what every engineer should know about earthwork specifications · this course is intended to help...

A SunCam online continuing education course

What Every Engineer Should Know About Earthwork Specifications

by

F. C. “Bud” Budinger PE illustrations by Ethan Hageman

What Every Engineer Should Know About Earthwork


www.SunCam.com Copyright 2020 F. C. Budinger PE of 28

Contents

Introduction to Earthwork …………….………….… 3

Earthwork Requires Teamwork ………………….… 3

Who Pays the Controller ? ………………………….. 5

What Can Go Wrong With a Fill ? …………………. 5

What Kind of Material Are We Placing ? ………… 6

Soil Maximum Density ……………………………… 7

Why Are We Placing This Fill ? ……………….…… 13

Soil Relative Density ………………………………… 14

What Should We Specify ? …………………………… 16

What Do We Expect of This Fill ? …………….……. 17

How Much Compaction Do We Want ? ………….… 18

Do We Need Additives in Our Fill ? ………………… 21

Do We Need Reinforcement in Our Fill ? ………….. 21

Surface to Support the Fill …………………………… 22

Surface of the Source ………………………….……… 22

Quality of Fill Material …………....………………… 23

What to Expect From Compacted Fills …….…..…… 24

Trench Backfill ……………………………….……… 24

Shrinkage …………………………………………..… 26

Should We Compact Footing Excavations ? ………… 27

Summary ………………………………………..….…. 28

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Introduction to Earthwork …………….…………..…..……………… This course is intended to help you tailor your earthwork specifications to your specific project, its needs, and its soil type. While Standard Specifications may be easy, one size does not fit all.

This course builds upon, and assumes that, you have taken the previous course #369, What Every Engineer Should Know About Soils, which addresses basic soil problems. Volume calculations are beyond the scope of this course, but shrink and swell are discussed. Earthwork Requires Teamwork …………….………………………… There are many topics to consider when moving soil and writing Earthwork Specifications, not the least of which is Teamwork. We seldom get a suitable fill at a reasonable cost unless these three players work together as a team.

The Designer has to make the decisions: o How much fill? o How to treat the surface to support the fill? o What material: native or import? o How much Compaction?

The Constructor has to move and compact the material: o How much moisture? o How thick a lift can be compacted? o What equipment works best? o How do we know when we have enough compaction?

The Controller has to answer that last question plus a few others: o Do we have enough compaction? o Do we need more or less water? o How can we assure the owner that this fill was placed as specified?





How can we expect to get a good, safe, economical fill, if these three do not work together as a team? Unfortunately, all too often, they tend to see each other as adversaries:

o “Are specs the be followed to the letter, or are they just guidelines?” o “I’ve been doing it this way for 20 years. Don’t tell me there is a better way.” o “The designer is too conservative. We don’t need that much compaction.” o “I can just compact the hell out of the top foot. Who’s to know what’s below?

While the laboratory technician is there to police the contractor, he or she is also there to help the contractor. Yet, some earthnovers have a tough time viewing the technician as a helpful team member. But, they both have the same objective; a fill as specified with minimal time and cost. The technician can be particularly helpful because he or she has a much broader perspective than does the contractor.

o The technician has intimate knowledge of the soil’s reaction to increasing moisture. o The technician has helped other contractors compact this same soil. o The technician has seen many different methods used on this soil. o The technician knows which method is most efficient. o The technician can help find the ideal moisture to more efficiently compact this soil. o The technician can determine when the contractor need apply more no effort. o The technician can assure a contractor that pumping will stop as pore pressure dissipates.

When the laboratory and the contactor work together, the fill goes in as specified with minimum time and effort. When I present this material in seminars, I am often asked, “How can you get people to spend two days listening to you talk about soil compaction?” My response;

“Next time you drive by an earth moving project, count the number of big yellow machines and figure that each one costs about $100 per hour. If you counted 10 machines, that is about $8000 per day:

o If they do a poor job, it can be done in one day. ………………… $8,000 o If they do a good job efficiently, it can be done in one week. .….. $40,000 o If they do a good job inefficiently, it may take 2 or 3 weeks. …… $100,000 o If they do a good job and no one believes it, it may take a month. .. $160,000”

This difference in cost is what brings people to my seminars. Could it be what has you taking this course. Who wants to pay $100,000 for a job that could be done for $40,000?





Who Pays the Controller ? …………………….…………..………….. This may be the most critical section in this course. We count on the testing lab to assure us that the fill is placed as specified. But the lab can do no more they are hired to do. The lab must be loyal to its client who is ultimately in control.

o The lab can be hired to test only as directed by the client. o The lab can be hired to take a few tests at random. o The lab can be hired to test only the top of a deep fill. o The lab can be hired to assure a good fill placed as specified.

If we want a quality fill and are willing to pay for it, control must remain in the hands of the owner or designer. Upon completion, the lab should be able to report that they were on site enough to verify that the fill was placed and compacted as specified with no uncompacted lifts.

If we merely want a piece of paper that shows good test results at minimal cost, it can be had by putting control in the hands of the contractor who can limit testing to a few well-compacted zones. There are, of course, many contractors (seldom the low bidder) who will settle for noth-ing less than a good job strictly in accordance with the specifications. What Can Go Wrong With a Fill ? …………………..……………… We have all heard Murphy’s Law, “Whatever can go wrong, will.” All too often it may appear that Murphy has it in for those of us who move earth. It doesn’t have to be that way. Murphy can be defeated with good teamwork. That means thorough specifications, conscientious con-struction, and tight control; that is to say, Teamwork.

When fills fail, more often than not, they have had passing test results. How can that be? In most such cases, the good test results were obtained on the surface of a thick fill. People seem to have difficulty understanding the depth limits of compactive effort. Conventional equipment cannot densify lifts much thicker than one foot.

When the laboratory is only called out to test occasionally, we often end up with well compacted lifts on 4’ centers, with 3’ of uncompacted soil between them. In such cases, the native grade is often ignored with fill being placed on a loose native soil, which may contain organic debris.

For pavements, surface compaction is critical. But surface density has little effect on footings founded well below finish grade. In cases where a subdivision abuts a bluff, neighbors often toss their yard wastes over the edge on vacant lots. A builder then levels one of these lots with fill placed over the debris. He then hires a lab to test the finish grade. When footings on the uncom-pacted fill fail, the lawyers ask, “How can we have this disaster? The fill tested good?”





In the previous SunCam Soils Course, I introduced a few approximate constants that have served me well over the last 50 years. I call them “Bud’s Rules of Thumb” or BROTS. They are numbered consecutively with those from the previous course (#369).

BROT #10 Conventional Equipment Cannot Densify Lifts Much Thicker than 1 Foot.

What Kind of Material Are We Placing ? …………….……………….. Every site is unique; so is every fill source. The first thing we must address is the nature of our fill material which could be a mixture of several of the following;

o Boulders. (+ 12”) Rocks of boulder-size are usually reserved for Rip-Rap or gravity re-taining walls called Rockeries. They must be carefully placed to interlock, but cannot be compacted or tested by traditional methods. With good design, Boulders can retain earth or armor stream banks.

o Cobbles. (3” – 12’) Rocks of this size can be compacted but cannot be tested. Compac-tion must be specified by a procedure such as several passes with a heavy Grid Roller over lifts about twice the size of the average rocks. While Cobbles can provide good structural support, they require some sort of filter material such a Gravel at the surface,

o Gravel. (#4* – 3”) A clean, rounded, uniform, coarse material like Pea Gravel can be end-dumped at 90% of its maximum density and does not need compaction. Clean, well-graded Gravels without Sand can be compacted, but cannot be tested and require a proce-dure spec. Most Gravels contain enough Sand to be compacted and tested by traditional methods. Vibratory tools usually work best. Clean Gravels can provide excellent struc-tural support, but, without cohesion, they must be safely sloped or contained.

o Sand. (#200 - #4) Clean Sands can be compacted to nearly 100% by placing them satu-rated in a confined area (trench backfill) and expelling the excess water with heavy vibra-tion. In an open, fill Sands are still best compacted with vibration at high moisture con-tents, but can easily turn quick. While clean Sands without cohesion can provide good structural support, they must be safely sloped or contained.

o Silt. (0.005 mm - #200) Clean Silts are best compacted by kneading with rubber-tired heavy equipment. Lacking significant cohesion, they can readily start pumping at high moisture contents. While compacted Silts can provide adequate support for light struc-tures, they are quite compressible and can contribute to high settlements.

