AASHTO (LRFD) Specifications for Nominal Driving Resistance of Driven Steel Piles – Discussions with Dynamic

Test Results

Sujan K. Bhattacharya, Ph.D., P.E.Golder Associates Inc., Atlanta, GA

Purpose and Goal• Purpose In Atlanta area which is situated in Piedmont geology, driven steel H

piles are often founded in Partially Weathered Rock (PWR) materials. The purpose of this presentation is to show that piles can be founded in hard PWR considering that the structural strength limit state conditions control the design.

• GoalThe pile size and length can be estimated using Maximum Factored

Strength Limit State Load (MFSLSL) = Maximum Factored Structural Resistance (MFSR) in the bidding phase or early design phase when no load test data is available. The goal here is to present this point of view with dynamic load test results.

Geology of the Project Site• Project Site is in Piedmont

Geology near Brevard Fault Zone; see Figure 1 – Weathering Profile

• Note, Partially Weather Rock (PWR) forms the upper relic of relatively sound bedrock (Mayne et. al, 2000).

• PWR is locally characterized with SPT N ≥ 100 (Thompson et. al, 2012)

• Hard PWR which is believed to behave like bedrock, can be characterized by 50/5” ≤ N ≤ 50/1”

FIGURE 1: TYPICAL WEATHERING PROFILE(Sowers and Richardson, 1983)

MetamorphicIgneous

Definitions• Maximum Factored Structural Resistance (MFSR) The maximum load that can be applied using a Resistance Factor = 0.5 to steel

yield stress, e.g., for HP12x53 steel with Area = 15.5 in2and steel grade Fy = 50 ksi, MFSR= (15.5 x 50)*0.5 = 387.5 kips.

• Maximum Factored Strength Limit State Load (MFSLSL) This is the load that is computed by the project structural engineer

• Nominal Driving Resistance (NDR) The NDR is the resistance of soils that is mobilized when piles are driven into

the ground; NDR =MFSLSL/ϕdyn where, ϕdyn is the dynamic resistance factor (AASHTO 2014, Table 10.5.5.2.3-1)

Design Procedure• During Bidding Phase or Early Design Phase of a Design Build (DB) Project,

generally no data from Dynamic Measurements such as PDA and CAPWAP are available.

• In the early phase, pile size and length can be estimated based on computed preliminary MFSLSL if we know piles will be tipped in hard PWR or rock; pile sizes can be chosen so that MFSLSL ≤ MFSR. For Example, if MFSLSL = 360 kips, use HP12x53 if Boring Logs show PWR with N=50/5” at the pile bearing strata. Pile size should not be chosen based on NDR = 360kips/0.65 = 554 kips for which HP14x89 size will be required.

• This approach is based on considering hard PWR similar to rock with a very high geotechnical capacity; practical refusal is encountered by the piles – a condition when, structural strength limit state controls the design (AASHTO 2014, Article 10.7.3.2.3 and FHWA 2016, Volume I)

Design Procedure (Continued)• When structural strength limit state controls the design, driving

resistance is not mobilized; MFSR controls the design. Bearing materials such as, hard PWR should be considered equivalent to practical refusal materials.

• If NDR is used for choosing pile size and length in hard PWR, the design can be potentially too conservative

• two resistance factors ( factors of Safety): one applied to steel yield stress and the other, i.e., the dynamic resistance factor ϕdyn to NDR can likely result an overly conservative design when driving resistance does not control the design.

Discussions of Wave Theory• When Piles are driven by a pile driving hammer, a compressive wave is

generated at the top of the pile• According to Wave Theory, it can be shown that compressive waves are

reflected to the top by a refusal medium resulting insignificant or no dynamic resistance of the refusal materials at the pile tip

• This condition can be probably considered as “Free End Condition” (Salgado, 2008); see Figure 2 in the next Slide.

• If the bearing materials are not practical refusal materials such as soils and soft PWR, the pile will experience a resistance from surrounding soils and PWR. This resistance is called the driving resistance and the condition of fixity of the pile tip can be probably considered a “Fixed End Condition”.

Pile tip on rock

Pile

Tip on Rock; Force=0, velocity is doubled –Free End

Tip on Soil; Force=doubled, velocity is zero – Fixed End

Figure 2: Illustration of Fixed and Free End Conditions for Wave Propagation

Pile tip on soil

Pile

Discussions of Wave Theory (Continued)

rock

Dynamic Measurements of Capacity• AASHTO emphasizes the importance of dynamic measurements using

signal matching techniques for static pile capacity• Case Method is commonly followed for dynamic and static pile capacity

determinations using Pile Driving Analyzer (PDA) and CAse Pile Wave Analysis Program (CAPWAP).

• Force and velocity measurements are recorded by PDA apparatus in the field and a soil and hammer model is used by CAPWAP during analysis in the office; force is first assumed, velocity is used as the input and the force is calculated and compared with the assumed value; the iteration continues till the measured and calculated values match.

