Capacity Methods Capacity Methods

11.2 Capacity Methods

The biggest challenge for a 8G user is to be able to accur ately predict pile capacity. If the user makes a mistake, it could potentially turn into a very costly one. However it is also the most rewarding when the 8G can reduce expensive static testing or determine that length and cost of a pile foundation can be reduced. Unfortunately, it is impossible to give guidelines that apply to all situations. In general, the following capacity methods immediately for every blow in real time. These solutions require that the pile be linear elastic and that the cross section be uniform along the pile length. For non-uniform piles, the CAPWAP program can accurately model non-u niform piles, and should be used for ca paci ty det ermina tion (Sec tio n 11. 2.3 on pag e 139 ). Even for uniform piles,

state-of-practice would require confirming any Case Metho d result with CAPWAP .

135 135

11.2.1.1 RSP Method

Figure 11.2:

Figure 11.2: RSP calculation is made from the initial time markers noted on the graphRSP calculation is made from the initial time markers noted on the graph The RSP method uses empirical damping factors JC determined from soil type (implies the soil is properly identified by grain size, and that the soil at the boring is similar across the site). The empirical study included data primarily from restrike (or end of driving in sands) with moderate blow counts. Unfortunately, the RSP method is sensitive at low blow counts; small JC changes can make large capacity changes. For large soil quakes, the full toe resistance may not be fully active at 2L/c unless a time delay is used. For concrete piles with 2 peaks (from non-uniform pile cushion compression), selecting the second peak (use a delay DL) usually gives a better solution. View RT-RS curves and adjust JC until a “flat” curve is obtained; however as shaft friction increases, this technique becomes less reliable. Details on how the PDA-S program calculates the RSP method are detailed in Appendix A Sec tio n A.4. 5. The RSP method is generallyThe RSP method is generally historical and now rarely used directly.

historical and now rarely used directly.

11.2.1.2 Shaft Resistance Estimation

The PDA-S program is able to calculate a very rough estimate of the shaft resistance in a pile using the Case method. For shaft friction, the SFT computation makes no allowance for damp ing. The SFR quan tity has a crud e corr ection ba sed on the curr ent JC selection.

This method may also be used with quantities SF0 through SF9 where the last number reflects the Case damping selection (note that SF0 and SFT are essentially the same quantity).

Add itio nally the PDA-S c an calculat e whe re th e s haft resistan ce is being d ev elo ped a long the length of the pile. This may be presented numerically with the SFL1 through SFL9 quantities where the last number indicates the shaft resistance developed over a percentage of the length of the pile in ten percent increments ( i.e.SFL5 will calculate the

136 136

shaft resistance in the upper 15m of a 30m pile). This computation uses the damping constant JC

Figure 11.3:

Figure 11.3: Shaft Resistance distribution versus depthShaft Resistance distribution versus depth These calculations are perhaps better suited for graphical presentation and can be done so using the ‘Rs Distribution’ window. This may be accessed by clicking the vertical

graph button and selecting the ‘Rs’ tab. This graph, Fig ure 11. 3, will plot shaft resistance versus depth with a calculation of total resistance, shaft resistance (and percentage of total) and end bearing (and percentage of total).

Please keep in mind that as with all Case Method Capacity Estimations, they are only intended for uniform driven piles. CAPWAP analysis should always be performed to confirm estimations and ultimately will yield much more reliable results when properly performed. For details on how the PDA-S program calculates Static shaft resistance please see Appendix A Sec tion A.4. 9.

11.2.1.3 End bearing Capacity Estimation

Similar to the SFR quantity the PDA-S program is able to calculate a rough estimation of the end bearing with the EBC quantity. The EBC quantity has a crude correction based on the current JC selection. This method may also be used with quantities EB0 through EB9 where the last number dictates the Case damping selection. Details on how the PDA-S program calculates static end bearing are noted in Appendix A Sec tio n A.4 .9 .

137 137

11.2.1.4 RMX Method

Figure 11.4:

Figure 11.4: The RMX method accounts for soil elasticity and pile toe displacementThe RMX method accounts for soil elasticity and pile toe displacement The RMX method searches for the maximum resistance during the entire blow and thus overcomes some of the limitations of the RSP methods for small or large blow count, or high quake situations. Many find a JC of 0.7 for RMX (or RX7) gives a goo d first estimate.

