751.36 Driven Piles

751.36.1 General

Accuracy Required

All capacities shall be taken to the nearest 1 (one) kip, loads shown on plans.

Maximum Specified Pile Lengths

Steel.....................	No Limit
Cast-In-Place.........	No Limit

Steel Pile

All steel in steel piling shall be A36 unless Seismic Design requires A572 (50 ksi ) steel for bending stresses or Drivability Analysis requires 50 ksi piles.

Test Pile

Length shall be pile length + 10’.

When test piles are specified to be driven-in-place they shall not be included in the number of piles indicated in the “PILE DATA” Table.

Load Test Pile

When Load Test Pile are specified, the nominal resistance value shall be determined by an actual load test.

For preboring for piles see Sec 702.

Preliminary Geotechnical Report Information

The foundation can be more economically designed with increased geotechnical information about the specific project site.

Soil information should be reviewed for rock or refusal elevations. Auger hole information and rock or refusal data are sufficient for piles founded on rock material to indicate length of piling estimated. Standard Penetration Test information is especially desirable at each bent if friction piles are utilized or the depth of rock exceeds approximately 60 feet.

Geotechnical Redundancy

A nonredundant pile group is a pile group of less than five piles. Resistance factors should be reduced by 20% for nonredundant pile groups. Greater reductions (additional 20%) should be considered when single pile supports an entire bridge pier.

751.36.2 Steel Pile

HP Size

Section	Area
HP 10 x 42	12.35 sq. in.
HP 12 x 53	15.58 sq. in.
HP 14 x 73	21.46 sq. in.

The HP 10 x 42 section should generally be used unless a heavier section produces a more economical design or required by a Drivability Analysis. The same size pile must be used for all footings on the same bent. Pile size may vary from bent to bent.

Shell Cast In Place Pipe Pile (CIP) Size

Diameter	Wall Thickness
14 inch	0.25 inch
16 inch	0.375 inch

The wall thickness noted above should be considered as minimum wall thickness to avoid damage during construction. External collapse index is diameter/wall thickness. For current construction practices, a minimum collapse index of 400 is recommended.

Minimum tip elevation must be shown on plans. Criteria for minimum tip elevation shall also be shown. The following information shall be included on the plans:

“Minimum Tip Elevation is required _______________.” Reason must be completed by designer such as:

• for lateral stability
• for required tension or uplift pile capacity
• to penetrate anticipated soft geotechnical layers
• for scour
• to minimize post-construction settlements
• for minimum embedment into natural ground

Pile Tips

Pile tip reinforcement shall be used if specified on the Design Layout. Use of pile tips should be indicated if directed by the Geotechnical report. The need for pile tips should also be reviewed if 50 ksi is required pile strength for design loadings.

751.36.3 Design Procedure

• Structural Analysis
• Geotechnical Analysis
• Drivability Analysis

Design Procedure Outline

• Determine foundation load effects from the superstructure and substructure for Service, Strength and Extreme Event Limit States.
• If applicable, determine scour depths, liquefaction information and pile design unbraced length information.
• Determine if downdrag loadings should be considered.
• Select preliminary pile size and pile layout.
• Perform Pile Soil Interaction (DRIVEN) Analysis. Estimate Pile Length and pile capacity.
• Based on pile type and material, determine Resistance Factors for Structural Strength $\,(\phi _{S})$ .
• Determine:
  • Maximum axial load effects at toe of a single pile
  • Maximum combined axial & flexural load effects of a single pile
  • Maximum shear load effect for a single pile
  • Uplift pile reactions
• Determine Nominal and Factored Structural Resistance for single pile
  • Determine Structural Axial Compression Resistance
  • Determine Structural Flexural Resistance
  • Determine Structural Combined Axial & Flexural Resistance
  • Determine Structural Shear Resistance
• Determine method for pile driving acceptance criteria
• Determine Resistance Factor for Geotechnical Strength $\,(\phi _{G})$ .
• If other than end bearing pile on rock or shale, determine Nominal Axial Geotechnical Resistance for pile.
• Determine Factored Axial Geotechnical Resistance for single pile.
• Determine Nominal pullout resistance if pile uplift reactions exist.
• Check for pile group effects.
• Check Drivability of pile using the Wave equation
• Review Pile Soil Interaction (DRIVEN) Analysis and pile lengths
• Show proper Pile Data on Plan Sheets.

