1. 1 INTRODUCTION
The high compressibility and low shear strength of Mexico City lacustrine clays, together with
regional subsidence due to pumping of water from the subsoil and high seismic activity, have
made of this city (A soil Mechanics paradise according to K. Terzaghi) a full-scale laboratory
where it has been possible to ponder the influence of many factors on foundation behavior in
extreme conditions. This document presents an overall picture of the subsoil of Mexico City
and of the problems encountered in excavations and foundations design and construction. The
multiple geotechnical hazards and corresponding serviceability and failure limit states to be
considered in design are examined. The special conditions prevailing in Mexico City led to the
adoption of different types of excavation and foundation techniques in which an impressive
amount of creativity was involved. These systems are briefly described herein and their respec-
tive merits are evaluated.
2 THE SUBSOIL OF MEXICO CITY
The urban area of Mexico valley can be divided in three main geotechnical zones (Marsal &
Mazari, 1975): Foothills (Zone I), Transition (Zone II) and Lake (Zone III). 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 er-
ratically distributed.
Excavations, Foundations and Geotechnical Hazards: The case of
Mexico City
G. Auvinet
Research Professor, Institute of Engineering, National University of Mexico, Mexico City
ISSMGE Vice President for North America
gauvinetg@iingen.unam.mx
ABSTRACT: An overview of the main geotechnical hazards that must be taken into account in
the design and construction of deep excavations and foundations in soft soils is presented. The
paper focuses on the case of the lacustrine zone of Mexico City. The solutions commonly
adopted for excavations and foundations of buildings in these very difficult soft soil conditions
are examined. It is concluded that most geotechnical hazards can be dealt with by using ration-
ally the geotechnical information available and performing analyses based on sound soil me-
chanics principles.
2. Figure 1. Soil profile in the Lake Zone of Mexico City (Marsal, 1975)
In Figure 1, a typical soil profile corresponding to the Lake Zone is presented. The water ta-
ble is close to the surface. Three clayey layers are to be distinguished, denominated upper
(Formación Arcillosa Superior, FAS), lower (Formación Arcillosa Inferior, FAI) and deep de-
posits (Depósitos Profundos, DP). The clays of FAS are separated from FAI by a hard layer
(Capa Dura, CD), a sandy clayey stratum, some 3m thick, lying at a typical depth of 30 to 35m.
Generally, FAS is covered by a desiccated crust and/or an artificial fill several meters thick.
Average values of index properties for borehole Pc 28 are presented in Table I.
Table I. Typical average values of index properties in Lake Zone
(Borehole Pc-28, Marsal, 1975)
PROPERTY FAS CD FAI
Water content, % 270 58 191
Liquid limit wL, % 300 59 288
Plastic limit, wP, % 86 45 68
Density of solids, Ss 2.30 2.58 2.31
Initial void ratio, e0 6.17 1.36 4.53
Unconfined compressive
strength, qu, kN/m2 85 24 160
The spatial variations of soil properties in the lacustrine zone have been assessed forming a
data base consisting of more than 10,000 boreholes. This data base was incorporated in a Geo-
graphical Information System focused on geological and geotechnical problems. Interpolations
of data have been performed using estimation and simulation geostatistical techniques
(Auvinet, 2002). The present geotechnical zoning of Mexico City is presented on Fig. 2. An
updated zoning will be available in 2014.
3. Figure 2. Geotechnical zoning of Mexico City (GDF, 2004)
Due to extraction of potable water from deep aquifers and to other factors, since the begin-
ning of the XXth century, the lacustrine zone of Mexico City has suffered a general subsidence
that in some locations has exceeded 13m. Recent data show that subsidence rate tends to de-
crease in certain areas. However, in newly developed urban zones such as the center of Texco-
co Lake and of former lakes of Xochimilco and Chalco, in the south of the valley, the consoli-
dation process is only in its first stage and the subsidence rate attains 40cm per year. There is
no end in sight for this phenomenon since close to 70% of the potable water of Mexico City is
directly obtained from the subsoil of the valley. Differential settlements associated to general
subsidence induce soil fracturing. This is becoming a severe problem in some zones of Mexico
City. Soil fracturing is also induced by other mechanisms, including hydraulic fracturing
(Ovando et al., 2012).