* #4 sieve = 4 meshes /inch; slightly less than ¼”.





o Clay. (Finer than 0.005 mm) Due to their high plasticity and cohesion, Clays are difficult to compact. They require high pressure and thin lifts. A heavy sheepsfoot roller usually works well. While compacted Clays can provide good support for light structures, they are often expansive and can lead to damage from heave.

o Peat. (Organic) Due to its organic content peat cannot be compacted as a structural fill.

These discussions assume that the fill comprises single soil types. Things get much more com-plex when we start mixing soils. Unfortunately, soil mixing is generally unavoidable. Soil Maximum Density ………………………………………………… As Dry Density can vary from 70 pcf to 140 pcf, it becomes obvious that we cannot specify com-paction as density in pounds per cubic foot. If we were to specify a dry density of, say 100 pcf, it would be problematic in that some soils cannot get that dense while others would be dangerously

loose at 100 pcf.

The solution is to specify compaction as a percentage of the maximum dry density for any given soil or mixture of differing soils.

There is no shortage of methods to determine the maximum den-sity of soils. I know of at least 13 and there may be more. As can be seen in the list to the right, some agen-cies and state DOTs have de-vised their own criteria.

>>>





To keep things simple here, let us deal with only 3 methods to determine maximum density:

o Standard Proctor ASTM D - 698 3 Lifts, 5½#, 12” Drop 12,375 ft-lbs/ft3 o Modified Proctor ASTM D-1557 5 Lifts, 10 #, 18” Drop 56,250 ft-lbs/ft3 o 3 Lift Modified Proctor California 3 Lifts, 10 #, 18” Drop 33,700 ft-lbs/ft3

While they differ in procedure, nearly all methods involve compacting the soil in molds at vary-ing moisture contents.

For any given soil, the compacted density varies significantly with moisture content. Water lubricates the soil particles permitting more density with any given degree of compactive effort. More moisture results in higher compaction until a point is reached where the water begins to de-velop high pore pressures.

By compacting a sample at several moisture contents and plotting the results, we can in-terpolate an optimum moisture at which we could have ob-tained the maximum density for that soil.

Soon after R. R. Proctor devel-oped the Standard test in 1933, it became obvious that its re-sults approached, but did not achieve the maximum density for any soil. Consequently, it was modified to increase the energy level by 4.5 times to yield a true maximum density.

As the Standard Proctor does not produce a true maximum density, it should have been discarded years ago. But,

how can we reject anything with a name like “Standard”? So, unfortunately, it is still with us and is all-to-often used interchangeably with the Modified Proctor even though their results are quite different.





BROT #11 Although Seldom True, 95% of a Standard Proctor Is Often Equated With 90% of a Modified Proctor.

The 3 Lift Modified, Modified Proctor, while only used in California, is presented herein be-cause it demonstrates an interesting phenomenon. With 3 lifts the California method develops only 60% of the energy of the Modified Proctor’s 5 lifts. However, the results are nearly the same. This suggests that most soil’s maximum density can be obtained above 30,000 ft-lbs/ft3.

BROT #12 Compaction Specs Should Be Written as a Percentage of a True Maximum Density, ASTM D-1557 or Equivalent Thereof.

A full understanding the moisture / density relationship is quite complex and beyond the scope of this course. Here, we are more interested in useful information than in academic knowledge. Suffice it to say that . . . . . . . . Like climate, the relation-ship between soil density, en-ergy applied, and moisture content are functions of more complex variables than we can fully comprehend:

Yet, try as we may, we just cannot simplify it enough. Even if we limit it to just the ASTM procedures, there are 2 of them, each with 3 or 4 methods (D-1557-78 includes a method D) in 2 mold sizes.

The reason for differing methods is to accom-modate different aggregate sizes. Necessarily, ¾” gravel does not compact well in a 4” mold.





Even through the energy is equated, the results are not. When we have gravel larger than ¾”, a rock correction becomes necessary. Rocks (+ ¾”) weigh on the order of 170 pcf, while even the heaviest base course might weigh in at about 140 pcf. One rock in a 6” mold could skew the re-sults. Hence, they must be excluded from the test, but are included in the fill and can skew the field test results.





