Second International Workshop on Micropiles (1999 IWM) in Ube City,
Yamaguchi University Japan

Preface

The Loma Prieta Earthquake in the San Francisco Bay area on October 17^th 1989 and the Northridge Earthquake in Los Angeles on January 17^th 1994, prompted a vast review of all bridges and freeway structures by the California State Highway Department (CALTRANS). It was found that with an inventory of 11,000 bridges, most require seismic upgrading.

Seismic forces apply dynamic horizontal and vertical loads on structures. They generate eccentric bending moments in the foundations, resulting into twisting of the structure and opening of the joint between footing and ground. This opening of the joint can be eliminated or reduced by post-tensioning, (pre-compressing).

The dynamic seismic shocks shall be resisted by the elastic (spring system) post-tensioned strand tendons, eliminating or minimizing uplift of the footing.

Con-Tech Systems developed this method and tested the same with Caltrans. The challenge was to develop a top anchor head, which withstands stress reversals. The tie-down anchors also had to be prefabricated and shipped coiled for low headroom applications under bridges. Strand anchors are ideal to fit this envelope. This method was then used in a number of Caltrans bridge retrofit projects in the LA and San Francisco areas.

A 16"dia. pipe pile is driven through the overburden and socketed into the harder load bearing strata. The pipes are cleaned out to within 24" of the pile tip, a 6"dia. PVC pipe is centered inside the pipe and concreted in. This PVC pipe serves as a drill guide to drill a 5"dia. anchor hole to a specified depth.

The double corrosion protected CTS tie-down strand anchor, with a bond length and a permanent unbonded length, is installed through the PVC pipe into the 5"dia. anchor hole and grouted. After the grout reaches sufficient strength, the anchor is tested, tensioned and locked off on top of the pipe pile or footing. The amount of lock-off force is governed by the allowable max. elastic movement during seismic occurrences. Another factor is the allowable additional load on the footing, which is introduced during post-tensioning of the tie-down anchor

During one of the retrofit projects, (the Interstate Highway I-80 Viaduct, El Cerrito SOH Bridge in the San Francisco Bay area) it was noticed that the driven pipe piles, which were load tested and accepted right after installation, started to fail (settle) during the testing of the strand anchors. The testing applied an external vertical load to the pile.

It was discovered, that either the clean-out drilling operation inside the pipe pile or the drilling of the hole for the tie-down anchor disturbed the ground under the pipe pile tip and therefore reduced the pile’s load carrying capacity.

The project was stopped and Caltrans, after reviewing the problem, ordered an extensive Pile Load Test Program

Caltrans asked for comparison tests using 62mm DSI bars, tested but not locked off, versus the conventional, as specified, pipe pile/strand anchor tie-down.

We requested that Caltrans also include the TITAN pile in this program, which they accepted. The specifications and limitations for this test program allowed a max. elastic movement under compression and or tension of 0.5" at 200 Kips (100 tons) in tension and 400 Kips (200 tons) in compression. We proposed the TITAN 73/53 bar with 4 / 0.6" high strength strands inside as a comparison test to the DSI bar proposal. Our aim was to achieve as much skin friction as possible and also to reduce elastic strain during loading conditions. The steel area of our system, compared to the DSI system, is 50% less. This gave us the original idea to introduce internal post-tensioning of the hollow TITAN bar to achieve the same stiffness. The high skin friction is achieved due to the rotary grout injection drilling, which is a standard feature of the TITAN system. The internal pre-stressing was achieved in pushing the 4 strands into the grout filled Titan 73 bar. The lower 10’ of the strands are bare to develop bond inside the TITAN bar and the upper length is unbonded by encapsulating the individual strands in corrosion inhibiting grease and high-density polyethylene sheath. After the grout reaches a min. strength of 3000PSI, the strands are then tensioned to 145KIPS with a mono or multiple strand jack, before the actual pile is load tested.

Figure 2: Internal post-tensioning of hollow Titan 75/53 micropile with 4 / 0.6" high strength strand. Caltrans seismic retrofit project highway I-80 El Cerrito Viaduct, Albany, CA., field test.

Piles 3C, 3D, and 3F were supplied by Con-Tech Systems and Pile 3E was supplied by DSI.

Pile 3C	was the as specified driven pipe pile with CTS pre-stressed strand anchors, but installed purposely incorrectly by over cleaning the pipe pile.	Figure 3: Micro pile field test arrangement. Caltrans seismic retrofit project highway I-80 El Cerrito Viaduct, Albany, CA.
Pile 3D	was the same as 3C, but installed using extra precautions, not to disturb the ground under the pile
Pile 3E	was a driven pipe pile with an unstressed 62mm DSI-GEWI bar inside.
Pile 3F	was also inside a driven pipe pile with the TITAN 73/53 bar and 4/.6 post-tensioned strands inside, but about 10 feet shorter than the other piles.

