A deep foundation system that is gaining popularity in areas with difficult soil, contaminated soil and due to its speed of construction is steel screw piles.

At first glance screw piles look like the typical helical pier.  However, there are significant differences that make a dramatic change in capacity.  First, the screw piles are installed as a single continuous piece in lieu of sections bolted together.  Second, the machinery used to install screw piles is larger and capable of producing a higher torque.

The components of a screw pile are simple; a steel shaft, steel helix at bottom of shaft, and a connection plate at the top of the shaft.  Screw piles can vary in size to accommodate different design loads.  The steel shaft size is a function of axial load, torsion during installation and thickness due to corrosion.  The helix is a plate that is shaped just like the threads of a wood screw and engages the soil to distribute the axial load and it locks into place to provide tensile loads as well.

One of the first questions people have is, “How long will it last until failure from corrosion?”  There are several factors that go into corrosion of steel; oxygen, moisture, dissolved salts and other corrosive chemicals.  Since there is very little oxygen in the soil, corrosion is limited and can be controlled by using thicker material to allow for expected corrosion.  The life of steel screw piles can be 50 years to an indefinite period depending on the type of structure and life span needed.

I designed foundations in Stapleton where the site was located over the old terminal and contaminated soil was discovered.  If typical straight shaft drilled piers were used the spoils from drilling the hole would have had to been bagged and disposed of properly.  To avoid bringing contaminated soil up to the surface steel screw piles were used.

I list a lot of PROS below and for good reason.  I have been very impressed with screw piles and the wide range of uses of them.  EVstudio was recently visited by Alpine Site Services for a lunch and learn.  Mike Oliveira discussed how screw piles are installed and the many advantages to their system over other typical foundation types.  What really impressed me was the speed of placement of screw piles.  Alpine can place 30 to 50 screw piles in one day!

Another great advantage to screw piles is that they can be placed at an angle to provide bracing and tension resistance that is aligned with the force of the tension.  An example is a typical counterfort that braces a foundation wall.  You can see the angled screw pile above.

PROS
Fast Installation
Can be Proof Tested
Negligible Uplift Pressure from Swelling Soil
High Capacity up to 500kips
High Tensile Resistance
Made from a reusable material (LEED point)

CONS
Specialized System
Large Equipment
Concerns with Corrosion

Learn more about different foundation types.

Originally posted 2009-10-10 00:21:43.

Related posts:

1. Straight Shaft Drilled Concrete Pier Foundation
2. Spread Footing Foundation
3. Wall on Grade Foundation

Jim and I went by Pink Fog to inspect the steel connections on the stair to the loft.

Due to existing conditions, we designed a stair that would avoid placing a post in front of a window. The design required both cantilevered upper floor framing and a cantilevered stringer. In order to make this work, we utilized a unique structural design with a bent frame to handle the torsional force and the cantilevers.

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1. Residential Spiral Stairs – Guidelines, Criteria and Dimensions
2. Installing New Doors at an Exit Stair – Joy Burns Arena at the Ritchie Center
3. Commercial Stair Design – Guidelines, Criteria and Dimensions

Why? well, we're talking about commodities. The prices change and sometimes rapidly. Right now, steel has been skyrocketing. In 2002, the question of steel versus wood as a structural choice was easy - steel all the way. That has changed in more recent years, however you'll see that it has actually caught up with lumber (which has also skyrocketed in its own markets in recent years)

First of all, you buy steel by weight. The industry works in dollars per ton, so take your 20 foot long w10x49 steel i-beam and multiply like this:

Figure the weight of the steel: 20’ (length of beam) x49#/ft (from the spec W10x49 - the 10 is inches height and the 49 is pounds per foot) = 980 lbs.

Convert to tons: 980lbs/2000lbs/ton = .49 tons

Multiply by the price per ton: .49 tonsx$2000/ton = $980 (today the average price is $2000 per ton - or $1 per pound -- Isn't that convenient?)

Local suppliers may charge additional fees for their services.This is material only – not labor to install.Spanning 20 feet with wood could easily be a 24” deep glu-lam or deeper (depends on the load configuration). Today, you can source a beam like that for around $40 to $50 per lineal foot, so at the high end, the cost of wood or steel would be a wash. However, lumber and steel prices cycle and vary (see below).

"Prices for shop-fabricated structural steel that is primed and delivered to the job site increased nearly 40% during 2004 caused by global influences, real and imagined. Prices then settled down to an annual increase of about 8% for 2005, 2006, and 2007. We forecast that fabricated structural steel prices will increase about 10% during 2008, having already increased 5% year to date."

Source: The Means Report

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