Analysis and design of foundations in Mexico City, 20 years after the 1985 earthquake

G. Auvinet Instituto de Ingeniería, UNAM, Mexico

ABSTRACT: The paper presents a general panorama of the solutions commonly adopted for foundation of buildings in the difficult soft soil conditions of the lacustrine zone of Mexico City, 20 years after the 1985 earthquake. The most common types of foundations are discussed together with the design criteria generally adopted, most of which have been included in Mexico City building code (2004). Special foundation systems developed specifically for Mexico City subsoil conditions are also examined. RESUMEN: Este trabajo presenta una revisión general de las soluciones comúnmente adoptadas para la cimentación de edificios en las condiciones difíciles de la zona lacustres de la ciudad de México, 20 años después del macrosismo de 1985. Se presentan los tipos más comunes de cimentaciones con los criterios de análisis y diseño mas aceptados, la mayoría de los cuáles han sido incorporados al Reglamento de Construcciones de la Ciudad (2004). Se examinan asimismo los sistemas especiales desarrollados para las condiciones especiales del subsuelo de la Ciudad de México. Keywords: Foundations, soft soils, earthquake, analysis, design, reliability 1 INTRODUCTION The high compressibility and low strength of the lacustrine clays of Mexico City, together with the existence of regional subsidence and high seismic activity, have made of this city a full-scale laboratory where it has been possible to ponder the influence of many factors on foundation behavior. This document presents an overall picture of the subsoil of Mexico City and of the problems encountered in design of foundations, which depend upon the type of structure to be built and the kind of foundation adopted. The multiple serviceability and failure limit states to be considered in design are examined. The special conditions existing in Mexico City have led to the adoption of different types of foundations and to the development of specific solutions in which an impressive amount of creativity was involved. These foundation systems are briefly described herein and their respective merits are evaluated. The lessons learned from the 1985 earthquake induced some significant changes in the analysis and design techniques of all types of foundations in Mexico City. Many factors are now better evaluated and taken into account. 2 SUBSOIL OF MEXICO CITY

The urban area of Mexico valley can be divided in three main geotechnical zones (Marsal and Mazari, 1959): Foothills (Zone I), Transition (Zone II) and Lake (Zone III). Figure 1 shows the three zones as defined in the present building code. In the foothills, very compact but heterogeneous volcanic soils and lava are found. These materials contrast with the highly compressible soft soils of the Lake Zone. Generally, in between, a Transition Zone is found where clayey layers of lacustrine origin alternate with sandy alluvial deposits erratically distributed. Due to exploitation of underground water for supply to the population and to other factors, in the course of the present century, Mexico City has suffered a subsidence that in some locations reaches 10m. Recent data show that the rate of subsidence tends to decrease in certain areas. However, in newly developed urban zones such as the center of the Texcoco Lake and of former lakes of Xochimilco and Chalco, in the south of the valley, the consolidation process is only in its first stage and the rate of subsidence attains 30cm per year (DGCOH, 1994). Since 1985, the geotechnical zoning of Mexico City (Fig. 1) has been updated taking into account a data base consisting of more than 10,000 boreholes and pits. This data base was incorporated in a Geographical Information System focused on geological and geotechnical problems.

Figure 1. Geotechnical zoning of Mexico City (2004)

Figure 2. Seismic zoning of Mexico City (2004) In the same way, the seismic zoning has been modified (Fig. 2). The lacustrine zone has been divided in four subzones according to the soil response observed during seismic events using a web of more than 100 accelerometers

