Discussion of “Assessment ofReinforcement Strains in Very TallMechanically Stabilized Earth Walls”by Armin W. Stuedlein, Tony M. Allen,Robert D. Holtz, and Barry R. ChristopherMarch 2012, Vol. 138, No. 3, pp. 345–356.
DOI: 10.1061/(ASCE)GT.1943-5606.0000586
Peter L. Anderson, P.E., M.ASCE1; andRobert A. Gladstone, P.E., M.ASCE2
1Vice President, Technical Development, The Reinforced Earth Company,133 Park St., North Reading,MA 01864 (corresponding author). E-mail:[email protected]
2Executive Director, Association forMetallically Stabilized Earth, P.O. Box9142, McLean, VA 22102. E-mail: [email protected]
In the course of analyzing data collected from two exceptionallytall mechanically stabilized earth (MSE) walls constructed for thethird runway at Seattle-Tacoma International Airport (SeaTac), theauthors failed to understand three fundamental points regardingMSE wall design:1. The authors ignore both the mandated design friction angle and
the fact that the actual backfill internal friction angle was notknown when the wall was designed. Instead, they assess theaccuracy of the MSE design methods using selected in-placevalues of the soil friction angle, 40� and 44�, which are sub-stantially higher than the mandated design friction angle, 37�.
2. The authors also ignore the fact that MSE walls are designedfor the finished condition, not for the partially constructedcondition. Instead, they condemn the validity of the CoherentGravity and Simplified design methods based on those meth-ods’ underprediction of reinforcement loads at intermediatestages of construction. Postconstruction stresses in the soilreinforcements at SeaTac are consistent with those predictedby the Coherent Gravity and Simplified methods using themandated design friction angle, 37�.
3. The earth pressure distribution behind an MSE wall is trian-gular, not trapezoidal as assumed by the K-Stiffness method.
Internal Friction Angle (w)
In the vast majority of projects, the MSE wall designer cannotpredict the friction angle, w, of the actual wall backfill because thefill is not selected until after the design has been completed andfabrication of wall materials is underway or even complete. There-fore, a design value of 34� is assumed forw based on this value beingthemaximum value permitted byAASHTOwithout project-specificbackfill data. Designing with w equal to 34� and then constructingwith w greater than or equal to 34� simply offsets irregularities anddeficiencies in construction quality control.
Because of the unusual height, loading, and design criticality ofthe SeaTac MSE walls, the geotechnical consultants mandated thedesign be completed using an internal friction angle of w5 37�. Thecontractor was required to use backfill meeting or exceeding thisfriction angle requirement. The critical role of this backfill in meetingthe design requirements is evidenced by the outright rejection, on the
basis of inadequate strength, of 3 of the 19 backfill samples submittedfor approval.
Had the authors of this paper been designing this structure, theywould have been required to use w5 37� in their design; no modi-fication of this friction angle would have been permitted. Therefore,if any comparisons are to be made using various design methods,such comparisons should be made using the design friction angle of37�. Similarly, any comparison of the measured results to the pre-dictions of the Coherent Gravity or Simplified methods should bemade using w5 37�, that is, by comparing the actual design to themeasured results. Use of a higher friction angle for this comparisonis not relevant and not correct because 37� was the mandated andagreed-upon design value.
Design for Finished Condition
MSE structures are designed for their finished loading conditionand for any temporary loading conditions that may exceed thefinished loading condition during construction, such as a tempo-rary surcharge to induce settlement of the foundation soils. MSEstructures are layered systems, with each layer designed to sup-port all of the structure and surcharge to be built above it. Usingthe 45.7-m-tall SeaTac wall cited by the authors as an example, thedesign called for 28 reinforcements per 1:53 1:5-m panel at thebottom of the wall. After the first panel was fully reinforced andbackfilled, those reinforcements were clearly more than sufficient tocarry the remaining load yet to be applied by the additional 44.2 m ofthe finished structure plus surcharge. No intermediate (construction)load would exceed the design capacity of those 28 bottom-panelreinforcements. Therefore, comparing the results of any design me-thod to the measured results for a partially constructed structure iscompletely irrelevant. The partially constructed structure is overlyreinforced for this condition because the design for this section is forthe final constructed condition.
Triangular, not Trapezoidal, EarthPressure Distribution
The authors discuss the trapezoid-shaped horizontal earth pressuredistribution assumed by theK-Stiffness method. A trapezoidal earthpressure distribution is typical of braced excavations that are con-structed in a top-down, excavate-and-brace process. MSE walls areconstructed in layers, from the bottom up, and the measured re-inforcement tensions in the SeaTac structures clearly depict a tri-angular horizontal earth pressure distribution. A triangular earthpressure distribution is the classical approach to earth-retainingstructures subject to rotation (overturning) and is confirmed by nu-merous research studies to be the correct hypothesis for MSEstructure design. Use of a trapezoidal earth pressure distribution, asassumed by the K-Stiffness method, is clearly not correct for MSEretaining wall design.
Conclusion
Data collected from two exceptionally tall MSE walls at SeaTacairport are entirely consistent with the mandated design frictionangle (37�), with the mandated design methods (Coherent Gravityand/or Simplified, per AASHTO), with postconstruction loadingconditions, and with the expected triangular earth pressure
distribution behind any steel-reinforced MSE wall. The authors’analysis of these walls does not correctly address these fundamentalpoints regarding MSE wall design.
Closure to “Assessment of ReinforcementStrains in Very Tall Mechanically StabilizedEarth Walls” by Armin W. Stuedlein,Tony M. Allen, Robert D. Holtz, andBarry R. ChristopherMarch 2012, Vol. 138, No. 3, pp. 345–356.
DOI: 10.1061/(ASCE)GT.1943-5606.0000586
Armin W. Stuedlein, Ph.D., P.E., M.ASCE1; Tony M. Allen,P.E., M.ASCE2; Robert D. Holtz, Ph.D., P.E.,Dist.M.ASCE3; and Barry R. Christopher,Ph.D., P.E., M.ASCE4
1Assistant Professor and Loosley Faculty Fellow, School of Civil andConstruction Engineering, Oregon State Univ., Corvallis, OR 97331(corresponding author). E-mail: [email protected]
2State Geotechnical Engineer, Materials Laboratory, Washington StateDept. of Transportation, State Materials Laboratory, P.O. Box 47365,Olympia, WA 98504.
3Professor Emeritus, Dept. of Civil and Environmental Engineering, Univ.of Washington, Seattle, WA 98195.
4Geotechnical Consultant, 210 Boxelder Lane, Roswell, GA 30076.
The writers thank the discussers for their interest in our paper thatdescribed the analysis of reinforcement loads inferred from thestrains measured in two very tall mechanically stabilized earth(MSE) walls. Although we did present design performance metricsfor the SeaTac Third Runway MSE walls, our main objective wasnot to assess MSE wall design practice; rather, the accuracy of de-sign methods was investigated to identify possible advantages andlimitations when applied to very tall MSE walls. We focused ouranalysis on modeling the actual behavior of the walls, and that iswhy we used measured soil parameters to assess the accuracy ofthe methods for very tall walls.
The following will address the three concerns of the discussers,specifically:1. The friction angles selected for comparison;2. The validity of making comparisons of reinforcement loads at
intermediate stages of construction; and3. The use of trapezoidal earth pressure distributions associated
with the K-Stiffness method.
Use of Friction Angle for ReinforcementLoad Calculations
Contrary to the discusser’s statement, “The authors ignore. . .themandated design friction angle [of 37�],” we did not ignore thedesign-mandated friction angle of 37� in our comparison of the pre-diction methods to the loads in the reinforcement as estimated fromthe measured strains. In the paper’s subsection Comparison of Re-inforcement Loads to Actual Wall Design, we used the design-mandated friction angle f5 37� to compare the mean, maximum,and minimum design and measured reinforcement stresses as a per-cent of the yield stress of the steel. The design reinforcement stresseswere computed using theCoherentGravitymethod as described in theAASHTO (2010) specifications. As shown in Table 3 of the paper, the
stresses were predicted with reasonable accuracy using the design-mandated friction angle f5 37�.
The discussers appear to have misunderstood the objective of thepaper, stated clearly several times throughout, which was to assessthe accuracy of reinforcement load prediction methods using actualstrength parameters. We understand the basis for the AASHTO(2010) specifications limiting the friction angle to 34� when thebackfill cannot be specified in advance [recall that both Allen andChristopher were directly involved in the development of thoseAASHTO (2010) specifications]. Calibration of empirical designmethods such as the AASHTO Simplified method or the CoherentGravity method is based on measured input parameters, not con-servative values selected for design purposes. Using conservativelyselected or presumptive design values for shear strength does notprovide a constant baseline of comparison for assessing the accuracyof these formulations to predict the maximum reinforcement load,Tmax. The use of measured peak friction angles for design modelcalibration leads to a design method that provides the engineera clear baseline against which the consequences of choosing a con-servative design value can be compared. Furthermore, consideringthat both methods were calibrated using measured friction angles, itwould not be correct to assess their accuracy using parameters thatwere not consistent with their original development. Thus, we haveassessed the accuracy of these methods using our best estimate ofstrength parameters determined from large-box direct shear testresults corresponding to contractor submittals that were available tothe first writer during the design and construction of the SeaTacMSE walls.
