chapter 12 special soil problems - … can lead to construction errors that are ... grading and the...

FM 5-410

CHAPTER 12

S p e c i a l S o i l P r o b l e m s

Misunderstanding soils and their proper-ties can lead to construction errors that arecostly in effort and material. The suitabilityof a soil for a particular use should be deter-mined based on its engineering characteris-tics and not on visual inspection or apparentsimilarity to other soils. The considerationspresented in the following paragraphs willhelp reduce the chances of selecting thewrong soil.

AGGREGATE BEHAVIORGravel, broken stone, and crushed rock are

commonly used to provide a stable layer, suchas for a road base. These aggregate materialshave large particles that are not separated orlubricated by moisture; thus, there is not asevere loss of strength with the addition ofwater. They remain stable even in thepresence of rain or pending within the layer.As a result, these soils can be compacted un-derwater, such as in the construction of ariver ford.

SOIL-AGGREGATE MIXTURESConstruction problems can arise with cer-

tain mixtures of soil and aggregate. Theseinclude dirty gravel, pit-run broken stonefrom talus slopes, clays used in fills, andothers. If there are enough coarse particlespresent to retain grain-to-grain contact in asoil-aggregate mixture, the fine material be-tween these coarse particles often has only aminor effect. Strength often remains higheven in the presence of water. On the otherhand, mixtures of soil and aggregate in whichthe coarse particles are not continuously

in on tact with one another will lose strengthwith the addition of water as though no ag-gregate particles were present. Highplasticity clays (PI > 15)in sand and gravelfills can expand on wetting to reduce grain-to-grain contact, so that strength is reduced anddifferential movement is increased. Thechange in texture from stable to unstable soil-aggregate mixtures can be rather subtle,often occurring between two locations in asingle borrow pit. To assure a stablematerial, usually the mixture should cont-ain no more than about 12 to 15 percent byweight of particles passing a Number 200sieve; the finer fraction should ideallyshow low plasticity (PI < 5).

LATEITES AND LATERITIC SOILSLaterites and lateritic soils form a group

comprising a wide variety of red, brown, andyellow, fine-g-rained residual soils of light tex-ture as well as nodular gravels and cementedsoils. They may vary from a loose material toa massive rock. They are characterized bythe presence of iron and aluminum oxides orhydroxides, particularly those of iron, whichgive the colors to the soils. For engineeringpurposes, the term “laterite” is confined to thecoarse-grained vermicular concrete material,including massive laterite. The term“lateritic soils” refers to materials with lowerconcentrations of oxides.

Laterization is the removal of siliconethrough hydrolysis and oxidation that resultsin the formation of laterites and lateritic soils.The degree of laterization is estimated by the

Special Soil Problems 12-1

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silica-sesquioxide (S-S) ratio (SiO2/(Fe2O3 +Al2O3)). Soils are classed by the S-S ratios.The conclusion is—

An S-S ratio of 1.33 = laterite.An S-S ratio of 1.33 to 20 = lateriticsoil.An S-S ratio of 20 = nonlateritic,tropical soil.

LateritesMost laterites are encountered in an al-

ready hardened state. In some areas of theworld, natural laterite deposits that have notbeen exposed to drying are soft with a clayeytexture and mottled coloring, which may in-clude red, yellow, brown, purple, and white.When the laterite is exposed to air or dried outby lowering the ground water table, irre-versible hardening often occurs, producing amaterial suitable for use as a building or roadstone.

Frequently, laterite is gravel-sized, rang-ing from pea-sized gravel to 3 inches minus,although larger cemented masses are pos-sible. A specific form of laterite rock, knownas plinthite, is noteworthy for its potentialuse as a brick. In place, plinthite is softenough to cut with a metal tool, but it hardensirreversibly when removed from the groundand dried.

Lateritic SoilsThe lateritic soils behave more like fine-

grained sands, gravels, and soft rocks. Thelaterite typically has a porous or vesicular ap-pearance. Some particles of laterite tend tocrush easily under impact, disintegratinginto a soil material that may be plastic.Lateritic soils maybe self-hardening whenexposed to drying; or if they are not self-hardening, they may contain appreciableamounts of hardened laterite rock or lateritegravel.

