Design Guides 3.10.1 - LRFD Pile Design

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					Design Guides                                                   3.10.1 - LRFD Pile Design

3.10.1 LRFD Geotechnical Pile Design Procedure

Reference the AASHTO LRFD Bridge Design Specifications third addition with ’05 and 06’

   1. The Geotechnical Engineer evaluates the subsurface soil/rock profile and develops a
       pile design table provided to the structural engineer in the Structure Geotechnical Report
       (SGR). The pile design table will contain a series of Nominal Required Bearing (RN)
       values, the corresponding Factored Resistances Available (RF) for design, and the
       Estimated Pile Lengths. When multiple pile types are being considered, a pile design
       table will be provided for each pile type to allow the structural engineer to select the most
       economical and feasible pile type, size and layout.

            a. The Nominal Required Bearing (RN) represents the sum of the nominal tip
               resistance (qPAP) and nominal side resistance (qSASA) the pile will experience
               during driving (RN = qPAP + qSASA)

                   i. Nominal Tip Resistance is determined by multiplying the nominal unit end
                      bearing resistance (qP) of the soil or rock layer below the pile by the pile
                      end bearing area (AP).

                      The nominal unit end bearing resistance should be determined as follows:

                          For granular soils, the qP may calculated as:

                                 0.8N160 D b
                          qp =               ≤ ql

                                              ⎡          ⎛ 40 ⎞⎤      q l = 8N160    (sands)
                          where:                         ⎜ σ ' ⎟⎥ N & q = 6N1
                                       N160 = ⎢0.77log10 ⎜     ⎟
                                              ⎣          ⎝ v ⎠⎥  ⎦      l      60    (nonplastic silt)

                                      D=      pile diameter or width (ft.)
                                      Db=     depth of penetration into soil (ft.)
                                      N=      field measured SPT blow count (blows/ft.)
                                      N160 = SPT blow count corrected for overburden pressure
                                              and 60% hammer efficiency (blows/ft.)

Nov. 2006                                                                             Page 3.10.1-1
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                                     σv’ =   effective vertical soil pressure (ksf)

                       For cohesive soils, the qP should be computed as:

                       qP = 9 QU

                       where:        QU =    average unconfined compression strength of the
                                             soil (tsf)

                       Note that QU is input in tsf and qP is output in ksf.

                       For rock, the qP should be computed as:

                       For shale and sandstone qP = 27 ksi = 3888 ksf
                       For limestone and dolomite qP = 36 ksi = 5184 ksf

                       These values are to be used for driven H-piles only, not for design of
                       piles set in rock.

                   The pile end bearing area should be determined as follows:

                       Steel piles           AP = the cross-sectional area of steel member
                       Metal Shell piles AP = the horizontal end plate area
                       Precast piles         AP = width squared end area
                       Timber piles          AP = Assume a 7 in. diameter tip to calculate end

                       Do not make any adjustment to the above end areas when pile shoes
                       are used.

                ii. Nominal Side Resistance is determined by multiplying the nominal unit
                   side resistance (qS) of the soil layer by the effective surface area of the
                   pile (ASA)

                   The nominal unit side resistance should be determined as follows:

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                For granular soils, the qS should be computed as:

                For Hard Till,

                qs = 0.07N160                                      (N160 < 30 )
                q s = 0.00136N12 − 0.00888N160 + 1.13
                               60                                  (30 ≤ N160 )

                Very Fine Silty Sand,

                q s = 0.1N160                                      (N160 < 30 )
                                ⎡ (N160 −175.05 )2 ⎤
                                ⎢                  ⎥
                q s = 42.58 e   ⎣                  ⎦
                                                                   (30 ≤ N160 < 74)
                q s = 0.297N160 − 10.2                             (74 ≤ N160 )

                Fine Sand,

                q s = 0.11N160                                     (N160 < 30 )
                q s = 0.3256N160 +              − 12.51            (30 ≤ N160 < 66)
                q s = 0.329N160 − 9.91                             (66 ≤ N160 )

                Medium Sand,

                q s = 0.117N160                                    (N160 < 26)
                q s = 0.00404N12 − 0.0697N160 + 2.13
                               60                                  (26 ≤ N160 < 55)
                q s = 0.356N160 − 9.1                              (55 ≤ N160 )