The best estimate of the mean peak friction angle for the selectedbackfill at the specified level of compaction was 43� (for the rangein normal stresses mandated in the design specifications). To beconsistent with AASHTO (2010) specifications, we capped themaximum direct shear friction angle at 40� for use with the Coher-ent Gravity and Simplified methods, and the corresponding planestrain friction angle required for the Ehrlich and Mitchell andK-Stiffnessmethod at 44� as appropriate. The reason that we appliedthis cap, even considering our desire to use measured values as aconstant baseline of comparison, is that at higher friction angles it iswell known that the loads in steel reinforcement in MSE wallscorrelate very poorly to the friction angle (Allen et al. 2001), and notcapping the friction anglewould have compromised the comparison.In fact, this is the very reason for the mandated cap in the AASHTO(2010) specifications. Thus, we disagree with the discussers that useof the measured friction angles for this comparison, although higherthan the project mandated maximum friction angle of 37�, “does notcorrectly address these fundamental points regarding MSE walldesign”, because “use of a higher friction angle for this comparisonis not relevant and not correct, since 37� was the mandated andagreed-upon design value”. The relevance of the use of measuredfriction angles is discussed in more detail by Bathurst et al. (2011) intheir closure to similar comments made by the discussers to Bathurstet al. (2009).
Prediction of Reinforcement Loads at IntermediateStages of Construction
The discussers argue that “comparing the results of any designmethod to themeasured results for a partially constructed structure iscompletely irrelevant. The partially constructed structure is overlyreinforced for this condition, since the design for this section is forthe final constructed condition,” i.e., it is irrelevant to compare theperformance of a design model at intermediate stages of construc-tion, because the MSE wall will be safe, if not overly so, at
intermediate stages (presumably against internal stability). A gooddesign model should be accurate for all stages of construction; thatis, a design model that considers all aspects of soil-reinforcementbehavior should be accurate regardless of confining pressure,compaction effort, reinforcement spacing and stiffness, as well aswall geometry and height. Some of the methods assessed in thecomparisons presented by the writers, including the CoherentGravity and the Simplified methods, do not address some keydesign parameters such as reinforcement stiffness. The inter-mediate construction stages are useful to consider to determinethe empirical limits of the design methods, to expand their em-pirical basis, and for the development of improvements to thedesign methods so that a broader range of conditions can be ac-curately predicted. Furthermore, the ability to predict rein-forcement strains during construction is critical to assess theperformance of a wall as it is being constructed, where suspiciousincreases in reinforcement strain could be identified and appro-priate remediation efforts determined (Stuedlein et al. 2010, 2012).It should not be surprising that the most sophisticated and theo-retically robust model, that of Ehrlich and Mitchell (1994), gen-erally outperformed all other methods at all intermediate and finalconstruction stages.
Use of Trapezoidal Earth Pressure Distributions
The discussers state “a triangular earth pressure distribution is theclassical approach to earth retaining structures subject to rotation(overturning) and is confirmed by numerous research studies to bethe correct hypothesis for MSE structure design”, but un-fortunately they do not give any specific references to make theirargument that the earth pressure distributions in MSE are tri-angular. Empirical data suggests, however, that triangular dis-tributions are often not observed inMSEwalls. This is illustrated inthe original Federal Highway Administration report on reinforcedsoil structures (Christopher et al. 1990) and in Allen et al. (2001);both provided the basis for the AASHTO Simplified method. Tostate that the load distribution in all steel walls under operationalconditions must be triangular is overly simplistic. Such an asser-tion fails to recognize the many influences on the distribution offorces in MSE walls, including locked-in horizontal stresses fromcompaction (Ehrlich and Mitchell 1994; Stuedlein et al. 2010,2012), the effect of restraint of thewall toe (Huang et al. 2010) frompartial burial and base friction, and variations in reinforcementstiffness and spacing as a function of depth (Christopher et al.1990). Separately, Miyata and Bathurst (2012) describe theworking stress behavior of MSE walls constructed in Japan, whichconfirms the possibility of nontriangular reinforcement loaddistributions.
The development of the K-Stiffnessmethod (Allen et al. 2004)for steel-reinforced walls was based on the empirical data avail-able at the time of its calibration. The trapezoidal distributionrecommended for use with the K-Stiffnessmethod appeared to fitthe available reinforcement load data, none of which were derivedfrom MSE walls as tall as SeaTac walls. As we pointed out in theDesign Method Limitations section of the paper, the height andglobal reinforcement stiffness of the SeaTac walls were well be-yond the range of conditions considered in the empirical databaseused in the development of the K-Stiffnessmethod. Nevertheless,we reiterate that the K-Stiffnessmethod yielded reinforcementload predictions that were conservatively safe for design. The paperrecognizes these limitations, and furthermore recommends that theK-Stiffness method be modified to better model tall MSE walls.
Because one of the main objectives of the paper was to assesswhether available methods could accurately predict the observedreinforcement distribution of very tall MSE walls, we must dis-agree with the discussers that we have not properly addressed theissue of reinforcement load distribution.
Concluding Remarks
We appreciate the opportunity to state our understanding of steel-reinforced MSE wall behavior. Although the discussers believewe did not address the as-designed performance of the wall, wepointed to the portion of the paper that in fact addressed their con-cern. We have also noted that the assessment of the accuracy ofdesignmodels is useful.We disagree about the efficacy of an internalstability design model to predict reinforcement strains at in-termediate construction stages. Empirical data collected over severaldecades appears to contradict the discusser’s claim that all MSEwalls exhibit triangular earth pressure distributions. We have alsorecognized that the K-Stiffnessmethod distribution did not modelthe two very tall SeaTac walls very well, primarily because theheight and stiffness of these two walls were well beyond the em-pirical database used to develop the method, and we have recom-mended improvements to that method be made. In summary, wereject the discusser’s assertion that we have incorrectly treated ourassessment of the behavior and performance of the SeaTac MSEwalls.
Allen, T. M., Bathurst, R. J., Holtz, R. D., Walters, D., and Lee, W. F.(2004). “New method for prediction of loads in steel reinforced soilwalls.” J. Geotech. Geoenviron. Eng., 130(11), 1109–1120.
Allen, T. M., Christopher, B. R., Elias, V., and DiMaggio, J. A. (2001).“Development of the simplified method for internal stability design ofmechanically stabilized earth (MSE) walls.” Washington State DOTResearch Rep. WA-RD 513.1, Washington State DOT, Olympia,WA.
Bathurst, R. J., Nernheim, A., and Allen, T. M. (2009). “Predicted loads insteel reinforced soil walls using the AASHTO simplified method.”J. Geotech. Geoenviron. Eng., 135(2), 177–184.
Bathurst, R. J., Nernheim, A., and Allen, T. M. (2011). “Closure to ‘Pre-dicted loads in steel reinforced soil walls using the AASHTO simplifiedmethod’ by Richard J. Bathurst, Axel Nernheim, and Tony M. Allen.”J. Geotech. Geoenv. Eng., 137(12), 1307–1310.
Christopher, B. R., et al. (1990). “Reinforced soil structures.”Vol. 2., Designand construction guidelines. FHWA Rep. FHWA-RD-89-043, U.S.DOT, Federal Highway Administration, Washington, DC.
Ehrlich, M. and Mitchell, J. K. (1994). “Working stress design method forreinforced soil walls.” J. Geotech. Engrg., 120(4), 625–645.
Huang, B., Bathurst, R. J., Hatami, K., and Allen, T. M. (2010). “Influenceof toe restraint on reinforced soil segmental walls.” Can. Geotech. J.,47(8), 885–904.
Miyata, Y., and Bathurst, R. J. (2012). “Analysis and calibration of defaultsteel strip pullout models used in Japan.” Soils and Foundations, 52(3),481–497.
Stuedlein, A. W., Allen, T. M., Holtz, R. D., and Christopher, B. R. (2010).“Factors affecting the development of MSE wall reinforcement strain.”Proc., 2010 Earth Retention Conf., GSP208, ASCE, Reston, VA,502–511.
Stuedlein, A. W., Allen, T. M., Holtz, R. D., and Christopher, B. R. (2012).“Assessment of reinforcement strains in very tall mechanically stabilizedearth walls.” J. Geotech. Geoenv. Eng., 138(3), 345–356.
Discussion of “The State of the Practice inFoundation Engineering on Expansive andCollapsible Soils” by William N. Houstonand John D. NelsonProceedings of the Geotechnical Engineering State of the Art and Practice:Keynote Lectures from GeoCongress 2012 (GSP 226), pp. 608–642.