LocationLaterites and lateritic materials occur fre-

quently throughout the tropics andsubtropics. They tend to occur on level orgently sloping terrain that is subject to very

little mechanical erosion. Laterite country isusually infertile. However, lateritic soils maydevelop on slopes in undulating topography(from residual soils), on alluvial soils thathave been uplifted.

ProfilesThere are many variations of laterites and

lateritic profiles, depending on factors such asthe—

Mode of soil formation.Cycles of weathering and erosion.Geologic history.Climate.

Engineering ClassificationThe usual methods of soil classification, in-

volving grain-size distribution and Atterberglimits, should be performed on laterites orsuspected laterites that are anticipated foruse as fill, base-course, or surface-coursematerials. Consideration should be given tothe previously stated fact that some particlesof laterite crush easily; therefore, the resultsobtained depend on such factors as the—

Treatment of the sample.Amount of breakdown.Method of preparing the minus Num-ber 40 sieve material.

The more the soil’s structure is handled anddisturbed, the finer the aggregates become ingrading and the higher the Atterberg limit.While recognizing the disadvantage of thesetests, it is still interesting to note the largespread and range of results for both lateritesand lateritic soils.

Compacted Soil CharacteristicsParticularly with some laterites, break-

down of the coarser particles will occur underlaboratory test conditions. This breakdown isnot necessarily similar to that occurringunder conditions of field rolling; therefore,the prediction of density, shear, and CBRcharacteristics for remolded laboratoryspecimens is not reliable. Strength underfield conditions tends to be much higher thanis indicated by the laboratory tests, providedthe material is not given excessive rolling.Field tests would be justified for supplying


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data necessary for pavement design. Thelaboratory CBR results for laterite showhigher values after soaking than initiallyfound after compaction, indicating a tendencyfor recementation. For lateritic soils, thepresence of cementation in the grain struc-ture also gives the same effect. In addition,the field strengths may be higher than thosein the laboratory, although in a much lesspronounced fashion. If the soil is handledwith a minimum of disturbance and remold-ing, laboratory tests do furnish a usable basisfor design; however, lesser disturbance andhigher strengths may be achieved under fieldconditions.

Pavement ConstructionThe laterized soils work well in pavement

construction in the uses described in the fol-lowing paragraphs, particularly when theirspecial characteristics are carefully recog-nized. While the AASHTO and the Corps ofEngineers classification systems andspecifications are the basis for lateritematerial specification, experience by high-way agencies in countries where laterites arefound indicates that excellent performancefrom lateritic soils can be achieved by modify-ing the temperate zone specifications. Themodified specifications are less exacting thaneither the AASHTO or the Corps of Engineersspecifications, but they have proven satisfac-tory under tropical conditions.

Subgrade. The laterites, because of theirstructural strength, can be very suitable sub-grades. Care should be taken to providedrainage and also to avoid particle break-

down from overcompaction. Subsurface in-vestigation should be made with holes atrelatively close spacing, since the depositstend to be erratic in location and thickness.In the case of the lateritic soils, subgrade com-paction is important because the leachingaction associated with their formation tendsto leave behind a loose structure. Drainagecharacteristics, however, are reduced whenthese soils are disturbed.

Base Course. The harder types of lateriteshould make good base courses, Some areeven suitable for good quality airfield pave-ments. The softer laterites and the betterlateritic soils should serve adequately for sub-base layers. Although laterites are resistantto the effects of moisture, there is a need forgood drainage to prevent softening andbreakdown of the structure under repeatedloadings. Base-course specifications forlateritic soils are based on the following soiluse classifications:

Class I (CBR 100).Class II (CBR 70 to 100).Class III (CBR 50 to 70).

Gradation requirements for laterite andlaterite gravel soils used for base courses andsubbases are listed in Table 12-1. Atterberglimits and other test criteria for laterite basecourse materials are listed in Table 12-2, page12-4.

Subbase. Subbase criteria are listed inTable 12-1, and Table 12-3, page 12-4. Thesecriteria are less stringent than those found inTW 5-330, which limits subbase soil materials


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to a maximum of 15 percent fines, a LL of 25,and a PI of 5.

Surfacing. Laterite can provide a suitablelow-grade wearing course when it can be com-pacted to give a dense, mechanically stablematerial; however, it tends to corrugateunder road traffic and becomes dusty duringdry weather. In wet weather, it scours andtends to clog the drainage system. To preventcorrugating, which is associated with loss offines, a surface dressing maybe used. Alter-natively, as a temporary expedient, regularbrushing helps. The lateritic soils, beingweaker than the laterites, are not suitable fora wearing course. Their use for surfacingwould be restricted to—

Emergencies.Use under landing mats.Other limited purposes.

