Archive for the ‘Ground Improvement’ Category

MICRO PILE REINFORCEMENT SYSTEMS

December 31st, 2009

 MICRO PILE REINFORCEMENT SYSTEMS and CORROSION PROTECTION.
Horst Aschenbroich, Dipl. Ing. President and CEO CON-TECH SYSTEMS LTD, Delta BC, Canada

 

Introduction:

Since mankind started to design and build structures for different usages and environ­ments, foundation systems to support such structures had to be developed in order to match the architectural and structural needs. With the ever-increasing urban expansions, it is not always possible to find good supporting ground at or close to surface level. Therefore, foundations other than spread footings were designed to transfer compression loads down to a suitable load-bearing stratum.

Higher and slender structures subjected to wind and seismic loads need foundations capable to support compression as well as uplift and lateral forces. Instead of large, mass concrete foundations, which require large areas and mass excavations, smaller and deep­er drilled shaft or pile foundations became a more economical alternative, in which steel reinforcing systems embedded in concrete and cement grout are the major component.

Micro Piles belong in this category of foundation elements. They are very simple but unique in design and construction and are becoming more and more popular.

The evolution of Micro Piles

Since its original conception in the 1950’s by Dr. Fernando Lizzi, a number of micro pile sys­tems using steel-bar reinforcement / cement grout combinations with or without steel pipe casing, have been developed.

Lizzi’s idea was, to produce a foundation system consisting of small pile groups, which form a reinforced soil mass like the root system of a tree. He called these PALI RADICE or “ROOT PILES” (see Figure 1).

Further developments using different installation methods and reinforcing systems made it necessary to capture them all under a general heading, first “MINI-PILES”, which was later changed to “MICRO PILES”.

With the creation of the International Workshop for Micro piles (IWM), first in North America and later internationally, MICRO-PILE became a household name in the Geotechnical and foundation industry. They are mainly used as Friction Piles to take tension and / or com­pression loads.

 

Figure 1: Pali Radice or Root Pile foundation examples (after FHWM-SA-97-070, 2000)

What is a Micro Pile?

A generally up to 300mm diameter, drilled and grouted pile with a centrically placed steel reinforcing member consisting of single or multiple bars.

Why are Micro Piles such a unique foundation system?

They can be placed with relatively small drilling equipment, single or in groups, under lim­ited access and low headroom conditions. They can be installed, for instance as the Titan IBO system, using rotation boring only. This reduces or eliminates the risk of structural damages caused due to vibrations, by otherwise used heavy percussion and pile driving methods, especially inside or in close vicinities of buildings.

 

Figure 2: Typical micro pile sections, left with solid bar reinforcing, right with hollow bar reinforcing or casing (after FHWM-SA-97-070, 2000).

 

Figure 3: Threadbars, All-Thread Bars and hollow bars (left to right).

The reinforcing materials are simply single solid or hollow bars with continuous outside threads, which can easily be spliced and coupled to any required length.

The intent of this presentation is to introduce, to designers and specialized foundation-engineering contractors, the different types of reinforcing systems and corrosion protec­tion methods available for drilled and grouted Micro Piles.

 

Pile Types and Reinforcing Systems

During the evolution process of developing Micro-Piles over the past 40 years, besides different drilling equipment, a variety of continuously threaded reinforcing bars and grout­ing systems have been successfully introduced.

The “GEWI PILE” System

When I first introduced the Dywidag Threadbar System in North America (in 1967), I had the opportunity to propose this bar as a post-tensioned single bar reinforcement in Tension Piles for the Bonnybrook Sewage treatment plant extension in Calgary Alberta, Canada.

Approximately 1500 piles were required to support large sewage aeration tanks against uplift. This became the first application (worldwide) for Dywidag bars in Piles and in the geo­support industry. A drill-through Diesel Hammer was used driving casing through the over­burden, cleaning out the casing with an inner drill steel and advance-drilling the same into the underlying bedrock. In the free stressing length, the bars were corrosion protected by a shop applied heat-shrink sleeve with inner asphalt coating (Yellow Jacket). As an addition­al bond breaker, a metal sheath was placed over the coated bar. The drill hole and casing was tremie-grouted with cement grout. Each pile was stressed to a test load and locked off at a design load equivalent to the uplift force.