The figure to the right shows graphically the limits of the 3 ASTM D-1557 methods. >>>

As can be seen from the drawing at the left, larger molds allow room for some of the compactive effort to displace soil not directly under the hammer. Conse-quently, we get higher densities in smaller molds. <<< In my experience, we see higher densi-ties with the Cal 216 (due to its confine-ment), even though it uses less energy





On the previous page, we see typical Proctor curves for several different soil types. The saturation curve is for SpG = 2.65, and would vary somewhat with differing SpGs. Even a cursory examination of the Proctor curves reveals several interesting phenomena:

o Optimum moisture is consistently about 85% of saturation. o The higher we go in density, the steeper the curve. o Coarser soils are heavier. o Finer-grained soils are lighter.

Unlike the other soils, the curve for a clean Sand shows an inflection due to apparent cohesion. Lacking cohesion, sand can be compacted to near its maximum when completely dry. As we add moisture, it enables apparent cohesion to work against our compactive effort; that is until we get enough moisture to begin lubrication. Why Are We Placing This Fill ? ………………………….…………… What is our intended purpose? To what end are we specifying this fill? Is this fill intended to:

o Get Rid of Excess Material o Level Ground for Farm or Lawn o Provide Drainage o Limit Seepage o Provide a Toe Surcharge to Balance a Sliding Slope o Improve Loose Native Soil o Build a Soil-Reinforced Retaining Wall o Backfill Against a Basement or Cantilevered Retaining Wall o Backfill a Utility Trench o Support Structures o Support Pavement

Each of these requires a different material and a diffeent degree of compaction. One size does not fit all. The first item is often a source of problems. Why compact waste material? Because someday, someone will build on it. If an old uncompacted fill is recognized, it can be excavated and replaced properly, or it can be compacted in-place with Deep Dynamic methods.

Unfortunately, it is seldom recognized which invariably leads to serious troubles. In California, the various jurisdictions require grading permits for all fills. Even waste dumps must be com-pacted and supervised by an independent testing laboratory.





Soil Relative Density ……………………………………… If maximum density is a bit complex, we must also consider Relative Density an entirely dif-ferent way of addressing compaction.

Instead of looking at percent of maximum density, Relative Den-sity considers the differ-ence between maximum density and minimum density. Thus 50% rela-tive density defines a fair degree of compac-tion.

Relative Density method determines the maximum (ASTM D-4253) with a vibrating table and the minimum (ASTM D-4254) by free-falling dry soil one inch into a mold.

ϒR = Maximum Relative Density ϒo = Minimum Relative Density

As ASTM D-4253 compacts with vibration, it is only applicable to relatively clean sands which compact well with vibration.

While the Relative Density method is seldom used, it introduces a confusion factor in many of our specifications. When we use terms like “Relative Density” or “Relative Compaction” to mean percent of maximum density by the standard or modified Proctor, we confuse any bidder familiar with the Relative Density method. When we see specs that call for 90% relative density, what does the writer mean? A relative density of 90% is nearly 100% of a modified Proctor maximum.





The word “relative” has no place in specifications unless they are written for the Relative Den-sity method. Consider the misunderstanding injected if we were to specify 70% Relative Den-sity which equates to about 92% of maximum density. Some contactor is going to think that no compaction is required and bid accordingly. How can we get a good fill when the contractor faces bankruptcy by compiling with specifications that misled him?

BROT #13 The Word RELATIVE Has No Place in Earthwork Specifications, Unless We Mean the Relative Density Method (ASTM D-4253 & 4254)





What Should We Specify ……………………………………………… In the course of controlling compacted fills over many years, I have been amazed at how many specs address items that should be left to the contractor’s discretion and how many fail to address critical items. What should we include in our specifications?

o Soil Type. YES. If the native soils are at all suitable, they should be used. If not, we should specify a material that serves our purpose. Often, import material has dramatically different properties than the native. Differences in permeability, expansion, and frost susceptibility can cause major problems, particularly in trench backfill. Pavements can be disrupted when the backfill is the only material not to expand or suffer frost heave.

o Moisture. YES? But only in rela-tion to the optimum moisture for the given material; not as a percent moisture. As can be seen on the chart to the right, >>> physical characteristics of the fin-ished fill vary considerably with the moisture at which it is com-pacted; over optimum or under op-timum. If any of these characteris-tics are important, we should spec-ify that the fill be compacted over or under optimum to gain the de-sired characteristic.

o Surface Preparation. YES. If the native surface is not properly pre-pared, the fill could slide on the sloping surface or settle as the loose native top soil compresses under the load of the fill.

o Equipment. NO & YES. Unless we are willing to take responsibility for the resulting compaction, the choice of equipment should be left to the contractor who may have suitable, if not the most efficient, machines. If we force him to rent something different, he will have to increase his bid. Only in cases where the material cannot be tested, should we specify a compaction procedure and assume that it will get the required compaction.





o Number of Passes. NO & YES. Unless we need to specify proof rolling and are willing to take responsibility for the resulting compaction, the procedure should be left to the contractor and the controller. We only need enough passes to achieve the specified compaction. Only in cases where the material cannot be tested, should we specify a procedure and assume that it will get the required compaction.

o Lift Thickness. NO & YES. Lift thickness is a function of load and width of the compact-ing tool. A tool with wide tires at high pressure (like a scraper) can usually handle lifts as thick as the tire width (1’±). Tools with smaller, higher pressure application (like a sheeps-foot) cannot handle lifts much thicker than twice the width of its individual feet. While lift thickness is best left to the contractor and laboratory, there is always the danger of placing lifts too thick to be adequately compacted. It is usually best to specify thin lifts and leave the definition of “how thin” to the laboratory.

o Percent Compaction. YES. Even though compaction is critical, too many spec writers call for much more than is needed. While high compaction of 95% is required for subgrade to support flexible pavement, it is seldom necessary elsewhere. The difference in cost between 90% and 95% can be considerable and should be incurred only when necessary. With expan-sive soils, high compaction is detrimental and should be limited with specs such as, “at least 87% but not more than 92%”.

o Shrinkage. NO. When we compact loose soil from the pit into dense soil in the fill we are going to lose volume. While necessary for estimation, inclusion of shrinkage factors in the specifications can lead to unnecessary claims.

o Field Testing. YES & NO. Fills that are not tested are seldom adequately compacted. Some spec writers spell out the type and frequency of testing. This should be left to the la-boratory as they know what works best for the soil and the contractor’s procedures. Most critical is enough observation and testing to prevent the inclusion of buried, poorly com-pacted lifts which can compress under the load of the fill above.

Often the technician and contractor will work together at the beginning to establish a proce-dure that gets the specified compaction at the least cost. The lab then need only verify that the successful method is used throughout with no uncompacted lifts buried. Specifications should assure that the lab is there to test the surface to receive fill and all subsequent lifts to subgrade.

o Who Pays for Testing. YES. He who pays the laboratory controls the compaction. The specs should assure that such control is in the hands of those concerned with the fill’s quality.





How Much Compaction Do We Want ? ……………………………… Keeping in mind BROT #4 and considering that 5% more compaction may double the cost, we should specify no more compaction than we need to satisfy the purpose of the fill. The following are rough guidelines and do not consider soil type. We should always adhere to the recommen-dations of our Geotechnical Engineer.

BROT #4 More Density Is Not Always Desirable.

o Lawn or Garden (top 2 feet) ……….…….. 70% of ASTM D-1557 o Volume Stability ………………………….. 80% of ASTM D-1557 o Minimize Expansion ……………………… 80% of ASTM D-1557 o Cantilevered Retaining Wall* …….……… 85% of ASTM D-1557 o Soil Reinforced Retaining Wall ..………… 90% of ASTM D-1557 o Seal a Lagoon …………………………….. 90% of ASTM D-1557 o Stability to Support Structures ……………. 90% of ASTM D-1557 o Stability to Support Rigid Pavement …..… 90% of ASTM D-1557 o Stability to Support Flexible Pavement ….. 95% of ASTM D-1557

*Non-Structural BROT #14 We Should Not Specify More Compaction than Needed.

When compacting to reduce water loss, more density does reduce permeability, but not by much. The cost of increased compaction might be better spent importing a finer-grained soil. Grain size has far more effect on permeability than does density.

Buildings are heavier than trucks, so why do we want more compaction to support traffic than to support structures? Buildings are full of air while trucks are full of heavy stuff. Footing pres-sures for buildings are in pounds per square foot (psf) while traffic loads are in pounds per square inch (psi). The pressure applied to the pavement is the same as the tire pressure. The heaviest structural pressures that might bear on fill could be as high as 3000 psf which equates to 21 psi, while truck tires are inflated to about 100 psi.