Stabilization. The laterite and lateriticsoils can be effectively stabilized to improvetheir properties for particular uses. How-ever, because of the wide range in lateritic soilcharacteristics, no one stabilizing agent hasbeen found successful for all lateriticmaterials. Laboratory studies, or preferably

field tests, must be performed to determinewhich stabilizing agent, in what quantity,performs adequately on a particular soil.Some that have been used successfully are—

Cement.Asphalt.Lime.Mechanical stabilization.

Laterite and lateritic soils can still performsatisfactorily in a low-cost, unsurfaced road,even though the percent of fines is higherthan is usual in the continental UnitedStates. This is believed to be due to thecementing action of the iron oxide content.Cement and asphalt work best with materialof a lower fines content. When fines are quiteplastic, adding lime reduces the plasticity toproduce a stable material. Field trials makeit possible to determine the most suitablecompaction method, which gives sufficientdensity without destroying the granule struc-ture and cementation. Generally, vibratorycompaction is best. Laboratory test programsfor tropical conditions do not ordinarily in-clude freeze-thaw tests but should check theinfluence of wetting and drying. The finer-grained lateritic soils may contain enough


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active clay to swell and shrink, thus tendingto destroy both the natural cementation andthe effect of stabilization.

SlopesThe stiff natural structure of the laterites

allows very steep or vertical cuts in the hardervarieties, perhaps to depths of 15 to 20 feet.The softer laterites and the lateritic soilsshould be excavated on flatter than verticalslopes but appreciably steeper than would beindicated by the frictional characteristics ofthe remolded material. It is important toprevent access of water at the top of the slope.

CORAL“Coral” is a broad term applied to a wide

variety of construction materials derivedfrom the accumulation of skeletal residues ofcoral like marine plants and animals. It isfound in various forms depending on the de-gree of exposure and weathering and mayvary from a hard limestone like rock to acoarse sand. Coral develops in tropical oceanwaters primarily in the form of coral reefs, butmany of the South Pacific islands and atollsare comprised of large coral deposits. As ageneral rule, living colonies of coral arebright-colored, ranging from reds throughyellows. Once these organisms die, theyusually become either translucent or assumevarious shades of white, gray, and brown.

TypesThe three principal types of coral used in

military construction are—Pit run coral.Coral rock.Coral sand.

Pit-Run Coral. Pit-run coral usually con-sists of fragmented coral in conjunction withsands and marine shells. At best, it classifiesas a soft rock even in its most cemented form.The CBR values for this material may varybetween 5 and 70. This material seldomshows any cohesive properties. In general,pit-run coral tends to be well graded, but den-sities above 120 pcf are seldom achieved.

Coral Rock. Coral rock is commonly foundin massive formations. The white type is veryhard, while the gray type tends to be soft, brit-tle, and extremely porous. The CBR valuesfor this material vary from 50 to 100. Den-sities above 120 pcf are common except insome of the soft rock.

Coral Sand. Coral sand consists of decom-posed coral rock that may be combined withwashed and sorted beach sands. Generally, itclassifies as a poorly graded sand and seldomwill more than 20 percent of the soil particlespass a Number 200 sieve. The CBR values forthis material vary between 15 and 50. Be-cause of the lack of fines, compaction isdifficult and densities above 120 pcf are un-common.

SourcesCoral may be obtained from—

Construction site cuts.Quarries.Wet or dry borrow pits worked inbenches.

Rooters should be used to loosen the softerdeposits, and the loosened material should bemoved with bulldozers to power shovels forloading into trucks or other transport equip-ment. Rooting and panning are preferable insoft coral pits or shallow lagoons. Wherecoral requires little loosening, draglines andcarryall scrapers can be used. Occasionally,coral from fringing reefs and lagoons can bedug by draglines or shovels, piled as acauseway, and then trucked away progres-sively from the seaward end. Hard coral rockin cuts or aggregate quarries requires consid-erable blasting. In both pit and quarryoperations, hard coral “heads” are often foundembedded in the softer deposits, presenting ahazard to equipment and requiring blasting.