DOI: 10.1061/9780784412138.0023
Dennis Shallenberger, P.E., G.E., M.ASCE1
1President, Earth Systems Pacific, 4378 Old Santa Fe Rd., San Luis Obispo,CA 93401. E-mail: [email protected]
Introduction
The discusser addresses the authors’ research and the conclusions theyderived from it. The headings used subsequently identify those ele-ments of the authors’ paper the discusser is addressing. The authorsbegin their paper by defining legal issues; however, the standard ofcare definition they cite does not apply to professionals, underminingthe paper’s foundation. The professional standard of care—applicableto all professionals in theUnited States, not just engineers—comprisesthat degree of care ordinarily applied by peer practitioners practicingin the same general area (as defined by the court) at the same time andunder similar circumstances. A judge or jury determines the standardof care on a case-by-case basis. Given that definition, the authors’contention that “just applying general field investigation techniques,laboratory tests, and design procedures that have been used in the past,albeit in a widespread fashion, does not necessarily meet the standardof care, when health and safety are an issue” is seriously flawed. Infact, the “general field investigation techniques, laboratory tests, anddesign procedures that have been used in the past, albeit in a wide-spread fashion” establish the standard of care. The limiting factor—“. . .when health and safety are an issue”—is not a limiting factor atall because, when it comes to engineering services, health and safetyare always issues; i.e., engineers are legally and ethically required topreserve and protect public health, safety, and welfare.
The authors state that a local standard of care would imposemoreduty-of-care burdens on professional engineers than would a na-tional standard of care. However, all standards of care are local,making “national standard of care” an oxymoron. The authors alsoseem to believe that the standard of care can be something less thanthe state of the practice, when, even by the authors’ definition, stateof the practice is the standard of care.
The authors’ introduction concludes with the following:
“In this paper the authors present what we consider should bethe state of practice for field investigation techniques, labora-tory testing, and foundation design on expansive and collaps-ible soils. We then discuss the state of practice that is beingfollowed and present recommendations for change.”
While anyone is at liberty to create a theoretical standard of care, theactual standard of care needs to reflect how professional practitionerswere ordinarily performing a given service at the same time and in thesame general area. Accordingly, the standard of care that is beingfollowed must be the current standard of care—what was ordinarilybeing done in 2012—given that “is” comprises the verb “to be” in thepresent tense. As such, the authors cannot possibly know what isbeing followed given that the most recent report reviewed was pre-pared in 2005, and some were as much as 35 years old.
Site Investigation
The authors state that in the Front Range area of Colorado, theColorado convention is to drill at least one boring per lot. Theauthors fail to cite a basis for this contention. Contrarily, in the areaof the country in which the discusser primarily practices, fellowengineers would consider that extent of drilling excessive, even ifexpansive soils were involved. In the same paragraph, the authorscomment, “For large commercial projects across the country it iscommon practice to space boreholes on a 30 m (100 ft) grid. Thisgives general coverage over the entire area.” In the discusser’s ex-perience, borings at 30 m (100 ft) grids by no means comprise whatis ordinarily done; depending upon the situation, common practicecomprises many other, additional approaches.
On page 613, the authors state that “A prudent designer shouldperform adequate analyses or investigation to establish within rea-sonable certainty the design active zone for the life of the structure.Reasonable certainty can, for example, be establishedwith some sortof risk-based analysis. . ..” The authors fail to acknowledge the factthat geotechnical engineers often must compromise their technicalpreferences according to their client’s budget. Furthermore, in thediscusser’s opinion, experience with a particular soil type in a spe-cific geographical area is far more important and yieldsmore reliableresults than numerical analysis.
Soil Investigation Practices
The authors note that they have “gained insight relative to geotechnicalinvestigative practices across the USA” because they, and a group ofcolleagues, have reviewed perhaps as many as 600–700 geotechnicalreports. However, the authors fail to describe their review methodol-ogies and they do not indicate if they were uniform and applied con-sistently. Even if they were, hundreds of thousands of unreviewedgeotechnical reports could provide alternative evidence. The discusseralso has questions about the geotechnical consultants in several statesthe authors’ interviewed.Who and howmanywere they?What is theirexperience? How many states does several comprise?
The authors state that the data presented in their Tables 1–6 areconsistent with 95% of the cases reviewed. While that may be thecase, those data may be inconsistent with the far greater number ofcases the authors and their colleagues did not review. The authorsseemingly failed to consider this possibility in their determinations.
The authors state, “The data in Table 1 is (sic) weighted towardsites with soils that are not particularly moisture sensitive.” Giventhat the paper’s focus is onmoisture-sensitive sites, what is the valueof Table 1 relative to the topic of the paper? Note, too, that Table 1identifies common remediation measures, yet cases reviewed inTables 2–5 identify an array of measures. As such, no single re-mediation measure or group of measures should be considered or-dinary, the authors’ contentions notwithstanding.
The authors wrote, “Over 300 geotechnical reports prepared forthe Front Range area of Colorado have been reviewed and compiledinto a database. These reports were prepared by a wide range ofdifferent geotechnical engineers during the period from 1978 to2005.” Given the facts stated by the authors, the discusser finds ir-relevant the research on which the authors’ paper is founded.1. The authors and their colleagues considered perhaps as many as
600–700 reports. Because more than 300 reports applied to theFront Range of Colorado, analysis of all other areas consideredmay be as few as 250 or even fewer reports, and most of thosestem from studies of sites that were not moisture-sensitive sites.
2. How can the authors not know how many reports were re-viewed? In fact, how many is perhaps as many as 600–700?
Is it 500 or 400? If the authors do not know, why do they notknow? Why do they fail to explain?
3. Howwere the reports reviewed?Were the reviewers assembledinto teams and given specific guidance on how to review reportsto ensure consistency? Given that the authors do not even knowhow many reports were involved, it is doubtful that they knowthe review protocols—if any—that the reviewers used.
4. As already noted, not even one report relates to 2011 or 2012.The newest is at least 6 years old; the oldest is 35 years old. Assuch, the authors have no basis for knowing what ordinarily isbeing done now, nor is it likely they have a basis of knowingwhat was ordinarily being done at any time in the past.
5. Far too much information is omitted for the research to betaken seriously. For example, it is not known how many dif-ferent geotechnical engineers authored the reports and there isno clue as to the meaning of the term wide range.
The representativeness of the authors’ other tables is also a con-cern, given that the data in Table 2 come from only one source, as dothe data in their Tables 3(a and b) and 4. The data in their Table 5come from two sources in southern California. How do the authors’data indicate what was ordinarily done in the few areas in question?
Comments on General Practice
In their comments on general practice, the authors state that theconvention in more than one state is to drill one boring on each lot;however, they fail to indicate how many states follow that con-vention and how—given that the convention is the standard of care—they came to learn that. They wrote that “it could be argued thatthe depth of borings and associated testing are often not deep enoughto allow appropriate design.” They then write, “This in turn meansthat the probability is high that the piers will be under-designed,resulting in heave or settlement, or over-designed resulting in ex-cessive costs to the client.” The authors cite no evidence that eitherunderdesign or overdesign is an actual problem, and they do notdefine what high probability means. Furthermore, “It could beargued. . .” is hardly a statement of fact, leaving open the reciprocalargument that depth of borings and associated testing are generallyappropriate and that the authors’ speculative concerns about un-derdesign and overdesign are not genuine concerns at all.
Laboratory Testing
The authors’ discussion of test and design methods makes nomention of how much various methods cost, the budgets practicingprofessionals must contend with, and clients’ willingness or abilityto pay. The authors also fail to address their favored methods’practicality and the efficacy of results versus those attained via al-ternative means; e.g., by securing the judgment of a geoprofessionalexperienced with the soil deposit involved.
Design Methods
In Section 4.1.2., the authors discuss depth and degree of wetting,writing that “The authors have had rather strong disagreements onthese issues in the past. . .” That being the case, the discussion leadsnowhere except to the conclusion that the standard of care is actuallyfar broader than the authors would have their readers believe. Theauthors conclude their discussion of depth and degree of wetting bynoting that groundwater can be a significant source of wetting. Theyfail to note that groundwater can also be a stabilizing factor thatprevents cyclic drying of the soil, thus helping tominimize the shrink/swell cycle. In fact, when expansive soils are stabilized naturally orartificially much of the costly, time-consuming drilling, sampling,testing, and analysis advocated by the authors become unnecessary.
Summary and Conclusions
The authors conclude, in part, that “for every billion dollars wasteddue to underdesign there are probably 5 billion dollars, or evenmore,wasted in overdesign.” Probably? In fact, the authors fail to cite evenone datum to support their statement, making it nothing more thanunsupported speculation.