Resistance Factor for Structural Strength $(\,\phi _{S})$

Pile structural resistance factor for axial resistance in compression and subject to damage due to severe driving conditions where use of a pile tip is necessary:

Metal Shells - 0.60

H-Piles - 0.50

Pile structural resistance factor for axial resistance in compression under good driving conditions where use of pile tip is not necessary:

Metal Shells - 0.70

H-Piles - 0.60

Pile structural resistance factor for combined axial and flexural resistance of undamaged piles:

Axial resistance factor for H-Piles - 0.70

Axial resistance for Metal Shells - 0.80

Flexural resistance factor for H-Piles or Metal Shells - 1.00

Resistance Factor for Geotechnical Strength $\,(\phi _{G})$

The Geotechnical Resistance factor is dependent on method of pile driving acceptance criteria during Construction.

Method	$\,\phi _{G}$
Gates Formula	0.4
Dynamic Pile Testing on 1 to 10% piles	0.65
Other methods	Refer to AASHTO

Gates formula is not considered accurate for pile loading exceeding 600 kips or 300 tons. When pile loading exceeds 600 kips, use wave equation analysis and geotechnical resistance factor of 0.4.

See Structural Project Manager or Liaison for use of Dynamic Pile Testing. Dynamic Pile Testing is recommended for projects with friction piles.

Downdrag & Losses to Geotechnical Strength (kips)

Downdrag, liquefaction and scour all reduce the available skin friction capacity of piles. Downdrag $\,(DD)$ is unique because it not only causes a loss of capacity, but also applies a downward force to the piles. This is usually attributed to embankment settlement. However, downdrag can also be caused by a non-liquefied layer overlying a liquefied layer. Review geotechnical report for downdrag and liquefaction information.

Preliminary Structural Nominal Axial Design Capacity (PNDC) of an individual pile (kips)

The PNDC were calculated with the assumption that the piles are continually braced. This includes the portion of piling that is below ground or confined by solid wall encasement. For portions of piling that are not continually braced, the PNDC must be calculated taking the unbraced length into account.

Steel Piles

$\,PNDC=0.66^{\lambda }F_{y}A_{S}$

Since we are assuming the piles are continuously braced, then $\,\lambda =0$. If designing a pile bent structure, scour exists or liquefaction exists then pile shall be checked considering the appropriate unbraced length.

$\,F_{y}$	is the yield strength of the pile
$\,A_{S}$	is the pile area of steel

Shell Cast In Place Piles (CIP Piles)

$\,PNDC=0.85f'_{c}Ac+F_{y}A_{st}$

$\,F_{y}$	is the yield strength of the pipe pile
$\,A_{st}$	is the area of the steel pipe (deducting 12.5 % ASTM tolerance and 1/16 inch corrosion where appropriate.)
$\,f'_{c}$	is the concrete compressive strength at 28 days
$\,Ac$	is the area of the concrete inside the pipe pile

Maximum Load during pile driving = $\,0.90(f_{y}A_{st})$

Steel Shell is ASTM 252 Grade 2 (35 ksi) or Grade 3 (45 ksi). ASTM 252 allows “the wall thickness at any point shall not be more than 12.5% under the specified nominal wall thickness.” AASHTO recommends deducting 1/16” of the wall thickness due to corrosion. Area of steel shell used in design equations should deduct 12.5% and 1/16” where applicable.

Steel HP Piles

Section	Area	Structural Nominal Compression Resistance		Structural Factored $\,(\phi =0.5)$ Compression resistance
		$\,F_{y}=36ksi$ (kips)	$\,F_{y}=50ksi$ (kips)	$\,F_{y}=36ksi$ (kips)	$\,F_{y}=50ksi$ (kips)
HP 10x42	12.4	446	620	220	310
HP 12x53	15.5	558	775	275	380
HP 14x73	21.4	770	1070	385	535