After the 1985 earthquakes, an extensive research program was undertaken to obtain a better
knowledge of dynamic properties of Mexico City clays. Shear modulus attenuation curves were
obtained and modeled and the degradation and residual strains due to cycling loading have been
assessed. Important advances have also been registered in the modeling of site effects on
ground motions and of soil-structure interaction in seismic conditions (Romo & Auvinet, 1992;
Romo, 2002). The present and future influence of the subsidence process on the properties of
Mexico City clays has been evaluated and important consequences such as a significant modifi-
cation of the ground seismic response can been foreseen.
From the above, a list of the main geotechnical risks can be established for excavations or
foundations in Mexico City soft soil:
- Large short-term and long term deformations of the soil induced by loading or unloading
(foundations, excavations).
- Shear failure of the soil due to its low shear strength (slopes, superficial foundations)
- General subsidence effects on excavations and foundations (negative skin friction, appar-
ent protruding)
- Soil fracturing due to different mechanisms (affecting excavations and foundations)
- Seismic site effects (affecting excavations and foundations)
4. 3 EXCAVATIONS
Many different techniques have been and are being used for large excavations in Mexico City
clays. Few open excavations with slopes are executed these days, since they can only be per-
formed in large lots where interference with existing constructions is not a problem. Generally,
excavations can only be made using some sort of lateral support system.
a) Support elements
- Berlin Walls. Application of support systems constituted by anchored or unanchored soldier
piles and wood lagging is limited to very shallow (<3m) excavations in Mexico City. When the
FAS clay is reached, risk of base failure or extrusion of soft layers at the base of the wall, be-
fore the boards can be placed, becomes unacceptable.
- Sheet Piles. Wood, steel and concrete sheet piles driven in the soil before the excavation
starts have been commonly used to support excavation walls in Mexico City. Difficulties are
sometimes encountered to drive the sheet piles and pre-boring may be required, especially in
the Transition Zone. Seepage of water and/or intrusion of soil through joints between adjacent
elements can also be a problem. Due to their flexibility, wales are used to support the concen-
trated forces transmitted by the struts.
- Cast-in-place Diaphragm Walls. This technique consists of casting concrete in trenches sta-
bilized with slurry. Diaphragm walls were introduced in Mexico City for the construction of the
first lines of the subway system in 1967 and have received a wide acceptance since then.
- Precast Walls placed in Slurry Stabilized Trenches. This is an alternative to diaphragm
walls that is increasingly popular. Precast wall elements are placed in a trench stabilized with
bentonite cement grout. The main advantages of this technique are the quality of the wall sur-
face and its lesser final thickness as compared to diaphragm walls. Its main limitation is the
maximum weight of the precast elements that can be handled.
b) Excavation and Bracing Techniques
- Central core method. This technique consists of driving vertical support elements such as
steel or concrete sheet piles along the outer limit of the construction area and excavating only
the central part of this area, leaving a peripheral berm in place. The central part of the substruc-
ture is then built and used to support the struts that are progressively placed against the sheet
piles, while short stretches of the berm are removed. The rest of the substructure can then be
built. This technique is generally limited to wide excavations no deeper than 6m due to berm
stability problems. It can also be used with diaphragm walls.
- Wall-to-wall bracing. Wall-to-wall bracing is used when the existing horizontal space is not
sufficient to use the central nucleus method and for very deep excavations. The bracing struc-
ture consists generally of tubular struts. In some instances, cast-in-place transversal walls have
been used as supporting structure. Those walls are then partially or totally demolished during
excavation and substructure construction (Ponce, 1985).
- Anchors. Prestressed anchors are not used in the Lake Zone due to the low strength of the
soft clay. On the other hand, they have been quite useful in the Transition and Foothills Zones
where walls can be tied back in firm soils. The main advantage of this technique is that the sub-
structure can be constructed free of obstacles. However, legal aspects concerning anchors in-
vading the subsoil of contiguous lots are still a problem and have been a cause of serious con-
flicts.
- Use of the substructure as a bracing system. In a limited number of cases, the substructure
itself has been used as a support system. This requires driving sheet piles or casting walls in
5. trenches before the excavation starts and performing underground excavation while the sub-
structure is being built from the ground down.
