winturbines:CTS Wind Turbines - Con-Tech · PDF fileCombination Spread Footing with Micropiles (CSFM) as “Settlement Brake” for Wind Turbine Towers can reduce spread footing size

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Technical Information

www.contechsystems.comToll Free: 1 888 818 4826 e-mail: [email protected]

CTS/TITAN Injection Bore (IBO) Micropiles

For the efficient foundation ofwind turbines mobile phone towerstransmission lines

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2www.contechsystems.com

Toll Free: 1 888 818 4826 e-mail: [email protected]

CTS/Titan micropiles are installed using a standard, rotary percus-sive drill rig in a one visit drilling operation. The installation proce-dure uses a grout flush to stabilise the annulus, consequentlyeliminating the use of casings. This type of installation is free fromvibration and has a low noise level. Titan micropiles require small-er holes and work with small rigs which means less drill spoil andlower mobilisation costs.

The grout body takes care of the radial friction in the soil, the stiff-ness against buckling and the corrosion protection. By usingspacers (centralisers) at each coupler, a minimum grout cover of20mm is achieved. The soil mechanics of the Titan micropiles, incomparison to standard reinforced concrete piles (DIN 4014),show very little settlement due to their very good soil friction (typi-cal settlement less than 5mm under design load). Due to excellentmechanical bond between the grout body and the soil, the move-ment required to activate the friction is in the range of just a fewmillimetres.

The hollow steel section is far superior to a solid rod of the samecross section and steel quality with respect to bending, shear andsurface friction.

The material for the production of Titan micropiles is approvedmicro alloy constructional steel

In the thread production process, the material strength of themicro alloy steel is increased by cold forming without causingbrittleness.

Titan micropiles already comply with the draft of the futureEuropean standard for micropiles CEN/TC 288/WG 8.

As with reinforced concrete, these ribs induce an equal crack dis-tribution in the grout. Excavated pile grout bodies reinforced withTitan micropiles have shown that up to 1.25 of the design load thecharacteristic crack widths are below the permissible value of 0.1mm (0.004”) as required for reinforcement .

This proves that the system complies with DIN 4128 9.2 and thatsingle corrosion protection of minimum 20 mm (0.79”) cover, aswith reinforced concrete, is sufficient.

Jet Grouting

In order to install a working micropile within plastic claysand/or silty (SPT 3-4) conditions, Ischebeck Titan jetgrouted micropiles can be used.

The system involves installing the pile without grout forthe first metre and then injecting a grout mix with a w/cratio in the order of 0.8, at a grout pump pressure of upto 100 bars (1450 psi).

A grout body in the order of 400 mm (15 3/4”) to 600mm (23 5/8”), with a compressive strength of 12 MPA(1.7 ksi) can be achieved in these ground conditions.

The 40/16 Ischebeck Titan micropile, together with a110 mm (4 3/8”) hardened clay drill bit with adaptednozzles, is used for this application.

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Bond lengths for tension or tension and compression piles

L = Tw x Fsπ x D x Tult

Tw - Safe working loadFs - Factor of safetyπ - 3.142D - Drill bit diameter x enhancement factorTult- Skin friction 150 kN/mm2 (22 ksi)

Example:

100 kN (225 lbf) tension load in sand with 110 mm (4.3’’) drill bit

100kN x 3π x (0.11 x 1.5) x 150 kN/m2

300.0077.76

3.86m (12.7’) long bond length

Compression only

Compression only piles have the ability to spread the load over thesteel section and the grout body as a composite pile.

Load taken on grout (safe)

Area of grout x strength of grout4

40 N/mm2 (5.8 ksi) grout after 28 days

(Area of grout - Area of bar) x 40 N/mm2

4

For 200 mm (7.9 “) drill bit - (No enhancement factor)

(2002 - 522 ) x π x 404 4

(40,000 - 2,704) x π x 104

= 293kN (66 kips)

Load taken on steel

Yield strength of steel2

Yield of 52/26 micropile is 730kN (164 kips)

7302

= 365kN (82 kips)

Total safe working load of pile in compression only

Load taken on grout + load taken on steel

293 + 365 = 658kN (148 kips)

To calculate load carrying capacity

The load bearing length, L, of the pile is determined by grout bodydiameter, D, the ultimate soil friction, Tult, surface of the groutbody and the safety factor.

End bearing capacity of Titan micropiles can be ignored.

Skin friction values should be taken from site investigations andtests.

Calculation of CTS/TITAN IBO Micro Piles

Until recently, the design of foundations for windturbines has not been given the attention itdeserves. In general, approx. 90% of wind tur-bine foundations are spread footings, and insome rare cases, driven piles are used.Compared to typically designed spread foot-ings, cost savings of as much as 25% can berealized by reductions in the concrete masswhen using combination spread footings withmicropiles. This cost savings can represent asmuch as 2% of the overall wind turbine cost.

The innovative design of the CombinationSpread Footing with Micropiles (CSFM) usingTITAN Injection Bore Anchors should not beconfused with a conventional pile foundation,

such as driven concrete piles which distributeloads into deeper, stronger soil layers or rock.

