Section 4: Piling
Anchor: #i1038085Overview
Piling design should consider skin friction and may consider point bearing as well. Because piling has small tip areas and is generally placed in softer soil, the point bearing contribution is modest and is often disregarded in design.
Calculate total allowable skin friction by multiplying the perimeter of the pile by the unit value for allowable skin friction derived from Figure 5-1, Figure 5-3, or laboratory data or a combination thereof. The maximum recommended value for allowable skin friction for piling design is 1.4 tons per square foot (TSF). Accumulate skin friction along the length of the pile beginning at the previously defined disregard depth and continuing down to the tip of the pile. If using point bearing, calculate total allowable point bearing by multiplying the area of the pile times the unit value for allowable point bearing derived from Figure 5-2, Figure 5-4, or laboratory data. If softer layers exist within two shaft diameters of the proposed tip, use allowable point bearing values based on the softer layers. Displacement piling refuses to advance when it encounters material with TCP values harder than 100 blows/12 in. On refusal, assume that the piling has developed the maximum allowable service load for the pile.
Take care when designing piling in areas with shallow hard or dense soils. If piling cannot be driven through these areas, the contractor will need to pilot hole or jet the piling to achieve the desired penetration.
Anchor: #i1054271Wing Piling
Found wing piling in similar founding material as abutment shafts to minimize the potential for differential settlement.
Anchor: #i1054308Steel Piling Special Considerations
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- Grade Separations:
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- Foundation elements for grade separations are subject to potential vehicular impact the use of steel sections in a trestle configuration in those potential impact zones is highly discouraged. Instead for grade separations, steel H piling can potentially be used under pile footings for interior bents or for abutments.
Anchor: #LMPSYWUB - Stream Crossings:
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- Foundation elements for stream crossings are subject to scour, drift impact and have a higher propensity for corrosion. For stream crossings, steel piling needs to be analyzed for potential corrosion over the life span of the structure and need to be evaluated for both axial and lateral loadings under the scoured condition. Steel piling that have been evaluated for the above conditions and found to be acceptable could be used for trestle bents. However, the steel piling must be coated to a minimum depth of 15 feet below the maximum predicted scour elevation. Steel piling can be used to support pile footings as long as the footing is embedded at a depth below the maximum predicted scour depth thus minimizing the risk of exposure. Piling used in a footing configuration must be coated a minimum distance of 15' below the bottom of footing. Piling can be used for foundation elements for abutments.
Difficult Driving
If it is necessary to advance the piling through a strong or stiff layer where refusal is possible, a pile penetration note may be required. A typical note may read, “The contractor’s attention is drawn to the hard material in the soil profile, jetting and/or pilot holes may be necessary to advance the piling to the required penetration depth.”
Anchor: #i1032323Service Loads
See the following table for maximum piling length and structural loads recommended without conducting a detailed structural analysis. Many soils are not capable of developing these maximum loads. Before final structural design, review the soil information to verify the ability of the foundation to develop desired maximum loads.
|
Size |
Maximum Length |
Abutments and Trestle Bents |
Footings (per Pile) |
|---|---|---|---|
|
16 in. |
85 ft. |
75 ton |
125 tons |
|
18 in. |
95 ft. |
90 tons |
175 tons |
|
20 in. |
105 ft. |
110 tons |
225 tons |
|
24 in. |
125 ft. |
140 tons |
300 tons |
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Dynamic Monitoring
Dynamic monitoring of a pile during driving can be accomplished using a Pile Driving Analyzer (PDA) testing system. PDA testing measures the strain and acceleration in the pile as a result of the impact of the hammer. PDA testing of a pile can help to determine the stresses in the pile during driving and monitor the pile for damage or integrity. The capacity of the pile and time dependent changes in capacity (if a restrike is undertaken) can be obtained if the PDA testing data is used with the Case Pile Wave Analysis Program (CAPWAP).
Not all piling will require dynamic monitoring. However, for critical structures, projects with a large number of piling, or in difficult soil conditions PDA testing should be considered for use. Consult with the Geotechnical Branch to determine if a specific project might be considered as a candidate for PDA testing.