Blasting hard coral rock differs somewhatfrom ordinary rock quarrying since coral for-mations contain innumerable fissures invarying directions and many large voids. Theporosity of the coral structure itself decreasesblasting efficiency. Conventional use of


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low-percent dynamite in tamped holesproduces the most satisfactory results.

Shaped charges are ineffective, especiallywhen used underwater; cratering charges, al-though effective, are uneconomical. Usuallyholes 8 to 12 feet deep on 4- to 8-foot centersare required to get adequate blasting effi-ciency.

UsesCoral may be used as—

Fills, subgrades, and base courses.Surfacing.Concrete aggregate.

Fills, Subgrades, and Base Courses.When properly placed, selected coral that isstripped from lagoon or beach floors or quar-ried from sidehills is excellent for fills,subgrades, and base courses.

Surfacing. White or nearly white coral withproperly proportioned granular sizes com-pacted at OMC creates a concrete like surface.The wearing surface requires considerablecare and heavy maintenance since coralbreaks down and abrades easily under heavytraffic. Conversely, coral surfaces are ex-tremely abrasive, and tire durability isgreatly reduced.

Concrete Aggregate. Hard coral rock, whenproperly graded, is a good aggregate for con-crete. Soft coral rock makes an inferiorconcrete, which is low in strength, difficult toplace, and often of honeycomb structure.

ConstructionConstruction with coral presents some spe-

cial problems, even though the uses arealmost the same as for standard rock and soilmaterials. Since coral is derived from livingorganisms, its engineering characteristicsare unique. Use the steps discussed in the fol-lowing paragraphs to help minimizeconstruction problems:

Whenever coral is quarried from reefsor pits containing living coral

deposits, allow the material to aerateand dry for a period of 6 days, if pos-sible, but not less than 72 hours.Living coral organisms can remainalive in stockpiles for periods of up to72 hours in the presence of water. If“live” coral is used in construction,the material exhibits high swell char-acteristics with accompanying loss ofdensity and strength.Carefully control moisture contentwhen constructing with coral. In-creases of even 1 percent above OMCcan cause reductions of 20 percent ormore in densities achieved in certaintypes of coral. For best results, com-paction should take place between theOMC and 2 percent below the OMC.Maintenance on coral roads is bestperformed when the coral is wet.When added to coral materials, saltwater gives higher densities, with thesame compactive effort, than freshwater. Use salt water in compactionwhenever possible.Hard coral rock should not be used asa wearing surface. This rock tends tobreak with sharp edges when crushedand easily cuts pneumatic tires.When constructing with hard coralrock, use tracked equipment as muchas possible.

DESERT SOILSDeserts are very arid regions of the earth.

Desert terrain varies widely as do the soilsthat compose the desert floor. The desertclimate has a pronounced effect on thedevelopment of desert soils and greatly im-pacts on the engineering properties of thesesoils. Engineering methods effective for roador airfield construction in temperate or tropi-cal regions of the world are often ineffectivewhen applied to desert soils.

Because of wind erosion, desert soils tend tobe of granular material, such as—

Rock.Gravel.Sand.


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Stabilization of the granular material is re-quired to increase the bearing capacity of thesoil. The options available for stabilizingdesert soils are more limited. The primarymeans of stabilizing desert soils are—

Soil blending.Geotextiles.Bituminous stabilization.

Chemical admixtures generally performpoorly under desert conditions because theycease hydration or “set up” too quickiy andtherefore do not gain adequate strength.

If a source of fines is located, the fines maybe blended with the in-place soil, improvingthe engineering characteristics of the resul-tant soil. However, before using the finematerial for blending, perform a complete soilanalysis on the material. Also, consider un-usual climatic conditions that may occurduring the design life of the pavement struc-ture. During Operation Desert Storm, manyroads in Saudi Arabia were stabilized withfine material known as marl. The roads per-formed well until seasonal rains occurred;then they failed.

Geotextiles can be used alone, such as thesand grid, or in combination with bituminoussurfacing. The latter is the most effective.

Bituminous treatment is the most effectivemethod of stabilizing desert soils for tem-porary road and airfield use.