Soon after, in the early 1970’s,, Dywidag started to market the grade 60 and grade 75 rein­forcing steel thread-bar system, called GEWI Bars, which lead to the development of the

 
 

Table 1: GEWI pile bar steel properties (courtesy DSI).

“GEWI PILE” by Dr. Thomas Herbst, who was at that time chief of the geotechnical devel­opment department. GEWI originates from the German word GEWINDE or THREAD. These piles are installed using open or cased hole drilling methods. The GEWI BAR forms the concentric reinforcing element. The drill hole is filled with cement grout. In order to increase the grout to soil bond capacity of the pile, especially in cohesive soils, post-grout tubes are installed at the outer perimeter of the grout body. Post-grouting can be repeated several times until the required pressure or skin-friction is achieved.

 

Figure 4: GEWI Pile (typical) with standard and double corrosion protected reinforcing bar (after FHWA-SA-97-070, 2000)

 

Figure 5: Typical post-grouting system (after FHWA-SA-97-070, 2000)

The “PIN PILE”System

The PIN PILE is a development in the 1970’s by the Nicholson Construction Company USA. This method uses an outer pipe casing to stabilize the drill hole and an inner drill rod for cleaning out the casing or drilling further into harder ground. After placing the centric rein­forcing element, single or multiple bars (see figures 6 and 7) and filling the casing with cement grout, the casing is slowly pulled under constant pressure grouting and partly left in the ground as additional reinforcement to increase bending moment and / or lateral load capacities and to prevent grout loss in grounds with large voids. Post-grout systems can be used with these piles as well.

The Threadbar or All-Thread Bar systems (tables 2 – to 5) are supplied by the ADSC Associate Members

CON-TECH SYSTEMS, (CTS)

DYWIDAG SYSTEMS INTERNATIONAL, (DSI)

WILLIAMS Form Engineering

 

Figure 6: Pin Pile installation sequence (FHWA-SA-97-070, 2000). Figure 7: Single (left) and multiple bar (right) micro pile reinforcing.

 
 

Table 2: Properties of cold-rolled grade 75 (yield) All-Thread bars.

 

Table 3: Properties of cold-rolled grade 150 (ultimate) All-Thread bars.

 

All-Thread Bar Steel Properties Hot-Rolled Grade 75 (yield), ASTM-A615

 

Nominal Diameter

Steel Area

Load Capacity Ultimate Yield Load

Major Thread Diameter D

Weight

28 mm

616 mm2

405 kN

 319.0kN

32mm

4.83 kg/m

 

 

 

  

 

lbs/lf

32 mm

804 mm2

524 kN

 416.7kN

36mm

6.31 kg/m

 

 

 

  

 

 

 lbs/lf  

40 mm

1260 mm2

821 kN

 648.3kN

45mm

9.87 kg/m

 

 

 

  

 

lbs/lf

50 mm

1960 mm2

1285 kN

 1008.4kN

55mm

15.4 kg/m

 

 

 

  

 

 

 lbs/lf  

63.5 mm

3167 mm2

2215 kN

 1760.0kN

68mm

24.68 kg/m

 

 

 

  

 

lbs/lf
 

Table 4: Properties of hot-rolled grade 75 (yield) All-Thread bars.

 

All-Thread Bar Steel Properties Hot-Rolled Grade 96 (yield), ASTM-A615

Load Capacity

Nominal Major Thread Steel Area Weight Diameter Diameter D Ultimate

Yield Load

28 mm 616 mm2 490kN

410kN

32mm 4.83kg/m

1 1/8 in 0.95in2 110.2K

92.2K

1.26in 3.25lbs/lf

30 mm 720 mm2 575kN

480kN

34mm 5.65kg/m

1 1/4 in 1.12in2 129.3K

107.9K

1.34in 3.80lbs/lf

35 mm 962 mm2 770kN

640kN

39mm 7.55kg/m

1 3/8 in 1.49in2 173.1K

143.9K

1.54in 5.07lbs/lf

43 mm 1466 mm2 1170kN

980kN

47mm 11.51kg/m

1 5/8 in 2.27in2 263.0K

220.3K

1.85in 7.73lbs/lf

57.5 mm 2597 mm2 2080kN

1740kN

62mm 20.38kg/m

2 1/4 in 4.03in2 467.6K

391.2K

2.44in 13.69lbs/lf

63.5 mm 3167 mm2 2540kN

2120kN

68mm 24.38kg/m

2 1/2 in 4.91in2 571.0K

476.6K

2.68in 16.38lbs/lf

Table 5: Properties of hot-rolled grade 96 (yield) All-Thread bars.