scattered in the different zones of the city and the results of microtremors measurements. 3. TYPES OF FOUNDATIONS 3.1 General design considerations Foundations of buildings in the lacustrine zone of Mexico City must be designed taking into account the high compressibility and low shear strength of the thick soft clays layers of the subsoil. Design must also consider, among other factors, the general subsidence induced by pumping of water from the deep strata, the fracturing of the soil frequently observed in some areas, and the site effects that induce a strong amplification of the seismic waves that periodically affect Mexico Valley subsoil. Using the terminology of the Federal District (Mexico City) Building Code (G.D.F., 2004), it can be said that foundations must present an adequate safety against a large number of limit states that can be divided as follows: a) Failure limit states: flotation, plastic local or general displacement of the soil below the foundation and structural failure of footings, slabs, piles, drilled shafts or other foundation elements. b) Serviceability limit states: vertical average movement, settlement or emersion with respect to the surrounding ground, average tilting and shear deformation induced in the structure. Security against these different limit states must be guaranteed under different load combinations including extreme and average static loading but also accidental conditions including wind and seismic actions. It is accepted that foundations should be designed mainly for the following load combinations: a) Permanent loading plus variable loads with average intensity. This combination should be used to compute long-term soil deformation and to evaluate the excavation required for load compensation. b) Permanent loading plus the most critical variable loads with maximum intensity plus other variable loads with instantaneous intensity. This combination should be used to assess failure limit states. c) Permanent loading plus variable loads with instantaneous intensity plus accidental loads (earthquake or wind). With this combination, failure limit states and serviceability limit states (including transient and permanent soil deformations) should be evaluated. Estimation of each of the loads entering these combinations is far from trivial. Too often, design is in fact based on

preliminary estimations of the permanent, live and accidental loads. Even when a careful final review of the actual loads is made, design loads retain generally a large random component. For Mexico City buildings, Meli (1976) has suggested that the coefficient of variation of permanent loads is approximately constant and typically equal to 0.08. The coefficient of variation of live loads is larger and is a function of the area on which they act; yet it generally has only a small effect on the coefficient of variation of the total load. Not included in these considerations is the case of gross variations of live load due to changes in the utilization of the buildings or occurrences such as flooding of the basement of compensated buildings, which further increase the uncertainty on the loads at the foundation level. All these factors make it highly commendable that a reliability analysis be performed for design of important buildings.

Estimation of the seismic loads for foundation design is also a complex matter. Design spectra cannot be developed readily nor reliably. In the last years, however, some important advances have been registered towards a more rational approach to this problem through a better understanding of site effects and dynamic soil-structure interaction. (Romo, 2002). 3.2 Main types of foundations. The solutions adopted for foundation of buildings on soft soils in Mexico City have evolved progressively since the pre-Columbian and colonial periods due to the necessity of building increasingly larger, higher and heavier constructions (Fig. 3).

Figure 3 Types of foundations in soft soils 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 a lesser extent, bored piles and drilled shaft for heavier buildings, especially in the transition zone. A number of special systems, including friction piles, have also been used or developed. In many cases, the choice between these different solutions is not obvious and their functional and economical advantages and inconveniences have to be carefully compared. In all cases, the selected foundation must meet the safety requirements imposed by the building code. It must be recognized that, with some notable exceptions (Zeevaert, 1973), foundation design in Mexico City before the macro-earthquakes of 1985 was almost exclusively aimed at controlling the magnitude of total and differential settlements or the apparent emersion of foundations in static conditions. The lessons learned during the 1985 earthquake

made it necessary to review the traditional foundation systems taking systematically into account the seismic factor. 3.2.1 Surficial and compensated foundations Foundation on masonry footings or general raft, sometimes with short wood piles, was the first system tested by the founders of the city, with very little success at that as attested by the spectacular problems registered in the foundation of the “Templo Mayor”, the main pyramid of the Aztecs, and of many heavy colonial temples such as the Metropolitan Cathedral and the Vera Cruz, Profesa, Santísima and Loreto churches, to name only a few. It is now accepted that surficial foundations on footings or surficial mats are only acceptable for light constructions occupying a relatively small area. It must be taken into account that a load of only 20kPa applied on a large area of the lake zone can be expected to induce a total settlement close to 1m with differential settlements of about 50cm. Moreover, these

