01 G Auvinet - Subsidence Mexico City

 

Land subsidence in Mexico City  El hundimiento de la ciudad de México G. Auvinet  Laboratorio de Geoinformática, Instituto de Ingeniería, UNAM, Mexico

ABSTRACT: Land subsidence induced by deep well pumping has been affecting Mexico City since the end of the nineteenth century. This phenomenon damages the drainage and transport systems and other services of the city and generates severe foundation engineering problems. In some parts of the urban area, subsidence rate attains values as high as 400 mm/year. Geotechnical scientist Nabor Carrillo (1948) was the first to establish a clear correlation between subsidence and piezometric drawdown. Groundwater pumping from the thick aquifer system underneath the city is now about 52.2 m3/s, representing 72% of potable water provided to the city dwellers. Recently, a reassessment of this  problem has been undertaken in order to define better mitigation measures. The paper presents an updated evaluation of the accumulated subsidence from 1862 to our days and a review of its consequences. For a better evaluation of subsoil conditions, spatial variations of the soil profile and mechanical properties have been assessed processing a large data  base by geostatistica geostatisticall methods. These elements cast a new light on the phenomenon and offer better perspectives for a realistic predictive model. Finally, some foundations techniques developed to deal with subsidence are briefly presented.

RESUMEN: El hundimiento inducido por el bombeo en pozos profundos ha afectado a la ciudad de México desde fines RESUMEN: del Siglo XIX. Este fenómeno ocasiona daños en los sistemas de drenaje y de transporte así como en otros servicios  públicos y causa serios problemas de ingeniería de cimentaciones. En algunas partes del área urbana la tasa de hundimiento alcanza 400mm/año. Nabor Carrillo (1948) fue el primero en establecer una clara correlación entre hundimiento y abatimiento piezométrico. El volumen de agua bombeada del acuífero ubicado de bajo de la ciudad es actualmente de aproximadamente 52.2m3/s y representa 72% del agua potable que se proporciona a los habitantes de la capital. Recientemente se ha emprendido una reevaluación de este problema para definir mejores medidas de mitigación. En este trabajo se presenta una evaluación actualizada del hundimiento acumulado desde 1862 hasta nuestros días y se revisan sus consecuencias. Para una mejor evaluación de las condiciones condiciones estratigráficas, se ha determinado determinado la variación espacial del espesor de los estratos y de las propiedades índice y mecánicas recurriendo a la geoestadística. Estos nuevos elementos arrojan una nueva luz sobre el fenómeno y ofrecen mejores perspectivas para el desarrollo de un modelo  predictivo mas realista. Finalmente, se comentan brevemente algunas técnicas de ingeniería de cimentaciones desarrolladas para suelos sometidos a subsidencia regional.

1  INTRODUCTION

In 1925, Roberto Gayol informed the Mexican Society of Engineers and Architects of Mexico (Sociedad ( Sociedad de  Ingenieros y Arquitectos de México) México) that he had reached the conclusion that Mexico City was sinking. He also suggested that the main cause of this phenomenon was the drainage of water from the subsoil by the works he had himself designed and constructed at the beginning of the XXth century (Gayol, 1929). José A. Cuevas, pioneer of soil mechanics in Mexico, accepted this conclusion and years later asked Mexican Scientist Nabor Carrillo to undertake a formal analysis of the influence of the extraction of underground water on the settlements of the city. Carrillo (1948) established that loss of pressure in the aquifers induces a change in the effective stress state

within the soil inducing a consolidation process and surface settlements. According to Roberto Gayol, in 1891, the elevation of a  benchmark close to the west tower of the metropolitan cathedral was 2238.8m over the sea level; in 1966 the elevation of this same point was 2233m, a difference of 5.8m! (Marsal, 1992). The problem of Mexico City subsidence was thoroughly reviewed by Marsal and co-workers in the 1950’s and 1960’s (Marsal et al.  al.  1951, 1959; Hiriart, 1969) and by Zeevaert (1962). Other studies on regional subsidence, especially in the Texcoco Lake area, were  presented by Murillo Murillo (1990). Originally a mere curiosity, the subsidence of Mexico City has become a source of serious safety hazards and damages to installations and constructions. It reduces the drainage efficiency and augments the flooding risk. The

 

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transport systems and other public services of the city are affected. Subsidence can also be related to soil fracturing and is one of the main causes of foundation engineering  problems. The combined effect of subsidence and earthquakes poses a severe threat to the conservation of architectural monuments in Mexico City (Ovando, 2008) At this stage, it seems thus useful to review fresh evidences and measurements of Mexico City subsidence, to examine its consequences, to assess progress made in the modeling of the phenomenon and to evaluate the techniques that have been developed to deal with it in geotechnical engineering practice.

