Special Foundatifgfgon Works Curs Sem2 09

8/12/2019 Special Foundatifgfgon Works Curs Sem2 09

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drilling ;- grab-, chisel- and bailer borings ;- driving of lances ;- vibrating of casing or drill pipes.

In unstable ground full borehole penetration may require:- the use of drilling muds, grouts or foams ;- temporary casing ;

- direct insertion of sleeve pipes ;- progressive stabilisation as the borehole advances.

Grout placement

The method of grout placement will be determined by the ground condition, the works requirementand the type of grout used. The basic approaches are the following

a) injection in unsupported boreholes in stable ground ;

b) injection via sleeve pipes previously placed in a temporarily cased borehole, in unstable ground ;

c) injection through the drill string in unstable ground, generally considered as a pre-grouting phase and to be followed by approaches a) or b) ;

d) compaction grouting is usually performed through a casing retrieved during upstage grouting.

Soil grouting can be achieved with casing, grout sheath, pierced casing, and sleeve pipes.

Sleeve pipes which are permanently seaied into the ground by use of a support mix (sleeve grout)allow a repeated use of the injection points.

Large openings (voids, cavities, etc.) are generally filled under gravity, either directly, or via atremie pipe extending to the base of the opening.

Packers are used to isolate a grouting stage. Packers are either passive, mechanical or pneumatic,and have to be long enough to minimise the risk of grout bypass through the medium being grouted.

Packers shall ensure tight sealing between the grout hole wall and the injection pipe at maximumgrouting pressure.

Grouting sequences

Descending or downstage grouting is commonly reserved for the treatment of unstable rock. Ifseveral holes are grouted using downstage grouting, the uppermost stage in all holes is drilled andgrouted before drilling and grouting the next stage in all neighbouring holes.

Upstage grouting is only used in open holes in stable rock or if the aim is compaction grouting.

Multistage grouting, using sleeve pipes is generally used in soils and sometimes in unstable rock.

Combinations of these techniques are possible.


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13. JET GROUTING

The jet grouting method is covered by the EN 12716.

Definitions

jet groutingthe jet grouting process consists of the disaggregation of the soil or weak rock and its mixing with,and partial replacement by, a cementing agent; the disaggregation is achieved by means of a highenergy jet of a fluid which can be the cementing agent itself

jet grouted elementvolume of soil treated through a single borehole. The most common elements are:- jet grouted column : a cylindrical jet grouted element (Fig.1 a) ;- jet grouted panel : a planar jet grouted element (Fig.1 b).

jet grouted structurean assembly of jet grouted elements which are partially or fully interlocked. The most commonstructures formed are:- jet grouted diaphragm : a wall structure (Fig.2 a) ;- jet grouted slab : a horizontal structure formed by essentially vertical jet grouting (Fig.2 b) ;- jet grouted canopy : a structure formed by horizontal jet grouting - see 3.8 below (Fig.2 c) ;- jet grouted block : a three-dimensional structure.

single systemthe jet grouting process in which the disaggregation and cementation of soil are achieved by a highenergy jet of a single fluid, usually a cement grout (Fig.3 a)

double (air) systemthe jet grouting process in which the disaggregation and the cementation of soil are achieved byone high energy fluid (usually a cement grout) assisted by an air jet shroud as a second fluid (Fig.3 b)

double (water) systemthe jet grouting process in which the disaggregation of the soil is achieved by a high energy water jetand its cementing is simultaneously obtained by a separate grout jet (Fig.3 c)

triple systemthe jet grouting process in which the disaggregation of the soil is achieved by a high energy water

jet assisted by an air jet shroud, and its cementing is simultaneously obtained by a separate grout jet(Fig.3 d)

horizontal jet groutingtreatment performed from a horizontal or sub-horizontal borehole (within 20 from the horizontal

plane)

jet grouting rigrotary rig able to automatically regulate the rotation and translation of the jet grouting string and tool

jet grouting string

jointed rods, with simple, double or triple inner conduit, which convey the jet grouting fluid(s) to themonitor