The authors also put forth the following with regard to sitecharacterization:
“In addition to advancing the hole to sufficient depth andtesting samples at those depths is the need for careful loggingof the soil profile. Author Nelson asserts that an importantelement of careful logging is inspection of continuous corewhen practical. Granted that taking core during samplingincreases the cost of investigation, that cost is minimal whencompared to potential cost of damages. It is the responsibilityof the engineer to educate the client as to the need to un-dertake that cost. Author Houston asserts that continuouscoring is not practical in a great many cases.”
The discusser finds the wording to be both curious and alarming.The authors state that they have almost diametrically opposed opin-ions about the value of continuous-core inspection. That being thecase, why would they write, “It is the responsibility of the engineer toeducate the client as to the need to undertake that cost [of continuous-core inspection]” and not something such as “Author Nelson believesit is the responsibility. . ..”? The importance of a statement such as “Itis the responsibility of the engineer to educate the client as to the needto undertake that cost” should not be minimized. An attorney or un-ethical expertwitness could easily take such a statement out of contextand use it to allege that highly qualified engineers, practicing ina nonnegligent manner, actually were negligent because they failed tofulfill their professional “responsibility. . .to educate the client as tothe need to undertake that cost” even though “continuous coring is notpractical in a greatmany cases.”The authors’ final sentence reads, “Inthe authors’ experience with litigation cases involving piers, it hasbeen seen that, until recently, in almost no cases were design calcu-lations performed, by either the geotechnical or structural engineer. Inthe cases where this criticism applies, a change in the state of thepractice is needed.”First, in the discusser’s experience, the standard ofcare always requires the structural engineer to perform design cal-culations. Depending upon the project and the geotechnical engi-neer’s experience with the soil deposit of concern, the geotechnicalengineer may or may not perform design calculations. Second, casesthat wind up in litigation get there because, allegedly, the engineersinvolved failed tomeet the standard of care. Thosewho fail tomeet thestandard of care do not establish the state of the practice.
Discussion of “The State of the Practice inFoundation Engineering on Expansive andCollapsible Soils” by William N. Houstonand John D. NelsonProceedings of the Geotechnical Engineering State of the Art and Practice:Keynote Lectures from GeoCongress 2012 (GSP 226), pp. 608–642.
DOI: 10.1061/9780784412138.0023
Ji H. Shin1 and Michael J. Byrne21General Counsel, Earth Systems, Inc., 895 Aerovista Pl., Suite 102, San
Luis Obispo, CA 93401 (corresponding author). E-mail: [email protected]
2Partner, Gogick, Byrne & O’Neill, LLP, 80 Main St., Suite 140, WestOrange, NJ 07052.
The authors describe the changing state of the practice of foun-dation design for expansive and collapsing soils over the pastdecades. In their paper, the authors (1) define the standard of careand state of the practice based on their professional experienceand (2) “. . . present what they consider to be the appropriatepractice for field investigation techniques, laboratory testing andfoundation design . . .” This discussion is limited to a review of thelegal issues the authors raise about the standard of care. On p. 609,the authors wrote
“The Standard of Care represents that standard that anengineer should be following in order to provide the ap-propriate care to protect public health and safety. The LegalDictionary (Law.Com, 2012) defines the standard of care asfollows:
‘The watchfulness, attention, caution and prudence thata reasonable person in the circumstances would exercise. Ifa person’s actions do not meet this standard of care, then his/her acts fail to meet the duty of care which all people (sup-posedly) have toward others. Failure to meet the standard isnegligence, and any damages resulting there from may beclaimed in a lawsuit by the injured party. The problem is thatthe “standard” is often a subjective issue upon which rea-sonable people can differ.’”
In fact, the authors chose the wrong definition, because it in-correctly describes the “standard that an engineer should be fol-lowing in order to provide the appropriate care to protect publichealth and safety,” which is the standard of care applicable to pro-fessionals, including geologists, architects, physicians, and others,not just engineers. Another on-line source, USLegal (2012), pro-vides a correct definition
“Standard of care refers to the degree of attentiveness, cautionand prudence that a reasonable person in the circumstanceswould exercise. Failure to meet the standard is negligence,and the person who fails to meet the standard is liable for anydamages caused by such negligence. The standard is notsubject to a precise definition and is judged on a case by casebasis.”
“Certain standards for professionals are established by practiceof similar professionals in their community. . ..”
The discussers’ contention about professional standard of care(versus standard of care in general) is reinforced by published ju-dicial decisions that discuss the formulation and application of theprofessional standard of care. The following are examples of ap-propriate professional-standard-of-care clauses that are consistentwith such published precedents:1. Article 2.2 of AIA Document B101-2007 [American Institute
of Architects (AIA) 2007]: “The Architect shall perform itsservices consistent with the professional skill and care ordi-narily provided by architects practicing in the same or similarlocality under the same or similar circumstances. The Archi-tect shall perform its services as expeditiously as is consistentwith such professional skill and care and the orderly progressof the Project.”
2. Judicial Council of California (CACI 2012) Civil Jury Instruc-tionsNo. 600 for professional negligence: “[A/An] [insert typeof professional] is negligent if [he/she] fails to use the skill andcare that a reasonably careful [insert type of professional]would have used in similar circumstances. This level of skill,
knowledge, and care is sometimes referred to as ‘the standardof care.’ [You must determine the level of skill and care thata reasonably careful [insert type of professional] would use insimilar circumstances based only on the testimony of the ex-pert witnesses [, including [name of defendant],] who havetestified in this case.]”
The professional standard of care is also discussed in “Recom-mended practices for design professionals engaged as experts inthe resolution of construction industry disputes” (InterprofessionalCouncil on Environmental Design 1989), a document endorsed byASCE, National Society of Professional Engineers, AmericanConsulting Engineers Council, American Society of Heating, Re-frigerating and Air-Conditioning Engineers, and some 35 additionalorganizations
“7. The expert witness should testify about professionalstandards of care* only with knowledge of those standardswhich prevailed at the time in question, based upon reasonableinquiry.”
“COMMENTARY
When a design professional is accused of negligence, the trierof fact must determine whether or not the professionalbreached the applicable standard of care. A determination ofthe standard of care prevailing at the time in question may bemade through investigation, such as the review of reports,records, or opinions of other professionals performing thesame or similar services at the time in question. Expert wit-nesses should identify standards of care independent of theirown preferences, and should not apply present standards topast events.”
“*Standard of care is commonly defined as that level of skilland competence ordinarily and contemporaneously demon-strated by professionals of the same discipline practicing in thesame locale and faced with the same or similar facts andcircumstances.”
Standard of care is not an engineering concept. It is a legalconcept, developed as a means to evaluate professional perfor-mance. It applies to engineers, geologists, architects, and otherdesign and environmental professionals, as well as other pro-fessionals, like accountants and physicians. Standard of care hasbeen reinforced through decades of consistent applicationthroughout the United States. The concept provides flexibility, anessential quality when it comes to actions of negligence againstdesign professionals because flexibility allows the courts tomaintain the discretion they need to consider the particular cir-cumstances of each case, including factors such as project type,geographic locality, and the time period when the work wascompleted.
Because a trier of fact—a judge or jury—is not alwaysequipped to understand the technical issues associated with moststandard-of-care issues in design-professional claims, the lawrequires the involvement of expert witnesses. These expert wit-nesses are often retained to explain technical and professionalissues and to offer opinions about whether a professional’s per-formance was in keeping with the ordinary performance of peerprofessionals performing at the same time, under the same orsimilar circumstances, in the same court-defined community.However, these expert witnesses are not allowed to testify aboutthe legal definition of the standard of care because they are notexperts when it comes to legal matters. For the same reason, it isinappropriate for the authors to cite engineers’ opinions as
authority on the legal definition or meaning of the professionalstandard of care.
In State-by-State Guide to Design and Construction Con-tracts and Claims, the discussion by Dodd and Findlay (2012) onprofessional negligence and the standard of care confirms thisprocess
“Expert witnesses are often more than willing to opine whathe or she personally thinks is “the standard of care.” How-ever, the trial judge, not the expert, should instruct the jurywhat is the legal definition of the standard of care, based uponcourt rules, administrative regulations, statutes, case law, andpattern jury instructions, etc. Unless the expert is a lawyer, theexpert is not qualified to opine what is the legal definition ofthe standard of care. What a particular expert personallythinks the legal definition of “the standard of care” should beand/or what “the state of the practice” should be is irrelevant;it is what was actually typically done in practice, at the time inquestion, by the design professional community, as a whole,that is relevant. The expert should only be allowed to opinewhat the design professional community, as a whole, wouldcustomarily do under the same or similar circumstances, atthe time and locality as the services were originally providedby the defendant.”