foundations are vulnerable to movements induced by adjacent buildings. Some of the problems faced when using surficial foundations can be managed recurring to compensated or “floating” foundation. The well-known compensated foundation technique consists of designing the foundation, generally a box-type structure, in such a way that the mass of excavated soil will be comparable to the mass of the building (Cuevas, 1936). Theoretically, if both weights are equal, the soil below the foundation is not submitted to any net stress and no significant settlement should develop. When the weight of the soil is smaller than the weight of the building, the foundation is partially compensated; in the opposite case, it is overcompensated. In practice, even perfectly compensated foundations undergo some absolute and differential vertical movements due to soil elastic deformation, to soil disturbance during construction and to static soil-structure interaction thereafter. Furthermore, constructing this type of foundation is not straightforward since a deep excavation in soft soil is generally required with the associated problems of stability of earth slopes or support systems and to bottom expansion or failure (Auvinet & Romo, 1998). Water tightness of the foundation is also a critical factor for compensated foundations; in many cases, this type of foundation must be equipped with a permanent pumping system to control infiltrations. Settlements and soil deformation The methods for estimating absolute and differential settlements of shallow or partially compensated foundations in static conditions have not progressed much during the last years. The standard procedure consists of determining the vertical stress increments induced in the soil by the construction using elasticity theory and estimating the corresponding strains from one-dimensional laboratory consolidation tests. Results obtained by this widely used method are slightly on the conservative side and can be considered as adequate at least to detect the possibility of grossly excessive settlements. Stress increments are now easily determined using computers instead of traditional tools such as Newmark's chart. The development of closed solutions for stresses induced by linearly loaded polygonal areas (Damy and Casales, 1985; Rossa and Auvinet, 1991) has also been helpful. In highly compressible soils, the computed settlements are of course extremely sensitive to uncertainties on load magnitude and eccentricity. Mexico City Building Code specifies that total settlement of foundations should not exceed 30cm (first criterion) and, for concrete structures, differential settlement per unit length between any two points should be less than 0.004 (second criterion). Accepting that the maximum allowable settlements constitute a critical

threshold, and that the combination of permanent plus mean live loads is a random variable, the reliability of typical buildings on compensated foundations was computed by Auvinet and Rossa (1991). A very low reliability index β was obtained, especially regarding the second criterion. Reliability decreases with the magnitude of the compensation (weight of soil excavated). If a load factor of 1.1 is introduced in the compensation calculation, reliability improves only slightly for the second criterion. Accepting a differential settlement twice as large (0.008, third criterion), as was proposed by some engineers, does not increase significantly the reliability either. Introducing load eccentricities in two perpendicular directions as additional independent random variables leads to a further decrease of reliability. It can thus be concluded that compensation, theoretically an ideal solution, can in fact be unreliable, especially when the loads are not known with precision. It must also be stressed that overcompensated foundations tend to present upward movements due to elastic strains and volumetric expansion of the unloaded soil. On the other hand, the unloaded soil below an overcompensated foundation moves into the recompression range of the compressibility curve while the soil in the surroundings remains on the virgin branch. As a consequence, the regional subdidence process less affects the preconsolidated soil below the foundation than the soil located in the surrounding area, and an apparent emersion of the foundation is observed (Diaz Cobo, 1977). This has led to spectacular protruding of some light structures such as underground parking lots and underpasses in the city. Bearing capacity The bearing capacity of shallow and compensated foundations under static vertical loads is rarely critical, since the design is generally governed by soil deformation. Moreover, it can be estimated with good accuracy using for example the well-known Skempton formula. Verification of the bearing capacity of a particular foundation can thus be made checking the following inequality:

vRcci pFcNAFQ +≤∑ / (1)

where A area of the foundation c soil cohesion (undrained shear strength)NC Skempton coefficient pv vertical stress within soil at foundation depth Fc load factor, as specified by building code FR strength reduction factor, idem Methods for estimating the bearing capacity under seismic conditions are not so satisfactory. As a matter of fact, the present state of practice consists of comparing the maximum load on the foundation, estimated assuming rigid, elastic or visco-elastic behavior of the soil, to the static bearing capacity. The effect of the earthquake is represented by an

