Deep Vibro Technique (keller, 2006)

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Deep Vibro Techniques

Brochure 10 - 02 E

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Deep vibro techniques present flexible solutions for soil improvement. They are mainly used under foundations of structures that are to be constructed on soils of low bearing capacity. Keller developed the depth vibrator (patented in 1934), which was originally used to compact granular soils such as sand and gravel. Today Keller improves a variety of granular and cohesive soils employing a wide range of depth vibrator models and techniques.

Content 3

Overview of deep vibro techniques

4

Vibro Compaction

6

Vibro Replacement

8

Structural Foundation Elements

10

Special Applications

11

Quality Control

12

Keller Branches

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Overview of deep vibro techniques

The subsoil Usually the soil conditions are described in a soil investigation report. If the properties of the existing soil cannot fulfil the requirements set by the proposed loading conditions, deep vibro techniques offer an economical solution for the ground improvement. They can be carried out to almost any depth.

The third technique creates structural foundation elements in the ground which will allow comparatively high loads to be safely carried by soils where no adequate lateral support for Vibro Replacement columns can be mobilized.

The principle of the vibro process

The execution For all techniques the vibro process starts with the penetration of the oscillating depth The depth vibrator vibrator into the ground to the required The cylindrical depth vibrator is typically beimprovement depth. Subsequently, the tween 3 m and 5 m long and weighs approximately 2 tons. The core element of the vibrator vibrator is then removed as required by the is an electrically driven eccentric weight which employed technique to either compact the induces the horizontal oscillation of the vibra- soil from the bottom up, to build a stone column or to construct a structural tor. The vibrator string is assembled with the foundation element. vibrator and extension tubes to suit the improvement depth and suspended from a crane The benefits or mounted on a custom built base The deep vibro techniques present a very machine (i.e. the Keller vibrocat). versatile ground improvement method that can be adjusted to a wide variety of ground The techniques The depth vibrator is used for 3 distinct tech- conditions and foundation requirements. Its execution is comparatively fast even if large niques which differ both in their soil improvevolumes of soil are to be improved and ment and in their load transfer mechanism. subsequent structural works can follow very The foundation design is therefore frequently developed in close cooperation between both quickly. The soil improvement enables the contractor to utilise standard shallow the consultant’s geotechnical and structural footings which, in turn, leads to additional engineers and Keller. savings. The Vibro Compaction technique compacts Another advantage is the environmental granular soils with negligible fines content by friendliness of the deep vibro techniques, rearrangement of the soil particles into a as natural and in situ materials are used. denser state. The Vibro Replacement technique builds load In addition, only a comparatively small bearing columns made from gravel or crushed quantity of soil is removed in the process. stones in cohesive soils and granular soils with high fines content. Limits of application for deep vibro techniques

Sieve passing [% by weight]

Clay

Silt

Sand

Transition zone

Gravel

Cobbles

100

100

80

80

60

60

Vibro Replacement Vibro Compaction

40

40

20

20

0

0,002

0,006

0,02

0,06

0,2

0,6

2,0

6,0

20 60 Grain size [mm]

0

3

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The Vibro Compaction process in granular soils

Flexible coupling

Water or air supply

Electric motor

Eccentric Weight

Nose Cone

Compaction below raft footings

Geotechnical aspects Under the influence of the induced vibration, the soil particles within the zone of influence are rearranged and compacted. The range of this zone depends on the vibrator used, the soil and the method employed. The volume reduction of the compacted soil can reach values in the order of 10% depending on the soil conditions and the intensity of the compaction effort.

Density of the soil before

after

h

Compaction below single footings

4

The foundation concept The range of compaction for an individual point is governed by several parameters. Keller is able to draw upon a wealth of experience to propose a suitable foundation concept. The optimum arrangement of the vibro compaction points is usually best achieved by an on-site trial, where different compaction grids and methods can be tested and evaluated. After compaction, high loads can be safely carried and can reach foundation pressures of up to 1MN/m2.