State of the Practice
The discussers polled the attorney members of the Legal AffairsCommittee of ASFE (ASFE 2012)/The Geoprofessional BusinessAssociation; individuals practicing throughout the United States.The discussers inquired about the authors’ belief that state of thepractice is different from standard of care. The discussers asked theattorneys if, in their respective jurisdictions, therewas a difference inthe law between state of the practice and professional standard ofcare. Their response was that the two terms mean exactly the samething in the context of evaluating the professional performance ofdesign professionals.
State of the Art
The authors’ comments about state of the art also miss the mark.The law does not require design professionals to apply state-of-the-art practices, and any such mandate would be devastating tothe engineering profession. State-of-the-art practices, by defini-tion, are not time tested and set out unreasonable expectations ofperfection. As a result, their application would invariably createa high risk of failure to satisfy the state-of-the-art standard, andevery failure would bring with it a near certainty of negligence-based litigation that would over time discourage engineers fromusing professional judgment to create, analyze, and enhance theprojects for which they are engaged. Reliance on the time-testedprofessional-standard-of-care concept avoids such problems. Itdoes not require perfection, thus permitting civil engineers toapply the professional judgment that is essential to society’sgrowth, and which is becoming even more significant in light ofthe challenges being posed by a changing environment. As such,the professional standard-of-care concept encourages technicalevolution while requiring professional performance by imposingsevere penalties on those who fail to meet their duty of care.“Those who hire such persons [engineers] are not justified inexpecting infallibility, but can expect reasonable care and com-petence. They purchase service, not insurance” [Gagne v. Bertran,43 Cal.2nd 481 (1954)].
Professional Standard of Care for VariousEngineering Tasks
After opining about the meaning of legal terms, the authors set forthwhat they portray as the standard of care applicable to various en-gineering services. However, any such portrayal lacks substancebecause the standard of care applicable to a given service is, to thediscussers’ knowledge, established only by a trier of fact, in lightof the evidence presented and the testimony of fact and expert wit-nesses. Theoretically, the standard of care could otherwise be de-termined by amassing empirical data indicating the ordinaryperformance of all like professionals performing the same serviceunder similar circumstances at the same time in the same area.Absent the role of judgment vested in the trier of fact, and dependingon the services, area, time, and number of practitioners involved, thiswould require the consistent review of hundreds (more likely,thousands) of reports and/or the conduct of dozens (more likehundreds) of interviews (assuming practitioners’ recall would betotally reliable and unaffected by hindsight), and that would applyonly to one highly defined area; e.g., a state or area of a state. Toauthoritatively identify a standard of care prevailing nationwide,unquestionably thousands (more likely tens of thousands) of reportswould have to be reviewed and/or hundreds (if not thousands) ofpractitioners would have to be interviewed. Accordingly, the authorsshould be asked to share the data and correlations they relied on toprove that the scope of service they propose for projects involvingexpansive and collapsible soils would lead to (or has already led to)improved safety or would (or already has) decreased the amount ofoverdesigned or underdesigned projects.
Summary
Unless otherwise qualified, engineers—be they professors or prac-titioners—lack the expertise required to opine authoritatively aboutlegal issues, especially those such as standard of care, whose ap-plication is not at all confined to engineering practice. The authors’paper demonstrates this lack of expertise, in part because the defi-nitions and theories they offer conflict with applicable law. Perhapseven more important, the authors seem to miss sight of the purposebehind standard of care. In fact, if the authors’ definitions and the-ories were to be accepted, they would eliminate the professionalprotections applicable law now affords to professionals and thosewho rely upon their services. That situation, in turn, would lead tothe creation of a destructive legal environment where all pro-fessionals—not just engineers—would be forced to contend withlitigation because they failed to achieve unrealistic standards that farexceed what now is reasonably required. The legal system has itright by defining the professional standard of care as what was or-dinarily being done in practice, at the time in question, by peerprofessionals active in the same general geographical area, anddealing with similar circumstances.
References
American Institute of Architects (AIA). (2007). “Standard form of agree-ment between owner and architect.” AIA Document B101-2007, Wash-ington, DC.
ASFE. (2012). “ASFE to the rescue . . . maybe.” ASFE NewsLog, 43(2), 1.Dodd, M., and Findlay, J. D. (2012). State-by-state guide to design and
construction contracts and claims, 2nd Ed., Aspen Publishers, NewYork.
Interprofessional Council on Environmental Design (ICED). (1989).“Recommended practices for design professionals engaged as experts inthe resolution of construction industry disputes.” ICED c/o ASFE, Inc.,Silver Spring, MD.
Judicial Council of California (CACI). (2012). Judicial Council of Cali-fornia civil jury instructions (CACI). Judicial Council of California,Sacramento, CA.
Closure to “The State of the Practice inFoundation Engineering on Expansive andCollapsible Soils” by William N. Houstonand John D. NelsonProceedings of the Geotechnical Engineering State of the Art and Practice:Keynote Lectures from GeoCongress 2012 (GSP 226), pp. 608–642.
DOI: 10.1061/9780784412138.0023
William N. Houston, Ph.D., P.E., L.S., M.ASCE1; andJohn D. Nelson, Ph.D., P.E., D.G.E., F.ASCE2
1Professor Emeritus, School of Sustainable Engineering and the Built En-vironment, Arizona State Univ., Tempe, AZ 85287-5306 (correspond-ing author). E-mail: [email protected]
2Professor Emeritus, Colorado State Univ., Fort Collins, CO 80523; and CEOand Principal Engineer, EngineeringAnalytics, Inc., 1600 Specht Point Rd.,Suite 209, Fort Collins, CO 80525. E-mail: [email protected]
Introduction
Two discussions were presented in response to the writers’ paper.The discussion by Shin and Byrne focused primarily on legal issuesrelated to the paper subject. The discussion by Shallenberger touchedon both legal and engineering issues of the paper. The closure pre-sented subsequently is divided into three sections. The first sectionaddresses legal issues and the second section addresses engineeringissues related to the subject of the paper. A third section addressesa few specific discussion criticisms that were not addressed in the firsttwo sections.
Legal Issues
The writers began their paper with a legal definition of the standardof care and definitions of the state of the practice and the state of theart. The writers consulted numerous attorneys and other sources (asnoted in the list of references) when developing their definitions ofstandard of care and, in fact, received assistance from their attorneys.The writers were informed by their attorneys that the state of thepractice and the state of the art were not legal terms and have nostanding in a court of law. Accordingly, the writers defined theseterms in more or less the same way they are commonly defined forpapers presented at a conference.
Discussers Shin and Byrne place great emphasis on the legaldefinition of the standard of care, and they find fault with the defi-nitions that were presented in the paper. They then proceed to supplyan alternate definition and assert that the writers’ definition is in-correct. In preparing this response, thewriters carefully reviewed thedefinitions and associated discussions given by these discussers andsaw no big differences, compared with the writers’ presentation. Infact, it is appreciated that the discussers present a comprehensivediscussion of definitions that apply to the standard of care, and thewriters feel it complements the limited scope presented in the paper.
Both discussions present a definition that is frequently quotedas “what a reasonable engineer practicing in the same location at thesame time would have done.” A similar definition that is often
presented uses the termprudent engineer. The difficultywith the afore-mentioned definition is that it does not clarify what a reasonable(prudent) engineer is, nor does it define how to determine what thatengineer would have done. Circular reasoning would state that areasonable (prudent) engineer would have followed the standardof care.
Discusser Shallenberger notes that a judge or jury determines thestandard of care on a case-by-case basis, and states that “Engineersare legally and ethically required to preserve and protect publichealth, safety and welfare.” The writers concur wholeheartedly withthose statements. However, it is important to realize that in cases oflitigation it is the duty of the engineers who are testifying expertwitnesses to aid the judge or jury in determining the technicalaspects of what constitutes the standard of care on a case-by-casebasis. The engineer does not provide a definition of standard of care.Instead, the engineer opines on whether certain actions met thestandard of care and presents those arguments to the judge or jury inorder to persuade them in their decision making.
The writers strongly disagree with the assertion by all of thediscussers that the state of the practice and professional standard ofcare means “exactly the same in the context of evaluating the pro-fessional performance of design professionals.” Of course, the stateof the practice in a community provides the rawmaterial fromwhichthe standard of care is eventually developed. However, the distinc-tion arises from the fact that there is always a spectrum of practicewithin a given community. The community within which the stateof the practice is being assessed could be a city, a county, severalcounties, or a region as big as southern California. The size of thecommunity selected obviously affects the range in the level ofpractice within the community. However, for any community se-lected, the level of practice will range from the lowest level or worstpractice to the highest level or best practice. The inclination may beto assume that what gets chosen eventually as the standard of carewill fall in between the two extremes; however, this result is notguaranteed by the process. As stated by the writers in their paper andby the discussers, the judge puts forth the general legal definition ofthe standard of care, expert witnesses flesh it out by giving theiropinions relative to the case at hand, and finally the judge and/or jurymust weigh all evidence and testimony and arrive at a final decisionrelative to the standard of care. Thus, the expert witnesses playa major role in the process. For example, if there are two expertwitnesses and perfect recollection, and unusually high integrity, thenit may be expected that both experts will describe essentially thesame level of practice as the standard of care. However, those withsome experience in these matters know that one or more of thepreceding conditions are usually absent and the opinions differ.Typically, the expert hired by the defendant has tended to focus on orvery near the low end of the spectrum representing the state of thepractice and the expert hired by the plaintiff tends to focus on or verynear the high end of the spectrum; that is, the best practice in thecommunity.