Dynamic Measurements of Capacity, Continued• According to Case Method (Rausche et. al, 1985), the total resistance

is first calculated and then, the dynamic resistance is calculated using a damping factor (Jc).

• The dynamic resistance is subtracted from the total resistance of the bearing materials to obtain the static pile capacity.

• The output from CAPWAP shows the total capacity including skin friction and end bearing together with stresses in the pile, maximum and minimum strokes required by the hammer and the dynamic properties, such as damping, quake and transferred energy.

PDA/CAPWAP Test Results and Discussions – Table 1• Five PDA and CAPWAP tests

have been selected to show pile capacity in hard PWR in Table 1.

Bridge,BentNo.

Pile Size & MFSR (kip)

Tip Elevation (ft)/Length (ft)

NDR (kip)

Bearing Materials-

PWR &Top of PWR EL

Auger Refusal

Elevation (ft)

1CAPWAP Results (kip)

Side Shear

End Bearing

2Total Capacity

Bridge A, Bent 10 HP14x117,

860 kip 940.45/25.30 802

PWR,N=50/1” topEl. 957.86,

936.36 228.00 822.00 1050.00

Bridge B-Bent 1 HP14x117,

860 kip 1062.02/24.10 818

PWR, N=50/5” to

50/4”, top El. 1082.86

1047.11 251.20 626.90 878.10

Bridge C-Bent 3 HP14x73,

520 kip 938.67/42.42 459PWR,

N=50/5” top El. 945.84

938.34 241.00 294.00 535.00

Bridge D-Bent 20 HP 14x117,

860 kip 919.75/17.80 852PWR,

N=50/5” Top El. 920.56

909.06 225.90 718.50 944.40

Bridge E-Bent 1 HP 12x53,

384 kip 979.26/47.98 378PWR,

N=50/3”, Top El. 984.73

957.23 180.20 400.00 582.20

1The CAPWAP Analyses were conducted using Jc Method based on End Of Initial Drive (EOID) Tests.2These capacities are not ultimate capacities; tests were terminated depending on the required minimum tip elevations and NDR=Maximum Factored Strength Limit State Loads/0.65, where, ϕdyn = 0.65 (AASHTO Table 10.5.5.2.3-1).

Discussions

• From above results, it is evident that total capacity of each pile exceeded both corresponding NDR and MFSR when piles were founded in hard PWR

• N values ranged between 50/5” to 50/1”. Therefore, It may be concluded that geotechnical axial capacities of these rock-like materials are high and comparable to rock.

• Table 2 in the next slide summarizes the penetration lengths into PWR strata, NDR, maximum stress, blow counts (blows/inch).

Discussions (Continued)

Bridge/Bent No. MFSLSL (kip) Maximum Stroke (ft)

Maximum Compressive Stress

(ksi)

Blows per inch

Penetration in PWR (ft)

1Beta Integrity Factor (BTA,

100%)

Bridge A, Bent 10 521 9.5 28.4 16 17.41 100

Bridge B-Bent 1 532 10.3 32.1 12 20.84 100

Bridge C-Bent 3 276 8.7 30.1 10 7.5 100

Bridge D-Bent 20 553.3 11.79 30.83 20 0.81 100

Bridge E-Bent 1 246 11.1 37.3 20 5.47 100

Table 2: Summary of PDA/CAPWAP Results

1100% BTA indicates no damage in any of the piles

Discussions ContinuedFrom above results it can be concluded that:• Actual geotechnical capacities (total capacities in Table 1) always exceeded the Maximum Factored Strength

Limit State Loads (MFSLSL) that were provided by the project structural engineer. These results indicated high strength of PWR materials which are probably comparable to rock with absolute refusal capacity.

• Average maximum stroke is 10.3 feet based on the values of Maximum stroke in Table 2. 10± feet stroke in PWR is common in Atlanta area and it indicates practical refusal or, imminent practical refusal (FHWA 2016, Vol II)

• Maximum compressive stress recorded is 37.3 ksi which was less than the tolerable limit, 45 ksi. • Blows per inch required to record the maximum capacity ranged from 10 to 20 blows per inch. Based on our

review of ICE I30 V2 Bearing Chart and comments, 10 blows per inch should be considered practical refusal and driving in excess of 20 blows per inch should be considered “improper use” and will void the hammer warranty. Therefore, blows per inch recorded for five tests indicate the bearing materials (PWR with 50/5” ≤ N ≤ 50/1”) can be considered “practical refusal materials”.

• Penetration of piles into the PWR layers (0.81 feet to 20.84 feet) presents varying consistencies of the PWR layers with SPT values as given in Table 3 (next slide). The thicknesses of the PWR layers that were penetrated by the piles, can be considered soft PWR.