2  EVIDENCES AND MEASUREMENTS OF LAND

SUBSIDENCE Some of the earliest topographical references on the lacustrine zone of Mexico City can be found in the scientific work of German geographer Alexander Von Humboldt (1803). The many surveys performed since then by a number of different public and private institutions have been compiled by Auvinet  Auvinet  et al.  al.  (2008). Most of them were  performed by Organismo de Cuenca. Aguas del Valle de  México (formerly  México  (formerly Comisión de Aguas del Valle de México and Comisión and  Comisión Hidrológica de la Cuenca Valle de México) México) and Sistema de Aguas de la Ciudad de México. A description of these studies can be found in other  presentations in this this same workshop workshop (Pineda, 2008). 2008). Most of the existing data have been captured and included in a Geographical Information System (Méndez et al., al., 2008). Processing these data allowed obtaining several valuable products and useful conclusions. a) 

Figure 1. 3D rendering of Mexico basin. The vertical scale of the lacustrine zone was exaggerated to enhance subsidence (Laboratorio de Geoinformática, LGI, 2008).

 b) 

Accumulated subsidence

Reviewing available historical data, it was concluded that the hydrological map established in 1862 by Francisco Díaz Covarrubias, by instruction of Fomento of  Fomento Minis Minister Manuel Silíceo, constitutes an excellent initial topographical reference.

Present topographical configuration of the lacustrine zone.

Fig. The 1 shows a 3D representation of Mexico relief. vertical scale was exaggerated for a Valley better appreciation of the lacustrine zone subsidence. The lowest  point of the valley is located close to the international airport, with an elevation of 2223.87m  above sea level (2005).

Figure 2. Accumulated subsidence of the lacustrine zone during the 1862-2005 period (m). (LGI, 2008).

Comparing elevations found on this map with those obtained from recent surveys, the accumulated subsidence for the 1862-2004 period can be determined (Fig. 2). In two points of the valley, located respectively close to the Cerro del Peñon  Peñon  and  Xico  Xico   hills, the accumulated subsidence exceeds 13.5m. c) 

Evolution of subsidence rate.

An updated graph of the evolution of the subsidence rate registered in several points of the downtown area (Catedral, Palacio de Minería and Minería and Alameda  Alameda Central  Central Park, Fig. 3) shows that subsidence rate has been far from constant during the last century. In the first third part of

 

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the twentieth century, the rate was about 4cm/year but after 1938 it increased to 14cm/year. In 1948, this rate more than doubled to 30cm/year. In 1967, however, a significant decrease was observed, attributed to restrictions imposed on the installation of new wells. The rate is now approximately steady, around 8cm/year, in this area.

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intensity of the phenomenon can be detected. The surface of the highest subsidence rate zones generally tends to shrink. (an exception to this trend can however be observed in the southern part of Chalco lake). Moreover, subsidence contours in the western part of the city slightly migrate eastwards, indicating that subsidence tends to level off in this area of the city.

Figure 3. Evolution of subsidence during the 1891-2005 period (LGI 2008).

The subsidence rate is highly variable from one shown site to another within the valley. The subsidence contours in Fig. 4 and 5 correspond to the 1992-200 and 2000-2004  periods respectively. respectively. Figure 5. Subsidence rate during the 2000-2005 period (LGI 2008).

3  CONSEQUENCES OF REGIONAL LAND

SUBSIDENCE Subsidence of Mexico City is a source of serious damages and safety hazards. It affects the drainage and augments flooding risks. It is also an obstacle to proper operation and conservation of transport systems and other public services of the city. Subsidence has also been related relat ed to many problems of soil fracturing. It is one of the main causes of foundation engineering problems. Some secondary effects such as progressive changes in index and mechanical properties of the consolidating soft clays and consequently in their seismic response have also  been detected by some researchers (Méndez, 1991; Ovando, 2007).

