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Characterization of Soil Permeability in the Former Lake Texcoco, Mexico

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Characterization of Soil Permeability in the Former Lake Texcoco, Mexico Open Geosci. 2019; 11:113–124 Research Article Open Access Norma Patricia López-Acosta*, Alejandra Liliana Espinosa-Santiago, and David Francisco Barba-Galdámez Characterization of soil permeability in the former Lake Texcoco, Mexico https://doi.org/10.1515/geo-2019-0010 Received September 20, 2018; accepted January 10, 2019 1 Introduction Abstract: The geotechnical subsoil conditions of the for- The construction of the New Mexico International Airport mer Lake Texcoco represent a complex sequence of highly (NAIM) has drawn renewed attention to the subsoil condi- compressible lacustrine clays interbedded with layers and tions of the former Lake Texcoco. This area overlies a se- seams of harder and more permeable materials. Although ries of soft clay and clayey silts, widely known for their the mechanical properties of these deposits have been ex- high water content and large compressibility, interlayered tensively studied in the past, the information about their by units of cemented clastic material [1]. In the past, sev- hydraulic properties is scarce. Currently, a comprehen- eral authors have studied the mechanical characteristics sive characterization of the hydraulic conductivity of this of the clayey deposits [1–6]. However little research has site has become necessary because of the construction of been conducted to evaluate their hydraulic properties. In the New Mexico International Airport (NAIM). The present particular, the available information about the permeabil- study describes a systematic evaluation of the hydraulic ity of the deeper strata is scarce. conductivity in the former Lake Texcoco through three A comprehensive hydraulic characterization of the different in-situ methods (well permeameter, LEFRANC area is important for the design of airport and hydraulic and piezocone dissipation test). The measurements, taken structures under construction. Hydraulic conductivity k af- from 155 locations, show a high spatial variability, with fects geotechnical analyses that involve water flow within ranges spanning more than two orders of magnitude. The the ground or earth structures, such as uplift pressure in results also reveal that the estimated permeabilities vary excavations, regional subsidence and the effects of rain in- significantly among methods. These discrepancies reflect filtrations on slope stability of lagoons and canals. Italso the scale dependency of the hydraulic conductivity in exerts a strong influence on the design of pumping sys- the area caused by soil heterogeneities. A comparison of tems and soft ground improvement methods. Despite its the presented results with previous studies demonstrates great importance, the accurate evaluation of this parame- that piezocone tests provide representative results for the ter remains a complex task because of soil’s inherent het- clayey formations, while LEFRANC tests better estimate erogeneity. Many authors have documented the great spa- the hydraulic conductivity of the permeable strata. Be- tial variability of k, finding coefficients of variation upto sides, CPTu tests yield more consistent values of hydraulic several orders of magnitude [7–9]. Furthermore, soil per- conductivity, with smaller dispersion than well perme- meability can vary with the measurement scale. Some field ameter and LEFRANC tests. features (e.g., fractures, cracks and pervious seams) create preferred flow paths that increase the value of k as the vol- Keywords: Hydraulic conductivity, Lake Texcoco, ume of tested material augments [10]. LEFRANC test, USBR test, CPTu test, Piezocone The methods to determine soil hydraulic properties in- clude laboratory and in-situ tests. Although the former are economical, relatively quick and allow controlling differ- ent boundary conditions, they require an appropriate sam- ple handling and provide just local k values. In-situ meth- ods, instead, evaluate a more representative volume of soil *Corresponding Author: Norma Patricia López-Acosta: Universi- under actual field conditions. However, most of them are dad Nacional Autónoma de México Mexico City, Coyoacán Mexico, costly, laborious and time-consuming. These drawbacks E-mail: [email protected] complicate the obtainment of large datasets required for Alejandra Liliana Espinosa-Santiago, David Francisco Barba- studying the spatial variability of the hydraulic conductiv- Galdámez: Universidad Nacional Autónoma de México Open Access. © 2019 Norma Patricia López-Acosta et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution alone 4.0 License. 