• It should be noted that subsurface conditions at each test location were reviewed using the boring data at or near the respective bents. Blow counts from the top to the bottom of the PWR layer at each test location are presented in Table 3:

Discussions Continued

• Table 3: Blow Counts in PWR Layers at Test Locations

Bridge/Bent No1Penetration in PWR

(ft)

N values in PWR

2Top of Layer Bottom of Layer - Auger Refusal

Bridge A, Bent 10 17.41 34-50/4” 50/1”

Bridge B-Bent 1 20.84 5-23-50/3” 50/1”

Bridge C-Bent 3 7.5 12-24-50/4” 50/5”

Bridge D-Bent 20 0.81 50/5” 50/1”

Bridge E-Bent 1 5.47 35-50/5” 50/1”1Penetration values are obtained from CAPWAP results

2The blow counts are based on borings located at or near the bents and they show the strength of PWR materials varied from top to bottom

Conclusion1) The geotechnical capacity can exceed the MFSR and NDR in hard PWR without exceeding the maximum allowable compressive stress

and without damaging the pile. Therefore, pile sizes can be estimated based on estimated maximum factored structural strength limitstate data in the bidding phase or, in the early design phase, when no load test data is available.

2) According to AASHTO article 10.7.3.2.2 (Piles Driven to Soft Rock), “soft rock that can be penetrated by pile driving shall be treated inthe same manner as soil for the purpose of design for bearing resistance in accordance with article 10.7.3.8”. It seems the word“penetrated” does not clearly explain what soft rock is; maybe combining with penetration resistance (N values) would characterizethe soft rock better. If we assume PWR can belong to the class of soft rock, then, from these test results, PWR with N values less thanN = 50/5” should be considered penetrable; 50/5” ≤ N ≤ 50/1” can be considered practical refusal media (Hard PWR) and notpenetrable.

3) Estimation of pile capacity in hard PWR considering the maximum design load = NDR, is probably a conservative approach. Tworesistance factors including one applied to the steel yield stress and the other (ϕdyn) applied to NDR are questionable; if we treat theresponse of hard PWR very similar to the response of rock, then, structural strength limit state conditions of the steel section shouldcontrol the design in hard PWR.

4) Therefore, the NDR calculated using NDR=MFSLSL/ϕdyn is probably not necessary when piles are tipped in hard PWR that can beconsidered practical refusal materials. From the perception of wave mechanics, the hard PWR appears to present a free endcondition; velocity of stress wave doubles and force diminishes to zero – a condition that is applicable for bedrock.

5) Modeling of hard PWR in static pile capacity calculation should follow the same approach as rock, i.e., use unit weight, and cohesion(c) instead of unit weight and friction angle (ϕ) that are commonly used in silty sand, sand and sandy silt layers in APILE – FHWAMethod.

6) More research needs to be conducted for modeling the capacity of PWR with N values ranging from N=100 to N=50/1” using moreload test and laboratory test results.

Reference• 1.0: American Association of State Highway and Transportation Official (AASHTO) LRFD Bridge Design Specification,

2014.• 2.0: Mayne, Paul W., Brown, Dan, Vinson, James, Schneider, James A. and Finke, Kimberly A., Site Characterization of

Piedmont Residual Soils at NGES, Opelika, Alabama, ASCE Geotechnical Special Publication (GSP) No. 93 – National Geotechnical Experimentation Sites (NGES) Ed. By Jean Benoit and Alan Lutenegger, ASCE, Reston, VA, April 2000, pp 160-185

• 3.0: Thompson, Robert W., Brown, Dan A. and Hudson, Aaron B., Load Testing of Drilled Shaft Foundation in Piedmont Rock, Lawrenceville, GA, ADSC SE Chapter Research Project, Lawrenceville Site Rock Load Test Report, January 19, 2012.

• 4.0: Sowers, G. F. and Richardson, T. L. (1985). Residual Soils of the Piedmont and Blue Ridge, Transportation Research Board, 919, National Academy Press, Washington D. C., 10-16.

• 5.0: Federal Highway Administration (FHWA) Publication No. FHWA-NHI-16-009, FHWA GEC 012, Design and Construction of Driven Pile Foundation, Volume I, July 2016.

• 6.0: Salgado Rodrigo, The Engineering of Foundations, McGraw-Hill, 2008. • 7.0: Rausche, Frank, Goble, George G., Likins Jr., Garland, E. Dynamic Determination of Pile Capacity, Journal of

Geotechnical Engineering, Volume III, No. 3, March,1985. • 8.0: Federal Highway Administration (FHWA) NHI Course No. 132021 and 132032– Design and Construction of Driven

Pile Foundations-Volume II, Publication No. FHWA-NHI-16-009, FHWA-GEC-012-Volume II, July 2016.

QUESTIONS?