The “TITAN / IBO – INJECTION-BORED MICRO PILE”

The successful construction of a Micro-Pile, involves a number of steps.

                Drilling,

                Placing of reinforcing steel.

                Grouting.

 

One of the latest developments is a system and method, which combines all in one single step installation.

This method uses hollow bars, sometimes in combination with inside solid bars or strand, which can also be post-tensioned (figure 10 and figure 14).

This Injection-Bored (IBO) pile is a joint development between the companies Friedrich Ischebeck Gmbh, Germany and Con-Tech Systems LTD, Canada. The goal was to pro­duce a drilled, grouted and reinforced Micro Pile following the original Root Pile idea by Lizzi. The pile totally integrates with the soil. It forms a foundation system of reinforced soil mass, in particular if placed in groups. The piles are drilled-grout-injected in one step, using the hollow bars as drill rods and grouting ducts with disposable special drill bits (fig­ure 12) and rotary drilling methods. The drill bits have jet openings allowing for pressure grout penetration while drilling. During drill advancement and grout injection through the hollow bars, with the aid of a flushing head, the drill cuttings are continuously flushed or

 

Figure 8: Exhumed TITAN Pile

tremied out by the cement grout. It is a clear advantage of this method that the drill hole is stabilized, and the ground cannot relax or cave, but to the contrary is grout penetrated and densified. Figures 8 and 13 show this on an exhumed pile.

The basic idea was, to produce a pile of very high capacity using small drilling equipment, which can operate in tight areas with limited overhead space inside buildings to underpin or seismic upgrade existing foundations (Figure 9).

 

Figure 9: Limited overhead installation of TITAN micro piles.

Figure 10: Typical TITAN/IBO micro pile details

 

Micro-Pile Reinforcement Systems and Corrosion Protection, Horst Aschenbroich, Con-Tech Systems Ltd.

  CTS-TITAN Hollow Bars Meets and Exceeds ASTM-A615 Specifications  
Bar Size Dout/Din mm Steel Area Load Capacity Ultimate Yield Test Design Design G.U.T.S. 80% G.U.T.S. 70% G.U.T.S. 60% G.U.T.S. Outside Diameter Effective Nominal Weight
                lbs./lf.
  mm 2 kN kN kN kN kN mm mm kg/m
30/16 382 220 180 176.0 154 132.0 26 30 2.02 3.00
32/20 445 260 210 208.0 182 156.0 28 32 2.30 3.42
30/11 446 338 280 270.4 236.6 202.8 26.2 30 2.35 3.50
40/20 644 510 430 408 357 306 36 40 3.60 5.35
40/16 879 660 548 528.0 462 396.0 36 40 4.64 6.90
52/26 1337 929 730 712.0 650.3 557.4 48.8 52 7.06 10.50
73/53 1631 1160 970 928.0 812 696.0 70 73 8.60 12.80
103/78 3146 1950 1570 1560.0 1365 1170.0 100 103 16.60 24.70
103/51 5501 3460 2726 2714.0 2422 2076.0 100 103 29.00 43.15
130/60 10100 7051 5555 2714.0 4935.514 4230.4 126 130 53.09 79.00
 

Table 6: Properties of CTS-TITAN hollow bars

MAI Hollow Bars Meets and Exceeds ASTM-A615 Specifications    
Rod Size Dout mm Steel Area Load Capacity Ultimate Yield Test Design Design G.U.T.S. 80% G.U.T.S. 70% G.U.T.S. 60% G.U.T.S. Diameter Inner Outer Weight  
                lbs./lf.  
  mm 2 kN kN kN kN kN mm mm kg/m  
R25N 330 200 150 160.0 140 120.0 12 25 1.75 2.60  
R32N 444 280 230 224.0 196 168.0 18 32 2.35 3.50  
R38N 761 500 430 400.0 350 300.0 19 38 4.03 6.00  
R51N 1217 800 630 640.0 560 480.0 34 51 6.45 9.6  
 