                Clean Medium to Coarse Sand,

                q s = 0.128N160                                    (N160 < 24)
                q s = 0 .00468 N12 − 0 .0693 N160 + 2.05
                                 60                                (24 ≤ N160 < 50)
                q s = 0.394N160 − 9.42                             (50 ≤ N160 )

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                       Sandy Gravel,

                       q s = 0.15N160                                      (N160 < 20)
                       q s = 0 .00861 N12 − 0 .217 N160 + 3 .91
                                        60                                 (20 ≤ N160 < 40)
                       q s = 0.6N160 − 15.0                                ( 40 ≤ N160 )

                       For cohesive soils, the qS should be computed as:

                               −1    3          2
                       qs =        Q u − 0.177Q u + 1.09Q u                (Q u ≤ 1.5 tsf)

                                     3          2
                       q s = 0.0495Q u − 0.347Q u + 1.278Q u − 0.068       (1.5 tsf < Q u < 2 tsf)

                       q s = 0.470Q u + 0.555                              (2 tsf ≤ Q u < 4.5 tsf) *
                       qs = 2.67 ksf                                       (4.5 tsf ≤ Q u ) *

                       *If QU > 3 tsf and N > 30, treat as granular and use Hard Till

                       Note that QU is input in tsf and qS is output in ksf.

                   The effective surface area of the pile (ASA) should be determined as the

                       Steel piles         ASA = 2 x (flange width + web depth) x length (for
                                                cohesive layers) or 1 x (flange width + web
                                                depth) x length (for granular layers)
                       Metal Shell piles ASA = the shell circumference x length
                       Precast piles       ASA = 4 x (width) x length
                       Timber piles        ASA = the circumference (assume 12 in. dia.) x

                iii. The Maximum Nominal Required Bearing (RN MAX.) that can be specified
                   for the standard pile types are limited by empirical relationships
                   developed to provide reasonable confidence that the dynamic pile

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                stresses caused by properly functioning pile hammers will not cause pile
                damage when installed in appropriate subsurface conditions.            These
                empirical relationships are as follows:

                   1. Metal Shell Piles:      RN MAX. = 0.85xFYAS

                       where: 0.85 = IDOT factor for Metal Shell piles relating acceptable
                                       dynamic stress to static nominal steel capacity
                               FY =    yield strength of the steel shell (45 ksi)
                               AS =    the steel shell area (in.2)

                   2. Steel Piles:             RN MAX. = 0.54xFYAS

                       where: 0.54 = IDOT factor for Steel H-piles relating acceptable
                                       dynamic stress to static nominal steel capacity
                               FY =    yield strength of the steel (50 ksi)
                               AS =    the steel cross-sectional area (in.2)

                   3. Precast Piles:           RN MAX. = 0.3xf’cxAg

                       where: 0.30 = IDOT factor for Precast piles relating acceptable
                                       dynamic stress to static nominal concrete capacity
                               f’c =   compressive strength of concrete (4.5 ksi for
                                       precast or 5 ksi for prestressed)
                               Ag =    area of 14 in. x14 in. square pile (196 in.2)

                   4. Timber Piles:            RN MAX. = 0.5xFcoxAP

                       where: 0.50 = IDOT factor for Timber piles relating acceptable
                                       dynamic stress to static nominal timber capacity
                               Fco =   base resistance of wood in compression parallel to
                                       grain (2.7 ksi)
                               AP =    average cross-sectional area near top of timber pile
                                       (assuming 12 in. dia =113 in.2 )

Nov. 2006                                                                      Page 3.10.1-5
Design Guides                                                   3.10.1 - LRFD Pile Design
                       Using the above relationships and material strengths results in the
                       following maximum nominal required bearings that can be safely specified
                       in the pile design table and on the Contract plans:

                                                                           Maximum Nominal
                                     Pile Designation
                                                                        Required Bearing (RN MAX.)
                             Metal Shell 12”φ w/0.179” walls                 256      kips
                             Metal Shell 12”φ w/0.25” walls                  355      kips
                             Metal Shell 14”φ w/0.25” walls                  416      kips
                             Metal Shell 14”φ w/0.312” walls                 516      kips