overturning moment and a base shear force at the foundation depth. These mechanical elements are in turn considered equivalent to an inclined resultant force with a certain eccentricity, e. Eccentricity is generally taken into account by substituting the actual width B of the foundation by a reduced equivalent width equal to B-2e (Meyerhof criterion). Accepting that static bearing capacity is representative for seismic conditions is implicitly equivalent to accepting a compensation of effects, namely, the increase of the soil strength in dynamic conditions on one hand (between 20 and 40% for Mexico City clay according to Jaime, 1988) and, on the other hand, its decrease as a result of cyclic loading when the deviator cyclic stress exceeds a critical threshold of about 0.85c (Córdoba, 1986; Díaz, 1989). The partial mobilization of the shear strength of the soil by the seismic waves and the inertia forces in the foundation soil, that can contribute to failure (Rosenblueth, 1985; Cordary, 1987) have been neglected in the most recent versions of Mexico City building code. The reduction suggested by Terzaghi for sensitive soils is not taken into account either. Much research is needed on this problem, which happened to be relevant during the 1985 Mexico earthquake. As a matter of fact, as proposed by Romo (1990), the bearing capacity should be evaluated in two different conditions: during the earthquake, when dynamic effects are present, and immediately afterwards, when the pore pressure induced by the shaking may have reduced the available soil strength. The bearing capacity problem under eccentric loading has been reexamined recently (Pecker and Salencon, 1990; Auvinet et al., 1996) using the plastic flow theory. Considering parameters N (vertical load), T (base shear), M (overturning moment) and Fx (inertia force within the potentially sliding soil mass), the cinematically admissible mechanism that leads to the best inferior limit of the bearing capacity is determined. The results are presented in a normalized form in terms of the vector:

⎭⎬⎫

⎩⎨⎧

=cBF

cB

MBcT

BcNF x

2 (2)

where B is the width of the foundation and c is the undrained shear strength of the soil. In the space of normalized variables, a domain is defined where the behavior of the foundation is expected to be satisfactory. The overturning moment can be divided in a normal force N and a load eccentricity e, so the results can also be presented in the space of normalized (N, T, e, Fx). It can be established that for N/Bc < 2.5 ands a safety factor larger than 2 under central loading, the effect of the inertia forces can be neglected. On the contrary, for low safety factors, these forces induce a drastic reduction of the bearing capacity. Moreover, the results indicate that, in certain conditions, the B-2e criterion for eccentricity, generally considered as overconservative, can actually be unsafe.

Within the limitations of the present methods, it can be shown that bearing capacity in seismic conditions is principally a problem for slender structures. Auvinet and Rossa (1991) have shown that, for Mexico City conditions and considering the local seismic coefficient as a random variable, the reliability index regarding possible overturning rapidly decreases to unacceptably low values when the slenderness ratio of the structure increases. During the 1985 earthquake, for a small number of surficial and partially compensated foundations, punching of the soil and tilting were observed (Auvinet and Mendoza, 1986). This type of behavior could be traced to excessive contact pressures at the foundation level in static conditions, loading eccentricity, and infiltration of water in the substructure. Some cases of structural collapses of the substructure were also observed showing that designers wishing to use compensated foundation tend to structurally underdesign the substructure to gain some weight. It was also obvious that compensation is a poor solution for slender buildings submitted to large overturning moments since it may lead to an unstable equilibrium. 3.2.2 Foundation on point-bearing piles. Precast or cast-in-place end-bearing piles embedded in a deep hard stratum are an apparently obvious solution for foundations on soft soils. Moreover, this solution has proven to be more reliable than other types of foundation in seismic conditions in Mexico City. However, foundations on point-bearing piles may present some serious problems and their design faces many difficulties. The bearing capacity of the hard layer in which the piles rest is a first source of uncertainty. The shortcomings of classical analytical methods for evaluating this capacity have long been recognized. Most of them assume rigid-plastic behavior of the soil ignoring the essential role of soil deformability. Bearing capacity estimations thus tend to be based principally on in situ tests (cone penetration test, pressuremeter) or on loading tests. Heterogeneity of these hard strata is difficult to assess and may originate tilting of buildings with such a foundation (Ovando et al, 1988). Another source of uncertainty is the scale factor that should be considered when piles of large diameter are used, to take into account soil deformability, curvature of the Mohr envelope for high confining pressures, and progressive failure. Some of the scale factors proposed in the literature (Meyerhof, 1983) lead to unrealistically low bearing capacities. In consolidating subsoils, negative skin friction develops on the pile shaft, reducing its net bearing capacity (Auvinet & Hanell, 1981). Moreover, an apparent emersion of the structure is generally observed and damage can be induced in adjacent buildings supported by other types of foundation. Consolidation has also the effect of separating the slab of the substructure from the soil. In that condition, the head of the piles is no longer confined and can be structurally vulnerable

