Q & A with a Ram Jack Engineer: Part I
Our live engineering webinar, “Helical Design: Code Compliance and Effects of Seismic Activity” held in March was a huge success thanks to those of you who either watched online or attended an in-person event with your local Ram Jack experts. We received many questions for our engineering team during the webinar. This is the first of a series of those questions and answers.
If you weren’t able to attend or participate in the accredited webinar, you can register for the recorded online session to earn your PDHs or contact your local Ram Jack for a free Lunch & Learn with the accredited webinar.
When you say hard soil, what is the blow count of the hardest soil it (helical piles) can penetrate?
The penetration of helical piles into hard soils varies based on pile diameter, max torque rating of pile, helical plate type, and tip type. As the pile diameter increases, so does the torque rating of the pile. Consequently, additional torque may be applied to embed the piles into the hard layer. In addition, the use of a modified helical plate (the helical plate diameter starts out small at the tip and increases in diameter along its length) will help the pile torque into a hard layer. Typically, a soil blow count value of 50 or greater would prevent a helical pile from penetrating further. However, pre-drilling a hole the size of the pile shaft would help the pile penetrate a couple of feet into such hard soil layers where capacity could be achieved by end bearing or by a combination of end bearing and skin friction.
Can you walk us through the typical inspections (and frequency) for helical anchor installation? What are manufacturer's recommendations versus code requirements? Are they the same?
A typical inspection for helical piles will include verification of manufacturer, equipment used, pile depth, helix plate configuration (size and thickness), pile diameter, final installation torque, and any information required in the construction documents prepared by the design engineer. This is referenced in Section 1705.9 of the IBC. In most cases the manufacturer’s recommendations match the code requirements. In some cases, the design engineer might require the inspection to be performed by a third-party inspection firm.
- Does this require Special Inspector to tension loads?
Technically, yes. Unless the exceptions listed in Section 1704.2 of the IBC are met.
With a threaded connection between sections, can a pier be removed?
In some cases, they can be removed. However, the torque being applied to the pile will weld the faces of the threaded connections together through friction.It creates a very rigid connection with no eccentricity but makes it difficult to get them apart. If removal of the piles is known, it's better to use a thru-bolt connected pile.
Thermoplastic coating - how durable is this during installation?
The thermoplastic coating is in compliance with ICC Acceptance Criteria AC228
What is your recommendation if you reach the maximum torque for your anchor before reaching the required embedment depth.
Our recommendation would be to provide the information to the geotechnical engineering firm who performed the soils investigation at the site. Since helical piles have a relatively smaller cross section than other pile types, the effects of potential soil heave, down drag, etc. are minimal. If the pile depth is unacceptable, we reduce the number of helical plates on the pile or use a smaller helix diameter configuration on the lead section so that the pile can be embedded further. If this doesn’t increase the embedment depth enough, the pile diameter could be increased to obtain a higher torque rating or a pilot hole could be pre-drilled for the pile to the required depth. There are a number of options available.
Do you know the cost difference between the Ram Jack helical pile and a standard concrete pier?
That’s difficult to answer. There are many site variables that could affect cost. In some cases a standard concrete pier could be more economical. However, the cost difference is going to be parameters such as installation time, time saved on waiting for concrete to cure, equipment required for installation, space/clear height limitations, field condition, site access, height of water table, whether the concrete pier holes need to be cased, whether the concrete needs to be trimied, presence of VOCs/contaminants, etc. These conditions wouldn’t affect a helical pile but would have an impact on the cost of a concrete pier.
Can you explain the use of the piles for light poles?
Ram Jack has designed helical piles for light poles in the past. Drawings can be provided upon request. Typically, axial loads are not the issue. The moment and shear force will drive the design of the pile. Larger diameter pile shafts are typically used.
Explain more about the 5 plate optimum?