The standard of care does not lend itself to quantification asa number; however, a mathematical analogy is potentially helpful.The state of the practice in a community is a spectrum ranging fromlow to high, sometimes with a wide range. The state of the practicecan be thought of as a continuous function with an upper and lowerbound. The standard of care is analogous to a single value within thecontinuous function—a value finally arrived at by judge and/or jury.
Because of the aforementioned points, the writers find that theydisagree with the discussers on three points in particular. (1) Thewriters consider it to be misleading and/or inaccurate to refer to thestate of the practice as a fixed quantity or a particular, knowable levelof practice. Even within a given community the state of the practiceis a spectrum. (2) It is likewise misleading to refer to the standard of
care as if it were cast in stone. Even though it may become fixed ina particular case, a different set of experts and a different judge andjury could well arrive at a different standard of care—even withexactly the same facts and circumstances. (3) The writers think it isan oversimplification and is misleading to assert that the state of thepractice is exactly the same as the standard of care, for the reasonsstated previously. Furthermore, the writers strongly disagree withanyone who contends that the standard of care is equivalent to thelowest level of practice in a community. If that should turn out to bethe case in a particular instance it is because one or more expertshave asserted that it is so and has prevailed. In this case, the experthas made a conscious decision to espouse this position and thewriters regard it as unprofessional. To say that an engineer onlyneeds to do the worst that has been done previously in an area, evenif the performance of that practice has been poor, should not betolerated within the profession. Self-perpetuation of poor practicedoes not serve the community well, leads to an overall decline in thelevel of practice, and should not be construed by experts to be thestandard of care.
Another point that seems to have been troublesome to the dis-cussers and some other practitioners has to do with the writers’contention—particularly for projects where public health and safetyare major issues—that practitioners should keep abreast of the stateof knowledge in geotechnical/foundation engineering. DiscusserShallenberger took issue with the writers’ phrase “where publichealth and safety are amajor issue” and asserted that all geotechnicalprojects involve public health and safety. While this may be tech-nically true, it is obvious that some projects like dams and nuclearpower plants involve public health and safety in a major way.
Two examples from actual court cases illustrate that the state ofpractice is not necessarily the standard of care. The first case in-volved tanker ships sailing up and down the east coast of the UnitedStates [The T.J. Hooper, 60 F.2d 737, 739–40 (2nd Cir. 1932)].Some ships foundered with large loss of life, and a major contributorto the cause of the failure was the fact that ship owners were notequipping the tankers with emergency radios. As a consequence,crewswere drowning in storms that were predicted but ofwhich theywere not warned because of the absence of radios. When sued, theship owners’ defense was that they could not have breached thestandard of care because the standard of practice of all ship ownerswas not to equip the tankers with radios. The court rejected thatdefense, stating that no industry can itself set the standard of care byacting in accordance with a negligent standard of practice.
A second case involved the failure of builders to install structuralfloors over expansive soil (Peterson et al. v. Mission Viejo). In thatcase, the plaintiffs alleged that the builders failed to meet the stan-dard of care because they installed slab-on-grade floors in basementsover expansive soils instead of structural floors. At the time, builderstypically were not installing structural floors in house basements inColorado. The builders vigorously argued that because the standardof practice was not to install structural floors, they could not havebreached the standard of care by which the law judges their conduct.Court after court rejected that defense, the case went to trial, MissionViejo lost, and the industry settled the remaining cases. Thereafter,structural floors became commonplace, just as radios on ships didafter the shipping industry lost the same argument in the tanker case.
Both of the aforementioned cases demonstrate clearly that justdoing what everyone else is doing locally is not necessarily ade-quate. In fact, the reason for many states to require professionaldevelopment hour credits is the recognition that the profession needsto keep abreast of current technology. If that technology is not goingto be used, the profession will not be providing the best services thatit can. Thus, the standard of care as demonstrated by the two casescited previously would not be followed. It was pointed out in the
writers’ paper that a reasonable engineer is expected to use theknowledge that is available, and hence is expected to keep abreast ofthe state of knowledge in geotechnical engineering and to use theresults of research, provided these results have been thoroughlyreviewed and vetted and that a consensus exists relative to theirapplicability and value to the practice.
Engineering Issues
Up to this point, the writers’ have focused primarily on what couldbe called legal issues relating to the state of the practice and thestandard of care because that was the primary focus of the discussers.However, the paper contained numerous other components, someof which were arguably important. After completing the paper,the writers were actually hopeful and slightly optimistic that somereaders may be persuaded to go beyond the current typical state ofthe practice in a particular community and affect an upgrade in theirpractice. This optimism may have been unrealistic. The suggestionto do so and a path for accomplishing this objective came at the endof the paper, which was already too long and thus allocated a smallspace. In the next few paragraphs this idea will be further explainedand hopefully further clarified.
Thewriters realize that it is very difficult to change the state of thepractice in a community. It is similar to a huge massive train that ismoving at a snail’s pace. It will not be significantly slowed, accel-erated, or given a new direction by a blast of wind. The writers arenot naive enough to think that they can single handedly upgradegeotechnical practice significantly by publishing a paper or two.Likewise, an individual consultant or expert witness can do verylittle alone; however, if all in the profession, or even most, were tomake an effort to upgrade the practice, this would have an impact onthe profession. It is for this reason that the writers have appealed tofuture expert witnesses to avoid choosing the lowest level of practicein a community as the standard of care. All geotechnical consultingfirm principals will eventually reach the point of retirement andsome may reflect on their legacy. Surely, it would enhance thefeeling of pride and accomplishment if retiring expert witnessescould say that they did what they could to upgrade the state of thepractice in their communities. Therefore, the writers would askprincipals to consider what it is, if anything, that they are doing toupgrade the practice.
Proposal for a Major Upgrade in Practice
When a geotechnical consultant recommends a foundation schemeor a site remediation scheme, the result must fall into one of the threecases. (1) The design is unconservative, leading to failures or highrisk of failure and loss of safety. This error will be called Type I. (2)The design is about right, which is self-explanatory. The about rightdesign could well comprise a fairly wide range. (3) The design isoverly conservative and wastes the owner’s money. Constructioncosts are much higher than they need to be. This error will be calledType II. The writers could argue about the placement of the borderbetween about right and overly conservative. In fact, the writershave argued with each other about this in the past and probably willin the future. Nevertheless, there is some limit beyond which almosteveryone would agree that a design is overly conservative. Themainpoint here is that the writers contend good engineering requiresstriving for designs that are about right and will eliminate Type Ierrors and will eliminate or at least minimize Type II errors.
The simultaneous avoidance of Type I and II errors will gen-erally require more testing and analyses and more economiccomparisons of alternatives than that which the writers perceive to
be typical of the range in the state of the practice in most commu-nities. The writers realize that they have constructed a shoe that willnot fit all geotechnical firms and they do not ask anyone to wear theshoe if it does not fit. The commitment to avoid Type I and II errors,and thus serve the best interests of the owners while protecting thepublic safety, goes beyond the typical current state of the practiceand associated standard of care for the following reason. The typicalstate of the practice and associated standard of care speak only toType I errors. Geotechnical engineers who commit Type I errors arepunished regularly; however, who can remember the last time ageotechnical engineer was hauled into litigation for endorsing adesign that was overly conservative and subjected the owner toexcessive construction costs. By and large Type II errors go un-published and are buried with the foundations.
Assuming geotechnical engineers were inclined to consider un-dertaking the upgrade described previously, the first question thatarises is the following: Who will pay the costs of the additionaltesting, analyses, and study of alternatives? In the paper, the writersacknowledged that the geotechnical engineer is typically the victimin this dilemma, in that the entity hiring the geotechnical engineer isgenerally unwilling to pay for more work. A possible solution to thisproblem was introduced near the end of the paper and is furtherexplained in the subsequent section.
