10_4_fall
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http://www.braunintertec.com/0/pdf_files/10_4_Fall.pdfTRANSCRIPT
Vol. 13 Issue 4 PRACTICAL AND ENTERTAINING SINCE 1997 Fall 2010
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THE POCKET
Construction considerations for when temperatures drop
By Chris Kehl, PE
When the temperatures drop in the Midwest, there is a good chance it will eventually mean frost in the ground. For those involved in building and roadway construction, the cold
Chris Kehl, PE
A chill in the air and frost on the ground Cold weather can affect a wide range of construction materials. PVC (polyvinyl chloride) pipe can be particularly brittle in freezing temperatures, as evidenced by this drain pipe, which broke after being dropped.
threatens to thwart compaction efforts, heave and crack foundations and slow the curing of concrete and masonry construction.
Frost in the Soil Upon freezing, soils can experience positive volume change. Since soils are generally confined laterally, this volume change occurs vertically in the form of heave. Soils susceptible to heaving have two things in common: (1) they are generally fine-grained, consisting mainly of silt and/or clay, and (2) they are generally saturated or nearly so, the source of moisture from groundwater, moisture in the soil, precipitation or irrigation. Of course, frost-susceptible soils also have to be exposed to freezing temperatures for these changes to occur.
Sands and gravels are generally not frost-susceptible. Clays and clayey sands are only moderately frost-susceptible because they are not sufficiently permeable to accommodate much moisture transfer. Silts are generally the most frost-susceptible because they best meet the criteria for frost heave, but silty clays and silty sands can also be highly frost-susceptible.
When soil moisture freezes, it expands, but frost heave does not directly result from the expansion of liquid phase water to solid phase frozen water on soil particles. The vast majority of heave results from the creation of ice lenses that form within the soil, displacing it vertically as the lenses grow. The thermodynamics of
the process are complicated but favor the attraction of soil moisture to single sources whose vertical spacing and horizontal limits reflect the soil composition and abundance of moisture. Ice lenses may be numerous and only fractions of an inch thick, or less frequent but an inch or more thick, and still able to expand the thickness of the frozen soil by 5 to 10 percent. This can be significant with the depth of frost penetration varying from 1 to 3 feet in southern Iowa, 3 to 5 feet in southern Minnesota, Wisconsin and South Dakota, and 5 to 7 feet in northern Minnesota, northern Wisconsin and North Dakota.
The depth of frost penetration and the amount of frost heave that occurs can be estimated analytically, though the calculations are quite complex. Soils are rarely homogenous and weather patterns vary. Factors that can affect the rate and depth of freezing may include the time during which freezing temperatures are maintained, peak high and low temperatures experienced during the winter and the presence and depth of snow cover. Compacted soils, prepared building and roadway subgrades, and buried structures (foundations, utilities, etc.) can be protected from the unfavorable effects of freezing in several ways. Buildings can be enclosed and heated, foundations can be blanketed, and prepared subgrades can be insulated with additional sacrificial soil, straw or even snow.
See FROST - Continued on page 6
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See FOUNDATIONS - Continued on page 3
By Matt Glisson, PE
Part 1 of a two-part series: Deep foundations are perhaps the most robust option for supporting a structure. Sure, there are other options — shallow foundations (spread footings), intermediate foundations (rammed aggregate piers) and ground improvement methods — but none are as impressive in their own right or carry
Deep Foundations Part I: Serious Solutions for Serious Loads
depth. Geometrically, a deep foundation can be defined as having a length (depth of embedment) significantly greater than its width. Deep foundations can be drilled, driven, screwed or pushed into the ground. In some cases, they are actually excavated by hand or with typical earthwork equipment, concrete is then cast to fill the excavation. We choose to use deep foundations for many different reasons. The most common reasons include: • Penetrating relatively weak, near-surface soils. • Penetrating highly compressible soils, such as organics or weak clays at depth. • Penetrating deep, contaminated, near-surface soils. • Penetrating deep, uncontrolled fill. • Accommodating large loads.
One can classify deep foundations in many different ways based on their purpose, type, construction method, displacement or capacity. The table below is not an exhaustive list, but covers the most common deep foundation types in our region. In addition to material strength and diameter, typical load capacity also varies with subsurface conditions at the site. This concept leads us to the design of deep foundations.
a load like a pile. Besides, piles provide so many options of their own that there’s insufficient room here to discuss anything else. Deep foundations are more than we can handle in just one article! So, this is Part I, which will discuss the different types of deep foundations and some of what goes into designing them. Part II, in our next issue, will address quality control and integrity testing — in other words, construction.