Other techniques, such as driving precast substructures by inducing failure of the soil at the
base of the peripheral wall from within, have been tested but were only partially successful due
to the high sensitivity of Mexico City clay (Marsal, 1959).
For deep excavations in soft clays, and especially for tunnel shafts, some special techniques
have also been developed. These include the flotation (Auvinet et al., 2010) and the prefabri-
cated rings methods (Zemva, 2011).
c) Groundwater considerations for deep excavations
In saturated soft clays like those found in Mexico City, the water table subsides spontaneous-
ly as the soil is excavated due to negative pore pressures induced by unloading. Dewatering is
thus required only to maintain this condition and to control the flow of water, mainly through
pervious lenses, towards the excavation. It may also be necessary to eliminate uplift pressures
in sand layers close to the bottom of the excavation. Dewatering contributes to avoid effective
stress changes within the soil that cause volumetric expansions and have an unfavorable effect
on soil strength. It can also be used to give a favorable orientation to seepage forces within the
soil in order to improve the working conditions and the stability of the excavation.
Extraction of water should however be reduced to a minimum in order to avoid consolida-
tion-induced settlements of the surrounding area. In impervious clays, flow nets cannot be used
to design pumping systems since the flow condition is generally not a steady one. These sys-
tems are thus designed on an empirical basis, using local experience. They generally consist of
a series of wells of small diameter with jet-eductor pumps. Even for large excavations, the vol-
ume of water extracted rarely exceeds a few liters/s.
d) Analysis and design of deep excavations
Much of the experience in matter of deep excavations in the soft soils of the Mexico City
valley has been incorporated in the guidelines and requirements included in Mexico City Build-
ing Code and Complementary Norms for Design and Construction of Foundations (2004).
According to these documents, two types of limit states must be considered:
- Failure Limit States: collapse of slopes and/or supported or unsupported walls, failure of
the foundation of contiguous constructions, failure of the bottom of the excavation due to shear
stresses or to water pressure in pervious layers located below the excavation depth.
- Service Limit States: short and long term vertical and horizontal movements induced by un-
loading in the excavated area and its surroundings.
A minimum uniform vertical surcharge of 15kPa in the area surrounding the excavation must
be considered in the stability analysis.
From the point of view of Failure Limit States for design of deep excavations, the main con-
siderations are the following:
- Slopes
Stability of excavation slopes must be verified using limit analyses considering kinematically
admissible potential failure surfaces. Cracking of soil induced by unloading must be taken into
account and attention must also be paid to failure mechanisms involving extrusion of very soft
thin layers (Reséndiz and Zonana, 1969). Time has also been shown to be an important factor in
6. the stability of slopes as cohesion tends to decrease (Alberro, 1987). The resisting forces or
moments considered in the analyses must, in general, be reduced multiplying them by a re-
sistance factor of 0.6; however, a factor of 0.7 is considered acceptable when failure cannot af-
fect public facilities or contiguous constructions.
- Failure of bottom of excavation induced by uplift pressure.
When an excavation is performed in the upper clay layer, where some pervious horizontal
lenses are interbedded, it is essential to check that the water pressure in these lenses cannot in-
duce uplift failure of the bottom of the excavation. The minimum thickness of impervious mate-
rial above any lens should be:
h > (w/m)hw (1)
where:
h thickness of impervious layer
hw water pressure in lens (meters of water)
w unit weight of water
m unit weight of impervious material
When this condition cannot be satisfied with an ample safety margin (safety factor larger
than 2, to allow for cracks in the clay), water pressure in critical lenses must be abated by
pumping from vertical wells. This has become a common practice in Mexico City.
- Stability of supported walls.
In braced excavations, the main points to check are the stability of the bottom, the stability of
a mass of soil including the support elements and the structural capacity of struts.
Base shear failure is especially critical for deep excavations. The following condition must
be checked:
RcuCv FNcqFp (2)
where:
cu undrained shear strength of the soil below the level of the bottom of the excavation
Nc Skempton's bearing capacity factor, a function of the dimensions of the excavation
(Skempton, 1951)
pv vertical total stress in the soil at excavation depth
qFc superficial lateral loads affected by a load factor
FR resistance factor equal to 0.5 when public services or contiguous constructions can be
affected and to 0.7 when such a risk does not exist.