A maximum of five (5) helical plates was a misquote during the presentation. Actually, six (6) helical plates are the recommended maximum on a pile but aren’t necessarily the optimum. Helical plates are placed on a pile shaft from smallest to largest starting from the toe of the pile shaft. The plates are spaced three times the diameter from the lower plate. While the plates are placed on the pile shaft to track each other, the trailing plates each lose some efficiency as they extend up the pile shaft. The general rule is that once you get beyond the sixth helix plate, additional plates don’t provide enough benefit to justify the expense.
Were tests conducted on threaded connections between extensions or bolted connection of piles?
Yes, tests were conducted on threaded connection with the connections being located in the middle. One end of the shaft would be connected to a torque driver and the other end would be fixed with the threaded connection in the middle. It was found that the threaded connection provided the max torque capacity and were always stronger than the pile shaft. Consequently, the torque rating of the pile shaft controlled the torque rating of the system and not the strength of the connection as is the case with other connection types.
Do you have any good links on AC358?
ICC Evaluation Services started selling their Acceptance Criteria online a few years ago. Click here if you would like to order one. The current cost is $274 for an electronic copy of AC358.
Do you have data to validate the vibration claim?
I’m assuming you are referring to the Baker Hughes machine foundation. The testing was performed by the owner and machine manufacturer. A verbal report of the test was provided to Ram Jack but not a written report. However, the machine weighs more than 1,000 kips, generates 500 kip dynamic lateral loads, costs several million dollars and has been in operation for more than three (3) years. If they had any issues with vibration of the foundation, they likely would have contacted Ram Jack.
Union County - was the shotcrete wall supported independently with soil nails or something, or was the total soil load supported by the tiebacks on the helicals?
The reinforcement for the shotcrete wall was tied to the vertical piles which were braced/anchored with helical tiebacks.
Are not all piles custom designed?
Ram Jack has ESR recognition on our standard product sizes which are the 2⅜” and 3½” piles and its related brackets. The capacities listed in the ESR report are for standard conditions that you expect in the field. Ram Jack manufactures and keeps these material in stock so that there is no delay during ordering material. A custom pile design would include piles that have excessive unbraced length due to post excavation, liquefaction zones, fluid soils, piles extending above grade, etc. These conditions could warrant the use of a specific custom pile diameter, bracket configuration, material type, protective coating, etc.—which can be manufactured by Ram Jack at its manufacturing facility.
Foundation Solutions Software: If you have a deep boring and capacity is achieved up high, will the software check the depth of the layer you achieve torque in to determine whether you could punch through to a weaker layer once the pier is loaded?
The software calculates capacities in one-foot (1 ft.) intervals. Therefore, if the designer is concerned about the piles punching through a respective layer, the pile capacities per feet can be verified and the piles installed accordingly. It should also be noted that the capacity provided at a particular depth is the average capacity calculated over a three-foot (3 ft.) depth from each individual helical plate. This helps the software to avoid terminating the pile in a thin lens of hard/dense soil. However, it is imperative that the software results be reviewed by a professional engineer before it is approved for installation.
Can CPT Sounding meta-data be inputted directly into the Helical Design Software or does Sounding data have be converted to N-values?
Currently this feature is not available in the software. The CPT results would have to be converted into equivalent SPT “N” values before it can be used in the software. Another option would be to manually enter the soil properties and bearing capacity factors estimated from the CPT data.
Can you discuss corrosion protection of the piles?
Ram Jack applies a thermoplastic coating to all products for corrosion protection. This is applied at our manufacturing plant. Galvanization can also be provided upon request. The thermoplastic coating carries a 50 year service life. Chapter 6 of Ram Jack’s Engineer Manual, found in the Ram Jack Technical Manual, discusses corrosion as it relates to steel piles installed in soil. This manual provides an excellent resource with references.
Is there a maximum helical pile depth?
No, there’s not a maximum embedment depth. For economic reasons, you typically want to stop the embedment depth as quickly as possible once the minimum embedment depth has been reached and a bearing stratum capable of supporting the structural loads is reached. However, we have encountered some locations where soft soil has extended very deep and helical piles were installed to over 100 feet. However, the average embedment in most locations in North America is around 20 to 30 feet.