diffractometer. The first set of diffractograms confirmed the presence of allophane in each sample. Then, allophanes were removed using 0.5 N NaOH and heating according to Hashimoto and Jackson (1960). The coarse size fraction was removed from each sample and the remaining fine-size fraction was dispersed for 30 seconds using an ultrasonic sounding regulated to 30 % of its capacity. Finally, the dispersed aliquot was used in the X-Ray diffraction using its natural orientation. 3. RESULTS AND DISCUSSION

3.1 Stratigraphy and physical properties Figure 2 shows five main clayey layers interbedded with pyroclastic lenses. A surface saline layer with abundant vegetal roots and weathering, a black silty-clay with lapilli and ash lenses, a gray olive silty-clay with small fractures, and two fractured layers, a red silty-clay and a brown silty-clay. The contacts between layers show both transitional and sharp changes. As can be observed in figure 3, variation of the parameters is clearly stratigraphy-dependent. Dominant grain size corresponds to silt (Figure 3a). Maximum silt content of the sequence is 90 %. The sediments are characterized by a high gravimetric water content, with maximum and minimum values between ca. 350 % and ca. 50 % (350 % indicates 3.5 g of water are present in each gram of solid particles). Red and brown sediments between 0.80 and 2.2 meters depth have the highest clay content in the sequence of ca. 44 %. This layer also has the highest values in other measured parameters with the exception of the solid density. The decrease in clay content below 2 m depth is also reflected in the measurements of other parameters. The variability in red and brown layers might also be attributed to the sediments transported into the observed cracks or fractures. This process possibly takes place during groundwater flow. Righi and Meuner (1995) observed that the presence of montmorillonites in clayey sediments can contribute to the formation of fractures resulting from differential variations in moisture content. Changes in moisture can imply changes in volume and during volume adjustment internal stresses are created and generate shear planes. Such changes in moisture can occur during groundwater extraction, when water is removed from aquitards and flows through small lenses and fractures that are possibly acting as channels for pore-water evacuation.

A lens of lapilli located between 3.75 and 4.0 meters depth, which was deposited during a volcanic eruption 12,800 years ago (Lozano-Garcia et al., 1993) has the lowest density in the sequence and consists predominantly of sandy and gravely materials (Figure 3a). Generally, density is dependent on the degree of crystallization in sediments (Gama-Castro et al., 2000); therefore, poorly crystallized lapilli probably contribute to the low density in the black sediments and may affect the values of other parameters. Below the lapilli layer, the brown sediments represent a zone of transition between dark and gray-olive layers and increasing values of water content, density and porosity. On the other hand, the gray-olive sediments show little variation in parameter values.

Figure 2. Stratigraphic characteristics of the top 15 meters in the Chalco uppermost aquitard.

3.2 Contribution of Atterberg limits Liquid and plastic limits represent the gravimetric water content at which sediments behave as a viscous liquid or are deformed plastically. Results in figure 3c indicate that under natural moisture, sediments deform as a plastic material at several points in the analyzed sequence. This observation emphasizes the plastic behavior of the aquitard under natural conditions, a behavior independent of the applied stress related to groundwater pumping. 3.3 Mechanical behavior We document the mechanical behavior of the analyzed sequence during and after the application of stresses by evaluating the compressibility index. In Chalco Valley effective stress is influenced by the decrease of the pore pressure during groundwater pumping. High values of compressibility index indicate that relatively low stresses are sufficient to reduce the volume of the aquitard. This property in the sediments of Chalco Valley has been reported in some studies (Rudolph and Frind, 1991; Ortega, et al., 1999). High compressibility along with high void ratio can also be an indicator of the presence of allophanic clays (Wesley, 2001). In addition, fractures observed in these sediments play an

Figure 3. Physical and mechanical properties of the Chalco sediments

important role in compressibility because they allow pore water to drain more rapidly during the application of stress (Figure 4).

Figure 5 shows a comparison of the compressibility obtained from different types of sediments: brown, red and grey-olive. High compressibility values represented by a pronounced slope were obtained for both sediment types. Undisturbed samples of black sediment were not obtained due to the occurrence of ash and lapilli lenses. However, high compressibility values are expected in this layer as a result of the volcanic lenses. The mechanical behavior of brown and red sediments is presented in figure 5a. Fractures along with volcanic ash lenses observed in these sediments can contribute to a more rapid deformation during consolidation. Curves representing brown a red sediments show high compressibility as do curves for gray-olive sediment (figure 5b). However, curves for the gray-olive sediments are closer to each other than in figure 5a, indicating more homogeneity in both mechanical response and physical properties.

Figure 4. Examples of clay filled fractures in red and brown sediments. Groundwater flow through these discontinuities was observed during field work (largest photography). Smaller fractures were also observed in laboratory work (smaller photography).