under the combined effect of overturning moment and base shear (Auvinet & Mendoza, 1986). As recognized in Mexico City building code, when estimating the forces induced in piles by negative skin friction, the following elements should be taken into account

1) the shear stress developed on the shaft of a pile cannot be larger that the limit soil shear strength determined in CU triaxial tests under a confining pressure representative of the conditions of the soil in situ.

2) this limit shear stress can only be reached when the

soil attains the corresponding required shear deformation.

3) the axial force developed in a pile due to skin

friction within a pile group cannot be larger than the weight of the soil located within the tributary area of the pile.

4) the unloading stresses induced by the skin friction

within the soil cannot be larger than those that are sufficient to stop the consolidation process that originates the skin friction in the first place.

Curiously, many of the methods available to take into account the negative skin friction do not consider all of the above conditions, especially the last one. It seems that the best way to take into account all of these factors is using numerical (finite element) modelling as shown in a companion paper presented in this workshop (Auvinet and Rodríguez, 2002). As mentioned above, foundations on point-bearing piles presented generally an acceptable behavior during the 1985 earthquake. However, some cases of structural damages in the upper part of the piles were detected. They were attributed to load concentration in the perimeter of the structure due to the overturning moment and to the base shear. 3.2.3 Special deep foundation systems Objective Special deep foundation systems have been developed with the principal aim of avoiding both excessive settlement and apparent emersion associated to consolidation of the upper clay formation. Some systems also allow controlling the load transmitted to each pile. Foundation systems The different systems all have in common the inclusion in the piles of a “fuse” (an element presenting large deformations when a critical load is exceeded) allowing the construction to follow the regional subsidence. In Table I, the

principal systems have been regrouped according to the position of this fuse (in the upper or lower part of the pile, or both). The type of fuse used is characteristic of each system.

Table 1 Principal types of special foundations.

Type Fuse in lower part

Fuse in upper part

Friction piles X Piles with penetrating point X P3 piles X Telescopic piles X Negative skin friction piles X Control piles X Overlapping piles X X

Another solution, not included in the above table, consists of using piles placed within a flexible case (Támez et al, Sedesol, 1992). These piles are designed to avoid overloading of point bearing piles by negative skin friction. a) Friction piles Friction piles are generally used to transfer stresses induced by shallow or partially compensated foundations to deeper, less compressible layers of the subsoil, and to reduce settlements. Not so often, they constitute the main foundation system and the stability of the structure is made dependent on the bearing capacity of the piles. A clear distinction must be established between these two types of design (Fig. 4; Auvinet & Mendoza, 1987)

Type I: Design in terms of bearing capacity In this first type of design, the number and dimensions of the piles are selected with the aim of guaranteeing that they will be able to support the load from the structure under static as well as dynamic conditions, with a safety factor generally larger than 1.5. In areas affected by regional subsidence, this type of friction pile is submitted to complex loading conditions (Fig 4a). It has been shown (Reséndiz & Auvinet, 1973) that negative skin friction can develop on the upper part of the piles while positive friction develops in the lower part. A "neutral" level can then be defined where no relative displacement occurs between soil and piles. The position of the neutral level can be approximately determined by a simple equilibrium equation (Reséndiz and Auvinet, 1973):

