Section 3: Drilled Shafts

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Consider both skin friction and point bearing in drilled shaft design. Calculate total allowable skin friction by multiplying the perimeter of the shaft by the unit value for allowable skin friction derived from Figure 5-1, Figure 5-3, or laboratory data or any combination thereof. Apply a reduction factor of 0.7 to allowable skin friction values derived from Figure 5-1 or from laboratory testing. Do not apply the reduction factor to allowable skin friction values obtained from Figure 5-3. Accumulate skin friction along the length of the shaft beginning at the previously defined disregard depth and continuing down to the tip of the shaft. Calculate total allowable point bearing by multiplying the area of the drilled shaft 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 for the softer layers. If drilled shafts are to be tipped in very hard material that is overlain by soft strata, the skin friction contribution of the softer strata may be disregarded in design. However, do not ignore the contribution of significant amounts of competent material in order to tip in rock. In many areas of the state, rock is overlain by thick layers of material that can support considerable loads.

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Belled Shafts

Belled drilled shafts are no longer used as a foundation element for bridge foundations. Therefore, do not use belled shafts for bridge foundation design.

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Standing Water

Drilled shafts installed in lakes or rivers require use of a casing placed from above the water surface to a minimum embedment into the river or lake bottom. Do not define the top of the drilled shaft in the normal manner (a set distance below finished grade). Define the top of the drilled shaft as 1 to 2 ft. above the normal water elevation. If the water level is variable, add a provision allowing the top of the drilled shaft to be adjusted based on water level at the time of construction. Allow casing required for construction to remain in place at the option of the contractor. Typically, casings left in place look no worse than the stained concrete shaft that will be visible if casings are removed. If casing is to be left in place, disregard skin friction along the length of the casing. If permanent casing is used in standing water, consideration should be given to painting the portion of casing extending above the mud line.

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Wing Shafts

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

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Service Loads

See the following table for maximum drilled shaft service loads recommended without conducting a detailed structural analysis. Before final structural design, review the soil information to verify the ability of the foundation to develop desired loads.

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24 in.

175 tons

30 in.

275 tons

36 in.

400 tons

42 in.

525 tons

48 in.

700 tons

54 in.

900 tons

60 in.

1,100 tons

66 in.

1,350 tons

72 in.

1,600 tons

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Layout Notes

When drilled shaft capacity depends heavily on penetrating a specific hard layer, add a plan note instructing the contractor and field personnel of the penetration requirement. If no specific penetration into a hard layer is required, no plan note is necessary:

  • Hard founding layer at depth: When a hard founding layer is expected to be present more than three shaft diameters below the surface, specify a minimum penetration of one shaft diameter on the plans. Increase this minimum penetration if additional skin friction is required to fulfill the design requirements.

    Typical notes on bridge layouts:

    • "Found drilled shafts a minimum of one shaft diameter into hard rock," or
    • "Found drilled shafts at the elevations (lengths) shown or deeper (longer) to obtain a minimum one drilled shaft diameter penetration into hard rock."
  • The designer can use the control of elevation or length if elevations are not called out on the layout. Expand the words "hard rock" to distinguish the type of material anticipated. Although not a common practice, the first note allows a drilled shaft to be shortened if rock is encountered at higher than anticipated elevations, and it requires the shaft to be lengthened if rock is not encountered where expected.
  • Rock at surface: When rock is present at or near the surface, consider load-carrying capacity along with the stability of the superstructure on the foundation. For these shafts, a minimum shaft length of three shaft diameters is recommended. That is, a minimum three-diameter shaft length, not a three-diameter penetration into rock.

    A typical note on bridge layouts reads, "Found drilled shafts at the elevation (length) shown or deeper (longer) as necessary to obtain a minimum of three shaft diameter penetration into hard rock."

    The designer can use the control of elevation or length if elevations are not called out on the layout. Expand the words "hard rock" to distinguish the type of rock. This note does not allow a drilled shaft to be shortened from plan length, but it does require lengthening if rock is not encountered at the expected elevation.

Plan notes should be specific as to the type of material to be penetrated. If more than one material is likely to be encountered, it is acceptable to have multiple descriptions, such as “into dense sand, sandstone, and/or shale.” Avoid using vague terms such as “hard strata” or “founding material.” In stream or river environments, the channel flow line and estimated depth of scour should be considered in determining the final shaft length and necessary penetration.

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