1 Penetration

The process

Extension tube

Equipment and execution The compaction of granular soils is most economically attained with vibrators oscillating at a comparatively low frequency to achieve optimum compaction of the soil particles. The vibrator is typically suspended from a crane. The penetration of the vibrator and, to a certain extent also the compaction process, is aided by water flushing with jets of variable pressure. The pressure pipes and jets form an integral part of the vibrator string. The compaction is carried out from the bottom of penetration upwards in predetermined pull out steps and compaction intervals. The compaction result is dependant on the effectiveness of the vibrator and the soil conditions.

At full water pressure the oscillating vibrator penetrates to the design depth and is surged up and down as necessary to agitate the sand, remove fines and form an annular gap around the vibrator. When at full depth the water flow is reduced or stopped.

The layout of the compactions points can be adjusted in such a manner that soil volumes of any size are compacted. The achieved degree of compaction can be easily and economically verified using a range of different tests.

? Natural or man made deposits of sand and gravel are frequently not dense enough or are too inhomogeneous to allow a proposed structure to be safely and reliably founded. With Keller’s depth vibrators the soil density can be increased and homogenized independently from the groundwater table.

Depth

A B

0

B

-1 -2 -3 -4 -5 -6 -7 -8

...

-9

3 Backfilling

4 Finishing

The compaction is carried out in steps from the maximum depth of penetration upwards. It encompasses a cylindrical soil body of up to 5m diameter. The increase in density is indicated by an increased power consumption of the vibrator.

Around the vibrator a crater develops which is backfilled with sand, which is either imported (A) or taken from the existing soil (B). For this purpose a volume of up to 10% of the treated soil volume is required.

After completion of the compaction, the surface is relevelled and, if required, compacted with a surface vibratory roller.

As early as 1939 a compaction depth of 35 m was reached on a site in Berlin. Nowadays maximum compaction depths beyond 50 m have been achieved.

...

2 Compaction

-31 -32 -33 -34 -35

Vibrator in a compaction crater

...

Special applications With depth vibrators, slender elements such as dolphins, soil anchors or steel profiles can be sunk into sandy soils and securely anchored.

-48 -49

A further field of application is the densification of wall zones and excavation bases to reduce their permeability.

-50

5

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Vibro Replacement in granular soils with high fines content and in cohesive soils

Flexible coupling

Electrical motor Stone feeder pipe

Eccentric Weight

Nozzle

Geotechnical aspects Insofar as any compaction can be achieved in mixed or fine grained soils through horizontal vibrations and soil displacement (which depends mainly on their degree of saturation), this improvement should be evaluated in the same manner as for Vibro Compaction. The pure Vibro Replacement process, however, does not assume any compaction in the surrounding soil. The improvement relies on the higher stiffness and higher shear strength of the stone column.

The process

Extension tube and stone feeder pipe (material storage)

Design diagram for Vibro Replacement Improvement factor

Air chamber and lock

Equipment and execution For the construction of Vibro Replacement columns the bottom feed process is frequently employed, which feeds coarse granular material to the tip of the vibrator with the aid of pressurized air. To optimize the performance of this process and to accommodate the specialized equipment, Keller has developed the vibrocat base unit which guides the vibrator on its leader and allows the exertion of an additional pull-down pressure during penetration and compaction. The Vibro Replacement process consists of alternating steps. During the retraction step, gravel runs from the vibrator tip into the annular space created and is then compacted and pressed into the surrounding soil during the following re-penetration step. In this manner stone columns are created from the bottom up, which act as a composite with the surrounding soil under load.

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ϕS = 45.0˚ ϕS = 42.5˚

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2 Charging

The vibrocat positions the vibrator over the required location of the compaction point and stabilises itself using hydraulic supports. A wheel loader fills the skip with aggregate.

The skip is lifted and empties its contents into the air chamber. Once the air lock is closed, the material flow towards the vibrator tip is assisted by pressurized air.

µB = 1/3

ϕS = 40.0˚

5

ϕS = 37.5˚

4

ϕS = 35.0˚

3 2 1

1 Preparation

2

3

4

5

6

7

8

9

10

11

The allowable bearing pressure that is achieved after the improvement is typically in the range of 150 to 400 kPa.

Area ratio A / A S

6

1

0,8

1600 900 400

0,6

225 100 64 36 16 9 4

0,4 0,2

1

0

4

8

12

16

20

24

28

Depth ratio t/d * s ∞ = settlement of a theoretical infinite load area

No. of stone columns

Settlement ratio s/s ∞ *

Settlement evaluation for single footings

The foundation concept While the compaction of the surrounding soil can be easily verified by soundings, the improvement effect of the Vibro Replacement can only be checked by in situ load tests. Keller has developed a reliable design method which uses the geometry of the columns and the friction angle of the column material as input parameters. For the foundation design, the improved ground is treated like normal subsoil.

? Mixed grained and fine grained soils frequently do not possess a sufficient bearing capacity. For fines content in excess of 10% to 15% a sufficient compaction result without imported material cannot be expected. For these cases the Vibro Replacement technique is a viable option. This technique is also suitable for the treatment of coarse fills such as rubble, building debris and slag heaps.

Depth 0 -1 -2 -3 -4 -5 -6 -7 -8

...

-9

4 Compaction

5 Finishing

The vibrator displaces the soil and is lowered to the designed depth, aided by the compressed air and by the vibrocat’s pull-down.

After reaching the maximum depth the vibrator is pulled up slightly, causing the aggregate to fill the cavity so created. During re-penetration the aggregate is compacted and pressed into the surrounding soil.

The stone column is built up in alternating steps up to the designed level. During the final levelling, the surface requires to be re-compacted or a blinding layer is required as an alternative.

The Vibro Replacement technique was developed in the late 1950s. Without any special modifications the bottom feed setup the vibrocat can install columns up to 20 m depth.

...

3 Penetration

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View of the cut off level after Vibro Replacement

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Benefits of working with the bottom feed vibrator: • The aggregate is always fed directly to the tip of the vibrator, creating a continuous column. • Only a single penetration is required. • The collapse of the hole is not possible even in critical soils. • The leader ensures the verticality of the columns. • No water is required, eliminating the necessity to dispose of any mud otherwise created.

-16 -17 -18 -19 -20

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Structural Foundation Elements

Grouted Stone Columns (VSS) and Premixed Grouted Stone Columns (FSS)

Installation of grouted stone columns with in-situ grouting using cement slurr y

Equipment and execution These foundation elements are built in the same manner as described for the Vibro Replacement process. For the grouted stone columns (VSS), the gravel is fed into the ground and during the same process mixed in-situ with cement grout. This creates a solid column once the grout has set. For premixed grouted stone columns, a special coarse grained concrete mix typically ranging between strengths C15 and C20 is installed. It behaves identically to the stone material, allowing the same compaction and displacement effects in the surrounding soil. Geotechnical aspects The load bearing behaviour of the structural foundation elements is largely identical to the behaviour of piles. The foundation concept For Grouted Stone Columns and Premixed Grouted Stone Columns Keller has the approval of the German supervisory board for construction.

The external load bearing mechanism that is used for the design of the soil improvement is very well supported by a large number of load test results as per DIN 1054. Depending on the soil conditions and the materials used, working loads of up to 600 kN can be routinely achieved. Grouted stone columns can be easily combined with the normal Vibro

Replacement method by eliminating the use of grout in the upper or lower section of the column as required, thus creating a buffer for the rigid grouted columns. These columns are called Partially Grouted Stone Columns.

Formation of the toe

Penetration

Vibrocat

Excavated grouted stone column

Pull down

Cement grout from mixer Material charging

Vibrator with stone feeding tube and separate grout pipe

weak strata competent strata

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Nozzle Gravel toe

Installation of the grouted stone column

? These methods are employed if the fine grained subsoil does not mobilize sufficient lateral support for the columns or when high organic contents are found which decompose and cause soil shrinkage. Another field of application is the founding of structures with high loads.