W + FN = FP + Cp + U (3) where W weight of the construction U water uplift pressure on the substructure (if any) Cp end-bearing capacity of piles

FN negative skin friction on the upper part of the piles FP positive skin friction on the lower part of the piles

When the neutral level is in a low position (large number of piles or high strength of the lower layers), the negative skin friction may induce significant compression in the piles. Moreover, with time, the head of the piles can be expected to protrude due to the consolidation of the surrounding soil located between the surface and the neutral level. When this design philosophy is adopted, the bearing capacity of piles must be estimated taking into account the possibility of group behavior. When the density of piles is high, soil friction available on the perimeter of the pile group plus its base capacity can in effect be smaller than the sum of the capacities of individual piles (Reséndiz and Zonana, 1969). For piles working in the conditions indicated in Fig. 4a, settlements cannot be calculated by oversimplistic methods such as the "2/3 rule". Depending of the position of the neutral level, the foundation can in fact present either settlement or emersion. Details of a more realistic method to estimate foundations movements adapted to these conditions were presented by Reséndiz and Auvinet (1973). Type II Design in terms of soil deformation In this case, only a limited number of piles are used with the principal objective of reducing the settlements of a partially compensated foundation (compensated foundations with friction piles; Zeevaert, 1956, 1962, 1973 y 1990). Since the number of piles is low, the neutral level generally coincides with the piles cap (Fig 4b). In that case, positive friction is mobilized along the full length of the piles, and the piles are in permanent failure state, which justifies the name of “creep piles” that they were given by some authors (Hansbo, 1984). In that case, the equilibrium equation is written:

W = Q + FP + Cp + U (8) where Q is the effective contact pressure at the interface of soil and slab. Problems similar to those discussed for compensated foundations may occur. Reliability is low against excessive settlements in static conditions (Auvinet and Rossa, 1991). Without any doubt, the foundations of this type were those that suffered most damages during the 1985 earthquake. 13% of all buildings between 5 and 15 stories, most of them on compensated foundation with friction piles, experimented settlement, tilting and, in one case, total failure. “Creep piles” piles cannot be expected to absorb cyclic loading during earthquakes, since soil-pile adherence is already fully mobilized, and can even decrease due to remoulding as cyclic shear stresses develop at the interface between soil and pile. Full scale experiments performed by

Jaime et al. (1990) have shown that piles fail when the combination of sustained plus cyclic loading exceeds the static bearing capacity during more than ten cycles. When the total loading exceeds this value by more than 20%, the subsequent sustained bearing capacity decreases to a value as low as 50% of the static capacity, while a penetration of the pile of 10 cm or more is observed. In the laboratory (Ovando, 1995) some direct shear tests of the soil concrete interface have also been performed. The results show that static friction can decrease significantly after cyclic loading

WT

a) TYPE I

U

FN

NEUTRALLEVEL

FP

CP

W + FN = FP + CP + U

WT

b) TYPE II

QL + U

FP

C P

W = Q L + FP + CP + U

W W

Figure 4. Friction piles For this type of design, it should then be clear that it is commendable to ignore the contribution of the piles to the global bearing capacity. The bearing capacity to be considered under seismic conditions should merely the capacity of the soil to take the slab contact pressure. The presence of the piles should only be taken into account in the static settlement estimation There have been a number of proposals aiming at increasing the efficiency of friction piles by modifying the shape of their cross section (triangular, H, etc.). Jaime et al. (1991) have shown that this is generally not achieved. Among the research work aiming at improving friction piles, the attempts to develop high adherence electro-metallic piles using electrosmotic treatment should also be mentioned (Tamez, 1964; Solum, 1966) To improve further the understanding of the behavior of friction piles, a foundation of this type has been instrumented