Analyses by the characteristic lines method have shown that safety against base failure can
be significantly improved (up to 44%) by preloading of struts (Alberro, 1987). For an accurate
evaluation of this risk and estimation of induced movements, step-by-step finite element model-
ing of the excavation and bracing, assuming that the soil presents an elasto-plastic behavior, has
been used (Romo et al , 1992; Rodríguez, 1998).
Thrust on struts can be estimated from measurements that were performed on actual struc-
tures (Rodríguez, 1969; Alberro, 1970). Pressure diagrams similar to those proposed for clays
by Peck (1969) are commonly used for design. However, it is always considered that for exca-
vations performed below the water level, the resultant thrust on braces should be at least equal
to the hydrostatic pressure.
7. - Stability of trenches for diaphragm walls.
Stability of trenches used for diaphragm walls can be verified by a number of limit analysis
methods (Nash, 1963; Alberro and Auvinet, 1984). For the impervious clays of Mexico City, it
has been shown that plain water or water mixed with local clay can be used instead of bentonite
slurry to stabilize the trenches (Santoyo et al., 1987). Formal reliability analyses (Arias, 1997)
suggest that the critical stability factors are the level of the slurry in the trench and the magni-
tude of the lateral vertical surcharge in the surrounding area. It must also be taken into account
that soil fracturing is a common problem during the construction of trenches. This phenomenon
can generally avoided by controlling the level of the slurry within the excavation.
- Stability of contiguous building foundations
Attention must be given to possible damage or failure of foundation of contiguous buildings.
Conversely, superficial footings must always be designed taking into account the eventuality of
an excavation being opened in contiguous lots. Accordingly, for footings close to lot limits, a
low resistance factor, of the order of 0.3, must be considered.
From the point of view of Service Limit States, the following aspects are particularly rele-
vant:
- Short and long-term movements associated to unloading
Very large short-term vertical soil movements induced by unloading have been registered in
excavations performed in Mexico City soft clay (Cuevas, 1936). These movements can be suc-
cessfully predicted using the theory of elasticity (Alberro, 1970). The short-term modulus to be
used for Mexico City clay in this type of calculation is typically around 5MN/m2
and Poisson's
ratio is close to 0.5. Vertical movements of several decimeters can be expected in the bottom of
large excavations. In order to reduce the impact of such movements on the foundation structure,
it has been found convenient to divide the excavation in small areas, following a symmetrical
sequence. The corresponding parts of the foundation structure are built immediately in order to
ballast the excavated areas and to reduce overall unloading and differential movements.
When the excavation remains open during a significant period, expansions associated to vol-
umetric changes can also become very important, especially if drainage is inadequate and water
can flow freely towards the excavation.
Permanent unloading of the soil for structures such as underground parkings, or subway cut
and cover tunnel, lead to additional long-term vertical movements due to interference with re-
gional subsidence. The subsoil of unloaded areas becomes preconsolidated and less susceptible
to this phenomenon. Progressively, these structures tend to protrude, sometimes by several me-
ters, above the surrounding ground (Díaz Cobo, 1977).
- Settlement of the ground around the excavation.
The short-term settlement of the surrounding area induced by excavation depends mainly of
the movements of the bracing system. Preloading of struts close to their design load has been a
common and efficient practice to control these movements.
For deep excavations in Mexico City clays, elasto-plastic finite element analyses have been
performed in order to check and extend the results presented by Mana and Clough (1981),
O'Rourke (1981) and Hashash and Whittle (1996) regarding soil movements near braced exca-
vations. The soil thrust/struts reaction ratio was singled out as the main factor that controls the
magnitude and path of the soil movement (Rodríguez, 1998).
8. 4 FOUNDATIONS
4.1 General design considerations
Foundations of buildings in the lacustrine zone of Mexico City have to be designed taking into
account the high compressibility and low shear strength of the thick soft clays layers of the sub-
soil. Design must also consider, among other factors, the general subsidence induced by pump-
ing of potable 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 periodi-
cally affect Mexico Valley subsoil.