What are Deep Foundations and When are They Used? In general, deep foundations, often generically referred to as piles, are long, slender, structural elements that transfer the load from a structure (building, bridge, embankment, etc.) through supporting media or to a supporting medium (soil and/or rock) at
Matt Glisson, PE
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Deep Foundation Design Today, most of the design methodology in the United States for deep foundations is from work published by the Federal Highway Administration (FHWA), since they have the money and interest to conduct research and present findings. Various trade groups in the deep foundation community also contribute to the design of deep foundations. In concept, the design of deep foundations is relatively simple. The sides of the pile provide resistance to load through friction between the pile and the soil it penetrates. The bottom of the pile provides resistance to load through bearing like a typical spread (shallow) footing. However, the mechanics at work are more complex in practice, especially due to the nature of soil and rock behavior and the inherent variability of soil and rock. When you add tension, moment and lateral loads to the compression loads that a pile supports, the evaluation of a pile’s capacity and the selection of the right pile type becomes even more difficult and iterative.
Deep foundation design starts with developing an understanding of the subsurface conditions at the project site, as well as knowledge about what is to be built, and the performance expectations and risk tolerance. Once subsurface conditions and building loads are established, suitable pile types for design can be selected based on how easy or difficult the installation will be. For example, driven piles or auger-cast piles may not be appropriate for a site where bearing soils contain cobbles and boulders mixed with weak or even competent soils. Likewise, drilled shafts may not be the best choice for a site with contaminated soils, the disposal of shaft spoils being potentially costly and difficult.
After identifying suitable pile types, the side and end bearing resistance of the different pile types and a range of lengths and widths or diameters is evaluated. This evaluation consists of developing a model of subsurface conditions and assigning soil or rock strength parameters that impact the friction or bearing capacities of different types of pile. With the models developed, next consider the different limit states to which the pile is subjected, such as strength in axial compression or tension, settlement and
Pile aren’t reserved for building structures alone. These H-piles were driven along the downslope edge of a road in Bismarck, N.D., to help stabilize a landslide that had damaged the road. Note the angle at which these piles were driven.
lateral deflection. Depending on the results of the limit state evaluations, the pile size, length or strength required to meet the performance and risk requirements of the project is adjusted.
Although it has most likely been going on throughout the previous steps, communication with the project team (the owner, structural engineer and contractor) reaches a critical point now. The team needs to evaluate the costs and performance of the different pile types. Depending on the outcome of the discussion, the group may choose to consider additional pile types or sizes or eliminate some from consideration. Hopefully, the team is able to identify the perfect pile type and size and we can move to the next step of the process, construction.
FOUNDATIONS - Continued from page 2
Steel reinforcement adds critical axial strength in compression and tension, and lateral strength to drilled shafts. The reinforcement cage is instrumented (see O-cell at bottom of cage at right) to evaluate load distribution in the shaft after construction.
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Ron Shaffer, PE
Geotechnical Reports: What they tell you and what they don’tBy Ron Shaffer, PE
When Jack Braun, our founder, started writing soil reports* in 1957, a typical report was a concise, half-page letter summarizing soil and groundwater conditions
at two to four shallow borings that also presented a few recommendations. The report format was suited to that simpler, direct construction industry. However, as the construction industry and built environment became more complicated, so have soil reports. Today, the same two to four boring report tends to be more complex, averaging between eight and 12 pages in length. These reports are carefully assembled after the geotechnical engineer analyzes the background information, classifies the soils returned from the borings, reviews laboratory test results and applies his experience and judgment to the project. The reports also discuss the limits of what can be expected from the evaluation, considering the scope and cost of the evaluation.
The Evaluation Process The first step in the geotechnical report process is typically developing an agreement between the geotechnical engineer (GEO) and the client. Frequently, a member of the client’s design/construction team assists in negotiation of the contract. A primary element of the contract is the agreed-upon scope of services. It is important that the GEO exercise his or her judgment and experience in developing — not just pricing or executing — the scope of services. His/her experience comes into play particularly with layout of the borings, the types/numbers of laboratory tests and specialty field tests, as well as the design and construction items to be addressed in the “Recommendations” section of the report.
Upon completion of fieldwork, the geotechnical report is assembled and tailored to the project. Usually the project is an improvement to a site the client owns, such as a building,
pavement, holding pond, etc. While members of the construction team may find the report useful, it is not prepared specifically for them. A contractor bidding on the project may have to conduct his or her own exploration, for example, to further map-out the soil or rock or groundwater conditions that he or she is particularly interested in learning more about.