                                      Steel HP 8x36                           286      kips
                                     Steel HP 10x42                           335      kips
                                     Steel HP 10x57                           454      kips
                                     Steel HP 12x53                           419      kips
                                     Steel HP 12x63                           497      kips
                                     Steel HP 12x74                           589      kips
                                     Steel HP 12x84                           664      kips
                                     Steel HP 14x73                           578      kips
                                     Steel HP 14x89                           705      kips
                                     Steel HP 14x102                          810      kips
                                     Steel HP 14x117                          929      kips

                                    Precast 14”x14”                           265      kips
                               Precast Prestressed 14”x14”                    294      kips

                                        Timber Pile                           153      kips

                       When the Nominal Required Bearing (RN) is specified to exceed 600 kips,
                       the use of the Standard Specifications Gates formula cannot provide
                       sufficiently accurate predictions of Nominal Driven Bearing as well as
                       assurance against pile damage during driving (See LRFD AASHTO Art.
              and In these cases, i.e. RN > 600 kips, General
                       Note #24 (See Section 3.1.3 of the Bridge Manual) shall be included on
                       the Contract plans. This note requires the contractor to conduct a wave
                       equation analysis to establish the pile driving criteria.

           b. The Factored Resistance Available (RF) represents the net long term axial
                factored pile capacity available at the top of the pile to support factored structure

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            loadings. It accounts for any reductions in geotechnical resistance that occurs
            after driving such as scour, downdrag (DD), or liquefaction (Liq.) and reflects the
            construction control resistance factor used to verify the nominal required bearing

                i. The Factored Resistance Available (RF) shall be calculated using the
                    following equation:

                       RF= RN(φG) - (DD+Scour+Liq.)x(φG) x(λG) – DDx(γp)

                       where:      φG =   the geotechnical resistance factor related to pile
                                          installation control
                                   λG =   the Bias factor between Gates Resistance and the
                                          IDOT Resistance Equations (1.0)
                                   γp =   the load factor for DD applied to the pile by soil

                    Appling the resistance factor (φG) to the geotechnical losses may appear
                    unconservative. However, AASHTO LRFD Article requires the
                    factored loads (RF + γpDD) be ≤ the factored resistance below the DD
                    layers. Thus, the factored resistance below the DD layers must be equal
                    to the RF + γpDD. The pile must be driven to a Nominal Required Bearing
                    RN equal to the nominal DD resistance to install the pile to below the DD
                    layer plus (RF + γp DD)/φG. Solving for the RF, the geotechnical losses
                    and RN are multiplied by φG.

                ii. The Nominal values of the downdrag (DD), Scour and Liquefaction (Liq.)
                    shall be calculated using side resistance equations provided above.

                       1. The value of DD is used twice (as shown in the equation for RF
                           above). Once to represent the decrease in side resistance and
                           again to account for the added loading applied to the pile. The
                           nominal value DD is only the resistance lost or the load applied
                           (since they are of equal magnitude), not the sum of both.

                       2. The nominal value of Scour represents the decrease in resistance
                           in the layers above the design scour elevation. The Scour term

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                           shall be taken as zero when calculating the Factored Resistance
                           Available (RF) to resist Extreme Event I seismic loadings.

                       3. The nominal value of Liq. represents the decrease in resistance in
                           the layers expected to liquefy due to the design seismic event.
                           However, non-liquefied layers above layers expected to liquefy will
                           settle and result in DD in those layers. Thus, the value of Liq.
                           shall be added to the value of DD (to represent the total decrease
                           in resistance) in the RF equation. The value of DD is used again
                           when it is multiplied by the DD load factor (to represent the
                           increase in load) in the RF equation. The term Liq. shall be taken
                           as zero unless calculating the Factored Resistance Available (RF)
                           to resist Extreme Event I seismic loadings.

                iii. Geotechnical Resistance Factor (φG) and Bias factor (λG)