When we see fills fail, even with good test results, it is only natural to assume that we should have specified more compaction. Hence, we see a general inflation of specified compaction. However, the cause of failures is almost always due to subsurface lifts that have been placed without enough compaction. A fill uniformly compacted to 85% will outperform a fill with only a few lifts compacted to 95%’





BROT #15 It Is Not the Degree of Compaction That Counts; It Is the Uniformity of Compaction That Counts.

Sometimes we see compaction specified as high as 100%, a density which cannot be obtained even in the laboratory. There is a way though; clean sands can be compacted to 100% by placing them saturated in a confined area (trench backfill) and expelling the excess water with heavy vi-bration. Specifying 100% limits the contractor to only this method.

BROT #16 We Should Never Specify 100% of a True Maximum Density.

Nearly every soil pa-rameter changes with increasing density as shown in the adjacent chart; and not always for the better.

>>> For the most part, more compaction is advanta-geous. We do want to see more Strength and less consolidation. But should we stop with those two. Or should we not consider other parameters?

Do we really want to see more: • Expansion • Capillarity • Saturation

Do we really want to see less: • Permeability • Stability (Pumping)





Of these soil parameters, our greatest concern is of-ten expansion which in-creases significantly with increased compaction.

The Bureau of Reclama-tion conducted an in-depth study to see just how much expansion is affected by compaction. >>> Note that at 95% of Mod-ified Proctor, we might see about as much as a 10% increase in volume, but at 80%, expansion it might be as little as 1%. In presenting compaction seminars all over the nation, I could not help but note how engineers in various parts of the country harbor vastly different attitudes pertaining to earthwork.

o In some areas, they would never specify less than 95% of a Modified Proctor In others, 90% of a 3-lift Modified Proctor is the norm. In some, 95% of a Standard Proctor is standard.

o In some areas, they would never consider building on fill, no matter how well compacted. In others, they routinely improve native soil by excavating it and recompacting it as fill.

o In some areas, they would never move soil without adding Lime. In others, they would never even consider using Lime.





Do We Need Additives in Our Fill ? …………………………………. As long as we are tearing up the soil and placing it elsewhere, we have an opportunity to change its characteristics with the addition of some non-soil component. There are a number of chemi-cals that can be added, including, but not limited to the following:

o Enzymes. As there are several suppliers of Enzymes that can provide extensive infor-mation on their products, I will not go into them here. Suffice it to say that they can re-duce plasticity, reduce shrinkage, and increase soil strength. As liquids, they can be added to the water to avoid the cost of mixing.

o Portland Cement. Soil Cement is the common term used for fill material impregnated with Cement to increase its strength. It is most commonly used in pavement subgrades to reduce the need for Aggregate Base.

But, while Cement increases strength, it also renders soil brittle. I have worked on pro-jects where frost heave had caused Soil Cement subbases to develop transverse cracks at regular intervals, something which a more flexible subbase could have resisted.

o Fly Ash. While most commonly used as a concrete additive, Fly Ash is occasionally added to earth fills or Soil Cement. Its pozzolan component renders the fill material more workable while increasing the strength and reducing shrink and swell.

o Lime. The most common additive is Lime, either slacked or hydrated. While Lime tends to cement the soil particles, its greatest advantage is in reducing plasticity. When slacked, Lime can dry-up very wet soils as the Lime hydrates.

I once recommended Lime in an area where it was never used. The contractor, after fighting tooth and nail, reluctantly added the Lime. He was delighted to see the wet Clay slide right out of his dump trucks after he had bid to rake it out with a backhoe. By re-ducing the plasticity, the 4% added Lime had taken all the stickiness out of the Clay.

Do We Need Reinforcement in Our Fill ? …………..………………. Would we place concrete without reinforcement? When we rip up material and replace it as fill, we create an opportunity to add reinforcement between the lifts. Why is this only done for re-taining walls? Could it be that, for most structures, settlement controls design rather than bear-ing. While Geogrid or Steel can reinforce shear planes, and reduce the risk of bearing failures, it has little effect on compression. With retaining walls, however, the failure mode is a diagonal shear plane which can be effectively stabilized with reinforcement.