Figure 4. Subsidence rate during the 1992-2000 period (LGI 2008).

The largest subsidence rate is observed in front of the Cerro del values Marqués exceedsin the 40cm/year. Comparable havehill beenand measured Chalco lake. Comparing Figs 4 and 5, a slight decrease in the

3.1   Drainage system. system.

The subsidence has provoked a strong alteration of the geometry of the drainage system of the city. Until the end of the XVIIIth century, the valley of Mexico was a closed basin, with a number of shallow lakes, amongst them Texcoco Texcoco   and  Xaltocan  Xaltocan   lakes. It  became an open basin when the  Nochistongo  Nochistongo   cut was

 

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completed in 1789. Progressively, the lakes were drained mainly through the Tequisquiac Tequisquiac   and Deep Drainage ( Emisor  Emisor Central  Central ) tunnels, and practically disappeared. The general subsidence of the lacustrine zone has induced progressive changes in the slope of the different components of the drainage system. This is particularly conspicuous in the case of the great superficial drainage canal (Gran (Gran canal ) leading to Tequisquiac Tequisquiac   tunnels (Fig. 6).

Figure 7. Pumping station on the Gran Canal (km 18+500, LGI 2007). 3.2   Flooding hazard

Uneven sinking of the soil surface forms local depressions where water accumulates during peak events of the rainy season, causing local flooding. At a larger scale, the general subsidence has created a  potentially risky situation in case of an accident in the deep drainage tunnel (Emisor Central) of the city. Due to subsidence, the original shallow lake (2m or so) could theoretically form again but would then present a larger depth (12m or so) and would flood part of the downtown area. Fig. 8 illustrates this remote but worrying possibility. This hazard and the increasing needs for further drainage capacity have been the main motivations behind the construction of a new deep drainage tunnel (Emisor Oriente) that has been initiated recently.

Figure 6. Evolution of longitudinal profile of   Gran Canal   drainage as a consequence of regional subsidence. (CONAGUA, 2007).

The Gran canal can no longer work by gravity. It has  been necessary to build pumping stations in order to raise the water up to an elevation allowing drainage through the tunnels (Fig. 7). A new station at km 11+600 of the canal was inaugurated at the beginning of 2008 (Auvinet, 2008). Some additional pumping stations will be soon be built.

Figure 8. Flooding potential in the downtown area (LGI, 2007). 3.3  Transport system

The urban transport system has been severely affected by differential settlements induced in the soil surface by regional subsidence. This has been particularly critical in the case of subway lines crossing transition zones between soft and firm soils (Fig. 9). The case of subway line “A” is discussed in a paper presented in this workshop (López Acosta et Acosta  et al ., ., 2008).

 

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A systematic monitoring of fractures in Mexico City subsoil has been undertaken. A Geographical Information System containing data obtained from surveys performed with GPS has been developed. More than 300 sites have already been evaluated. (Auvinet et al ., ., 2008). It should be stressed that not all cracks are related to regional subsidence since many other mechanisms may  play an important important role in their generation (Auvinet (Auvinet,, 2008). subsidence on foundati foundations. ons. 3.5   Effect of subsidence

Figure 9. Uneven subsidence effects in a subway sub way line (STC).

Linear structures such as those of the subway shown in Fig. 9 are submitted to large subsidence-induced vertical and horizontal movements. It has been observed that the most serious damages do not correspond necessarily n ecessarily to the  portions of the line where land subsidence reaches the highest values. Representing the longitudinal profile of vertical movements as a Fourier series, it can be shown that the structure of the subway is particularly sensitive to certain Long harmonics criticalare periods et the al . 2008). period with movements easily (Auvinet absorbed by structure while short period movements have no significant effects on the structure that can bridge these irregularities. 3.4  Soil fracturing

Another harmful effect of the regional subsidence has  been the development of large cracks in the soil in transition zones between lacustrine soft clays and firm soils such as volcanic tuffs or basaltic rocks (Fig. 10).