                   The geotechnical resistance factor (φG) relates to the method selected to
                   establish the final Nominal Driven Bearing resistance of the pile. The
                   engineering news formula (ENR) was used in the past for this purpose
                   but now the more accurate Gates formula will be relied on to indicate the
                   nominal resistance. The 0.4 resistance factor recommended for Gates in
                   AASHTO results in either an increase in the number of piles or much
                   larger hammers sizes. The 0.4 represents a factor of safety of 2.4 which
                   when combined with the average increase in load (due to larger HL-93
                   and load factors) which approaches 72% and results in an overall factor
                   of safety just over 4.0. Although the formal factor of safety on the ENR is
                   6.0, research has shown the IDOT resistance equations and the ENR to
                   only provide on average overall factor of safety of about 2.4. To make
                   use of the Gates formula and maintain only modest increases in
                   foundations (mainly due to the new live load model) a geotechnical
                   resistance factor (φG) of 0.50 will be used. This value also corresponds to
                   the ratio between the increase in load demand 1.72 and the ASD factor of
                   safety 3.5 recommended for Gates (1.72/3.5=0.49 say 0.50).

                   When scour, downdrag or Liq. reduce the geotechnical Factored
                   Resistance Available, the Bias factor shall be used to “adjust” the

Page 3.10.1-8                                                                      Nov. 2006
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                        geotechnical resistance factor to correctly capture the reliability of the
                        side resistance equation predictions.        It is also important to properly
                        account for bias when using the nominal side resistance and end bearing
                        equations to estimate the length where the Gates formula will indicate
                        Nominal Required Bearing.       Since the factor of safety used with the
                        resistance equations was 3.0 and the Gates factor of safety (using .50
                        should be between 3.0 and 3.5) a bias factor of 1.0 is recommended until
                        further research can be conducted.

                        If it becomes clear during the planning process that earthquake forces
                        may govern the foundation design, the SGR pile tables should report both
                        the Factored Resistances Available for Extreme Event I loadings by
                        setting the geotechnical resistance factor (φG) to 1.0, and for Strength
                        Limit State loadings by setting φG to 0.5.

            c. The Estimated Pile Lengths shall be calculated and provided in the pile design

                   i. Initially, the geotechnical engineer will evaluate the Nominal Required
                        Bearing (RN) values that would be expected as the pile is being driven
                        through each soil layer.       The Nominal Required Bearing typically
                        increases linearly as it passes though a soil layer while it may suddenly
                        increase or decrease as it enters a new layer (due to changes in end
                        bearing). Thus, the initial pile lengths investigated are commonly located
                        just above and just below each major soil layer to get an idea of how the
                        pile will drive and what Nominal Required Bearings and/or pile types are
                        feasible.   To calculate these lengths correctly, the bottom of footing,
                        bottom of pile encasement, and pile cutoff elevations should be known
                        with reasonable accuracy.      This is because the estimated pile length
                        includes portions of the pile which will be incorporated in the substructure.

                   ii. Then, layers expected to settle, scour or liquefy are identified and the DD,
                        Scour or Liq. is calculated such that the RN can be reduced to determine
                        the Factored Resistance Available (RF) at each preliminary penetration
                        investigated above.

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                    iii. The geotechnical engineer should discuss the preliminary estimated pile
                        length vs. Factored Resistance Available (RF) relationship with the
                        structural engineer and obtain any preliminary loading information to help
                        narrow the range of pile types and loadings to be included in the SGR pile
                        design table(s).        This usually involves obtaining total factored
                        substructure loadings and dividing by the maximum and minimum pile
                        spacings to estimate the possible Factored Loadings Applied (QF)
                        demand per pile, from which an expanded range of RF values can be

                    iv. Using the preliminary pile length vs. Factored Resistances Available
                        calculated above that fall near the range of preliminary RF values
                        calculated above, the final pile design table of estimated pile lengths vs.
                        both RN and RF, can be developed and provided in the SGR for use by the
                        structural engineer.

   2. The Structural Engineer calculates the controlling loadings applied to the foundation,
       evaluates the cost and structural feasibility of using various pile types and sizes
       recommended in the SGR while evaluating various pile layouts/spacings to select the
       most cost effective and feasible foundation design.

           a. Find Controlling Factored Loading Applied to the foundation:

                 Calculate the factored loadings as described in Section 3 of LRFD for all
                 applicable loading cases including extreme events. The factored service limit
                 states need not be evaluated for most typical highway structures as the strength
                 limit state pile design, following this procedure, will result in less than ⅛ in. axial
                 deflection at the top of the pile. In some cases, it may be clear which group
                 loading will control the pile design while in others several load groups will need to
                 be checked.      Each factored loading group under consideration should be
                 resolved into a factored shear, a factored moment, and a factored vertical loading
                 applied to the center of the foundation along both the longitudinal and transverse
                 axes of the foundation.