The Geotechnical Report The geotechnical report should contain the GEO’s interpretation of the site conditions as they impact the client’s project. The GEO will frequently discuss various risks associated with different construction alternatives. This discourse is necessary as geotechnical engineering cannot be as exact as other engineering disciplines. The GEO must interpolate the subsurface conditions between borings and other field sampling locations. Obviously, a good deal of judgment is necessary in the interpretation. The client and design/construction team need to understand the basis of the GEO’s recommendations, and this discourse accomplishes that by transitioning the reader from the results (deep soft soils and shallow groundwater, for example) to the recommendations (driven piles for building support in lieu of pursuing excavation and backfill to accommodate spread footings).
The geotechnical report will not typically address specific environmental issues, even though the geotechnical engineering company may have an environmental department. However, recently more geotechnical reports for projects in urban areas are at least making clients aware of environmental issues and addressing
Our geotechnical reports contain information that not only brings the project team together, but transitions them from design to construction. In addition to dealing with the structural “meat and potatoes” of construction (foundations, slabs and pavements), we help address the project’s civil needs (drainage control and infiltration) and contractor needs (moisture control, subgrade stabilization).
See REPORTS - Continued on page 5
* A check of our project archives revealed that Mr. Braun’s four-boring report cost about $480. Today, a four-boring geotechnical report would cost about $3,200, a sixfold increase. For comparison, in 1957 a Clark (candy) Bar cost $0.05. Today a Clark Bar — if you can find one — will set you back about $0.65.
Author’s Note
their general impact on design and construction. An example of this would be the challenges associated with the reuse of environmentally impacted soils. Regulations governing environmental issues, along with site-specific protocol for mitigating those issues, are addressed in companion Environmental Site Assessment (ESA) and other related documents.
The “final geotechnical report” is something of a misnomer. The typical geotechnical report likely will not detail every excavation and earthwork issue that could possibly come up during site development. Such a report would be nearly the size of a book and difficult to effectively understand. It is important, however, that the report at least address site- or area-specific geologic issue that will influence design and/or construction. For example, a report prepared for a site grading project in northern Minnesota should discuss frost issues but will generally not address seismic performance. The opposite holds true for the same project in, say, St. Louis, an active seismic area.
Keeping the Report in Perspective A geotechnical report should not be considered a specification; rather, it is a technical paper providing analysis and recommendations. Specifications can be developed from elements of the report, but it is not the purpose or intent of the report to be provided as a specification. Nonetheless, it is important that the geotechnical report be referenced or, preferably, made a part of construction documents so the various parties, contractors included, can review available information, opinions and recommendations. Providing only a segment of the geotechnical report or none of it may seem a safe alternative to giving out “too much information,” but this approach is usually viewed as short-sighted and not in the best interest of the overall project.
That said, the most important part of the report is the name and phone number of the engineer who prepared the document. If it is your project, give the engineer a telephone call and spend 10 minutes going over the report. Most engineers will welcome the call as an opportunity to stay in contact with the project. At the very least, you will come away knowing you interpreted the report as the engineer intended. Nearly 40 years ago, one of my college professors observed that the best report is a one-page letter followed up with a 10-minute conversation with the client. Most modern geotechnical reports won’t condense into one or even two pages, as important details would be lost. However, the 10-minute conversation is timeless advice.
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REPORTS - Continued from page 4
Ask the ProfessorBy Charles Hubbard, PE, PG
Charles Hubbard The title of this essay — brash, humorous and telling — came from an insightful technical
mentor of mine. It is not a criticism but a reminder that the analytical models we apply to technical problems are more than just a means of obtaining numerical solutions. They are also (and perhaps more importantly) tools by which we study and strive to understand the mechanisms that control structure performance.
In geotechnical engineering, analytical modeling involves the application of strength, stress/strain and/or hydraulic material properties and boundary conditions to slope stability, structure deformation and seepage problems under steady-state and transient conditions. Analytical models can predict structure performance for design and construction and replicate structure performance post-distress/failure.
Analytical models are textbooks in action. They reveal graphically how pore water pressure builds and dissipates beneath rising embankments. They demonstrate the influence of time on excavation bottom stability and retention structure deflection. As powerful and enlightening as they are, however, analytical models must be used with an awareness of what processes and responses we are attempting to mimic.
