As specified in AASHTO LRFD Bridge Design Specifications, designers are directed to α-method in clays and cohesive soil layers and β-method within sands and non-cohesive material. For the later, note that engineering judgement is necessary when determining drained friction angle for an individual layer. AASHTO LRFD Bridge Design Specifications Table 10.4.6.2.4-1 presents friction angle φf ranges according to measured (N1)60 values. When selecting an effective soil friction angle according to AASHTO LRFD Bridge Design Specifications Equation 10.8.3.5.2b-3, use an additional reduction factor of 0.9 to account for the lower end of the range of friction angles in granular material with significant fraction of fines, such that:

φ’f = 0.9 * (27.5 + 9.2 log [(N

1

)

60

])

Rock-socketing into competent foundation layers is a common practice throughout Texas. Throughout the state competent foundation layer will vary from very hard, intact, non-weathered bedrock; to very soft, friable with poor jointing conditions, and/or extremely fractured “bedrock-like” conditions; to Intermediate Geomaterials (IGMs, as defined in AASHTO Article 10.8.2.2.3) displaying characteristics of both rock and soil. Foundation designer is responsible for determining if (within socket) only side or end resistance can be considered in their determination of total resistance; or in cases of softer competent foundation, they can incorporate part or all of both. When encountering fractured strong rock, or softer cohesive IGMs such as shale and/or severely weathered limestone; note that alternative methods are specified by AASHTO and GEC-10 (2010 and 2018). In stratified or visibly jointed rock bearing layers, it’s difficult to determine how much of actual load will be transferred to base of the drilled shaft and in lieu of load testing at locations, practical design should assume that the axial load will be resisted entirely by side resistance.

When relying on a rock layer for capacity, minimum rock socket to use on any project is 1 diameter length into such rock. Should rock be located near the ground surface, shafts should be drilled at minimum 10 feet or 3 diameters in length (whichever greater).

Use Resistance Factors for Drilled Shafts per AASHTO LRFD Bridge Design Specifications Table 10.5.5.2.4-1 for Strength and Extreme limit states, unless otherwise specified in other approved methods outlined in FHWA-NHI-18-024, GEC-10 Appendix B.

Do not use belled shafts for bridge foundations.

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. Define the top of the drilled shaft in water as 2 ft. typical above the normal water elevation. If the water level is variable, add a provision allowing the top of the drilled shaft to be adjusted vertically based on water level at the time of construction. If casing is to be left in place, disregard side resistance 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.

Design micropiles in accordance with AASHTO LRFD Bridge Design Specifications Article 10.5 and 10.9. Additional background information on micropile design may be found in the FHWA Micropile Design and Construction Guidelines Implementation Manual, Publication No. FHWA-SA-97-070 (Armour. et al., 2000) or FHWA-NHI-05-039.

Found wing shafts in similar founding material as abutment cap shafts to minimize the potential for differential settlement. Maximum length of wing wall drilled shafts is limited to 30 x D.

Design foundations to resist factored maximum strength loading case and extreme event loading case. Coordination with the structural engineer is required to ensure clarity on derivation and location of loading to be designed for.

See the following Table 5-1 for maximum drilled shaft service loads recommended without conducting a detailed structural analysis. Foundation design outlined in this chapter must be followed to ensure the proper sizes are selected and embedment criteria is specified in the contract plans.

Drilled shaft reinforcement is to be designed for axial, lateral, and uplift load (included within nonstandard design checks). The reinforcement will follow the Common Foundation Details (FD) Standard, unless site specific designs are required which require alternate reinforcement. The longitudinal reinforcement for the drilled shaft will extend the full length of the shaft.

Where shaft foundations are placed adjacent to existing structures, the influence of the existing structure(s) on the behavior of the foundation, and the effect of the foundation on the existing structures, including vibration effects due to casing installation, should be investigated. In addition, the impact of caving soils during shaft excavation on the stability of foundations supporting adjacent structures should be evaluated. At locations where existing structure foundations are adjacent to the proposed shaft foundation, or where a shaft excavation cave-in could adversely affect an existing foundation, the design should require that casing be advanced as the shaft excavation proceeds.

Various testing methods are available to determine the integrity of drilled shafts, which are Crosshole Sonic Logging (CSL), Gamma-Gamma testing, and Thermal Integrity Profiling (TIP). TIP is the preferred testing method, as it is done during the curing of the concrete and does not delay construction. Other methods are approved based on the priorities of the project. Bridge Division has developed a Special Specification for TIP testing titled “Thermal Integrity Profiler (TIP) Testing of Drilled Shafts.”

TIP or other integrity testing should be considered for use under one or more of the following conditions:

Number and frequency of tests is at discretion of foundation engineer and dependent on site specific conditions and redundancy designed into the foundation system.

Consult with the TxDOT Bridge Division Geotechnical Branch to determine if a specific project might be considered a candidate for TIP or other integrity testing.

Label foundations on plan set bridge layouts with the following:

Typical notes on bridge layouts:

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. The final length of the drilled shafts should be based on both axial and lateral loading (if required). If the potential scour extends down to the top of rock, then the minimum embedment of the drilled shaft should be three shaft diameters or deeper to obtain the required axial and lateral capacity.

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 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.

Include the following information on geotechnical design reports for drilled shaft foundation design: