April 2019 Print

President's Report

After a long and snowy winter, spring is finally in the air. I love spring and to see all the flowers begin to bloom and leaves growing on the trees. This is a time of new beginnings, not only for nature but for our members. Many of our members are graduating this spring with their undergraduate or graduate degrees and beginning their careers. This is an exciting part of a graduate’s life as they transition from student to working professional. I had the opportunity last week to go to Utah State University Eastern to watch my daughter graduate with her Associates degree. She plans to continue her education at Utah State University in Logan and pursue her bachelors and master’s degree in Communication Disorders. I couldn’t talk her into studying engineering. Regardless of each person’s course of study I want to offer my congratulations on your accomplishments. As you finish your schooling and begin, or continue, your career I strongly suggest that you continue your membership in ASCE. Your degree is the beginning of your career and ASCE will help you to continue to grow and progress. ASCE offers fantastic networking leadership, service, and learning opportunities. I also encourage the younger members to join the ASCE Younger Member Forum (YMF) and become active in that organization. The YMF members are doing great things and I am excited to watch and support them as they continue to improve the engineering profession.

Our annual Utah Section meeting is coming up next month and I am looking forward to meeting with all of you. Our Region 8 Director, Tony Lau, will be attending the meeting and speaking to us. Tony has been involved with ASCE for many years and, as part of his current role as Region Director, sits on the National Board of Directors for ASCE. The annual meeting will be held on June 7th at the New BYU Engineering Building. We will have a breakfast for the incoming and outgoing officers at 9:30 and the annual Section Meeting will begin around 11:30. Please plan on attending the meeting and bring your co-workers as well. This is a great networking opportunity along with an opportunity to hear from our Region leaders and ask questions.

The ASCE Utah Infrastructure Report Card update is progressing. Our committee met a week ago to review our strategic plan and update our committee member list. This will be a team effort, so we will continue to look for committee members to help us with the report card. It is important that this be a collaborative effort with technical experts, government leaders, planners, and policy experts. Our next step is to schedule training with ASCE Societal staff that support State Infrastructure Report Cards. The goal of the report card update is to provide the Utah Legislature with the necessary information to fund Utah’s infrastructure needs. The last report card that was released in 2015 estimated a $70 billion investment in infrastructure over the next 40 years. As Civil Engineers we have the responsibility to provide recommendations to our government leaders on infrastructure needs so that we can maintain an improve the quality of life for our communities.

I’ll see you all at the annual Utah Section meeting.

 

Craig Friant, P.E., M. ASCE

ASCE Utah Section President

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Technical Article

Regarding Liquefaction Mitigation Provisions of ASCE 7-16

Travis M. Gerber, PhD, PE; Chief Engineer, Gerhart Cole

Ryan B. Maw, PE; Principal, Gerhart Cole and Chair of ASCE’s Utah Geo-Institute

Once upon a time in the very recent past (being 2010), ASCE Standard 7-10 “Minimum Design Loads for Buildings and Other Structures” required that an evaluation of liquefaction hazards, including total and differential settlement be performed. However, specific foundation design and performance requirements (apart from foundation ties in specific cases) were not provided. The standard only directed that a geotechnical study report discuss potential mitigation measures, which might include structural systems to accommodate anticipated displacements and forces as well as ground improvement. As a result, there has been a wide range of recommendations and professional opinions on means and methods of foundation assessment and design approaches to address seismic performance of structures on potentially liquefiable sites.

With this article, we wish to discuss some of the design and performance requirements relative to liquefaction as contained in the newer ASCE 7-16. We will discuss 1) the nature and origin of some of these requirements, 2) some of the challenges in applying the requirements, and then 3) identify means of overcoming such challenges while improving local engineering practices.

Within ASCE 7-16, specific guidance is now provided relative to design performance requirements for structures located on liquefiable sites. For example, the method for calculating the tie design strength has been added. Also now provided is an upper limit on the amount of horizontal ground displacement resulting from lateral spread, beyond which deep foundations are required. (For those readers who are interested, this upper limit is 18 inches for structure risk categories I or II [see Table 12.13-2 in ASCE 7-16 for thresholds of other risk categories]). There are also differential settlement thresholds provided for shallow foundations. These thresholds vary by structure type and are expressed in terms of a specified differential settlement threshold coefficient times the length of interest (L) over which the differential settlement is to be assessed.