I was reminded of this while helping my high school sophomore with her math. Like many young adults balancing academics, extracurricular activities, social networking, driving instruction, etc., she is often more focused on a solution to the problem at hand than understanding how the problem should be structured. Approaching our own problems this way casually risks forcing inappropriate or unrealistic solutions from them.
Analytical models are only as good as the information from which they are developed. They can be too simple given what we don’t know, or too complex given what we do know. Model complexity and reliability must also be supported by appropriate exploration and testing programs: You can’t expect to estimate pile deflection to the nearest tenth of an inch based on shear strength correlations with N-values.
Keep in mind, too, that numerical accuracy may not be the most important outcome of the modeling process. Yes, codes and specifications require that minimum values be achieved for design and construction. But knowing that the results are of an appropriate magnitude or scale alone will tell us if we have a good understanding of our problem and are therefore on track to a good solution.
“No Models are Good, Some are Useful”
of a building, it is common to extend foundations an additional 1½ feet for frost protection.
In the upper Midwest, frost heave can be a long-term (post-construction) nuisance unless measures are taken to limit its presence. Frost-susceptible soils can be removed from beneath or beside foundations and slabs and replaced with soils that are not frost-susceptible (generally sand having less than 5 percent of the particles by weight passing a #200 sieve). Drainage of the sand may be required so that water does not accumulate. The potential for soil saturation can be reduced by grading surfaces to slope away from buildings and exterior slabs and pavements, and limiting irrigation of lawns and landscaping near building perimeters and exterior slabs and pavements. In some situations using insulation to limit the penetration of frost below exterior slabs and pavements can also be considered.
Work with your geotechnical engineer to evaluate frost heave potential and develop plans for its mitigation. Unless global warming gives us a climate more like that which Honolulu experiences, we’ll have to protect our projects from freezing soils and temperatures.
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Cold Weather Construction Freezing soils are particularly troublesome when building or pavement construction commences in the late fall or early winter and freezing temperatures are sustained, or at least present, during the morning and/or evening hours of the day. Frozen soils cannot be blended or moisture conditioned. They cannot be uniformly compacted, assumed to be free of voids and thus of sufficient strength to support foundations, or immune to potentially unfavorable total and differential settlements. Surfaces to receive fill and exposed foundation, slab and pavement subgrades can freeze overnight.
Foundations constructed on frost-susceptible soils should be immediately backfilled or insulated until curing is complete; even frozen soils can be used as insulation provided they are removed and recompacted once thawed. Interior footings may be particularly vulnerable during construction, as they often do not extend below the depths to which freezing will occur during
FROST - Continued from page 1
Work with your geotechnical engineer to evaluate frost heave potential and develop plans for its mitigation.
construction. In the absence of artificial heat, it is often necessary to extend interior footings below the depth of frost penetration or insulate them once backfilled (insulation required over an area extending at least as far outside the outer edges of the footings as frost is suspected to penetrate below the footings).
Frozen soil isn’t the only construction material that we need to be concerned about during cold weather. Fresh concrete and masonry mortar require above-freezing temperatures for the chemical reactions to occur that allow these materials to gain strength and for their water content not to freeze. Strength gain essentially stops in freezing weather and unless concrete, grout and mortar have achieved sufficient strength prior to exposure to freezing temperatures, they can be irreversibly damaged. Usually, concrete must be protected until it has achieved a compressive strength of at least 500 pounds per square inch (psi) to be considered not at risk to frost-related damage. This may require protection for anywhere from a day to a week or more. Because concrete gains strength based on time and temperature, curing can be monitored through the use of cast-in-place-punch-out-cylinders (CIPPOC), which allow samples of the in-place concrete to be collected and tested at early ages or through embedded thermometers. At low compressive strengths, however, concrete’s ability to support construction loads may still be limited, so it is prudent to protect concrete as long as possible to accelerate its strength gain.
Springtime Blues Soils generally thaw from the surface down. As a result, excess moisture from snow and ice lenses initially cannot percolate downward through the soil profile. In their saturated and potentially heaved condition (the soils are not able to consolidate until the moisture has left), the soils can be very weak with their natural or compacted strength compromised to varying degrees. These are the conditions that drive local road restrictions. Under such conditions, it may not be possible to travel or work on building or pavement subgrades or to spread and compact fill. Basement and other below-grade surfaces exposed to freezing temperatures during the winter can take time and require artificial heat to speed thawing and drying if encapsulated by above-grade construction before thawing is complete.
