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Side by Side Comparison of Exposed H-Piles Proves Points are Essential and Economical

Points for all H-Piles Proved Essential by Exposed Soldier Beams on Adjacent Jobs

Excavation to Below Tips of Soldier Beam permits Full Comparison of Driven Piles

Three adjacent building projects in Minneapolis, MN provided an excellent comparison of methods of soldier pile installation. Excavation proved the value of Associated Pile & Fitting Corp’s. cast steel point protection for all H-piles. Soldier beams with points reached full depth economically and in excellent condition. The contractor found this point reinforcement so valuable in speeding excavation and protecting temporary H-piles against underground hazards that it is used on all similar projects.

All three of the multi-story structures rest on rock. H-piles were installed as soldier beams through mixed material around the periphery of the building areas. In contrast to the usual foundation piling, where the condition of the lower end of the pile is seldom known, excavation exposed the soldier beams to below their tips. For contractors there is interest in the economy of recovery for reuse of the full length of the pile, plus a minimum of repair work. Engineers should relate the condition of piles driven with and without points to piles to be used permanently under structures.

To install soldier beams at the first structure, a hole was cored and drilled to rock. An H-section was then set in place. For the second project, H-piles were driven. As excavation proceeded the lower portion of most piles had to be pieced out as they had been badly distorted and had not penetrated to rock.

At construction of the third building the lightest point (APF’s No. 75750) was used experimentally on some of the soldier piles driven in a small area planned for early excavation. Exposure of the piles showed those with the cast-steel end protection to be in almost perfect condition. Those installed without the points drove out of the planned location, and some were mangled. The contractor, at his own expense, used APF’s points on all remaining soldier piles around the 365x365-ft area.

Rock is at relatively shallow depth at all three building locations situated along the Nicollet Mall in downtown Minneapolis. Sand with boulders, in medium dense formation, overlies a few feet of hardpan above the rock. All areas described were excavated down into the rock to provide for underground parking. Excavation was carried out to the line of existing buildings or the adjacent streets in order to utilize the entire lot area.

The sandy, cohesionless soil had to be fully restrained in order to prevent flow and loss of ground support from under the buildings or streets. The soldier piles were installed before excavation started. Timbers were inserted horizontally in back of the front flange or attached to its face. The vertical beams were tied back by wires or rods grouted into the rock or soil and installed with horizontal wales. These ties eliminated need for sloping or cross-lot bracing, leaving the area completely open for earth removal and construction.

Holes Cored At Building
The first of the three structures to be built was a 14-story parking garage and office for Midwest Federal Savings and Loan Association. Excavation along one side, and part of another, was carried right up to adjacent buildings founded at a higher elevation. Structural engineers, Christiansen, Robertson – and soils engineers Shannon & Wilson – planned to minimize effects to existing structures and the owner wanted noise from construction kept to a minimum.

Spencer White & Prentis, foundation contractors, met these requirements by augering and drilling 24-in. diameter holes to a toe-hold in rock and setting HP 12x53 sections in position. This required no pile driving and assured placing the solider beams exactly where wanted. Pretest tie-backs were installed by drilling and casing a 4 in. hold to rock then drilling a 3 in. hold a few feet into the rock. Steel prestressing cables were grouted in to hold three lines of wales along the 45-ft. deep excavation.

Piles Damaged in Driving
Underground conditions at the second structure, the Federal Reserve Bank, were similar. Here the contractor, Carl Bolander & Sons Co. of Minneapolis, elected to drive 14 in. H-sections to rack at 6-ft. centers around the periphery of the full block structure. The piles were driven 23 to 30-ft. with a Link-Belt Model 520, double-acting, diesel hammer delivering a nominal 30,000-ft. lb. of energy. As excavation progressed, horizontal wales were installed and tied back with Bauer Earth Anchors.

It was expected that two rows of wales would be adequate for the walls around this excavation. However, as earth removal reached mid-depth, it was found that many of the solider piles, which were installed without reinforcement of the tip had driven out of position and were badly distorted. The lower portion of a large number of the piles had to be replaced. This was time consuming and increased the cost substantially as it necessitated the addition of a tow of wales and tie-backs.

Points Protect Pile Ends
For the third structure, the 57 story Investors Diversified Services Building, APF points were used on all by a few of the piles. These were driven at the start of the project for comparison. When the first small area was excavated, piles installed with protective points were found to be straight and in excellent condition. Those without reinforcement were badly distorted. Bolander then elected to use the APF Point 75750 for all piles. While the piles were exclusively for temporary shoring Turner, as general contractor, and Severud-Perrone-Sturm-Conlin-Bandel, engineers, endorsed the use of APF Points as they gave greater assurance of stability to the wall. The solider piles were spaced at 6-ft centers around the 365x365-ft area to be excavated.

The HP 14x89 piles were generally started with a No. 1 Vulcan hammer and driving was completed with the Link Belt 520 diesel hammer to the 35 to 48-ft depth required to rock. Along one side of the lot the soldier piles were driven as close as 8 in. to an operating TV cable. A McKiernan Terry V-10 vibrating hammer was used in the area for installation of the soldier piles, with final driving done by the Link Belt diesel.

Placing 3 or 4 in. thick wood lagging followed the excavation. Some timbers were tucked back of the outside flange of the H-pile. The Contact Sheeting method of attaching he lagging to the outside face was used for the major part of the work. Bauer anchors were placed nearly horizontally into the soil, with four rows of wales holding the wall at its greatest height.
As excavation of the rock surface exposed the pile ends, the value of the points was evident. While some piles had twisted little from passing boulders, none were so seriously out of position or distorted that they required repair. This was in considerable contrast to the installation at the nearby Federal Reserve Bank. There, piles driven through similar materials (but not as deeply) without points, were badly damaged. Many had to have their lower ends replaced and required an additional line of tie-backs.

Points for Economy and Safety
Contractors find that points are quickly and easily attached by just a simple weld across the width of the flange of the pile. They assure straight driving and ease of withdrawal of soldier beams. The possibility of loss of links of pile is virtually eliminated. It is quite unusual that foundation piles are exposed for examination of their full length. The condition of the lower sections of solider piles is a good indication of what can be expected with all foundation piles. Cost of protection with points is nominal and installed cost is substantially less, as well as demonstrably better, than plate and angle or other reinforcement of the tip.

Points provide low cost insurance for every H-Pile.