Proposal for the Procurement of Fundingfor Added Geotechnical Work
A project owner funds the creation of a project by directing moneyinto various pots, including (1) geotechnical investigations, testingand analyses, and study of alternatives; (2) structural design; (3)construction; (4) maintenance; and (5) risk. Any reasonable ownershould be interested in minimizing the expected life-cycle costs ofthe entire project; i.e., the sum of all money going into all pots. Forany specific project there is an optimal amount of geotechnical workthat tends tominimize the expected life-cycle cost sum. To the left ofthe minimum the extent of the geotechnical work is inadequate—meaning that (1) more testing is conducted to provide more certaintyof important material properties; (2) more analyses are conducted toestablishmore confidence in the expected response of the foundationsoils to load and wetting; and (3) more study of alternatives will leadto lower construction costs. To the right of the minimum a point ofdiminishing returns has been reached. This means that the saving intotal project costs is less than the cost of the added geotechnicalwork, in this range. The first writer has been studying this issueinformally for over a decade and has become convinced that, fora typical project, the optimal amount of geotechnical work is two orthree times the scope actually authorized. To the extent that thisconclusion is valid, the writers could be spending two or three timesas much for geotechnical work as they now typically do and theowner would save on average more than this amount in constructioncosts, sometimes considerably more.
The logic of this approach seems overwhelmingly sound to thewriters and to every geotechnical engineer to whom they havepresented it. However, if geotechnical engineers went to potentialclients and asked for 2.5 times the usual fee for upcoming projects,they would likely to be met with suspicion and skepticism, even ifarmed with the state of the practice paper and the closure and dis-cussion.Most owners are going to need some hard facts, such as casehistories. Thus, here is what needs to be done. Thewriters hereby askgeotechnical firms to research their files and attempt to provide atleast one case history that illustrates more geotechnical testing andanalyses (beyond which is typical) saved the owner substantialconstruction costs. Then, e-mail these case histories to the firstwriter, who promises to review them and possibly rank them and get
them compiled on a web page for all geotechnical engineers to use ifthey so choose. Armed with these case histories and the precedingdiscussion itmay be possible inmany cases to be able to showownersthat it is in their best interest to authorize a substantially expandedscope of work, compared with the typical scope of work. This re-search of files and submission of relevant and potentially helpful casehistories for inclusion on the web page represents a small, but im-portant, first step toward upgrade of geotechnical practice.
There is one remaining problem not addressed by the previousproposal. In a great many cases the geotechnical engineer is hired bya civil engineering consultant or a structural engineering or architectwhose fee is a percentage of the construction costs. In this case, thehirer of the geotechnical engineer has a conflict of interest in that itdoes not pay the geotechnical engineer to minimize the expectedlife-cycle costs of the project. Even if the hirer of the geotechnicalengineer is not paid by percentage, there is still a small conflict ofinterest if the hirer has a fixed fee out of which the geotechnicalengineer is paid. Obviously, it is important that the geotechnicalengineer be allowed to negotiate the scope of work with the owneror another party with a vested interest in minimizing the total life-cycle cost. In addition to the case histories, geotechnical firmscould e-mail either writer with their assessments of how serious/pervasive this problem is and/or any ideas they have for circum-venting the problem.
Response to Discusser Criticisms NotResponded to Earlier
Shallenberger Comments
The discusser devotes a large amount of space to the issue of state ofthe practice versus standard of care by assuming at the outset thatthey are identical. The writers do not accept this blanket equivalencefor reasons discussed at length previously. If the reader is persuadedby the writers, then the discusser’s comments are irrelevant, whichthe writers contend they are.
The discusser also takes issue with the number of reports thewriters reviewed in developing their Table 1 and the section re-garding the Front Range area of Colorado. First and foremost, thediscusser appears not to understand sampling theory. Specifically,once the size of the sample exceeds what is statistically a largesample (which is much less than 50), then additional sample sizecontributes almost nothing to the certainty associated with theconclusions drawn. Therefore, given that the writers’ sample sizewas far greater than a large sample, the exact size of the sample wasnot very important. Furthermore, it appears that the discusser failedto understand that once a sample is obtained, which is much largerthan a statistically large sample, the number of elements in theuniverse sampled is inconsequential. It does not matter if the numberof elements in the universe is 2,000 or infinity, provided the sampleis representative. Furthermore, the discusser’s complaints seem toimply that the writers reviewed these reports as a research projectespecially for the paper. In reality, the reports reviewed for theirTable 1 and the Front Range area of Colorado were reviewed by thewriters over the last several years; however, this was well before thewriters were selected to write the paper.
In an attempt to discredit the writers’ findings, conclusions, andopinions, the discusser claims their research is not current becausetheir reports ranged in age from 6 to 35 years. First, only the reportsfor the Front Range area of Colorado ranged in age from 6 to 33years. Furthermore, as stated (and shown) in the paper, the almostimperceptible rate of change in practices over the 27-year periodcovered would lead to the belief that the changes in the last 6 years
would be small. Also, it was completely unfair and inaccurate tolevy the age criticism at the entire database represented in the paper.For example, the writers’ Table 1 was based on about 200 reportsthat were known to have been written in the last 3 years, most in thelast 1.5 years—before the paper was submitted. The data in thewriters’ Tables 2–5 were based on input from engineers creditedadjacent to each table. All of these engineers have considerableexperience in geotechnical engineering practice, including forensicwork. All were encouraged to provide up-to-date data and thewritersare confident that they did. The discusser’s claim that the writers’findings are not current is not accurate or well founded.
Shin and Byrne Comments
Almost all of the discussers’ comments have been responded toearlier except the following. The discussers take issue with thewriters’ discussion/definition of the state of the art. First, the writersasserted that the state of the art was not a legal term and they pro-ceeded to define it as it would normally be defined for a conference,wherein one speaker is asked to present a state of the practice paperand another is asked to present a state of the art paper. The writersnever suggested that a practitioner need perform at the state of the artlevel to satisfy the standard of care. Thus, the discussers’ complaint,or the relevance of their prediction of dire consequences, is notunderstood if practitioners were required to perform at the state ofthe art level.
Discussion of “Predicting the Onset of StaticLiquefaction of Loose Sand with Fines” byRahman Md. Mizanur and S. R. LoAugust 2012, Vol. 138, No. 8, pp. 1037–1041.
DOI: 10.1061/(ASCE)GT.1943-5606.0000661
G. Mesri, M.ASCE1; and J. R. Funk, S.M.ASCE2
1Ralph B. Peck Professor of Civil Engineering, Dept. of Civil andEnvironmental Engineering, Univ. of Illinois at Urbana-Champaign, IL61801 (corresponding author). E-mail: [email protected]
2Graduate Student, Dept. of Civil and Environmental Engineering, Univ.of Illinois at Urbana-Champaign, IL 61801.
Undrained stability analyses of contractive soils, such as soft claysduring typical construction and loose sands during dynamic shear-ing by an earthquake, are commonly carried out using the undrainedstrength stability analysis (USSA), where undrained shear strengthis determined directly (e.g., for soft clays using the in situ vaneshear tests and for sands using the standard penetration tests; Arti-cle 20, Terzaghi et al. 1996). The alternative approach to undrainedstability analyses of contractive soils using the effective stressstability analysis (ESSA) in terms of shear strength parameters,such as the friction angle and effective stress at failure, is notcommon because of the difficulties associated with estimatingthe shear-induced pore water pressures and, for example, the fric-tion angle mobilized at yield (Article 20.3, Terzaghi et al. 1996).Nevertheless, information on effective stress strength parameters isvaluable for interpreting undrained shear strength of soils, includ-ing the measurements by laboratory and in situ tests used forUSSA. Therefore, the authors have made a valuable contributionby collecting and presenting data on hIS, which may be used tocompute the friction angle mobilized at yield of contractive sands,f9(mob, yield) that controls the triggering of liquefaction, in termsof the state parameter c.
However, accurate interpretation of state parameter c ofsands either directly from void ratio measurements or from in situtests such as the cone penetration test involves substantial uncer-tainties (Shuttle and Jefferies 1998; Shuttle and Cunning 2007;Ghafghazi and Shuttle 2008). Therefore, the discussers presentthe f9(mob, yield) data in terms of ðN1Þ60 of the standard penetra-tion test.
In a previous exercise to estimate in situ penetration resistanceðN1Þ60 for laboratory data on suðcritical; DSSÞ=svo9 of sands, whereDSS 5 direct simple shear, the first discusser, in the early 1990s,used suðyield, DSSÞ=svo9 versus the number of cycles Nc curveto determine suðyield, DSSÞ=svo9 at Nc 5 15 ðM5 71/2, FC# 5%Þ(where M 5 magnitude and FC 5 fines content), and suðcritical, DSSÞ=svo9 at very large Nc, where the curve leveled out[note that critical strength used in Terzaghi et al. (1996) has beentermed residual strength or steady-state strength by others]. Then,assuming suðyield; mobÞ=svo9 to be equal to suðyield; DSSÞ, ðN1Þ60was estimated from su ðyield; mobÞ=svo9 versus ðN1Þ60 relationshipof Seed et al. (1984), were used to estimate ðN1Þ60. Thus, it waspossible to correlate suðcritical; DSSÞ=svo9 from laboratory teststo ðN1Þ60 (see Figs. 20.66 and 20.71 of Terzaghi et al. 1996 andMesri 2007).