As specified by Federal District (Mexico City) Building Code (G.D.F., 2004), foundations
must present an adequate safety against a large number of limit states that can be divided as fol-
lows:
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 ele-
ments.
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 for different load combina-
tions including extreme and average static loading but also for accidental conditions including
wind and seismic actions:
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). This combination should be used to evaluate failure limit states and ser-
viceability limit states (including transient and permanent soil deformations).
Estimating the loads entering into 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, 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 loads due to changes in the use of the buildings or occurrences such as flooding of the
basement of compensated buildings, which further increase the uncertainty on the actions at the
foundation level. All these factors make it highly commendable that a reliability analysis be
performed for design of important buildings.
4.2 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 in-
creasingly larger, higher and heavier constructions (Fig. 3).
9. Figure 3 Types of foundations on soft soils in Mexico City
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. In many cases, the choice be-
tween 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 control-
ling 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 re-
view the traditional foundation systems taking systematically into account the seismic factor.
4.2.1 Superficial 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 spec-
tacular 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. The large settlements of the
Bellas Artes theatre, built in 1910 are also famous. It is now accepted that superficial founda-
tions on footings or superficial mats are only acceptable for light constructions occupying a rel-
atively small area. It must be taken into account that a uniform loading 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 move-
ments induced by adjacent buildings.
10. Some of the problems faced when using superficial 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 stress increment
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 over-
compensated.
In practice, even perfectly compensated foundations undergo absolute and differential verti-
cal movements due to soil elastic deformation, soil disturbance during construction and 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 earth slopes or support systems stability and to bottom expansion or failure
(Auvinet & Romo, 1998). Water tightness of the foundation is also a critical factor for compen-
sated foundations; in many cases, this type of foundation must be equipped with a permanent
pumping system to control infiltrations.
a) Settlements and soil deformation
The methods for estimating absolute and differential settlements of shallow or partially com-
pensated 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 but can be considered as adequate at least to detect the pos-
sibility 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 so-
lutions for stresses induced by linearly loaded polygonal areas (Rossa & Auvinet, 1992) has al-
so 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 to-
tal settlement of foundations should not exceed 30cm (first criterion) and, for concrete struc-
tures, 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 Ros-
sa (1991). A very low reliability index 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 ei-
ther. 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 com-
pensation, theoretically an ideal solution, can in fact be unreliable, especially when loads are
not known with precision.
It must also be stressed that overcompensated foundations tend to present upward move-
ments 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 preconsolidated soil below the foundation is less affected by the regional sub-
sidence process than the soil located in the surrounding area, and an apparent protruding of the
11. foundation is observed. This has led to spectacular emersion (more than 1m) of some light
structures such as underground parking lots and underpasses in the city.
b) 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. Verifica-
tion of the bearing capacity of a particular foundation can thus be made checking the following
inequality:
vRcci pFcNAFQ / (3)
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 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 representa-
tive 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 et al., 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. The partial mobilization of the shear strength of the soil by the seismic waves and the in-
ertia forces in the foundation soil that can contribute to failure, have been neglected in the most
recent versions of Mexico City building code. The reduction suggested by Terzaghi for sensi-
tive 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, the bearing capacity should be evaluated in two differ-
ent conditions: during the earthquake, when dynamic effects are present, and immediately af-
terwards, 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
(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 bear-
ing capacity is determined. The results are presented in a normalized form in terms of the vec-
tor:
c
BF
cB
M
Bc
T
Bc
N
F x
2 (4)
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 ex-
pected 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).
12. 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 bear-
ing 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 de-
creases to unacceptably low values when the slenderness ratio of the structure increases.
During the 1985 earthquake, for a small number of superficial and partially compensated
foundations, punching of the soil and tilting of the building were observed (Auvinet & Mendo-
za, 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 design-
ers 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 build-
ings submitted to large overturning moments since it may lead to an unstable equilibrium.
4.2.2 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, founda-
tions 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 un-
certainty. 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 es-
sential 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.
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 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 subsoil, negative skin friction develops on the pile shaft, reducing its net
bearing capacity (Auvinet & Hanell, 1981). Moreover, an apparent protruding of the structure is
generally observed and damage can be induced in adjacent constructions 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 structural-
ly vulnerable to the combined effect of overturning moment and base shear (Auvinet & Mendo-
za, 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 re-
quired 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.
