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| For Engineers and Design Professionals
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HELIX PIERS AND TIEBACKS, AN INTRODUCTION
Helix members can be installed either as piers (vertical) or as tiebacks (diagonal or horizontal). We've used them for new construction, residential and commertial additions, underpinning existing structures, bridge abuttments, and retaining walls. In most underpinning with no signs of lateral movement we typically use only vertical piers. Where there are lateral considerations we add diagonal helix members as tiebacks.
More and more contractors are finding helix piers to be more cost effective than other forms of deep foundations.
All you need to do is note on the plans the ultimate capacity requirement of the pier or tieback, e.g. "20 kips capacity".
Please number the piers and any tiebacks on the plans for easy reference. You can either create a table listing the pier numbers and their ultimate capacities, or write on the foundation plan next to the pier what the capacity requirement is. If you prefer, you can calculate and specify the exact minimum torque figures.
Installation of helix members as piers or tiebacks is monitored using a torque monitoring device. The manufacturer of the piers we use has provided us with a proven conservative torque ratio for installation purposes.
As background information, the torque factor for our 2 7/8" outside diameter round shaft helix piers is 9. It is 7 for our 3 1/2" round shaft piers. For our S-150s (1 1/2" square shaft) helix piers and tiebacks the torque ratio is 10. We take your capacity requirement specified, divide
by 9 (or 10 or 7 depending on the pier used) to determine the foot pounds of torque we must achieve in installation. By following this formula we are certain the strength of the soil is sufficient to support the load. More technical information is available on request.
Example: You've calculated a 26,000 lb. pier load, and you choose an factor of safety of 1.5 = 39 kips ultimate capacity pier. Simply note on the plans "39 kips ult. capacity" by that pier. In using the 2 7/8" round shaft pier we divide by 9 to find the foot pounds of torque we need to drive to, 4333 ft lbs.
Example: You've calculated the load on a tieback to be 13 kips, and you choose an safety factor of 2.0 = 26 kips ultimate capacity tieback. Simply note on the plans "26 kips ult. capacity". In using our 1 1/2" square shaft tieback we divide by 10 to find the torque we need to drive to, 2600 ft lbs.
To ensure we are in good soil, the least we usually install a pier or tieback in to is 18 kips, even if the loads don't require this much.
Please leave the size and number of helix blades (aka helices) of the pier to the contractor. Sometimes we will switch from one size blade to a smaller blade to achieve greater penetration (tiebacks typically) or to avoid cobbles. In other situations we will go to a multi-blade and bigger helice to reach torque sooner (piers typically) in order to save the owner money in extensions, particularly in low cohesion soils. As you know, soils logs are not always representative of the soils in the exact location of a pier or tieback. One of the beauties of helix is the we have known capacity at the time of installation... no guessing or hoping.
Some project engineers like to watch at least part of the installation, or visit the site at the end of installation to witness torque checks on piers they have randomly pointed out. ALternatively, you can refer helix installation observation to special inspection firm. If a soils engineer is on the project, the soils engineer normally oversees and certifies the helix installation.
For members in compression, there are many advantages of the 2 7/8" round shaft helix leads and extensions over the 1 1/2" square shafts helix piers. The round shaft piers have much more rigid connections, are verifiable in depth post installation by sticking a tape measure down the hollow shaft, and can be grouted post installation for greater capacity and strength. We use the 1 1/2" solid square shaft primarily for tiebacks or shorter vertical piers. We have had these made to our specifications of 5" overlap and two bolts at the couplings.
With our 3 1/2" round shaft pier we reach up to 86 kips capacity. We have installed 5 1/2" helix piers. We can install larger piers with greater capacity. All of our helix piers and tiebacks are galvanized.
After installing all the piers on a project, we look over the pier log to look for any anomalies concerning depth/length. We'll look suspiciously at any relatively shallow pier, and get back on it to ensure it "torqued up" in the expected strata, not in a thin lens.
We use a portable rig to install in tight access locations. We have installed helix piers and tiebacks on a hillside, under a deck, in the pouring rain. We have installed a 38 kips capacity pier through the floor of a linen closet.
Note: We would be happy to email you construction details for your plans. No need for you to reinvent the wheel. You are also welcome to use the following texts.
SAMPLE TEXTS
HELIX PIER INSTALLATION INSTRUCTIONS
The Helix Pier installation contractor shall familiarize himself or herself with the project site, and shall verify dimensions prior to commencement of work. Discrepancies shall be reported to the project engineer prior to work.
The Helix Pier installation contractor shall be experienced in the installation of Helix Piers, and shall be qualified by the helix pier manufacturer to install the specified helix piers.
The Helix Pier installation contractor shall use equipment capable of 1000 ft. lbs. of torque more than the maximum specified in these plans, and shall meet or exceed the driving torques and depths specified herein, unless Engineer-of-Record notes otherwise.
Piers shall be installed by use of a DP-1 Torque Indicator. The Helix Piers shall be installed using a drive head revolving at no more than 20 RPM. Constant down pressure shall be applied during installation.
The installing contractor shall notify the project engineer if the minimum specified torque cannot be achieved, or if the minimum depth, if specified, cannot be achieved, or if foundation/soil/structural conditions are found to be other than those presented in these plans or accompanying building documents.
The number of helices and their diameter required is a matter of soil conditions and torque installation requirements, and may be adjusted in the field as approved by the project engineer.
Upon completion of the project, the installing contractor shall submit a Helix Pier Log which shall indicate the following:
| Pier No. |
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Specified Torque |
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Installed Torque |
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Size(s) of Helices |
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Depth of Pier |
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Date Installed |
The torque reading at final depth should be used for helix pier capacity determinations. In the case where cobbles are present, torque values during installation often exhibit peaks. The torque reading between peaks should be relied upon, not the peak torque readings. In areas where it is difficult to differentiate between peaks, an average torque reading can be used.
The log shall bear the signature of the installer. Original copies shall be submitted to the project engineer, building official of the governing body.
HELIX INSPECTION
1. The materials specified in these plans involve no ICBO Report. No special inspection by an independent Special Inspector is required.
2. The Project Engineer shall witness the torque verification and shall prepare a report for the Building Inspector stating the specified torque has been achieved.
NOTICE TO OWNER
The installation of the helix piers and tiebacks will fix the area of the structure involved directly. The remainder of the structure may or may not move differentially with regard to the underpinned area. If movement of the non-underpinned area becomes apparent in the future, underpinning similar to that proposed should be provided.
SPECIFICATIONS
1. The work shall be performed in accordance with the applicable UBC, the rules and regulations of the governing body, & these drawings and specifications.
2. Concrete shall be min. 2500 PSI @ 28 days, and placed in accordance with the latest A.C.I requirements.
3. The reinforcement steel shall be ASTM grade 40 up to #4, grade 60 for #5 and above, and shall be installed in accordance with the latest A.C.I. requirements.
4. The contractor shall notify the project engineer (Engineer-of-Record) forty eight hours prior to requiring his observations.
5. Observations by the project engineer shall not preclude the inspection requirements of the governing bodies.
6. Install piers to minimum 1800 ft lbs torque, UNO.
REUSE OF PLANS PROHIBITED
No portion of these plans may be used in part or in full on any project other than the subject property of these plans containing the wet stamp and signature of the project engineer, without the written permission of the project engineer.
DRAINAGE AND MAINTENANCE GUIDELINES (BY OWNER)
Owner is responsible for establishment and maintenance of drainage system, unless noted otherwise in the plans or specifications. Water should be directed away from the foundation, not permitted to percolate into the ground near the foundation.
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