In typical load bearing soils i.e. moderatelydense sand, weathered rock, rock, etc. a CSFMfoundation with drilled and injection groutedTITAN Anchors is the most economical founda-tion alternative; installed according to DIN4128; with non load bearing soils, the CSFMfoundation is as economical as and technicallyequal to, a conventional pile foundation. Thismakes the CSFM foundation an economic anduniversal construction option for all types ofground conditions.(Text continues on page 6 ...)

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Combination Spread Footing with Micropiles as Foundation for Wind Turbinesusing TITAN Injection Bore Anchors in accordance with DIN 4128

Dipl.-Ing. Ernst F. IschebeckFirma Friedr. Ischebeck GmbH

Combination Spread Footing with Micropiles (CSFM)as “Settlement Brake” for Wind Turbine Towers can reduce spread footing size by approx. 25%.

Sample Calculation:Combination Spread Footing with Micropiles(CSFM) using TITAN 73/53 micropiles – indense layers of gravel and sand:

Micropile Spring Stiffness

Soil Spring Stiffness

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Structural Design SystemComparison of Spring Stiffness, for a Load of F=500kNSettlement and Load Distribution Micropile Length L= 10m

MicropileTITAN 73/53

Spread Footing onGravel/Sand

Cross Section A cm2 16.31 110x110=12.100

Mod. of ElasticityN/mm2

Stiffness ModulusSteel

210.000Gravel/Sand

200

Bearing Stress σo N/mm2 306 = 500/16.31 413 = 500/12.1

Stress Distribution

Settlement mm 6.1 20.5

Load Distribution withCSFM and EqualSettlements%

- 3

77 23

Design with appropriate safety factor:TITAN Micropiles take up to 100% of Tension ForcesTITAN Micropiles take up to 50% of Compression Forces

Sample Calculation for the design of a TypicalWind Turbine Foundation System with the fol-lowing parameters:- 750 kW Rated Output- 74 m Nacelle Height- Conicle Steel Tubular TowerDesigns with Spread Footing, Pile Foundation,and CSFM foundation options are compared.

1 mm = 0.03937 in 1 m = 3.281 ft 1 kN = 224.8 lbf 1 MPa = 0.145 ksi 1kN/m2 = 0.02088 kip/sqft

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Schematic Depiction of Pile Types with Settlement Behavior compared to Pile Resistance(External Load Capacity)

Set

tlem

ent

[mm

]

Pile Resistance R [kN]

c) Grouted Composite Pilesper DIN 4128 (i.e. TITAN,GEWI)

a) Drilled Piles per DIN 4014

b) Grouted Concrete Piles perDIN 4128

d) Special Piles: Ballistic Piles

e) Special Piles: Sectional Precast Piles

90 kN/m2

50 kN/m2

150 kN/m2

300 kN/m2

Ultimate skin friction qs in moderately dense and dens layers of gravel and sand

Stiffness modulus Ks = 65 ÷ 150kN/mm for settlement calculations i.e. Silt, Sand Service Load 400 kN; Settlement = 400/Ks = 400/70 = 5,7 mm

A large part of the concrete mass in a spreadfooting is designed as ballast to resist eccentricloads and overturning moments imposed onwind turbine towers. This additional requiredmass unnecessarily increases the systemweight of the wind turbine and raises the foun-dation load. By doweling into the underlyingsoils with the use of Micropiles, a smaller con-crete spread footing is needed. Concrete andexcavation costs are reduced and 50% of the

compression loads and 100% of the tensionloads are resisted by the micropiles.

The Combination Spread Footing withMicropiles (CSFM) using CTS/TITAN InjectionBore Anchors has been successfully used oncell towers and high voltage transmission linesfor many years.

(Continued on page 7)


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Combination Spread Footing with Micropiles(CSFM) continued ...

Additional advantages of this foundation systemcan be realized when certain project conditionsexist, such as;

1. When concrete availability and quality con-trol are problematic

2. Reduced number of daily trips to and fromthe construction site for delivery of concreteand removal of excavated soils saves streetsand the environment

3. No need to dispose of excavated soils whichmay be toxic or considered to be hazardousmaterial

4. For coastal or shallow offshore construction5. For very tight construction schedules6. For use with ever increasing wind turbine

sizes in the future7. For use in Permafrost8. Wherever soil quality is so poor, casing the

hole would otherwise be required9. For dynamic, cyclic, and seismic loading

Summary

A Combination Spread Footing withMicropiles (CSFM) acts structurally as a cou-pled spring system. The “Micropile” spring isconsiderably stiffer – E modulus = 210,000N/mm² for steel – than the “Soil” spring – Emodulus =200 N/mm². With equal settling ofmicropiles and soil, theoretically approx. 77% ofthe compression force is resisted by themicropile, so that the assumed value of 50% isalways on the safe side.


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winturbines:CTS Wind Turbines - Con-Tech · PDF fileCombination Spread Footing with Micropiles (CSFM) as “Settlement Brake” for Wind Turbine Towers can reduce spread footing size