Alt ho ug h temp tation ex ists, do not use damp in g fact ors les s than 0.4 with this RM X method without substantial proof from CAPWAP or a static test that a lower damping factor corr ela tes well. Fo r fr iction piles in clay (where high damp in g fact ors ar e norm ally

appropriate), the full resistance should be active during the first 2L/c cycle anyway (RSP

= RMX). Sensitivity to the damping factor can be studied by viewing multiple results (e.g. RX5 for JC of 0.5 and RX8 for JC of 0.8 etc). Details on how the PDA-S program calculates the RMX method are noted in Appendix A Sec tio n A.4 .6 .

11.2.1.5 RAU/RA2 Method

For uniform piles with zero shaft resistance, the RAU method is theoretically the perfect method as all theories are correct and the method is independent of a damping constant. It makes no difference if it is easy or refusal driving; the key is that the force and velocity must be proportional for the entire first 2L/c (implies good data) and the 2L/c must be correctly chosen.

The method RA2 has shown considerable promise in determining the ultimate load even for piles with li ttle to mo der ate shaf t resistan ce an d th is method also does not re quire

the selection of a damping factor. Results are generally in good (not necessarily great) agreement with results from CAPWAP and therefore the method deserves at least a casual consideration on every project. If RA2 differs from the damping factor methods (e.g. RX7), then investigate further. If the pile is driving through a layered soil, the RA2 method has the additional advantage that the damping factor does not need

138 138

adjustment. Again, 2L/c must be chosen correctly. Details on how the PDA-S program calculates RAU are noted in Appendix A Sec tio n A.4 .7 .

11.2.1.6 RSU Method

Figure 11.5:

Figure 11.5: The RSU method accounts for early unloadingThe RSU method accounts for early unloading For longer piles with high friction distributed along the shaft, the velocity may become negative prior to 2L/c and the upper soil layers begin unloading even prior to the loading of the lower soil layers. Because the total soil resistance is then not activated simultaneously, most all methods then underestimate capacity and the unloading method RSU may be beneficial (RSU attempts to determine how much friction has unloaded and adds it back into the equations as a correction factor). RSU uses the JC damping factor (RU7 is RSU with JC = 0.7). However, CAPWAP (or static test) should be performed as soon as possible to verify the correct procedures. Details on how the PDA-S program calculates the RPDA-SU method are noted in Appendix A Sec tion A.4. 8.

11.2.2 Damping Constant JC 11.2.2 Damping Constant JC

The damping constant JC applies only to the basic Case capacity computations RMX, RSP, and RSU. To change the damping factor type JC value (e.g. JC0.45 will make JC equal to 0.45 after approval on the ‘Pile Properties’ dialog box). This may be helpful when viewing resistance on the graph.

The capacity methods can be selected for certain damping factors by the quantity selection. For example RX5 is RMX with JC of 0.5. Using these specific quantities (e.g.

RX4, RX6, Rx8...) rather than the general RMX gives perhaps a more clear indication of

139 139

the sensitivity (e.g. select both RX4 and RX7). Similarly, RP5 and RU5 are RSP and RSU respectively with a damping factor of 0.5.

Shaft Friction resistance (SFR) and End Bearing (EBR) reflect the damping constant JC, while SF5 and EB5, for example, reflect the damping factor 0.5.

Based usually on the soil at the pile toe, the following are given as general guidance.

Recommended Case damping constant JC values for the RMX methods are:

• 0.40 to 0.50 fo r clea n san ds quakes are expected or observed. The RX7 method is equivalent to RMX with a damping factor of 0. 7. Cautio n i s g iv en fo r l ow blow co unts (h igh set p er blow) t o b e c on serv at iv e

as low blow counts are indicative of low capacity. It would be helpful to reduce the hammer energy to obtain a higher blow count (smaller set per blow). Many also compare results with the RA2 method (which is independent of JC).

Recommended Case damping constant JC values for the RSP methods are:

• 0.10 to 0.15 fo r clea n san ds

• 0.15 to 0.2 5 for silty sa nds

• 0.25 to 0.40 fo r silts

• 0.40 to 0.70 fo r silty c la ys

• 0.70 o r h ig he r f or cl ays

Generally, the RSP methods are rarely used because there are better methods available.

RSP sensitivity to JC increases for finer soils or at low blow counts. For long piles where the velocity goes negative before 2L/c, the unloading methods (RSU) may be appropriate and these RSP damping factors are then appropriate for the RSU methods also.

Unless grain size analysis is available, visual inspection of the soil may be misleading. A lower prediction results by selecting a higher JC. A soil plasticity index(P.I.) above 5 may imply larger JC values.

