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Section 4: Piling

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Overview

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.

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Wing Piling

Found wing piling in similar founding material as abutment shafts to minimize the potential for differential settlement.

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Steel Piling Special Considerations

  • Grade Separations:
    • 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.
  • Stream Crossings:
    • Foundation elements for stream crossings are subject to scour, drift impact and have a higher propensity for corrosion. The use of steel piling for interior bents in a trestle configuration may be considered when the scour analysis shows that the predicted scour for the life of the structure does not expose a pile length that nears this tolerable maximum exposed length, where there is no evidence or history of drift load, or in highly corrosive soil or water environments. Piling used in these configurations must be coated to a minimum depth of 15' 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.
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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.

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Service 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.

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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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