                 In some cases, the SGR pile table will not include the Factored Resistances
                 Available to resist Extreme Event I seismic considerations unless it becomes

Page 3.10.1-10                                                                             Nov. 2006
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               clear during the planning process that earthquake forces may govern the
               foundation design. A method to convert the factored resistances available in a
               pile design table from non-seismic to seismic is provided in Section 3.15 of the
               Bridge Manual.

               It should also be noted that the factored loadings (QF) are at the top of piling and
               shall not include any factored downdrag (DD) which may be present. The LRFD
               load groups specify that the portion of DD which applies a loading to the pile be
               included with loadings from other applicable sources.       However, it is IDOT’s
               standard practice to require that the DD loading and DD reduction in resistance
               for a pile (as well as other reductions in resistance such as scour and
               liquefaction) be taken into account by the geotechnical engineer and
               incorporated in the SGR pile design table. Consequently, it shall not be added to
               the LRFD group loadings as suggested.

            b. Pile Group Layout and Factored Loading Applied (QF) per pile:

               Evaluate various pile layout arrangements, using the preferred number of pile
               rows and pile spacing.    Using each of the group limit states that may control,
               identify the maximum and minimum factored loading applied (QF) per pile in the

               The minimum Factored Loading Applied (QF) should normally be greater than
               zero. In cases where this cannot be accomplished using an economical pile
               layout, such as for seismic design, the Factored Resistance Available (RF) in
               pullout should be calculated using a geotechnical uplift resistance factor of 0.20
               for non-seismic loadings and 0.8 for seismic, and the nominal side resistance
               equations. This calculation will provide the minimum tip elevation to be included
               on the plans, to ensure pullout resistance. The pile anchorage into the footing or
               substructure should also be modified and designed to carry the factored pull out

               The maximum Factored Loading Applied (QF) shall be less than or equal to the
               Factored Resistance Available (RF) provided in the SGR pile design table for the
               pile type and size under consideration.

Nov. 2006                                                                          Page 3.10.1-11
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                 The pile size or type should be adjusted as necessary to ensure QF < RF or the
                 pile layout (rows, spacing, number) may be modified and reanalyzed to
                 determine the most economical pile type, size and layout to be used.

           c. Check Structural Resistance of pile:

                 Completion of step 2b ensures that the piles will have the geotechnical factored
                 resistance available (RF) to support the factored loadings applied (QF). Showing
                 a nominal required bearing (RN) which is below the maximum RN            MAX   values
                 above will ensure that the pile has the structural resistance to withstand the
                 driving stresses caused by a correctly functioning hammer. However, the static
                 structural resistance of the pile shall also be calculated to verify it can carry the
                 combined axial and lateral loadings, considering any unbraced length either
                 above finished ground surface or above the design scour or liquefaction depth.
                 The structural resistance factors and equations to check this are provided in the
                 LRFD Specification. Only a portion of the necessary information is provided
                 below for reference:

                     i. The structural resistance factors (φS) to be used with the nominal
                        structural resistance equations are provided below.

                        Metal Shells    φS =   0.8 in compression and 1.0 in flexure [Art.,
                        H-piles         φS =   0.7 in compression and 1.0 in flexure [Art.,
                        Precast         φS =   0.75 to 1.0 in both compression and flexure [Art.
                                     , LRFD]
                        Timber          φS =   0.9 in compression and 0.85 in flexure [Art.,
                        See also the LRFD Specifications for applicable resistance factors for
                        Extreme Event loadings such as earthquake.

                     ii. The Nominal Structural Resistance (RNS) of piles below ground or
                        confined by concrete encasement is typically calculated assuming them
                        to be continually braced. For portions of piling that are not continually
                        braced (extending above ground, subject to scour, Liq., etc.), the Nominal

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                Structural Resistance should be calculated taking the unbraced length
                into account.      The following equations are provided in LRFD to
                accomplish this check.

