12 Ground improvement by deep vibratory methods
In a company catalogue, Johann Keller GmbH (1938) summarized the
advantages of sand compaction:
• Naturally deposited sand can be compacted to any depth.
• Artificial sand fill can be compacted over its full height in one
operation.
• Densification achieved is complete and permanent; the void ratio can
be as low as its theoretical minimum.
• The method is safe, fast, and economic.
These advantages were the reason that increasingly costly pile foundations
could be avoided and replaced by shallow foundations on compacted sand
with previously unheard-of bearing pressures. Of equal importance were
the increased shear strength and the reduced compressibility that accompany the high density of the compacted sand. These findings helped the
vibro flotation, or vibro compaction method, as it was also called, to gain
an exceptionally broad recognition during the few years before and even
during the war. Completed projects included warehouses and industrial
plants employing the effect of sand densification at depth to increase the
allowable bearing pressure, thus leading to smaller and more economical
footing sizes. In connection with water structures at the North and Baltic
Seas, the method was also frequently used to compact sand-fill behind walls
or dredged fill. Records of some of these projects survived the war and are
worth recalling, because interesting features of early vibro flotation works
are included.
Degebo was founded in 1928 as a private organization attached to the
Technical University Berlin. It accompanied most of these early vibro compaction works and was instrumental in introducing appropriate testing
methods for measuring the densification achieved in natural and artificially
placed sand deposits with vibro compaction (Muhs, 1949, 1969). In this
context, it was found that continuous dynamic penetration tests would not
deliver reliable density measurements with increasing depth of investigation
because the increasing weight of the steel rod rendered the transformation of the constant energy of the drop hammer increasingly less effective.
A static cone penetrometer was therefore developed that was capable of
measuring, irrespectively of depth, the point resistance of a cone attached
to the rod and being pushed into the ground. Figure 2.6 shows an early
measurement of the point resistance by such an electric cone penetrometer
to control the density of sand fill before and after vibro compaction.
In 1939, a comprehensive compaction trial was carried out for the foundation of the Great Hall in Berlin. Under the supervision of Degebo, the
newly developed method was successfully used in medium sand to a depth
of 20 m. Density increased considerably and bearing capacity was more
than doubled. To test the capability of the vibrator to penetrate to greater
depths, it was suspended from a 40 m high gantry and Degebo (1940)
In a company catalogue, Johann Keller GmbH (1938) summarized the
advantages of sand compaction:
• Naturally deposited sand can be compacted to any depth.
• Artificial sand fill can be compacted over its full height in one
operation.
• Densification achieved is complete and permanent; the void ratio can
be as low as its theoretical minimum.
• The method is safe, fast, and economic.
These advantages were the reason that increasingly costly pile foundations
could be avoided and replaced by shallow foundations on compacted sand
with previously unheard-of bearing pressures. Of equal importance were
the increased shear strength and the reduced compressibility that accompany the high density of the compacted sand. These findings helped the
vibro flotation, or vibro compaction method, as it was also called, to gain
an exceptionally broad recognition during the few years before and even
during the war. Completed projects included warehouses and industrial
plants employing the effect of sand densification at depth to increase the
allowable bearing pressure, thus leading to smaller and more economical
footing sizes. In connection with water structures at the North and Baltic
Seas, the method was also frequently used to compact sand-fill behind walls
or dredged fill. Records of some of these projects survived the war and are
worth recalling, because interesting features of early vibro flotation works
are included.
Degebo was founded in 1928 as a private organization attached to the
Technical University Berlin. It accompanied most of these early vibro compaction works and was instrumental in introducing appropriate testing
methods for measuring the densification achieved in natural and artificially
placed sand deposits with vibro compaction (Muhs, 1949, 1969). In this
context, it was found that continuous dynamic penetration tests would not
deliver reliable density measurements with increasing depth of investigation
because the increasing weight of the steel rod rendered the transformation of the constant energy of the drop hammer increasingly less effective.
A static cone penetrometer was therefore developed that was capable of
measuring, irrespectively of depth, the point resistance of a cone attached
to the rod and being pushed into the ground. Figure 2.6 shows an early
measurement of the point resistance by such an electric cone penetrometer
to control the density of sand fill before and after vibro compaction.
In 1939, a comprehensive compaction trial was carried out for the foundation of the Great Hall in Berlin. Under the supervision of Degebo, the
newly developed method was successfully used in medium sand to a depth
of 20 m. Density increased considerably and bearing capacity was more
than doubled. To test the capability of the vibrator to penetrate to greater
depths, it was suspended from a 40 m high gantry and Degebo (1940)