Table 7: Properties of MAI hollow bars

Figure 11: CTS-TITAN hollow bars

 

Figure 12: CTS-TITAN special disposable drill bits for various grounds

 

Figure 13: Section of exhumed TITAN / IBO micro pile

 

Densified Ground

Soil / Cement Mix Neat Cement Grout Hollow TITAN Bar

Two types of hollow bars are available (see tables 6 and 7)

The CTS-TITAN Hollow Bars (table 6, figure 11), supplied by CON-TECH SYSTEMS LTD, in sizes up to 130 mm, 5 1/8” diameter with tension design load capacities in excess of 400 Tons. These bars are rolled with special continuous TITAN Threads for excellent bar to grout bond development. The bond development and crack width distribution of TITAN bars in tension and embedded in cement grout, had been tested at the Technical University of Munich in Germany. The results show, that at 125% of the maximum allow­able design load, the maximum crack width in the grout is less than 0.1mm. This is still considered complete corrosion protection of the steel under the German Industry Norm (DIN). No additional corrosion protection is thus required.

For variable ground and load conditions, different drill bits (figure 12) are designed and available. For the TITAN IBO Micro Pile, venturi jet-grout holes in the drill bits allow the jet grouting pressure to over-ream and pressurize the drill hole. Because of the continuous tremie-cement grouting operation, 100% grout cover can be guaranteed (figure 13).

The MAI Hollow Bars (table 7), supplied by DSI. These bars are rolled with a standard continuous Rope Thread (R-Thread).

All hollow bars are generally supplied in 10 foot lengths, (a standard length of a drill rod for easier handling) spliced together with special couplers.

 

Figure 14: Internal post-tensioning of TITAN micro pile.

Another feature of the hollow bar is the possibility of adding an additional solid rebar inside the grout filled bar, or placing a strand tendon inside to apply an internal pre-stress force to control elastic movement of the hollow bar (figure 14).

A special type of pile is used in California by Caltrans to upgrade existing viaduct founda­tions for seismic events. This pile consists of a steel pipe casing drilled through the over­burden. DCP, Double Corrosion Protected Strand tendons are placed through the pipe and anchored into the bedrock below. The pile is then vertically post-tensioned and cast into the foundation (figure 15).

Figure 15: Post-tensioned piles for seismic upgrading of bridge foundations (CalTrans).

 

 

CORROSION PROTECTION

If, besides cement grout, additional corrosion protection is required, several methods are available:

1.) Hot Dip Galvanizing or Zinc-Metallizing

Steel bar components can be hot dip galvanized or metallized as per ASTM A­153(AASHTO M232). Zinc is a well known, common and relatively inexpensive coat­ing material for iron and steel. Zinc acts as a sacrificial anode, i.e. it corrodes in a corrosive environment and lets the steel play the role of the cathode. The high alka­linity of concrete and grout (pH > 12) dissolves the zinc to a certain extent, at pH < 12 a very low corrosion rate of zinc occurs due to the development of a passivation film on the zinc surface. This film will stabilize if atmospheric CO2 reaches the sur­face of the zinc coating. This is the reason for the known durability of zinc coatings under open-air conditions.

Galvanizing requires tight control of coating thickness to assure threadability. In most cases the thread inside the nuts or couplers has to be oversized, which could cause a reduction in load capacity. We have found that by using the metallizing method, oversizing the threads is not necessary. Both methods, if properly applied to the ASTM Standard, provide a good protective coating. 

2.) Fusion bonded Epoxy Coating

Epoxy Coating shall conform to one of the following: ASTM A-934, ASTM A-775, or AASHTO No. M284. Applying this coating requires oversizing of the hardware threads. Care must also be taken not to damage the coating.

3.) DCP, Double Corrosion Protection System. (Not for hollow bars)

This method is mostly shop applied to solid Threadbars or All-Thread Bars. This Type

1)

2)

3) of additional corrosion protection was part of the original Dywidag GEWI PILE and ground anchor development and has found worldwide acceptance.

Figure 16: Corrosion protection: 1) Hot dip galvanizing, 2) Epoxy coating 3) DCP (left top to bottom) and DCP detail (right)

ADSC Micro-Pile Seminar, Charlotte NC, November 13, 2001

 

The bar is centered using spacers and is fully encapsulated inside a corrugated PVC or HDPE sheathing. The annular space between bar and sheathing must be a mini­mum of 5 mm (0.2 inch) thick and shop cement grouted.