as described in another contribution to this workshop (Mendoza 2002) b) Pile with penetrating point. This type of pile (Reséndiz, 1964) was conceived to increase the bearing-capacity of friction piles with a controlled contribution of the point. The diameter of the point is smaller than the rest of the pile in order to facilitate penetration in the hard layer under the combined effect of loading and negative skin friction and to avoid emersion. The point can be made of reinforced concrete (Reséndiz, 1964; Ellstein, 1980) or steel (Reséndiz et al., 1968). In the latter case, the bearing capacity of the pile can be better controlled by using a point with a pre-established failure load. Flexibility of the point constitutes however a problem during installation of piles. c) Negative skin friction piles Those are simply point-bearing piles that penetrate freely through the foundation slab (Correa, 1961). They can contribute to reduce significantly the settlements due to negative skin friction that develops on the shaft of the piles under the combined effect of the structural load and the consolidation of the clay layer. Finite element modelling of this type of piles has been presented recently (Rodríguez, 2000). Spacing of the piles appears to be the most significant design parameter. d) Control piles The so-called “control piles” are similar to the previous ones but they are equipped in their upper part with a mechanism that controls the load received by each pile. Each pile can also be unloaded by removing the mechanism in order to correct any tilting of the building. Those systems have

sometimes been installed during the life of the structure as part of an underpinning process (González Flores, 1964, 1981; Auvinet, 1989). The different available control mechanisms have been reviewed by different authors (Martínez Mier, 1975; Correa, 1980; Aguilar, 1990; Rico, 1991). In Table 2 a list of the best known systems is presented. En seismic conditions some of these special systems can be vulnerable and suffer damage going from simple deformations to total collapse. Lack of maintenance can also be a problem. Several proposals have been made to improve the design of control piles (Aguilar y Rojas, 1990). Overturning of the loading frame can be avoided using a new type of anchors. The mechanism can also be transformed to absorb tensions. e) Telescopic piles These are tubular piles with a piston-like cylindrical point lying on the hard layer (Correa, 1969). The tubular portion of the pile is partly filled with sand. When sand reaches a certain level, an arching effect develops and both parts of the piles work as a unit. If necessary, sand can be removed to free the point and avoid emersion of the foundation. f) Overlapping piles This type of foundation (Girault, 1964, 1980) includes conventional friction piles (A Piles) together with negative skin friction piles (B piles) lying on the hard layer. This arrangement reduces the increment of stresses in the soil and the corresponding settlements. Emersion is also avoided.

Table 2 Principal types of control mechanisms for piles.

Mechanism Reference

Loading frame with deformable wood cubes González Flores, 1948; Salazar Resines, 1978 Loading frame with jack and automatic relief valve

A, Pilatovsky, cited by J.J. Correa, 1980

Metallic tensors P. González, 1957, cited by Aguilar, 1990 Metallic cap Aguilar, 1960, cited by Aguilar, 1990 Loading frame with flat hydraulic jacks W. Streu, 1963, cited by J.J. Correa, 1980 and

Aguilar 1990 Sand confined within a capsule J. Creixell and J.J. Correa C., 1975, cited by

Aguilar, 1990 Energy dissipator M. Aguirre, 1981; D. Reséndiz, 1976 Mechanical system of self control M.A. Jiménez, 1980 Mobil wedge P. Girault, 1986, cited by Aguilar, 1991 Communicating hydraulic jacks F. Zamora Millán, cited by A. Rico A., 1991 Constant friction cell E. Támez, 1988 Cell with teeth for transmission of tensions A. Rico A., 1991

