METHODS OF GROUND IMPROVEMENT - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 209

13 METHODS OF GROUND IMPROVEMENT
Soft soil is often improved prior to building on/in it. This can be motivated by
the otherwise excessive settlement of foundations, which would compromise
serviceability and would require excessive maintenance, and by saving on
otherwise heavy foundations. Many methods of ground improvement have been
developed, and quite often various choices are available in a given situation to
obtain the desired level of performance of the structure.
Ground improvement methods can be grouped according to how the soft soil is
treated: displacement, replacement, mass improvement and improvement by stiff
inclusions. Pile foundations essentially fit into the latter group, but are not usually
viewed as being a method of ground improvement. Rather, they allow to by-pass
the soft soils altogether and transfer the foundation loads to deeper more competent
strata (see Chapter 12). Biological treatment of soil is in the development stage and
methods of application could eventually fall into either the mass treatment category
or that of the stiff inclusions. They are treated separately here.
A GRAVITY DISPLACEMENT METHODS
In the gravity displacement method, the soft soils are displaced by failure of a
sand-fill, which is placed on the surface. The fill is applied rather quickly and is
increased until failure of the soft soil occurs. The latter is pushed aside and the fill
settles to replace it. This method was applied on a large scale in the Netherlands for
road and railway construction up to approximately the 1950's. It had a number of
disadvantages: the density of the settled sand was usually quite low, soft soil was
often trapped by the fill before it could be displaced, and it caused large lateral
displacements. Fill could penetrate horizontally into the soft soil and flow laterally
to quite large distances, resulting in unexpected heave of the surroundings.
(a) initial situation (b) final situation
Figure 13.1 Gravity displacement
In the present time where much old infrastructure is being widened or crossed by
new infrastructure, it is often problematic to deal with the loose sand of old gravity
displacements. An example is the crossing of the Gouda railway line in Rotterdam
by a tunnel for the RandstadRail light-rail to Zoetermeer. Any disturbance of the
loose sand by the tunnel construction could result in its densification and
consequently subsidence of the Gouda line (see also Fig 8.9).
Approximate analyses were performed with the finite element method to relate
increases of pore pressure in the sand to increases of settlement and shear