CIP Piles

Diameter	Wall Thickness	$\,A_{s}-12.5\%$	Maximum Driving Resistance Allowed $F_{y}=35ksi$	$\,A_{s}$ - (12.5% & 1/16")	Structural Nominal Axial Compressive Resistance $\,F_{y}=35ksi$ & $\,f'_{c}=3ksi$	Structural Factored $(\,\phi =0.6)$ Axial Compressive Resistance $\,F_{y}=35ksi$ & $\,f'_{c}=3ksi$
		in.2	kips	in2	kips	kips
14	0.25	9.47	300	6.79	600	360
16	0.375	16.15	500	13.12	925	550

Preliminary Factored Nominal Resistance (PFDC) of an Individual Pile (kips)

$\,PFDC$ = Factored Structural Nominal Resistance – Factored Nominal Downdrag Load

Pile Group Layout:

Preliminary Number of Piles Required = $\,{\frac {Total\ Factored\ Vertical\ Load}{PFDC}}$

Layout a pile group that will satisfy the preliminary number of piles required. Calculate the maximum and minimum factored load applied to the outside corner piles assuming the pile cap/footing is perfectly rigid. The general equation is as follows:

Max. Load =   $\,{\frac {P}{Total\ No.\ of\ Piles}}+{\frac {M_{x}}{I_{x}}}x+{\frac {M_{y}}{I_{y}}}y$

Min. Load =   $\,{\frac {P}{Total\ No.\ of\ Piles}}-{\frac {M_{x}}{I_{x}}}x-{\frac {M_{y}}{I_{y}}}y$

The maximum factored load per pile must be less than or equal to PFDC for the pile type and size chosen. If not, the pile size must be increased or additional piles must be added to the pile group. Reanalyze until the pile type, size and layout are satisfactory.

The minimum factored load per pile should preferably be greater than zero. If this cannot be practically satisfied, the factored pullout resistance of the pile shall be calculated.

Estimate Pile Length and Check Pile Capacity

Estimated Pile Length

Friction Piles:

The estimated pile length will be determined from Pile Soil Interaction (DRIVEN) Analysis. The factor of safety used for this analysis shall be discussed with the appropriate Structural Project Manager or Structural Liaison.

End Bearing Piles:

The estimated pile length is the distance along the pile from the cut-off elevation to the estimated tip elevation considering any penetration into rock. The estimated tip elevation shall not be shown on plans for end bearing piles.

The geotechnical material above the estimated end bearing tip elevation shall be reviewed to review the presence of glacial till or similar layers exist. If these layers are present, then a Pile Soil Interaction (DRIVEN) Analysis shall be performed to verify if pile resistance capacity is reached at a higher elevation due to pile friction capacity.

Check Pile Geotechnical Capacity (Axial Loads Only)

Check Pile Structural Capacity (Combined Axial and Bending)

Structural design checks which include lateral loading and bending shall be accomplished using the appropriate resistance factors.

Pile Nominal Axial Compressive Resistance (kips)

The required nominal axial pile compressive resistance must be calculated and shown on the final plans. The factored nominal compressive resistance will be used to verify the pile group layout and loading. The required nominal axial pile compressive resistance will be used in construction field verification methods of nominal axial compressive pile resistance.

Factored Nominal Resistance $\,(FNR)$ = Maximum Factored Load per Pile

Nominal Axial Compressive Resistance $\,(Rndr)$

$Rndr={\frac {Factored\ Nominal\ Resistance}{\phi _{G}}}$

Check Pile Drivability

Practical refusal is defined at 20 blows/inch.

Driving should be terminated immediately once 30 blows/inch is encountered.

If analysis indicated the piles do not have sufficient structural or geotechnical strength or drivability issues exist then consider

• increasing the number of piles
• using higher strength piles

Information to be included on the Plans

Bent No.
Type
Kind
Number
Approximate Length
Pile Driving Verification Method
Design Bearing or Nominal Axial Pile Compressive Resistance
Minimum Tip Penetration	(*)	(*)	(*)
Criteria For Minimum Tip Penetration
Pile Standard
Hammer Energy Required

Pile Driving Verification Method

• Modified Gates formula
• Dynamic Pile Testing
• Other

Criteria for Minimum Tip Penetration

• Scour
• Tension or uplift capacity
• Lateral stability
• Penetration anticipated soft geotechnical layers
• Minimize post construction settlement
• Minimum embedment into natural ground
• Other