3.4 Mineralogy The summary of clay mineralogy presented in table 1 indicates a predominance of allophane and montmorillonites in the clayey sequence. The origin and depositional environment of sediments play an important role in the mineralogy of the uppermost clayey aquitard in Chalco. According to Warren and Rudolph (1997), the allophanic and montmorillonitic clays in Mexico Basin are the product of weathering volcanic ash. However, montmorillonitic clay can also be produced from weathering of volcanic rocks like basalt and andesites (Foreman, 1955). In Chalco Valley it is possible that the volcanic ash observed in several lenses, and the volcanic rocks in the highlands surrounding the basin, contributed to the formation of these clayey minerals. Table 1 summarizes the mineralogy of the top 15 meters of sediments. The results indicate that allophanic and montmorillonitic clays are present in most layers. Mineralogy of the Chalco clays is important to the land subsidence process because they are responsible for the physical and mechanical properties observed, particularly the high compressibility values.

Figure 5. Graphics of compressibility grouped by depth and by type of sediments. Heavy pointed curves represent controlled-gradient consolidation test, whereas light pointed curves were provided by incremental tests.

Depth, m Layer Type of clay Observations 0.8 Red sediments Allo

1.30 – 1.60 Red sediments Allo, Mont Unclear presence of Mont. 1.60 – 2.00 Red sediments Mont 2.48 – 2.61 Brown sediments Mont 3.37 – 3.60 Black sediments Allo High content of Allo 3.60 – 3.80 Black sediments Allo High content of Allo 4.39 – 4.60 Black sediments Allo High content of Allo

4.60 – 4.80 Black sediments Allo, Mont High content of Allo, Mont. In incipient quantity or poorly crystallized

5.05 – 5.38 Black sediments Allo High content of Allo 6.50 – 6.80 Brown sediments Allo Feldspars 7.15 – 7.38 Brown sediments Allo Feldspars 7.60 – 7.80 Brown sediments Allo, Mont Feldspars, Mont. In incipient quantity or poorly crystallized 8.09 – 8.34 Brown sediments Allo Feldspars 8.34 – 8.59 Brown sediments Allo Feldspars 9.35 – 9.63 Gray-olive sediments Allo, Mont Feldspars

9.72 – 9.77 Gray-olive sediments Allo, Mont Feldspars, Mont, confirmed after heating at 490o, although, in incipient quantity or poorly crystallized

10.34 – 10.60 Gray-olive sediments Allo Feldspars 11.34 – 11.63 Gray-olive sediments Allo, Mont Feldspars, Mont. In incipient quantity or poorly crystallized 11.86 – 11.92 Gray-olive sediments Allo, Mont Feldspars, Mont. In incipient quantity or poorly crystallized 12.06 – 12.33 Gray-olive sediments Allo, Mont Feldspars, Mont. In incipient quantity or poorly crystallized 12.63 – 12.93 Gray-olive sediments Allo Feldspars, Mont. In incipient quantity or poorly crystallized 14.22 – 14.27 Gray-olive sediments Allo Feldspars 14.69 – 14.27 Gray-olive sediments Allo Feldspars

4. CONCLUSIONS

In this paper, the mechanical behavior and physical properties of the near surface aquitards and their role in causing land subsidence in the Chalco Valley are analyzed. The main process causing land subsidence in this valley is the consolidation of silty-clayey upper aquitards due the groundwater extraction from induced stresses caused by pumping. The physical and mechanical properties associated with the high content of montmorillonite and allophane in the clays are the predominant factors that influence the mechanical behavior of uppermost aquitard. High compressible porous clay sediments with elevated plasticity and high gravimetric water content characterize the shallow aquitards. In addition, susceptibility to fracturing, and the presence of gravel and sandy lenses, is probably contributing to the high rate of subsidence. Fractures and volcanic lenses likely act as pathways for water evacuation from imposed stresses and the resulting relaxation of pore pressure (consolidation). The formation of fractures is possibly a consequence of clay mineralogy and changes in water content. The conditions leading to the formation of these clayey minerals have prevailed throughout Quaternary times and are related to factors such as weathering of volcanic material (for instance, ash and basaltic and andesitic rocks); humid conditions, and warm temperature. Finally,

Table 1. Mineralogical results from X-Ray Diffraction. Allo=Allophane, Mont=Montmorillonite.

correlating properties at several locations and depths within the Valley would be helpful to better understand the land subsidence process in the Chalco sub-basin. ACKNOWLEDGMENTS

The authors thank Ricardo Carrizosa for his valuable help in the determinations of physical properties in the Geomechanical Laboratory, National Autonomous University of Mexico (UNAM). Determination and interpretation of X-Ray Diffraction results were performed by Dr. C. Hidalgo of the Colegio de Posgraduados. The corresponding author is grateful to Dr. Thomas Burbey for his valuable comments that improved the manuscript. REFERENCES

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