With what can we reinforce our fill?

o Geogrid. Most common by far, Geogrid comprises pre-stressed, high-density plastic grids resembling the wire-mesh used in concrete slabs. They can be rolled out between lifts to prevent a shear plane from developing. There are several manufacturers who can provide extensive information on their products.

o Filter Fabric. Although often used to separate differing fill materials, Fabrics could be used as reinforcement. However, they are not nearly as effective as Geogrid. When placed between Base Course and Subgrade, Filter Fabric acts as reinforcement by dissi-pating wheel loads.

o Steel. Although the original patent for Reinforced Earth™ used Steel Strips, Steel is sel-dom used today, probably because Geogrid is less expensive and not subject to rusting.

Surface to Support the Fill ……………………………….…………… Most critical and often overlooked is the native soil which must support the fill. At 120 pcf, earth fill is heavy. In most cases, one foot of fill weighs more than the structure to be built on it. If we want the fill to be well-supported, there are several things we must consider:

o The native surface is probably very loose.

o The native surface is very likely vegetated.

o The native surface may be sloping.

Loose surface soil can compress causing settlement of a well-com-pacted fill. Vegetation can decay causing more settlement. Rotting vegetation can act as lubri-cant to induce sliding on a sloping native grade.

If the surface contains much organic material, it is wise to specify that it be removed and stock-piled for later use in landscaping.

While it is good to specify compaction of the surface to support the fill, that is not always possi-ble. In some cases, the native soil is not only loose, but also saturated. When the soil voids are full of water, compaction can only turn it quick. In such cases, it is best to specify proof rolling until pumping is observed.





When the surface to support the fill slopes more than 4:1 (hori-zontal:verticle) sliding can occur. On slopping native grade, we should specify benching usually at equipment-width.

When placing imported gravel on soft (above the plastic limit)

soils, we should separate the two differing materials with filter fabric. A low-cost, non-woven fabric can prevent the gravel from disappearing into the mud and prevent the soft soil from work-ing its way up into the gravel. Surface of Fill Source ... Be it on-site or imported, the natu-ral surface material in the cut area is seldom suitable for placement in a structural fill. However, it of-ten contains organic material ide-ally suited for landscaping. We should specify stockpiling all or-ganic materials for later use. Quality of Fill Material . Material quality is critical. Is the native excavated material suita-ble? If not appropriate for the project, what kind of material should be imported?

Various material characteristics are described in the adjacent chart. >>>





We are often tempted to specify an ideal impor material. Yet, there may be a suitable, if not per-fect, material readily available at a much lower cost. All too often we see specs that call for re-moval of a good material to be replaced by a slightly better import at great expense.

BROT #17 Design to Fit the Site Rather than Alter the Site to Fit the Design.

What to Expect From Compacted Fills …………………….…………. A loose fill will stay that way until it experiences saturation after which it will consolidate to a stable density, usually around 80% of maximum. I have seen fills collapse after as much as 40 years.

BROT #18 Uncompacted Fill May Settle 10% of Its Height Upon Initial Saturation.

A common misconception is the be-lief that fills can never be as good as native soils. Nature likes a stable density around 80%. But we can arti-ficially densify soils to 90% or 95%, thereby increasing its strength and re-ducing its compressibility.

But soil particles that have been in intimate contact for hundreds of years develop bonds or minor cemen-tation that is necessarily destroyed by excavation. Yet, in most cases, the increased density of a well-com-pacted fill more that compensates for the loss of cementation.

In some areas with loose flood plain deposits, it is common practice to specify excavation and recompaction to stabilize collapsing soils.

Once we compact a fill, how long will it stay that dense? In accordance with Newton’s law, things stay the way they are until disrupted by some force. Barring man’s efforts, the only natu-ral force that can disrupt compaction is sliding, expansion or frost heave.





Yet, even well-compacted fills will experience some additional settlement over time. This may amount to as much as ½% over many years. I recommend crowning the fill to accommodate 1½ times any anticipated secondary compression. Trench Backfill ……………………………………………………….. Backfill of deep trenches presents a whole new set of challenges. Trench compaction is not easy:

o It may not be safe for an operator of hand tools. o The native soil may be too soft to provide enough reaction. o Compaction of the lower lifts could damage the pipe.