Figure 10. Soil fracture in transition zone (LGI, 200 2008). 8).

These cracks have created a serious safety hazard for the population of the city and constitute an obstacle to the development of certain urban areas. The problem of soil fracturing is addressed by Carreón in a paper presented in this workshop (Carreón, 2008)

Solutions for foundation of buildings on soft soils in Mexico City have evolved progressively since the preColumbian and colonial periods, due to the necessity of  building increasingly larger, higher and heavier constructions. The most common solutions used today include footings, rafts, and box-type foundations for relatively light constructions and precast driven point bearing piles and, to to a lesser extent, bored piles and drilled drilled shafts for heavier buildings, especially in the transition zone. Subsidence has a strong effect on the behaviour of all types of foundations in the lacustrine zone but especially on compensated foundations and end-bearing piles. Compensated foundations tend to protrude from the surrounding soil due to interference between the local unloading of the soil and the regional consolidation  process. On the other hand, negative skin fri friction ction develops on the shaft of end-bearing piles, reducing its net bearing capacity (Auvinet and Hanell, 1981). Moreover, an apparent protruding of the piles is generally observed (Fig. 11) and damage can be induced in adjacent buildings supported by other types of foundation. Soil consolidation has also the effect of separating the slab of the substructure from the soil. These problems are examined in more details in a companion paper presented in this workshop (Auvinet et al., al., 2008).

Figure 11. Protruding pile foundation foun dation (Texcoco).

 

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4  INTERPRETATION, MODELLING AND

INSTRUMENTATION

Similar models covering different soil profiles and drainage conditions were proposed by Marsal and Mazari (1959). The simplest model is represented on Fig. 13.

It is now largely accepted that Mexico City subsidence is a consequence of piezometric drawdown induced by deep  pumping. The clays layers (upper and inferior clay formations) found in the former lake zone constitute an aquitard confining the main deep aquifer (80 to 850m) where pumping takes place. Using a large database of borings performed in the valley available at LGI, UNAM it has been possible to reach a better definition of the clay aquitard thickness. Geostatistical techniques were used for interpolating the available data. This new information has contributed to improve the calibration of available geotechnical and hydrogeological models of Mexico City subsidence. On the other hand, available historical piezometric data have  been included in a Geographical Information System and can now be readily processed (Juárez et al ., ., 2008).

Figure 13. Marsal and Mazari model

According to this model, the evolution of the settlement associated to a given pore pressure drawdown ∆ p at the limit between aquitard and aquifer can be expressed as:

(1)

Depending of the aquitard thickness, the evolution of the settlement for a hypothetical total pore pressure drawdown at the base of this layer should be as indicated on Fig. 14.

Figure 14. Expected total settlement due to total pore pressure drawdown at the base of the aquitard (Aubert, 2008).

The total expected long term settlement should be: Figure 12. Contours of clay aquitard thickness, m (LGI, 2008). (2) 4.1  Geotechnical models

 H  (Fig. 15)  with  ∆ p = γ  

Attempts at modelling Mexico City subsidence have been undertaken since the first half of the last century (Carrillo, 1948). Terzaghi’s consolidation theory was used to explain the deformations of the highly compressible lacustrine strata found in Mexico City subsoil, as a consequence of piezometric drawdown induced by deep  pumping. Figure 15. Expected long term total settlement due to total pore  pressure drawdown at the base base of the aquitard (Aubert (Aubert,, 2008).

 

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According to this figure, for a 70m thick aquitard, the long term compression could be of the order of half its thickness (35m). This value should be considered as the maximum value that can be reached in the valley due to consolidation. Other processes could however induce further settlements. Theoretically, drying by evapotranspiration could progressively lead the soil to its contraction limit (a compression of about 65%). Most of the city is protected against drying by impervious pavement and by the perched water table that develops during the rainy season. Some evidences suggest however that this drying process is already active in the North West part of the valley (Coacalco; Auvinet, 2008) Evaluation of the present available data leads to some important conclusions: a) The pore pressure drawdown at the base of the aquitard is generally far from being total. As shown on Fig. 16, drawdown practically never exceeds 150kPa. This may correspond to total drawdown in the west part of the city but constitutes only a small drawdown percentage in the rest of the lacustrine zone.