11.2.3 CAPWAP Capacity 11.2.3 CAPWAP Capacity

The Case Method in the field using a damping factor JC allows a capacity estimate. In all cases, we highly recommend CAPWAP signal matching analysis of the data as a better way to estimate pile capacity. CAPWAP is a rigorous numerical analysis which m odels the pile and soil behavior. CAPWAP also produces a simulated static lo ad test curve. After the CAPWAP analysis, a JC value can be chosen to estimate the CAPWAP result (or a static load test failure load if the pile has been tested statically). It is important to realize that

140 140

careful CAPWAP analysis is standard practiceCAPWAP analysis is standard practice for the high strain dynamic pile testing method when assessing pile capacity.

The CAPWAP program can accurately model non-uniform piles, and should be used for capacity determination for all non-uniform pile cases. For all practical purposes, it is recommended that at least one CAPWAP be performed in order to confirm field methods. For larger projects, it i s recommended to perform CAPWAP for at least 30 to 5 0 percent of tested piles, although usually only for the data at the end of drive and/or begin of restrike. Inspecting resistance distribution, unit friction values, and end bearing determined by CAPWAP (for restrikes and end of drive) and comparing with soil boring and static analysis calculations often results in better recommendations of total capacity, optimum driving criteria or pile length. Further discussion of CAPWAP is beyond the scope of this manual and the user is directed to refer to a separate CAPWAP Manual, also published by PDI.

11.2.4 iCAP Capacity 11.2.4 iCAP Capacity

Figure 11.6:

Figure 11.6: iCAP results shown in the vertical graph as well as output quantitiesiCAP results shown in the vertical graph as well as output quantities The iCAP program, if installed on the 8G, generates blow by blow capacity solutions in real time and offers the user a higher level of confidence in determining pile capacity during testing. These iCAP capacities are generated by algorithms similar to the auto-CAPWAP feature available in the auto-CAPWAP program. The iCAP capacity results assume that the pile is uniform (cross section and modulus versus length). If the pile is not uniform, these results may not be reliable. The iCAP program may also be installed on a PC and operated from PDA-S program during remote data collection or post processing.

For more information on ICAP and its operation, please refer to “iCAP® Operation” on

141 141

11.2.5 Energy Method 11.2.5 Energy Method

Capacity calculations for the measured Hiley formula (also known as Energy Approach) showing capacity as a function of the maximum energy (EMX) and set per blow is accessed by the QUS, QUT and QBC quantities. This formula has been researched by Paikowsky and recommendations are to use it only with end of drive data. Further the factor ‘k’ mu st be set to va lues le ss th an 1. 0 (v al ues between 0.75 and 0. 5 ar e co mm on;

logic would suggest lower values for cohesive soils and/or for lower blow counts)

Each of the capacity calculations will use either the user defined set (through the CAPWAP adjustment window), measured DFN, or the blow count from the entered LP values. In use, the ‘k’ factor reduction must be applied to all three energy capacity calculations.

Add itio nally the PDA-S prog ram incl ude s the RQX ca paci ty estima tes where the programs reports the greater value of the Case Method RMX formula or the QUT value (assuming a k value of 0.5). This method may be of some value when testing larger diameter shafts, though CAPWAP confirmation is required.

This method is provided for convenience only; PDI does not endorse the use of this method. For further information regarding the Energy formula please see Appendix A Sec tion A.4. 10 . their in-house pile projects as it will detect most common pro blems, and reduce liability.

Some suspect cases are in reality due to poor hammer performance at the end of driving causing relatively high blow counts, and the hammer performing much better during restrike or redrive, resulting in relatively lower blow counts; the 8G can easily identify these cases by looking at the hammer performance indicator EMX.

Soils are difficult. Some of those difficulties encountered are (but not limited to):

• Sit e soi l var iab ili ty ma y cau se co mpl ica tio ns.

• Clean c oarse gr ained sa nds are generall y well suited t o dynam ic cap acity an alysis , even at the end of driving, since capacity changes with time are usually minimal. However, end bearing in larger diameter displacement piles, may be under-predicted at higher blow counts.

• Changes in water table an d effe ctive st resses bet ween tim e of test ing and the servic e condition will have an affect on the long term pile capacity. Particularly seasonal variations or if the site has been temporarily de-watered for construction. The geotechnical engineer should review these changes, as well as settlement concerns for the piles, and pile groups, as well as scour and other concerns when adapting the dynamic testing results into his design and installation criteria.