The sheathing must have sufficient strength to prevent damage during construction operations, shall be watertight, chemically stable without embrittlement, softening, and nonreactive with concrete. The minimum sheathing wall thickness shall be 40 mils. The material must conform to ASTM D-3350 polyethylene, Index No. 335520 C, Table 1, ASTM D-1248, and AASHTO No. M252 for HDPE or ASTM D-1784 Class 13464-B for PVC.

The encapsulation shall be fabricated from material with the following properties:

i Capable of transferring stresses from the grout surrounding the tendon to the grout in bond length

i Able to accommodate movements during testing and after lock-off;

i Resistant to chemical attack form aggressive environments;

i Resistant to aging by ultra-violet light;

i Non-detrimental to the tendon;

i Capable of withstanding abrasion, impact and bending during handling and instal­lation and

i Capable of resisting internal grouting pressures.

If steel bar couplers are used, they will be field installed with a double or multiple cor­rosion protection (DCP or MCP) system as per manufacturer instructions.

The cement grout inside the annular space between steel and corrugated sheathing is the most efficient element of corrosion protection. It must provide a proper alkalin­ity, low permeability, high resistivity, minimum to no shrinkage in both plastic and hardened states, proper fluidity, little or no segregation and no bleeding.

4.) Sacrificial Steel design method (see table 8 )

Is used primarily for oversizing pipe casing but can also be used for the pile rein­forcing bars. The ISCHEBECK Hollow TITAN Bars are tested in various non-aggres­sive, mild-aggressive and aggressive soils for loss of steel area over a 60 to 120 year design life (see table 8). This method is extensively used in Europe and presently started to be accepted in North America.

Sacrificial Steel Method

The data and information can be used to determine the sacrificial steel thickness, if no additional corrosion protection (metallizing, galvanizing, stainless steel) is used on CTS/TITAN IBO Bar anchors.

Corrosion Of Buried Metal

Taken from: TRL Report 380/1993 Applied to: Ischebeck TITAN hollow groutable anchors

60 Years

120 Years

 

Bar Size

Cross

Ground

Diameter

Reduced

% Loss

Diameter

Reduced

% Loss

 

Section mm2

Aggression

Loss mm

Area mm2

 

Loss mm

Area mm2

 

30/16

382

None

0.9

342

10.5

1.5

318

17.0

 

 

Mild

1.5

318

17.0

2.5

278

27.0

 

 

Aggressive

2.9

263

31.0

4.9

190

50.0

30/11

446

None

0.9

408

8.5

1.5

384

14.0

 

 

Mild

1.5

384

14.0

2.5

346

22.5

 

 

Aggressive

2.9

331

26.0

4.9

261

41.5

40/16

879

None

0.9

828

5.8

1.5

794

9.7

 

 

Mild

1.5

794

9.7

2.5

739

16.0

 

 

Aggressive

2.9

718

18.3

4.9

613

30.3

52/26

1337

None

0.9

1271

5.0

1.5

1226

8.3

 

 

Mild

1.5

1226

8.3

2.5

1153

14.0

 

 

Aggressive

2.9

1124

16.0

4.9

983

26.5

73/53

1631

None

0.9

1533

6.0

1.5

1469

10.0

 

 

Mild

1.5

1469

10.0

2.5

1415

13.0

 

 

Aggressive

2.9

1320

19.0

4.9

1112

32.0

103/53

3146

None

0.9

2998

4.7

1.5

2904

7.7

 

 

Mild

1.5

2904

7.7

2.5

2750

12.6

 

 

Aggressive

2.9

2688

14.6

4.9

2385

24.2

Table 8: Sacrificial steel method

References

Micro Pile Design and Construction Guidelines: Implementation Manual, US Department of Transportation – Federal Highway Administration, FHWA-SA-97-070, 2000.

Grouted Piles, DIN 4128 9.2, Deutsche Industrie Norm (German Industry Norm) Crack Width Distribution in TITAN Anchors, Technical University Munich, Institute of Civil Engineering, Prof. Dr. Ing. K. Zilch and H.H. Mueller.

Corrosion of Buried Metal, TRL Report 380/1983, Great Britain, 1983.

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