For lower risk category structures (i.e., I or II), the differential settlement threshold in the case of a multistory, relatively stiff (brittle) structure consisting of a concrete or masonry wall system is 0.005 L. The coefficient decreases (representing increasing stringency) to 0.003 and 0.002 for risk categories III and IV, respectively. Table 12.13-3 in ASCE 7-16 (Differential Settlement Threshold) showing the thresholds for other structure types and risk categories has been replicated herein for reference.

 

Risk Category

Structure Type

I or II

III

IV

Single-story structures with concrete or masonry wall systems

0.0075 L

0.005 L

0.002 L

Other single-story structures

0.015 L

0.010 L

0.002 L

Multistory structures with concrete or masonry wall systems

0.005 L

0.003 L

0.002 L

Other multistory structures

0.010 L

0.006 L

0.002 L

 

It should be noted that the differential settlement thresholds for low risk categories are less stringent than those typically assumed for non-seismic, service-state conditions. For example, assuming 25-foot spacing between columns, a differential settlement of one-half to three-quarter inch of settlement might be considered acceptable for many buildings under static loading conditions; this equates to a threshold of 1 in 600 to 1 in 400, (or 0.0017 L to 0.0025 L, respectively). Under the thresholds of ASCE 7-16 for risk categories I or II, the acceptable differential settlement in the case of the relatively stiff, multistory structure would be 1.5 inches over the same 25 feet. Acceptance of a greater amount of differential settlement as a liquefaction-related performance threshold relative to service-state conditions should be expected because: 1) the case represented is an extreme case where the performance goal is life-safety, not damage avoidance, and 2) these thresholds appear to inclusive of all settlement, in which case the settlement under static-loading would be added to the liquefaction-induced settlement.

In reviewing the range of thresholds provided in ASCE 7-16, one observes that for risk category IV structures (i.e., essential facilities), the differential settlement threshold is 0.002 L regardless of building type. This independence of type is due to the immediate/continued functionality requirements for such structures and the results of fragility studies which suggest the jamming of doors initiates at a drift limit of about 0.002. Apart from risk category IV, the remainder of the thresholds derives from the study of excavation-induced settlement of buildings by Boscardin and Cording (1989). In their study, Boscardin and Cording correlated observed horizontal extension, angular distortionm and degree of cracking damage for brick bearing wall structures. At low levels of horizontal strain and for cases corresponding to the self-weight building settlement, “moderate to severe” damage was found to occur when angular distortions ranged from 1/150 to 1/300 (which correspond to approximately 0.0067 L and 0.0033 L, respectively). As stated previously, the ASCE 7-16 threshold for a multistory, risk category I or II structure consisting of a concrete or masonry wall system is 0.005 L, which is in the middle of this moderate to severe damage range identified by Boscardin and Cording. Consistent with a higher risk category of III, the ASCE 7-16 threshold for the same building is a lower value of 0.003 L, which is essentially the boundary between Boscardin and Cording’s ranges for “slight” and “moderate” damage. One can complete the ASCE 7-16 matrix of thresholds by using thresholds for single story structures which are about 50% higher than for multistory structures, and using thresholds for more ductile structures which are about twice that of less ductile structures.

Beyond understanding the origins and interrelations between the differential settlement thresholds in ASCE 7-16, we find the fact that these performance criteria are based on differential settlements rather than total settlements to be both heartening and problematic. It is heartening in the sense that ASCE 7-16 presents design criteria which reflect structural performance as being dictated by the deformations and stresses induced by differential settlement. We believe that provisions of ASCE 7-16 help inform its users about limitations of shallow foundations. This, in turn, should help overcome a tendency which we believe exists within design and development communities of too frequently overlooking and/or dismissing issues of liquefaction-induced deformations (whether total settlement, differential settlement, or lateral spread) which can sometimes be viewed as being unwieldy in the interest of pursuing expedited, lower cost design and construction.