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monitorthe tool mounted at the end of the jet grouting string, to enable jetting of the fluids into the ground

nozzlea specially manufactured device fitted into the monitor and designed to transform the high pressurefluid flow in the string into the high speed jet directed at the soil

radius of influenceeffective distance of disaggregation of soil by the jet, measured from the axis of the monitor

spoil returnthe surplus mixture of soil particles and introduced fluids arising from the jet grouting process, andnormally flowing to the ground surface via the annulus of the jetting borehole

jet grouting parametersthe jet grouting parameters are defined:- pressure of the fluid(s) within the jet grouting string ;- flow rate of the fluid(s) within the jet grouting string ;- grout composition ;- rotation speed of the jet grouting string;- rate of withdrawal or insertion of the jet grouting string.

prejettingthe method in which the jet grouting of an element is facilitated by a preliminary disaggregation

phase, with a jet of water and/or other fluids

Prejetting is also widely known as prewashing or precutting.

fresh-in-fresh sequencethe sequence of work in which the jet grouted elements are constructed successively without waitingfor the grout to harden in the overlapping elements (Fig.4 a)

primary-secondary sequencethe sequence of work ,in which the execution of an overlapping element cannot commence beforea specified hardening time or achievement of predetermined strength of the adjacent elements

previously constructed (Fig.4 b)

jet grouted materialthe material which constitutes the body of a jet grouted element

reinforced jet grouting jet grouted columns reinforced by steel or other high strength material


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Figure 1 a) Jet grouted column Figure 1 b) Jet grouted panel

Figure 1 Examples of jet grouted elements

Figure 2 a) Jet grouted diaphragms


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Figure 2 b) Jet grouted slab Figure 2 c) Jet grouted canopyFigure 2 Examples of jet grouted structures

Key1 Monitor

Figure 3 a) Single system


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Key1 Monitor

Figure 3 b) Double (air) system

Key1 Monitor

Figure 3 c) Double (water) system


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Key1 Monitor

Figure 3 d) Schemes of jet grouting systems

Figure 4 a) Fresh in fresh sequence

Figure 4 b) Primary secondary sequence

Figure 4 Work sequencesMaterials

Mixes composed of water and cement are usually adopted.Hydraulic binders other than cement can be used.In water/cement mixes the water/cement ratio by weight should range between 0,5 and 1,5.Water reducing, stabilizing, plasticising, waterproofing or antiwashing admixtures can beadded to the water/cement mix.Other materials, such as bentonite, filler, fly-ash, can also be added to the mix.When bentonite is to be used in the mix, a water/bentonite suspension should be prepared beforeadding cement.


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Examples of applications

Jet grouting can be applied in either temporary or permanent works for different purposes.For example:

providing foundations for structures to be erected (Fig.5 a) ;underpinning existing foundations (Fig.5 b) ;creating low permeability barriers ;

creating retaining or supporting structures ;complementing other geotechnical works ;reinforcing a soil mass.

Figure 5 a) Foundation for structure to be erected

Figure 5 b) Underpinning existing foundationFigure 5 Examples of applications


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Execution

The execution of jet grouting works requires knowledge and experience in this type of construction.

The high pressure employed in the jet grouting process is to generate a high speed jet to disaggregate the soil and isnot intended to be applied to the surrounding soil.

Jet grouted column execution method

The phases of execution usually consist of:drilling a borehole of a predetermined length ;- introducing to the end of the borehole a monitor connected to the jet grouting string. This

is unnecessary in some cases as the string and monitor are used for drilling ;- jet ting of the disaggregating and cementing fluid(s) through the monitor, simultaneously

withdrawing and rotating the rods, with pre-established withdrawal and rotational speed, pump pressure and flow rate for each fluid.

Jet grouted panel execution method

The phases of execution are the same as defined for jet grouted columns, with the exception thatduring jetting the rods are withdrawn and not rotated. Alternatively the rods can be rotated aboutlimited angles. The resulting panel is placed in a plane on the drilling axis, or is formed by two ormore sections on planes intersecting the drilling axis (Fig.1 b).

Alternative execution methods

If required by soil conditions, alternative execution methods may be adopted, both for column or panel processes. Among alternatives the most usual is prejetting. An element can also beexecuted in sequential treatment for a given length from the borehole collar is completed andallowed to gain strength. Then, after redrilling the treated soil, the process is repeated at a deeperstage, and so on, until the design length of the treatment is reached.

Equipment 2 > 2Grout flow rate (I/min) 50 to 450 50 to 450 50 to 200 50 to 200

Special Foundatifgfgon Works Curs Sem2 09

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