                Metal Shell and Steel Piles

                   RNS=0.66λFYAS                              (Eq., LRFD)

                Precast Piles

                   RNS=0.85(0.85f’c(Ag-As)+fyAs)              (Eq., LRFD)

                Prestressed Piles

                   RNS=(0.85f’cAg)-(AsE(εpe-(εcu-εce)))       (Com. C5.7.4.4, LRFD)

                Timber Piles

                   RNS=CcoAgCp                                (Eq. 8.8.2-1, LRFD)

                Normally the Factored Geotechnical Resistance (RF) will control over the
                factored structural resistance (RNS x φS).       If the factored structural
                resistance is less than the factored loadings applied (QF), either the pile
                size or the number of piles shall be increased such that RNS x φS > QF.

                   1. If the size of the pile is increased, both the nominal required
                       bearing (RN) and the factored resistance available (RF) shown on
                       the plans should not change, since the QF demand would not

                   2. If the number of piles is increased, both the nominal required
                       bearing (RN) and the factored resistance available (RF) shown on
                       the plans should be decreased, since the QF demand would

Nov. 2006                                                                   Page 3.10.1-13
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                         Both approaches, either an increase in pile size or an increase in the
                         number of piles, results in RNS x φS ≈ QF , RF ≈ QF and RN          MAX   > RN.
                         However, if the end bearing resistance of the piles is much larger than the
                         side resistance, it is recommended that the piles be driven to their
                         maximum nominal required bearing (RN          MAX)   since the added installed
                         geotechnical resistance can normally be obtained with minimal additional
                         penetration/cost and may also be needed for future bridge rehabilitations.
                         The modified load-resistance relationships would then be RNS x φS ≈ QF ,
                         RF > QF and RN MAX = RN.

           d. The Pile Data to be included in the Contract plans shall include: i. the pile type
                 and size, ii. the nominal required bearing, iii. the factored resistance available, iv.
                 the estimated pile length, v. the number of production piles, and vi. the number of
                 test piles. In some cases, other information shall be provided and is discussed.

                    i.   The Pile Type and Size shall be provided so the contractor can bid and
                         furnish the piles required at each foundation location. Examples of typical
                         pile type and size callouts are as follows:

                                    Metal Shell –___ in. dia. x ____ in. walls with pile shoes
                                    Steel – HP ___x ___ with pile shoes
                                    Precast Concrete – 14 in. square prestressed
                                    Timber – 12 in. dia. treated

                         Note the items in bold are examples of parameters which may or may not
                         be specified for a project.

                    ii. The Nominal Required Bearing is provided in kips to instruct the
                         contractor as to the driven bearing the production piles shall be installed
                         to as well as assist the contractor in selecting a proper hammer size.

                    iii. The Factored Resistance Available shall be provided in kips.               This
                         value is not used by the contractor but documents the net long term axial
                         factored pile capacity available at the top of the pile for the current and
                         future design/rehabilitation work.       It documents any reductions in
                         geotechnical resistance that will occur after driving such as scour,

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                downdrag, or liquefaction.   It also reflects the resistance factor which
                documents the accuracy in the method of construction control used at the
                time of installation.

            iv. The Estimated Pile Length is provided to give contractors a bid quantity,
                helps determine the length of the test pile, and when no test pile is
                specified, this length becomes the length furnished by the contractor. It
                also is used as a reference by the inspectors to identify when the pile
                problems, such as lack of set up or improper hammer performance, are
                causing piles to stop short or run long. In some cases, a minimum tip
                elevation will be specified in addition to the estimated pile length.
                Normally, the minimum tip elevation will only be necessary when the piles
                have the potential to stop shorter than estimated resulting in inadequate
                lateral load strength or penetration below any geotechnical losses such
                as scour.

            v. The Number of Production Piles is the total number of production piles
                required at the substructure or foundation covered by the pile data. When
                test piles are specified, the number of production piles shall be decreased
                by the number of test piles since they will be driven in production

            vi. The Number of Test Piles shall always be stated. When no test piles
                are required, the designer shall specify zero test piles to document that
                the estimated pile length was made with sufficient confidence or added

Nov. 2006                                                                   Page 3.10.1-15