The discussers used a similar approach to estimate ðN1Þ60 forf9(mob, yield) data using Fig. 20.47 of Terzaghi et al. (1996) basedon the triaxial compression test data of Castro (1969) on bandingsand (a uniform, clean fine quartz sand with subrounded tosubangular grains, D50 5 0:16mm, minimum and maximum voidratio of 0.50 and 0.84, respectively, and critical friction anglef95 30�). The values of c, suðyield; TCÞ=svo9 and f9(mob, yield)in Table 1, where TC5 triaxial compression, correspond to the datain Fig. 20.47 of Terzaghi et al. (1996).
The following equations were used to compute suðyield; DSSÞ=svo9 and ðN1Þ60:
suðyield, DSSÞsvo9
¼ 0:64suðyield, TCÞ
svo9ð1Þ
ðN1Þ60 ¼suðyield, DSSÞ=svo9
0:011ð2Þ
For an explanation of Eqs. (1) and (2), reference is made to Eqs.20.34 and 20.38 in Terzaghi et al. (1996).
The c and ðN1Þ60 data in Table 1 lead to
ðN1Þ60 ¼ 212 131:25c ð3Þ
or
c ¼ 0:162 0:0076ðN1Þ60 ð4Þ
Eq. (3) is plotted in Fig. 1 together with the four data points in Table 1.
Eq. (6) is plotted in Fig. 2 together with the four data points inTable 1. Also plotted in Fig. 2 are the data points from the authors’Figs. 2, 3, and 5, using Eq. (5) to compute f9(mob, yield) from hISand Eq. (3) to compute ðN1Þ60 from c.
It is, in summary, possible to see values of effective friction anglemobilized at the triggering of liquefaction in terms of a more widelyrecognized soil state measure ðN1Þ60. According to Eq. (6), the mo-bilized friction angles of 10, 12, 16, 20, and 26� correspond to ðN1Þ60values, respectively, of 2, 5, 10, 15, and 20. For the sands in Fig. 2, thetypical values of critical friction angle are likely to be in the range of30–35�.
Fig. 1. Relationship between soil state measure ðN1Þ60 and state parameter c
Fig. 2. Friction angle mobilized at yield f9(mob, yield) as a function of soil state measure ðN1Þ60
Castro, G. (1969). “Liquefaction of sands.” Harvard Soil Mechanics SeriesNo. 81, Harvard Univ., Cambridge, MA.
Ghafghazi, M., and Shuttle, D. A. (2008). “Evaluation of soil state from SBPand CPT: A case history.” Can. Geotech. J., 45(6), 824–844.
Mesri, G. (2007). “Yield strength and critical strength of liquefiable sands insloping ground.” Geotechnique, 57(3), 309–311.
Seed, H. B., Tokimatsu, K., Harder, L. F., and Chang, R. M. (1984). “Theinfluence of SPT procedures in evaluating soil liquefaction resistance.”Earthquake Engineering Research Center, Rep. UCB/GERC-84/15,Univ. of California, Berkeley, CA.
Shuttle, D. A., and Cunning, J. (2007). “Liquefaction potential of silts fromCPTu.” Can. Geotech. J., 44(1), 1–19.
Shuttle, D. A., and Jefferies, M. (1998). “Dimensionless and unbiased CPTinterpretation in sand.” Int. J. for Numerical and Analytical Methods inGeomechanics, 22(5), 351–391.
Terzaghi, K., Peck, R. B., and Mesri, G. (1996). Soil mechanics in engi-neering practice, 3rd Ed., Wiley, New York.
Closure to “Predicting the Onset of StaticLiquefaction of Loose Sand with Fines” byRahman Md. Mizanur and S. R. LoAugust 2012, Vol. 138, No. 8, pp. 1037–1041.
DOI: 10.1061/(ASCE)GT.1943-5606.0000661
M. M. Rahman, M.ASCE1; and S. R. Lo21Lecturer, School of Natural and Built Environments and Barbara
Hardy Institute, Univ. of South Australia, Adelaide-5095, Australia(corresponding author). E-mail: [email protected]
2Associate Professor, Univ. of New South Wales at the AustralianDefence Force Academy (ADFA), Canberra 2600, Australia. E-mail:[email protected]
The writers thank the discussers’ interest in our paper. As stated bythe discussers, measuring the state parameter, c, in accordance withthe definition of Been and Jefferies (1985) is a challenging task. Thisis because, in the calculation of c, the critical state line (which ingeneral is a curve) needs to be established accurately. When we aredealing with sand with a range of fines content, a family of criticalstate lines (one for every fines content) needs to be determined, andthe experimental program required can be overwhelming. This isone of the benefits of the proposed concept of the equivalent granularstate parameter, cp. It can capture the effects of fines content andavoid having to determine a family of critical state lines (Rahmanand Lo 2008; Rahman et al. 2008; Rahman et al. 2011).
We agree that ðN1Þ60, or other suitable field testing results, may becorrelated empirically to various measures of liquefaction resistance.However, the role of laboratory studies cannot be overstated becausethey provide a rationale framework for identifying the relevant param-eters to be incorporated into the correlations, establishing physicallymeaningful functions for the correlations and sound warning signalson unreasonable generalization or interpretation of correlations.
The discussers proposed a linear correlation between ðN1Þ60 andc presented as Eq. (3) in their discussion. Based on the four datapoints presented for the Banding sand, the proposed correlationappears to be near perfect, rather surprising noting that “accurateinterpretation of state parameter c of sands either directly from voidratio measurement or from in situ tests such as the cone penetrationinvolves substantial uncertainties.”
Using the same Banding sand data, the discussers showed thatðN1Þ60 can be correlated to f9(mob, yield). Because hIS is
mathematically related to f9(mob, yield) by their Eq. (5) that wasused to obtain f9(mob, yield), the choice between hIS and f9(mob,yield) in establishing a correlation (rather than using alternativemeasures of liquefaction resistance) is a matter of preference. How-ever, referring to the onset of instability (or deviatoric strain soften-ing) as yield is somewhat misleading or confusing. For a normallyconsolidated loose sand, yielding occurs from the start of loading.
The discussers attempted to show that the correlation between hIS,orf9(mob, yield), and ðN1Þ60 also applied to our data sets.Because ourdata do not contain any information on ðN1Þ60, the discussers calcu-lated ðN1Þ60 using their proposed linear correlation (by making theadditional assumption that it applies equally to cp). Mathematically,this process simply equates to a transformation of the x-axes fromcp toðN1Þ60. Because there is a correlation between hIS and cp, as estab-lished in our article, any transformation of cp to X will always showa correlation between hIS and X, irrespective of the transformationequation and the physical meaning of X. Thus, this exercise, whichresults in our data points presented in Fig. 2 of the discussion, does notprovide any meaningful evaluation of the proposed correlation.
To illustrate this mathematical point, we deliberately proposeda wrong correlation between cp and ðN1Þ60 expressed as Eq. (3a):
ðN1Þ60 ¼ 2ðC12C2cpÞ ð1Þ
where C1 5 21 and C2 5 131:25 (i.e., the same as those used in thediscussion). Eq. (1) is referred to aswrong because it contradicts Eq. (3)in the discussion by a factor of two.We followed through the process ofconvertingcp to ðN1Þ60 but instead used the wrong Eq. (1).We plottedthe resultant data points in Fig. 1 usingf9(mob, yield) instead ofhIS forthe y-axes for easy comparison with the discussers’ Fig. 2. Noting thatboth figures were plotted with the same data source (Rahman and Lo2012), a similar-looking correlation is observed. Indeed, the best-fitcurve for our data points for Fig. 2 (of the discussers) and Fig. 1[using the wrong Eq. (1)] is the same. If we calculated the scatteraround the best-fit curve objectively with the RMS deviation (RMSD),we end up with exactly the same value of 1.808 for both figures.
References
Been, K., and Jefferies, M. G. (1985). “A state parameter for sands.” Geo-technique, 35(2), 99–112.
Fig. 1. Misleading correlation between f9(mob, yield), and ðN1Þ60based on transformation of the x-axes from cp to ðN1Þ60 by a wrongEq. (1); RMSD 5 RMS deviation
Rahman,M.M., and Lo, S. R. (2008). “The prediction of equivalent granularsteady state line of loose sand with fines.” Geomechanics and Geo-engineering, 3(3), 179–190.
Rahman, M. M., and Lo, S. R. (2012). “Predicting the onset of staticliquefaction of loose sand with fines.” J. Geotech. Geoenviron. Eng.,138(8), 1037–1041.
Rahman, M. M., Lo, S. R., and Baki, M. A. L. (2011). “Equivalent granularstate parameter and undrained behaviour of sand-fines mixtures.” ActaGeotech., 6(4), 183–194.
Rahman, M. M., Lo, S. R., and Gnanendran, C. T. (2008). “On equivalentgranular void ratio and steady state behaviour of loose sand with fines.”Can. Geotech. J., 45(10), 1439–1455.
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