Several proposals have been made to improve the design of control piles (Aguilar & Rojas, 1990). Overturning of the loading frame can be avoided using a new type of anchors. The mechanism can also be transformed to absorb tensions. g) Telescopic piles These are tubular piles with a piston-like cylindrical point lying on the hard layer (Correa, 1969). The tubular portion of the pile is partly filled with sand. When sand reaches a certain level, an arching effect develops and both parts of the piles work as a unit. If necessary, sand can be removed to free the point and avoid emersion of the foundation. h) Overlapping piles This type of foundation (Girault, 1964, 1980) includes conventional friction piles (A Piles) together with negative skin friction piles (B piles) lying on the hard layer. This arrangement reduces the increment of stresses in the soil and the corresponding settlements. Emersion is also avoided. 4. CONCLUSIONS Design and construction of foundations on Mexico City soft soils constitute a difficult challenge. Mexican engineers have developed a number of original solutions to solve this problem. This never-ending research received a new impulse after the 1985 earthquake.

5. REFERENCES AND BIBLIOGRAPHY

Aguilar, J.M. & Rojas, E. , 1990, “Importantes mejoras en los dispositivos de control de pilotes”, Memoria de la XVa Reunión Nacional de mecánica de suelos, San Luis Potosí, México.

Aguirre, M. , 1981, “Dispositivo para controlar hundimientos de estructuras piloteadas”, Publicación No 439, Instituto de Ingeniería, UNAM, México, D.F.

Ang, H.S. & Tang, W.H., 1984, 'Probability Concepts in Engineering Planning and Design. Decision, Risk and Reliability", Wiley & Sons, Inc.

Auvinet, G. & Hanell, J.J., 1981, “Negative skin friction on piles in Mexico City clay”, Proc. Xth International Conference on Soil Mechanics and Foundation Engineering, Stockholm, Sweden, Vol.2

Auvinet, G. & Mendoza, M., 1986, "Comportamiento de diversos tipos de cimentaciones en la zona lacustre de la Ciudad de México durante el sismo del 19 de Septiembre de 1985", Memoria, Simposio "Los sismos de 1985; casos de mecánica de suelos", Sociedad Mexicana de Mecánica de Suelos, pp. 227-240, México, D.F.

Auvinet, G. y Mendoza, M., 1987, "Consideraciones respecto al diseño de cimentaciones sobre pilotes de fricción", VIIa Reunión Nacional de Ingeniería Sísmica, Sociedad Mexicana de Ingeniería Sísmica, 19-21 de noviembre, Querétaro, México.

Auvinet, G. y Gutiérrez, E., 1989, "Instrumentación de un edificio en proceso de recimentación", Memoria, Simposio sobre recimentaciones, Sociedad Mexicana de Mecánica de Suelos, pp. 137-148, México.

Auvinet, G., and Rossa, O., 1991, "Reliability of Foundations on Soft Soils", Proceedings, Sixth International Conference on Applications of Statistics and Probability in Civil Engineering, CERRA-ICASP-6, June, pp. 768-775, Mexico, D.F.

Auvinet, G., Pecker, A. & Salençon, J., 1996, “Seismic bearing capacity of shallow foundations in Mexico City during the 1985 Michoacan

Earthquake”, Proceedings, Eleventh World Conference on Earthquake Engineering, (CDROM), Acapulco, Mexico, July.

Auvinet, G. & Romo M.P., 1998, "Deep excavations in Mexico City soft clay", Invited presentation, ASCE's 1998 National Convention, Boston, Massachusetts, USA. Geotechnical Special Publication N0 86, Big Digs around the world. pp. 211-229.

Correa, J.J., 1961, “The application of negative friction piles to reduction of settlement”, Fifth International Conference on Soil Mechanics and Foundation Engineering, Paris, France

Correa, J.J., 1969, “A telescopic type of pile for subsidence conditions”, Proc. Specialty session on negative skin friction and settlements of piled foundations, 7th International Conference on Soil Mechanics and Foundation Engineering, Mexico, D.F.

Correa, J.J., 1980, “Estado actual del conocimiento sobre pilotes de control”, Memoria de la Reunión Conjunta Consultores-Constructores Cimentaciones profundas, Sociedad Mexicana de Mecánica de Suelos, México, D.F.

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