Zeevaert, Leonardo. (1983). Foundation engineering for difficult subsoil conditions (2nd edition).
Estados Unidos de América. Van Nostrand Reinhold Company Inc.
Introduction
Isolated footings
Footings are understood formed by a rigid rectangular base of stone or concrete of dimensions: width
𝐵 and length 𝐿, in which the ratio of 𝐿/𝐵 will not exceed 1.5. The foundation structure will support the
column load. The bearing capacity of the footing may be estimated, and its dimensions selected;
thereafter, a forecast of the settlement is made. To illustrate the case of footing foundations, consider
a building with nine columns supported on isolated footings. In this case, the footings will work
independently of each other. Therefore, it is required that the differential settlements between footings
will not exceed the allowable total and differential settlement requirements. The differential settlements
may be reduced selecting properly the area of the footings, and at times, using the stiffness of the
superstructure. From the structural point of view, however, the superstructure should not be allowed
to take high secondary stresses induced by the differential settlements of the footings, except in very
special cases. Single footing foundations, in general, will be used only in soils of low compressibility
and in structures where the differential settlements between columns may be controlled by the
superstructure flexibility, or including in the design of the building joints or hinges that will take the
differential settlements and/or rotations, respectively, without damaging the construction.
Strap footings
Continuous Footings
When it is necessary to control within certain limits the magnitude of differential settlements between
columns supported on footings, and when soil deposits of medium or low compressibility are
encountered, it is recommended to use continuous footings. They may be defined as resisting
elements joining columns together by foundation beams. Continuous footings are arranged by joining
two or more columns together with beams. The vertical differential displacements may be controlled
via beam stiffness. The selection of the foundation beams, either running in one direction or the other
along column rows, depends largely on the layout of the column loads, and other functional
requirements concerning the structural and architectural design of the project. For heavier loads, and
when the project calls for stiffness in both directions, the foundation is given stiffness with beams in
both directions. In this case, it may be that the footing slabs will cover practically all the foundation.
This type of foundation using continuous footings is advantageous in soils of medium compressibility,
where it is necessary to control differential movements between columns. The foundation beams are
designed with the necessary stiffness to fulfill the differential settlements requirements.
Raft Foundation
When the loads are so large that continuous footings will occupy close to 50% of the projected area
of the building, it is more economical to use a continuous mat covering the entire area. The total load
in this case may be assumed uniformly distributed in the area covered by the building. The soil reaction
is determined on the basis of a safe bearing capacity. The total and differential settlements may be
investigated considering the stiffness of the raft or foundation slab is a matter of economy, compatible
with the allowable differential settlements. Flexibility is important to obtain economy; however,
restrictions in differential vertical displacements between columns may call for certain slab stiffness,
either by making it thicker or by placing foundation beams joining column rows. The beams can be
designed with the required stiffness to reduce differential displacements. This type of foundation may
be used generally in soil deposits of medium compressibility; however, in certain instances, the surface
raft foundation may be used in soils of high and very high compressibility, where large total settlements
may be allowed. This type of foundation may be used efficiently in reducing differential settlement.
Compensated Foundations
In soil deposits of medium, high and very high compressibility and low bearing capacity, compensated
foundations are indicated. This type of foundation requires a monolithic box foundation. When the
water table is close to the ground surface, water proofing is necessary to use the buoyancy effect in
designing the foundation. In the design of compensated foundations, it should be borne in mind that
the soil should be considered as a material of two phases, namely: a solid and a liquid phase.
Therefore, in a compensated foundation, the compensation is made by adding two effects: (l)
substitution of the submerged weight of solids, and (2) the buoyancy effect by the weight of liquid
displaced. Both effects are used to equalize the total weight of the building. The volume of the concrete
box forming the foundation structure and basements will displace a weight of liquid that, according to
Archimedes' principle, will contribute in floating the foundation up to this value, reducing the load
applied to the solid phase. The load taken by the solid phase will, however, deform the soil because
of the change in effective stresses induced in the soil structure. It should be investigated from the point
of view of bearing capacity of the soil and total and differential settlements, as previously discussed
for other foundations. A compensated foundation is designed usually with a stiff foundation structure;
therefore, differential settlements are controlled rather easily. The foundation structure is designed
either as a flat slab system or a slab-beam foundation system, joining the columns with beams in both
directions. This type of foundation, owing to the characteristics of the soils where it is constructed,
implies the necessity of knowing more accurately the stress-strain-time characteristics of the material,
in order to evaluate settlements because of changes in effective stresses in the solid phase. The load
of the building will be compensated by means of an excavation sufficiently deep to permit the obtention
of the necessary load capacity, and the reduction of the vertical displacements to magnitudes that will
be satisfactory. The differential settlements will be controlled giving the necessary stiffness to the
foundation structure. The mechanical behavior of the foundation is controlled by the solid phase
because of changes in effective stresses in the ground. The magnitude of settlements in this type of
foundation depends mainly on the ability of the foundation engineer in keeping the prestressed
condition of the soil mass when the excavations are performed, and thereafter as the soil is reloaded.
For design, it is important to know the basic concepts related with the hydrodynamic flow of water in
the subsoil. The dewatering of the excavation should be designed in such a way as to preserve the
original effective stresses. Therefore, the future behavior of the foundation will be a function of the
process of making the excavation and of the way the hydraulic conditions are controlled in the subsoil.