13. 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 fric-
tion in the first place.
Curiously, many of the methods available to assess 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 by using numerical (finite element) modeling (Auvinet & Rodríguez,
2002; Rodriguez, 2011).
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 fol-
low 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 flex-
ible case (Tamez in 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 compen-
sated 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 be-
tween these two types of design (Fig. 5; Auvinet & Mendoza, 1987)
14. WT
a) TYPE I
U
FN
NEUTRAL
LEVEL
FP
CP
W + FN = FP + CP + U
WT
b) TYPE II
QL + U
FP
CP
W = QL + FP + CP + U
WW
Figure 5. Friction piles
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 re-
gional subsidence, this type of friction pile is submitted to complex loading conditions (Fig 5a).
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
& Auvinet, 1973):
W + FN = FP + Cp + U (5)
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 low-
er layers), 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 sur-
rounding 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 indi-
vidual piles.
For piles working in the conditions indicated in Fig. 5a, settlements cannot be calculated by
ill-adapted methods such as the "2/3 rule". Depending on the position of the neutral level, the
foundation can in fact present either settlement or emersion. Details of a more realistic method
for estimating foundations movements adapted to these conditions were presented by Reséndiz
and Auvinet (1973) and are now implemented in software commonly used by designers in Mex-
15. ico City. A comparison between the results obtained using this analytical model and those given
by 2D and 3D numerical models has been presented by Rodríguez (2011)
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 in static conditions while avoiding pro-
truding (compensated foundations with friction piles; Zeevaert, 1973). Since the number of
piles is low, the neutral level generally coincides with the piles cap (Fig 5b). In that case, posi-
tive 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 (6)
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 & 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 foun-
dation 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 clay remoulding as
cyclic shear stresses develop at the interface between soil and pile. Full scale experiments per-
formed by Jaime et al. (1991) 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, direct shear tests of the soil concrete interface have also been performed
(Ovando, 1995). The results show that static friction can decrease significantly after cyclic
loading.
When this type of design is adopted, 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 consid-
ered 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 es-
timation
A number of proposals aiming at increasing the efficiency of friction piles by modifying the
shape of their cross-section (triangular, H, etc.) have been presented. Jaime et al. (1991) have
shown that this is generally not achieved. Among the research work aiming at improving fric-
tion piles, the attempts to develop high adherence electro-metallic piles using electrosmotic
treatment should also be mentioned (Tamez, 1964)
To improve further the understanding of the behavior of friction piles, a foundation of this
type has been instrumented (Mendoza, 2002). Most of the above considerations about Type II
foundations have been confirmed.
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
16. 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 fail-
ure 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 (Cor-
rea, 1961). They can contribute to reduce significantly the settlements due to negative skin fric-
tion 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 modeling of this type of piles has been pre-
sented (Rodríguez, 2001). Spacing of the piles appears to be the most significant design param-
eter.
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 sys-
tems 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 mecha-
nisms 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 go-
ing 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 & Rojas,
1990). Overturning of the loading frame can be avoided using a new type of anchors. The
mechanism can also be transformed to absorb traction forces.
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 dissipater 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
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.
17. Figure 6. Control pile
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 re-
duces the increment of stresses in the soil and the corresponding settlements. Emersion is also
prevented.
4.2.3 A new popular foundation system: stiff inclusions.
For housing projects consisting of low buildings for which earthquake is not a critical aspect ,
stiff inclusions are now being widely used to control settlements. An ISSMGE TC36 interna-
tional workshop “Stiff inclusions in difficult soft soil conditions” was organized (May 11-12th
2006) on this subject in Mexico City (TC36, 2006). This solution is economically attractive in
many situations.
5 CONCLUSION
Design and construction of foundations in Mexico City soft soils constitute a difficult chal-
lenge. Mexican geotechnical engineers have developed a number of original solutions to solve
this problem. This never-ending research received a new impulse after the 1985 earthquake and
is still very active nowadays. Using sound soil mechanics principles has been reiterately proven
to be the best way to deal with the numerous geotechnical hazards encountered in Mexico val-
ley.
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