STANDARD METAL POLE FOUNDATIONS
By Scott Hidden, P.E., Soils and Foundations Project Engineer Soils and Foundation Design Section Geotechnical Engineering Unit
How did we get to where we are?
Signals and Geometrics Section wanted to standardize metal pole foundation design. Contacted Soils and Foundation Design and Soils wrote new specification entitled DRILLED PIER FOUNDATIONS FOR METAL TRAFFIC SIGNAL POLES Due to variability and unknowns associated with the ground, elected to design standard foundations based upon site specific data, in other words, soil tests.
Advantages of Soil Tests
Design is based upon site specific data DEFENDABLE! Foundations should be smaller and less conservative. Finds problems before construction such as shallow rock and collapsing soils. Gives Contractor information to assist in foundation installation. Provides information about underground utilities.
Disadvantages of Soil Tests
Costs? (Maybe) Time for testing and administration. Testing can not be done until grading is complete. Specifications include requirements to ensure that design assumptions are met. Closer inspection is required. For example, design is not for steep sloping ground. Different than what we have always done. Change! - Who moved my cheese?
ATV Drill Rig
Standard Penetration Test (SPT)
ASTM D 1586 - 99, 140 lb. Hammer, 30 Drop, 3-6 Increments Sum of blows for last two increments (12) is called N-value or blow count. High N-value Dense, Stiff Soil Low N-value Loose, Soft Soil Sample can be retrieved from split spoon for classifying soil type. Very Common Empirical Test
Manual and Automatic Hammers
Split Spoon with Soil Sample
Example Soil Test Log and Selection Form
Not Applicable Everywhere !
Foundation Selection Table from Plans
Where can I find the foundation selection form?
Where can I find a list of firms to perform soil tests?
Section 1.3, Drilled Pier Construction
Specifications are written based upon criteria for horizontal and rotational movement (1 inch and 1 degree). DRILLED PIERS MUST BE CAST AGAINST UNDISTURBED SOIL! Pared Down Drilled Piers Special Provision used for Bridge Construction Two Pay Items:
Soil Test..Each Drilled Pier Foundation (____-inch (mm) diameter). Linear Foot (Linear Meter)
Signal Contractors do not want to drill this!
And Divisions do not want to pay to drill this!
What does the new specification say?
Requires steel casing when unstable, caving or sloughing soils are anticipated or encountered. Provide one continuous piece of steel casing that is clean smooth non-corrugated watertight steel .. 1/4 Minimum Wall Thickness with Outside Diameter Equal to Specified Size of the Pier Casing is temporary (must be removed) except when Contractor elects to make it permanent at his own cost provided it is installed correctly and with an exception.
Steel Casing
Permanent Steel Casing and Mast Arms
Any steel casing left in place will be considered permanent casing and must be installed before excavating or drilling such that the permanent casing is against undisturbed soil. Permanent steel casings are only allowed for strain poles and prohibited for mast arm poles. No additional compensation will be paid for permanent casing. If the Contractor chooses to use permanent steel casing, include all costs for permanent casing in the cost of the contract unit price bid for the Drilled Pier Foundation pay item.
Mast Arms
Plastic Bolsters and Spacer Wheels
Drilled Pier Concrete
Designate the concrete as Drilled Pier Concrete with a minimum compressive strength of 4500 psi (31.0 MPa) at 28 days. Produce a workable mix so that vibrating or prodding is not required to consolidate the concrete. When placing the concrete, make certain the slump is between 5 and 7 inches (125 and 175 mm) for dry placement of concrete or 7 and 9 inches (175 and 225 mm) for wet placement of concrete.
Concrete Placement
What is a dry or wet pour? How do we determine the pour type? Greater than 6 per half hour Wet Less than 6 per half hour Dry Do not dewater any drilled pier excavations unless the excavation is entirely cased down to tip. DO NOT FORCE A DRY POUR!
Wet Pour Sequence
Result of Forced Dry Pour
Safety First!
ANY QUESTIONS?
Hey, its not my job.