I have been called upon several times to investigate longitudinal cracks in pavements. Initially recent sewer trenches were suspect, but the cracks, though parallel to the trenches, were some distance away.

When testing revealed that the trench backfills had not been adequately com-pacted, it became obvious that nature was compacting them for us. Lacking support from the backfill, active earth wedges (typical of slope failures) were sliding into the trenches. Without good compaction

we can expect such cracking as well as settlement of the backfill and subsequent rutting.

How do we address these challenges? Various jurisdictions have developed their own solutions. In New York City, they do not compact the backfill at all and require the contractor to bring the settled trench back to grade and replace the pavement the following year after the backfill has collapsed. Others have come up with their own solutions, most of them involving backfill mate-rial that does not require compaction:

What materials can be placed without comaction?

o Gravel. A clean, rounded, uniform, coarse material like pea gravel can be end-dumped at 90% of maximum density. However, it may be 1000 time more permeable than the na-tive soil and create a French drain.





o Soil Cement. While a true soil cement requires compaction, it is often used to backfill trenches with little compaction. The cohesion from the cement usually compensates for the lack of density with hardness. Soil Cement could be far less permeable than the na-tive soil and disrupt groundwater flow.

o Slurry. A Sand Slurry with a bit of Cement can make backfill easy. But where does all that water go? Such Slurries are often called, “Flowable Fill” or “Control Density Fill”. But, as the Sand goes in at its buoyant unit weight, it is anything but dense, and relies on cohesion from the Cement for stability. Slurries also could be far less permeable than the native soil and disrupt groundwater flow.

I was once called upon to figure out why a contractor could not keep his sewer trench from flooding. He had started at the high end of the project and backfilled with clean sand which was easy to compact. But his trench was through silt below the water table. The low perme-ability of the native silt kept the trench dry enough for construction. But when the water fi-nally seeped in, the sand backfill became a French drain sending all the water right into his work area.

BROT #19 When Backfilling, We Should Consider the Difference in

Permeability Between Native Soil & Backfill. Shrinkage ……………………………………………………………… Earthwork volumes are calculated from survey data showing contours of the pit and fill before and after earthmoving. Such calculations are well presented in the SunCam Course #227, “Earthwork Basics and a Traditional Calculation Method” by Joshua Tiner, and are not covered herein.

However, we must face the fact that volumes change from pit > truck > fill. For rough estima-tion, the general relationships are presented below in terms of compaction as percent of maxi-mum density by ASTM D-1557:

o Natural Density of Native or Pit ………… 75% - 80% o Loose Density in Haul Truck …………… 60% - 70% o Compacted Density in Structural Fill …... 88% - 97%

The final fill volume is not known until the earthwork is completed and both the pit and fill have been contoured by the surveyor. In the meantime, the contractor has only the truck count to esti-mate fill volume.





The material is never looser than in the truck and never denser than in the compacted fill. So, necessarily, the truck count will show a much higher volume of material than the final survey. When payment is by the cubic yard, I have seen more than a few augments develop over this dif-ference. Should We Compact Footing Excavations ? …………………………. I see this specified all too often, “Compact the bottom of the footing excavation to 95%.” Why? As can be seen from the section below, a compacted zone at that level has little effect on bearing or settlement, and a negative effect on expansion.

A bearing failure would occur on a deep-seated plane as a foot-ing revolves around its edge to lift the adjacent soil.

Compression develops in a zone of influence extending 3 times the footing width below the footing.

So, how much do we gain by compacting just the few inches in the footing excavation?

If the entire fill is properly compacted and all loose soil is re-moved from the foot-ing excavation, why would we need addi-tional compaction?





I recommend that the best tool to be used in a footing excavation is a square-point shovel to re-move all disturbed material.

BROT #20 Compaction of Footing Excavations Has Little or No Effect on Foundation Performance.

Summary ………………………………………………….……………. It is tempting to use Standard Specifications which are well and good, but they may not always fit our project and its unique soils. Often designers will simply reference DOT Standard Specs which may use state standards rather than ASTM procedures. While DOT Specs may well as-sure adequate compaction, they may involve slower or more costly test procedures. BROT #21 One Size Does Not fit All. Specifications Should Be

Tailored to Our Project and Its Soils.


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