The Failure of Pile 3C was expected. Piles 3D, 3E and 3F passed. It is, however, very interesting to compare the results of these piles which passed the test.

As shown in the report, our proposal using the composite TITAN bar with post-tensioned strands, in pile 3F, performed best; even being the shortest pile it showed the smallest amount of movement.

The DSI pile 3E also passed.

Pile 3D passed marginally in compression, primarily due to the additional external load introduced during pre-stressing of the tie-down anchor on top of the pipe pile.

The distance between the test piles of only 3’-0", less than 1.0 m, helped the test result of the center pile 3E (untensioned DSI bar), especially due to the injection grouting used during the TITAN pile installation and the consequently ground consolidation. The piles should have been placed at least 8 feet apart

In Pile 3F, the internal pre-stress force pre-compressed the TITAN-MICROPILE internally without reducing its external load capacity. This pile did not shorten or elongate any more elastically until the external test load exceeded the internal pre-stress load. Therefore, the footing will also not move until this load is reached, which can then be considered the Design Load.

The TITAN-MICROPILE, compared to the DSI bar pile, had less movement and less deformation; even though it was 10 feet shorter and has only half the steel cross-section area.

The TITAN pile is therefore much more economical

In California, contractors must conduct a pre-qualification MICROPILE Test Program in order to be able to tender on Caltrans projects.

Caltrans primarily introduced this for the installation of composite casing / bar micropiles, where a 7"dia. smooth wall casing with a lost ring bit is drilled down for the total length of the pile with a field welded anchor plate on top. A solid steel reinforcing bar (CTS, Williams or DSI), is lowered into the casing. The casing is filled with cement grout and pulled back a distance under pressure grouting. The grouting operation also attempts to fill the small annulus between casing and drill hole. The installation of this pile type is very sensitive. In order to produce an acceptable product, the contractors have to prove to Caltrans their capability. After acceptance and before production-piles are installed in every Caltrans project, a sacrificial verification pile has to be drilled and tested. Also 10% of all production piles, or a minimum of 2 piles per footing, have to be load-tested to verify to Caltrans the quality of these piles.

Figure 4: Caltrans 105/53 micropile field test, Los Angeles CA. Contractor Malcolm Drilling Co. Inc.

A large California contractor, Malcolm Drilling Co., applied for their field test program. Knowing the many advantages of the TITAN-MICROPILE, they added this pile to the program. The test requirements are: Max. movement shall not exceed 0.500" @ 300 kips. Based on an old existing soils report, provided by Caltrans, MDC calculated the pile length for the composite casing pile at 60.0’ and for the Injection Bore TITAN pile at 30.0’.

We supplied the following materials:

TITAN 105/53 bars for 2 test piles, one tension pile and one compression pile, and TITAN 103/78 bars for the reaction piles. The test results for this test are most interesting.

• The TITAN Injection Bore pile, only 30’ long, reached 0.500" movement at 275 kip
• The composite casing/bar pile, 60’ long, reached 0.500" movement at 150 kips.

This confirms that the friction values indicated in the TITAN-MICROPILE brochure are at least 2 x the values for the composite casing/bar piles.

A new test had to be conducted at the same site with both pile systems lengthened by 30’, because, as mentioned before, the soils report was very old and did not indicate the additional land fill, which occurred at a later date.

• The TITAN Injection Bore pile was lengthened from 30’ to 60’ and the composite casing/bar pile from 60’ to 90’.
• Both piles went over 400 kips at 0.500" movement.

The load at Geo. Failure for both pile systems was around 600 kips, (300 T).

The Test Report concludes the following:

The main advantages of the TITAN Injection Bore MICROPILE System over other Systems include: The ability to transfer the load for the full length of the pile. A min. grout cover of 2" over the steel can be guaranteed. The fully threaded, hollow TITAN bar allows connections to be made in any lengths and adjustments made in the field without welding if necessary. No need to weld the footing anchorage. Can be installed in extremely low overhead applications; therefore most ideal for seismic retrofit work. Can be post-tensioned to reduce elastic elongation or shorten movements. With these field tests, we think that we were able to prove to Caltrans and the industry that the installation of the TITAN injection grouted MICROPILES, for seismic retrofit projects and with possible post-tensioning, is much simpler, safer and faster. The extensive pre-qualification processes, which are required for the other method, will be eliminated, which makes the TITAN pile much more economical.

Figure 5: Finished typical Titan 105/53 micro pile anchor head after testing.

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