DOI: 10.17603/ds2hm33
title: Helical Piles Under Seismic Loading
title: in RAPID: Large-Scale Shake Table Test to Quantify Seismic Response of Helical Piles in Dry Sand

description: Tests were conducted on ten piles consisting of nine helical piles and one driven pile which were embedded in dense sand confined with laminar box, using the UCSD Large High Performance Outdoor Shake Table (LHPOST). Either six or seven coupled strain gauges, depending on the pile length and helix configuration, were installed along each pile on the east-west faces.  These strain gauges were applied in couples, at the same elevation in order to record bending moments and axial forces.  These strain gauges were connected as quarter bridges to the data acquisition system. Strains with time at each strain gauge elevation under dynamic loads were recorded. 

The pile heads and soil, along with the laminar box, were also instrumented with accelerometers to record pile head movement and accelerations, shear wave velocity of the sand and laminar box accelerations. The experiment consists of five test days (five events); each event has its own specific model configuration. Pulse, white noise, Northridge California earthquake (1994) and Kobe earthquake (1995) (hereby called Northridge and Takatori respectively) with an amplitude of 50%, 75%, 100% were used as input motions. 

Day 1 was just a shake sequence of the instrumented sand bed and laminar box configuration. 

Day 2 consisted of shaking the model with instrumented single piles and no inertial weights.  This was done to understand the change in stiffness in the sand bed as well as the kinematic behavior of the piles. 

The experiment on Day 3 added various concrete inertial weights to each of the 10 piles to understand the dynamic behavior of single piles. The amount of loading on each pile is provided.  

Days 4 and 5 consisted of testing 8 helical piles tied together in groups of 4 each with two sand skids of different weights.  Skid 1 was over the smaller 3.5" piles and had roughly 14,000 lbs of total mass and Skid 2 was over the larger 5.5" piles and had roughly 22,000 lbs of total mass.  Day 4 used a "Fixed" connection steel detail with double bolts and Day 5 used a "Pinned" connection with one bolt.  The goal was to understand how changing the connection affected the group behavior.  While both the "fixed" and "pinned" connections were not truly "fixed" and "pinned", the connections fall somewhere between completely "fixed" and completely "pinned".
description: This Rapid Response Research (RAPID) project investigated the seismic behavior of helical piles by means of shake table tests on full-scale piles in the UC San Diego laminar soil box. Helical piles are deep foundation elements that look like, and are installed like, a large steel soil screw - they have a slender steel shaft with any number of round plates welded to the central shaft at the tip to provide support to the structure they hold. Helical piles are spun into the ground with a large torque motor and provide support through soil bearing on the plates and along the shaft. They come in many lengths and are often the foundation of choice for retrofitting existing buildings or new, urban construction, due to their small footprint and ability to create minimal disturbance to surrounding structures. 

Even though it is known from anecdotal studies (e.g., New Zealand and Japan) that piles with comparatively small cross-section and high anchoring capacity, such as helical piles, are beneficial for seismic resistance seemingly due to their slenderness, higher damping ratios, ductility, and resistance to tip uplift, building codes and current state of practice have not been adequately developed for this pile type because no quantitative data exist. Research of seismic behavior of helical pile supported structures is therefore imperative to generate necessary data that will help ensure that helical piles are being correctly applied in seismic areas and establish quantifiable benefits and/or limitations of helical pile use in seismic areas. This project will benefit people living in seismic zones by educating engineers with full-scale helical pile experimental data so that they better understand how to design a building system that is safer, more resilient and sustainable for individuals and the community.