On the other hand, the performance criteria of ASCE 7-16 are problematic in the sense that there are challenges associated with evaluating performance in terms of differential liquefaction-induced settlement. One such example is current, local geotechnical engineering practice in which foundation evaluations are typically presented with liquefaction-induced settlement expressed in terms of a total, likely not to exceed, settlement. Sometimes, a range of calculated settlements is presented, representing the number and type (e.g., test holes with standard penetration test (SPT) blowcounts or cone penetration test (CPT) soundings) of geotechnical study locations. It is uncommon for an explicit estimate of differential liquefaction-induced settlement to be presented in a geotechnical study. Yes, some statement regarding differential immediate or consolidation settlement under static service loads is usually provided (such as “differential settlements of up to three-quarters of the total settlement may be expected”), but similar statements regarding liquefaction-induced differential settlements are typically absent. As such, with ASCE 7-16’s differential settlement thresholds, changes are needed with respect to the type of information commonly presented in geotechnical engineering study reports.

Another particular challenge in using the differential settlement thresholds presented in ASCE 7-16 is obtaining a sufficient amount of spatially distributed subsurface data with which to make a reasonable estimate of differential settlement. With only one test hole or CPT sounding, one is limited to only calculating settlement at that particular location. Additional study locations (i.e., soundings or test holes) are needed to calculate settlement at other locations, and from such information a differential settlement can then be quantified. It should be noted that all of the study locations should be deep enough to capture the vertical extent of liquefiable strata as well as to allow for development of potential ground improvement alternatives if needed to satisfy ASCE 7-16 requirements. Owners, architects, and engineers need to be willing to invest the resources necessary to obtain the data required to assess liquefaction / lateral spread hazards and then evaluate mitigation strategies as needed. Here we note that ground improvement and deep foundation contractors need subsurface information to provide designs. It has been our experience that relegating the performance of any supplemental field studies to specialty contractors during the bidding period frequently leads to higher projects costs, adverse performance risks, and/or claims. We recommend that a more holistic approach be taken during initial design phases of a project, using phased field studies as necessary to 1) collect needed data, 2) allow the geotechnical engineer to perform meaningful evaluations of potential mitigation strategies and coordinate with the structural engineer to integrate this information into structural design, and 3) allow the specialty contractor to provide a final design with realistic costs at the time of bidding.

In returning to the matter of evaluating differential settlement, at the extreme and absent information on the spatial variability of settlement, one might assume that the amount of differential settlement equals the amount of maximum total settlement. While this approach in itself may be conservative, use of a single study point does not forcibly reveal information regarding site variability, and the representativeness of the single point cannot be directly evaluated. Hence, settlement calculated at one point (or only a couple of points) may not accurately quantify the maximum total liquefaction settlement at a site. We note that engineering empiricism may help fill data gaps. For example, as discussed in “Special Publication 117A, Guidelines for Evaluating and Mitigating Seismic Hazards in California” (CGS, 2008):

“Any prediction of liquefaction-related settlements is necessarily approximate, and related hazard assessment and/or development of recommendations for mitigation of such hazard should, accordingly, be performed with suitable conservatism. Similarly, it is very difficult to reliably estimate the amount of localized differential settlement likely to occur as part of the overall predicted settlement: localized differential settlements on the order of up to two-thirds of the total settlements anticipated should be assumed unless more precise predictions of differential settlements can be made.”

While acknowledging and potentially implementing this rule-of-thumb, it is important to remember the context of its use: the initial hazard assessment itself should be conservative.

It has been our experience that in some instances, site variability (and hence in a large measure differential settlement) is small. In other instances, site variability is extremely high, with no liquefaction occurring at one geotechnical study location and appreciable liquefaction settlement expected to occur at another location, both locations being within the footprint of the same structure. Such experience echoes the need for comprehensive geotechnical studies when liquefaction is potentially in play, and highlights the limitations of rules-of-thumb.