Theoretically, if one could make a substitution of load without changing the effective stresses and
hydraulic pressures, no vertical displacements would take place at the ground surface. Therefore, the
fundamental concept of this type of foundation is to achieve the minimum change in effective stresses
during excavation and construction of the foundation structure.
Compensated Foundations with Friction Piles
When a compensated foundation as described is not sufficient to support the load with the allowable
total settlement, in spite of designing the foundation with sufficient stiffness to avoid detrimental
differential settlements within the foundation itself, friction piles may be used in addition to the concept
of compensation. This case may be present in deposits of high or very high compressibility extending
to great depth. The piles will reinforce the upper part of the soil where a higher compressibility is
encountered. The applicability of this foundation calls for a soil that varies from very high
compressibility at the upper part of the deposit, to medium or low compressibility at the bottom. The
total settlement of this type of foundations depends greatly on the way the friction piles are driven,
their spacing and length, the procedure used to perform the excavations, and the control of the
hydraulic conditions of the subsoil. To reduce total and differential settlements one should observe
always the fundamental concept of minimum change in effective stresses. The benefit of the piles is
mostly achieved when they are driven before the excavation is made, making them work under tension
forces during the excavation, thus preserving the confined original state of stress condition in the
subsoil.
(Friction Piles)
Point Bearing Pile Foundations
When the loads to be supported are higher than those a compensated friction pile foundation can take,
then it will be required to find a deep-seated hard stratum of low to very low compressibility and high
shear strength, where piles can be driven to point bearing. One can distinguish two main cases of
point bearing pile foundations.
The first case is recognized when the hard stratum of convenient thickness is found underlain by
materials of medium compressibility. In these cases, the piles should be evenly distributed. After
solving the problem of point bearing of the piles in the hard stratum, there still exists the problem of
finding if the lower compressible soil stratum will have a safe bearing value, and if the total and
differential settlements will be within the allowable values specified for the foundation in question. This
type of foundation should be designed with sufficient stiffness to control differential settlements.
The second type of pile foundation is recognized when the point bearing piles rest in a firm deposit of
low compressibility extending to great depth. In this case, it is economical to use groups of piles to
solve the foundation problem. The columns will rest on single footings supported on the piles. The
piles driven in the firm stratum develop lateral friction contributing to the total bearing capacity. The
bearing capacity of the piles will depend mainly on the mechanical properties on shear strength of the
deposits in which they are driven, on the spacing of the piles, on the length of penetration into the
bearing stratum, and on the state of density and confinement of such stratum. The point bearing piles
may be driven in groups or using a uniform distribution, depending on the compressibility of the
deposits underlying their points. The point bearing capacity of the piles may be increased if special
points are designed, improving the mechanical characteristics of the deposits into which they are
driven.
In the design of point bearing piles, the phenomenon of negative skin friction and the effect this
phenomenon produces in the confinement of the deposit where these elements are bearing should be
taken into consideration. The phenomenon of negative friction is extremely important and may be
recognized when there is a downward relative movement of the compressible soil mass with respect
to the firm stratum where the point bearing piles are driven, originating on them drifting forces. This
phenomenon implies a load transfer of part of the weight of the soil mass to the piles, and
consequently, a reduction of the vertical confining stresses on the stratum where the piles are bearing.
Sometimes, the downward dragging forces may be large, forcing the point bearing piles to penetrate
into the supporting stratum. Therefore, it is important to consider properly the phenomenon known as
negative friction. The total and differential settlements of these foundations may be estimated
computing the compression of the strata underlying the point of the piles.
Pier Foundations
Pier foundations are used to support very heavy loads in buried soil deposits of very low
compressibility. Their load capacity is a function of the mechanical properties of the soil under the
base of the pier, and of the confining stress of the bearing stratum. Actually, the bearing capacity of
such an element is determined as a deep-seated isolated footing. The piers, column-like elements
cast in place, in most cases carry high loads of 500 tf or more; therefore, the compressibility of the
deposit on which they are resting should be very low, in order that they may be recommended. Pier
shafts may be used from diameters of about 1 m to larger diameters. The bearing capacity and the
base dimensions are also a function of the procedure used to perform the excavation, and of the way
the hydraulic conditions are handled. The density of the material where these elements are bearing
may be altered during excavations if an upward water flow is produced. Especially important is the
case when the material is a cohesionless fine sediment or when the cohesion is small, in which case
it is necessary to perform the excavation using a pneumatic system, introducing air under sufficient
high pressure to balance the flow of water toward the bottom of the excavation, preserving the natural
confining and density conditions of the bearing stratum. Usually, if precautions are taken in the
installation of these elements, the settlements will be very small. The settlement, however, may be
estimated knowing the stress-strain characteristics of the strata encountered under the base of the
piers. The negative friction on these elements may take large proportions: hence, it should be
estimated. When these rigid elements are used in seismic regions to support loads through deposits
of high and very high compressibility, it is necessary to investigate the effect of the horizontal motion
of the soil mass during earthquakes. The horizontal drift forces against the piers because of soil
displacement should not be overlooked. In occasions, rigid elements have been damaged because of
the strong horizontal motions produced by the earthquakes.
Sand Pier Foundations (subsoil improvement)