ARCTIC AND SUBARCTIC SOILSConstruction in arctic and subarctic soils in

permafrost areas is more difficult than intemperate regions. The impervious nature ofthe underlying permafrost produces poor soildrainage conditions. Cuts cause changes tothe subsurface thermal regime. Stability anddrainage problems result when cuts aremade. If the soil contains visible ice, or ifwhen a sample of the frozen soil is thawed itbecomes unstable, it is termed thaw-un-stable. Cuts into these types of frozen soilsshould be avoided; however, if this is not pos-sible, substantial and frequent maintenance

will undoubtedly be required. Cuts cangenerally be made into dry frozen (for ex-ample, thaw-stable) soils without majorproblems. Thaw-stable soils are generallysandy or gravelly materials.

Road and runway design over frost-susceptible soils must consider both frosteffects and permafrost conditions. Adjust-ments to the flexible pavement design mayberequired to counter the effects of frost andpermafrost.

Depths of freeze and thaw are usually sig-nificantly altered by construction. Figure12-1, page 12-8, illustrates the effect of clear-ing and stripping on the depth to permafrostafter 5 years. The total depth of thaw isstrongly influenced by the surface materialand its characteristics (see Table 12-4, page12-8).

Increasing the depth of the nonfrost-susceptible base course material can preventsubgrade thawing. The required base thick-ness may be determined from Figure 12-2,page 12-9. The thawing index is determinedlocally by calculating the degree days of thaw-ing that take place. Thawing degree daysdata is usually available from local weatherstations or highway departments. If thawingdegree days data is unavailable, they can beestimated from information contained inFigures 12-3 through 12-5, pages 12-10through 12-12.

Surface-Thawing IndexThe surface-thawing index may be com-

puted by multiplying the thawing indexbased on air temperature by a correction fac-tor for the type of surface. For bituminoussurfaces, multiply by a factor of 1.65; for con-crete, multiply by a factor of 1.5.

For example, design a road over a per-mafrost region to preclude thawing of thepermafrost. The mean air thawing index is1,500 degree days (Fahrenheit). Base coursematerial moisture content is 7 percent.Bituminous surface is to be used.


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If the subgrade is thaw-unstable, a granular1,200 degree days (Fahrenheit) x 1.6 = 1,920surface-thawing index (air-thawing index) xbituminous correction factor)

What is the thickness of the base coursematerial required to prevent thawing of thepermafrost? (See Figure 12-2).

150 inches of the base at 5 percent moisture;adjust for moisture difference

150 x 0,87 = 130.5 inches

Therefore, for this example, permafrost thaw-ing can be prevented by constructing a130.5-inch base-course layer.

If the subgrade is thaw-stable, a soil thatdoes not exhibit loss of strength or settlementwhen thawed may accept some subgradethawing because little settlement is expected.

embankment 4 to 5 feet thick is generally ade-quate to carry all but the heaviest traffic.Considerable and frequent regrading is re-quired to maintain a relatively smoothsurface. Embankments only 2 to 3 feet thickhave been used over geotextile layers thatprevent the underlying fine-grained sub-grade from contaminating the granular fill.Additional fill may be necessary as thawingand settlement progress. It is usuallydesirable to leave the surface organic layer in-tact. If small trees and brush cover the site,they should be cut and placed beneath theembankment. They serve as a barrier toprevent mixing of the subgrade with theembankment material. Figure 12-6, page12-13, and Figure 12-7, page 12-14, assist theflexible pavement designer in determiningthe depth of thaw and freeze, respectively,


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beneath pavements with gravel bases. Thecurves are based on the moisture content ofthe subgrade soil.

Ecological Impact of ConstructionThe arctic and subarctic ecosystems are

fragile; therefore, considerable thought mustbe given to the impact that constructionactivities will have on them, Figure12-8, page 12-15, shows the long-term (26 years)ecological impacts of construction activities inFairbanks, Alaska. Stripping, compacting,or otherwise changing the existing groundcover alters the thermal balance of the soil.

Unlike temperate ecosystems that canrecover from most construction activities in arelatively short period of time, the arctic andsubarctic ecosystems do not recover quickly.In fact, once disturbed, the arctic and sub-arctic ecosystems can continue to undergodegradation for decades following the distur-bance.

The project engineer must consider theselong-term environmental impacts, from theperspective of a steward of the environment,and the effects such environmental degrada-tion may have on the structure’s useful life.


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chapter 12 special soil problems - … can lead to construction errors that are ... grading and the...

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