Page 67 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
62 Geotechnical Engineering

Buoyancy therefore is an important consideration in geotechnical engineering,
and simply lowering the groundwater table, for example by pumping from wells,
will increase the weight of the emerging soil by approximately a factor of two.
The additional weight leads to additional compression of the underlying soil.
This is dramatically illustrated in Mexico City, where pumping from wells has
resulted in settlement that in some areas is measured in tens of meters.

Another option in areas of soft soils is to support structures on piles that extend
down to underlying hard strata or bedrock. In this case settlement of adjacent
unsupported areas can create the illusion that buildings are rising up out of the
ground. In places like Mexico City a common countermeasure is to support a
structure on hydraulic jacks that can be lowered to keep pace with the settlement.

4.2.3 Medieval Construction

Many medieval structures have survived not because the builders were better
engineers, but because manpower was many times slower than machines, and
centuries often were required to complete a castle or cathedral. Typically, the
foundation was simply large stones laid flat, and structures settled as they were
being built. Corrections then were made during building to compensate for
uneven settlement and tilting.

For example, courses of masonry in the famous Leaning Tower indicate that
as tilting occurred during construction and was compensated by increasing the
height on the low side. Tilting then pursued in a different direction because, as
soil under the low side was compressed, it became stronger and less compressible.
Had the corrections not been made, the increasingly eccentric loading would
have caused the tower to topple, and after each correction was made a new cycle
of tilting proceeded in a different direction. It was only after the tower was
completed that there was no further correction and an increasing danger of falling
over. The latest correction devised by English and Italian geotechnical engineers
involved adding a temporary surcharge load to the high side, which arrested
further tilting, and then augering soil out from under the high side to bring it back
to a safer position. What makes this tower so unique is because it survived;
most medieval towers collapsed.

4.2.4 Surcharging

Fourteenth century construction scheduling does not fit a modern mold, and
a simple procedure that can be used to reduce or control settlement is to preload
the soil with a weight and pressure that are equal to that of a proposed structure,
allow time for the soil to compress, and then remove the weight and build the
structure. Extra weight may be used to hurry things along, and normally does
no harm so long as it does not punch down into the soil sufficiently to cause
a shear failure. This method of construction is called surcharging.

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