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This trend has been represented by a dotted line on Fig. 18, together with the total settlement corresponding to ∆ p = 150kPa computed using eq. 2 (upper continuous line).

Figure 18. Computed Computed and observed subsidenc subsidence. e.

It can be concluded that the accumulated subsidence is significantly larger than predicted by eq. 1 and 2. A simple explanation is that an important part of subsidence corresponds to deformations of deeper layers within the aquifer. This conclusion is reinforced by measurements made in the Metropolitan Cathedral indicating that a significant percentage of subsidence is taking place at depth exceeding 100m (Santoyo, 2008). This also ex plains why subsidence rates of up to 20cm/year have been registered in sites where no pressure drawdown has been detected within the clay aquitard. Geotechnical modelling of subsidence could certainly also be improved by considering more realistic constitutive laws for the soil. Research in that direction has already been performed. An elasto-visco-plastic model was  proposed by Ovando and and Ossa (Ovando, (Ovando, 2007).

Figure 16. Pore pressure drawdown at base of the aquitard (Pérez, 2008).

models 4.2   Hydrogeological models

 b) The accumulated subsidence registered since

A number of hydrogeological models representing the

1862, a percentage of the aquitard thickness, presents strongas scattering but the average is fairly constant with aa value of about 20% (Fig. 17).

global and mechanical behavior of the aquitardaquiferhydraulic system have been developed. A review of these models can be found in a paper presented in this workshop (Cruickshank and Palma, 2008).

100 

   %90   ,    5    0    0 80     2      2    6    8    1 70    o    d    i   m60     i   r   p   m50    o   c   o    d   r 40    a    t    i   u   c 30    a    l   e    d   r 20    o   s   e   p 10    s    E

Some authors consider that in the west part of the city the aquifer is no longer confined by the clay aquitard. A non-saturated zone is assumed to exist below the aquitard. The available deep piezometric data do not support this hypothesis. In fact, a residual positive pore pressure is registered in all piezometers (Fig. 19).

0  0

10

20

30

40

50

60  

  Figure 17. Accumulated subsidence as a percentage of aquitard thickness. (Pérez, 2008). Espesor del acuitardo, m

 

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of a wider project dealing with all aspects of water in the Valley (Cruickshank and Palma, 2008; Auvinet and Méndez 2008). Support was also received from Conacyt and DGAPA (UNAM). Very valuable information was received from Sistema de Aguas de la Ciudad de México del Gobierno del Distrito Federal .

REFERENCES

Figure 19. Pore pressure drawdown within the aquifer (Pérez, 2008).

It would seem that static levels in deep wells found at an elevation located below the aquitard have been incorrectly interpreted as evidence of a deep water table within the aquifer. To clarify doubts about piezometric conditions prevailing in the aquifer and aquitard and contributions of the different strata to subsidence, it would be highly commendable to install, in different parts of the city, a series of deep (>200m) instrumented stations, including piezometers and deep benchmarks.

5  GEOTECHNICAL ENGINEERING IN MEXICO

CITY CONSOLIDATING SOILS A number of special systems have been used or developed for foundation of buildings in the lacustrine zone affected  by regional subsidence. The solutions range from friction and control piles to rigid inclusions. The principles and constructive aspects of these solutions are reviewed in a paper presented in this workshop (Paniagua, 2008). In many cases, the choice between these different solutions is not obvious and their functional and economical advantages and inconveniences have to be carefully com pared. In all cases, the selected foundation must meet the safety requirements imposed by the building code. Some attempts have also been made to control the effects of land subsidence by injecting or pumping water from the subsoil. The objective is mitigating differential settlements by controlling the pore pressure drawdown. Results obtained using this technique are discussed by Pliego (2008) in a paper also presented in this workshop.

6  ACKNOWLEDGEMENTS

Different research projects related to land subsidence in Mexico City have been sponsored respectively by Gobierno del Distrito Federal    asCity part watershed of a general study on geotechnical risks in Mexico (Auvinet et al . 2007) and by Fideicomiso by  Fideicomiso del Valle de México México as  as part

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