114 Ë Norma Patricia López-Acosta et al. ity. As an alternative, soil permeability can be estimated reclamation for urban expansion during the 20th century indirectly from direct-push probings carried out to deter- caused the gradual shrinking of the lakes, which today mine other geotechnical properties (e.g., piezocone and have practically disappeared [14]. Fig. 1 shows a geologic flat dilatometer tests). The simplicity, inexpensiveness and map of the Basin of Mexico. speed of these indirect in-situ methods enable the acquisi- The site stratigraphy corresponds to a typical Lacus- tion of a greater number of measurements. Nevertheless, trine Zone according to Mexico City Geotechnical Zon- most of the proposed interpretation theories are empiri- ing [16]. Soil strata consist of soft clays and clayey silts cal and their applicability is restricted to specific soil types interspersed with seams and layers of harder clayey silts and field conditions. with sands. As seen in Table 1, the soil strata are: a) Surface This paper presents a systematic evaluation of soil per- Crust (SC) of light brown fat clay of soft consistency with meability in the former Lake Texcoco through three dif- the presence of cracks; b) Upper Clay Formation (UCF), a ferent in-situ methods: well permeameter, LEFRANC and thick layer of very soft lacustrine clay interspersed with piezocone dissipation test. The characterization provides relatively thin seams of volcanic origin and sandy silt; c) the permeability values of previously unexplored strata Hard Layer (HL) composed of thin layers of hard sandy silt and applies univariate statistics to evaluate the influence with variable cementation; d) Lower Clay Formation (LCF), of the various measurement techniques. Accordingly, the a green brown clay interspersed with gray fat clay of the specific objectives of this study are: (a) to determine the same origin of the UCF; e) Deep Deposits (DD) formed by hydraulic conductivity of clayey and permeable strata of silts and sand interspersed with hard clays; f) Deep Clay the former Lake Texcoco using different in-situ methods; Formation (DCF), a soft to very firm clay layer with similar (b) to evaluate the ability of indirect in-situ methods to es- characteristics to the others; and g) Deep Stratified Forma- timate accurate and reliable values of k; and (c) to quantify tion (DSF) composed of stratified deposits of clay, sand and the scale effects in the determination of soil permeability silty sand. in the study zone. The thickness of the strata remains relatively con- stant, with exception of the UCF that increases from north- east to southwest (Table 1). Clayey formations are charac- 2 Methods terized by their extremely high water content (varying from 300 to 400 %, but in some areas of the UCF values of up to 600 % have been registered) and void ratios (between 2- 2.1 Geological and geotechnical context of 4 in the DCF, 3-6 in the LCF and above 6 in the UCF), high the study area plasticity index and extraordinary compressibility [1, 2, 4]. Anisotropy is an important factor in Texcoco’s soils due to ∘ ∘ The study area (bounded by latitude 19 28’12” – 19 33’36” the presence of permeable seams and their intrinsic ge- ∘ ∘ N and longitude 98 56’24” – 99 01’12” W) lies at the lowest ological formation process. Generally, clays have greater portion of the Basin of Mexico, formerly occupied by Lake horizontal permeability kh than vertical permeability kv, Texcoco (State of Mexico, Mexico). The Basin of Mexico is with ratios of kh/kv ranging from 1.1 to 10.0 [17–23]. Field or 2 a compound graben of 9600 km elongated in the NNE- laboratory estimates of the anisotropy of the former Lake SSW direction (about 125-km length and 75-km width). It Texcoco clays have not been obtained yet, however, con- is bounded on the north by the Pachuca Range, east by sidering their layering, a value of kh/kv between 2 and 5 the Río Frío and the Nevadas Ranges, south by Chichin- may be assumed [4, 18, 20]. autzin Range and west by the Ajusco Volcano and Las The groundwater table is close to the surface. Previ- Cruces Range [11]. The basin is an extensive plain with an ous pumping wells for brine extraction have caused a sig- average elevation of 2240 m above sea level and a sub- nificant depletion of the hydrostatic pressure influencing tropical climate [12]. It was originally open until 700,000 the stress condition at the site (Fig. 2). In-situ piezometric years ago when volcanic activity blocked its drainage re- measurements show that the drawdown is gradual in the sulting in the development of an extensive system of lakes: study area, beginning at a lower depth in the north (be- Zumpango, Xaltocan, Texcoco, Xochimilco and Chalco. tween 5 and 10 m) and deepening to the south (between During this period, lacustrine sediments, volcanic ash and 20 and 24 m). In addition, the subsoil of the site has a high other pyroclastic materials were deposited overlaying pre- salt content, even
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