Vibro Concrete Columns (BRS) Equipment and Execution Vibro Concrete Columns consist typically of pumpable concrete, grade C 25. The toe of the column is enlarged by repeated retraction and repenetration of the vibrator, however the shaft is built in a single pull due to the high internal strength of the concrete. Geotechnical aspects During the installation of Vibro Concrete Columns no particular effort is taken to densify any specific soil layer. As with other structural foundation elements, a high degree of improvement can be achieved at the toe of the column, thus attaining a particularly high capacity and low deformations under load.

Installation of Vibro Concrete Columns

The foundation concept For Vibro Concrete Columns Keller also has the approval of the German supervisory board for construction.

Vibro Concrete Columns are generally more slender compared to other structural foundation elements. Typical shaft diameters range between 40 cm and 60 cm. The capacity under working load reaches up to 800 kN depending on the ground conditions and on the possibility to enlarge the toe.

Excavated Vibro Concrete Columns

Cross Section of a Vibro Concrete Column

Penetration and toe formation

Preparation

Installation of the shaft

Pull down Vibrocat Concrete Pump Vibrator with concrete feeder pipe

Readymixed concrete

weak strata

Concrete nozzle

competent strata

Toe

9

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Special Applications

Multiple Vibrators and Offshore Compaction Vibro Compaction of large areas both onshore and offshore can be carried out with multiple vibrator assemblies.

dredged and replaced by sand

clay, silt sandstone

For Vibro Replacement offshore, such as for quay walls and bridge pillars, a gravel blanket can be placed which is then installed into the ground. Bottom feed systems are also available.

aggregate mud sand

Vibro Replacement – Top Feed Method In suitable ground conditions the Vibro Replacement process can be performed using crane hung vibrators similar to the Vibro Compaction setup. In this case water or air flushing is used. The flushing medium assists rapid penetration into the ground and stabilizes the annular around the vibrator. It also can be used to increase the column diameter by flushing.

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Quality Control

For all vibro techniques, electronic measuring devices can be employed to ensure and record constant high quality workmanship.

Display unit and CPU of the M4 measuring device

The measuring device To control the process, monitor the quality and for production records, the relevant construction parameters for each compaction probe can be measured, saved and printed as proof of production and quantities. The measurement device consists of • The display unit in the operator's cabin, • The CPU with data storage, • PC with printer at the site office, • Dot-matrix printer mounted on the base unit for real time printout (optional).

Process: Inventory: 9130517 Lot: 0 Date: 15.09.04 Weight: 1.5 kN/m³ Legend:

Vibration Process (3.0.0) Site: 1234173 Point: 241 Ref. No.: Time: 05:10:47 Interval:

15 4 sek Dep.: Consulting and Development

0

0 50 100 150

400

0

200

[A]

-40 -20 0 20 40

Power

[bar]

10

Thrust

[m/min]

15

Penetration Rate

[m ]

0

Depth

[sec]

5

Time

Load tests are a suitable option to verify the improvement of the soil

1

500

1000

1500

2000

2

Event

Time

Depth Electrical Susp. Energy Point [kVAh] [cbm]

No. Type Description

hh:mm:ss [m]

01 09 02 10

05:10:47 05:45:08

Point Start Point End

Total Time: 34.33 min

0.1 0.1

0.00 21.03

0.00 0.00

Net Weight [Ton] 2.98 2.79

Max. depth: 10.00 m

Total Inclination Inclination Weight Right/Left For/Back [Ton] [Deg] [Deg] 2.98 5.77

-0.2 -0.4

+0.3 +0.2

Rel. weight: 0.58 Ton/m

The measurement results During compaction a number of different site and production parameters are automatically recorded. Values such as time, depth, penetration/pullout speed, pull-down force and current can be graphically displayed and printed. If required, the energy consumption can be recorded.

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