Building Performance In Minnesota, the building code requires that heated structure foundations built south of a line extending roughly through
St. Cloud be placed at least 42 inches below the ground surface. North of St. Cloud the minimum embedment increases to 60 inches. Similar requirements exist in Wisconsin, Iowa and the Dakotas. In cold storage areas or outside of the heated portions
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By Jamison Langdon, GIT
Tiny bubbles, as described by Hawaiian crooner Don Ho in the song by the same name, are a good thing — when it comes to sparkling wine. But in the world of concrete, tiny bubbles can be a source of trouble.
Throughout the past few years, there has been a slight rise in the number of lower-than-anticipated concrete compressive strengths for exterior concrete. At first glance, everything
appears normal: the concrete temperature, slump and air content were within tolerance at the time of placement, and the samples were allowed to cure for the required amount of time before being transported to the laboratory. Seven days later, however, the seven-day compressive strength test results come back low — very low. The trouble? Tiny bubbles — really tiny bubbles. This situation occurs in a low percentage of low strength compression tests.
The traditional method of testing concrete for air content in the field is per ASTM (formerly the American Society for Testing & Materials) C 231, “Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method.” This method is most reliable when the size of the air bubbles in the concrete is greater than 50 microns. When the bubble size wanders below 50 microns, which is less than 0.002 inches or finer than a No. 200 sieve, the air meter may have difficulty detecting the bubbles. In the circumstances where numerous tiny bubbles are present, it is not unusual to determine an air content of seven percent in the field per the pressure method, while the actual air content may
Jamison Langdon, GIT
exceed 10 percent. In general, as the air content increases, the compressive strength will drop, so it is not surprising that a low seven-day compressive strength will be hard to reconcile. Although the strengths have been reduced, the increased air content should have little effect on the abrasion resistance, creep, permeability and the drying shrinkage and may improve the concrete’s freeze-thaw resistance.
Have these tiny bubbles been around for a while? Are they a new phenomenon? Such a phenomenon could be sourced to many things, and is not likely something new. The frequency of occurrence due to random issues (for example, algae in the water or detergents in the boiler) has probably been consistent over time. Most concrete we evaluate that is five or more years old tends to have air contents about 1 percent or so higher than what the technician’s air meter may have read.
The only means of detecting “bubble bloom” in the field is by also monitoring the density of concrete as it is placed. The next time the compressive strength of your exterior concrete is low, consider monitoring its density during placement. Low densities, or a drop in density, may indicate that the air content is higher than determined by the air meter. Comparing air meter results with density results may help you address the issue on the day of placement instead of seven or more days later.
The trouble with bubbles The tiny bubbles (the small dark circles) are visible with the help of a microscope.
Fly ash is a fine grained by-product of the coal combustion process. According to the Minnesota Pollution Control Agency (MPCA) Standing Beneficial Use Determination, combustible coal fly ash (CCFA) as defined by ASTM C618 can be used as a cement replacement when making concrete. In the past there have been case-specific, beneficial use determinations that allowed the use of fly ash for soil stabilization.
There has been quite a buzz in the fly ash market since the spill at the Tennessee Valley Authority’s (TVA) Kingston Fossil Plant in Harriman, Tenn. on Dec. 22, 2008. The spill resulted in a release of 5.5 million cubic yards of coal ash, which damaged property and flowed into the Emory River. The TVA is remediating the damage and estimates the cost will exceed $1 billion. In a reaction to the
Newsflash on fly ashBy Mike Bratrud, PG and Greg Bauer, PE
spill, the Environmental Protection Agency is drafting regulations designating coal combustion products (CCP) under Subtitle C of the Resource Conservation and Recovery Act of 1976 (RCRA) as hazardous waste, or under Subtitle D as municipal waste. This will make it difficult to place fly ash in potentially sensitive environmental settings and could also impact both the energy and construction markets by limiting its reuse as a construction product.
The addition or substitution of fly ash has the added benefit of reducing the carbon footprint associated with the production of Portland cement concrete. Based on the Portland Cement Association (PCA), each pound of cement used in the making of concrete produces almost one pound of CO2. Cement replacement with fly ash could have an enormous environmental impact, both on the air and in the volume of solid waste that would otherwise end up in landfills.
The question is if fly ash is classified more strictly as a hazardous waste, will the cost of managing it for beneficial reuse become too high or the opportunity to reuse it diminish?
Fly ash (the circles) under microscope.
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Charles Hubbard, PE, PG Braun Intertec Corporation 1826 Buerkle Road Saint Paul, MN 55110 Phone: 651.487.7060 [email protected]
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