Regardless of issues associated with site variability and the spatial extent of subsurface data, it should be recognized that evaluation of liquefaction-induced settlement is frequently over-simplified. Much of recent, local practice involves calculating the post-event consolidation (i.e., volumetric reduction / settlement from liquefaction triggering) that occurs as excess pore water pressures that develop in a loose soil matrix during seismic shearing dissipate. In reality, liquefaction induced settlement results from three mechanisms. The first mechanism is the aforementioned volumetric deformations (i.e., post-event consolidation). The second mechanism is shear-induced deformation which is a bearing capacity-type failure and/or soil-structure interaction which induces a downward “ratcheting” of the structure. The third mechanism is the physical loss of ground mass due to ejecta, commonly manifest as soil boils created as pressurized pore water exits the ground surface, causing internal erosion / transportation of soil particles. In the past, the engineering community’s ability to quantify the second and third mechanisms has been limited; hence, the limited focus on calculating and reporting of volumetric deformations. With a growing catalog of case histories (notably provided by the 1999 Koceli, Turkey earthquake, the 2010-2011 series of strong earthquakes in Christchurch, New Zealand, and the 2011 Tohoku, Japan earthquake), methods for accessing these mechanisms are more readily available. We encourage our local engineering community to engage in the use of these more complete liquefaction assessment methods.

In closing, we believe guidance provided in ASCE 7-16 helps better define acceptable magnitudes of liquefaction-caused lateral displacement and settlement for foundation systems. To meet the seismic performance objectives of ASCE 7-16, we believe comprehensive geotechnical field studies which include meaningful assessments of site variability are needed; changes in the types of information usually reported in geotechnical study reports are needed as well. It should be recognized that such efforts will likely require a greater commitment of resources in terms of budget and schedule by all parties involved in building projects.

References

American Society of Civil Engineers [ASCE]. (2010). Minimum Design Loads for Buildings and Other Structures. ASCE Standard ASCE/SEI 7-10.

American Society of Civil Engineers [ASCE]. (2016). Minimum Design Loads for Buildings and Other Structures. ASCE Standard ASCE/SEI 7-16.

Boscardin, M.D. and Cording, E.J. (1989). “Building response to excavation-induced settlement.” Journal of Geotechnical Engineering. ASCE. 115(1), 1–21.

California Geological Survey [CGS]. (2008). Guidelines for Evaluating and Mitigating Seismic Hazards in California. Special Publication 117A, California Department of Conservation.

 

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Northern Utah Branch Update

In April we enjoyed a great presentation by Michael Smith from ACEC. He was able to update us on all the current legislation related to Civil Engineering.

For May we have a technical presentation planned on the 16th. Bill Young (Logan City Engineer) and Jon Powell (JUB) will be sharing their learning experiences about roundabouts. This type of intersection is becoming more prevalent in modern transportation and this will prove to be an interesting presentation.

As we come to the close of a great year, I would just like to thank everyone who has participated in any of our events. Attendance numbers have been great and it has been a special opportunity to be able to associate with all of you.

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Central Utah Branch Update

Luncheon

The CUB met with the BYU Student Chapter for their Capstone presentations and luncheon.  The students presented projects ranging from design of structures to remote GIS based flooding estimations in the Dominican Republic.  The students presented their posters and answered questions.  The presentation was well attended by BYU students and the CUB members.

 

The upcoming luncheon for May will include discussion of challenges at the Mill Site Reservoir renovation project and changes in the water regulatory environment in the state of Utah.  The meeting will be held on May 16, 2019.  More details to follow.

 

Call for Presentation Ideas

CUB is still looking for presentation ideas for the upcoming months.  We are interested in having presentations on projects, research, historical information, and ethics discussions from across the civil engineering spectrum (geotech, transportation, water, structures, etc…).  Please contact Ben Willardson at [email protected], or call him at (801) 310-6153 if you are interested in presenting to the group.  We would love to hear from you.

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Utah Geo-Institute Chapter

Utah Geo-Institute (G-I) Chapter By Ryan Maw and Taylor Hall

It was great to see so many in attendance at the 2019 Cross Country Lecture by Paul Mayne, PhD, PE this past month. As we move towards a field work filled Summer, we wanted to extend an invitation to our members to participate in the upcoming ‘Recent Revisions to Seismic Ground Motion Provisions of IBC 2018 and ASCE 7’ workshop on June 10th. This event represents a unique effort, which the G-I is championing along with several other organizations, to address industry code changes that were recently voted into state law in March. Details on the event are attached to the Civil Source article and we strongly encourage attendance for what will surely be a significant event industry wide.

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Announcements

Payment for the event can be made through PayPal to [email protected] or by clicking the payment link https://www.paypal.me/ASCEUtahSection.           

                                               

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