EESC456-CHAPTER4.2

Question 1

Particle density

Answer 1

of minerals : 2.6 – 2.75 ton/m3

of organic matter : 0.1 – 0.3 g/cc

mass per unit volume of soil solids (in contrast to the volume of the soil, which would also include spaces between particles)

describes the weight of solid particles in a given volume of those solid particles,

same as the specific gravity of a solid substance.

The chemical composition and crystal structure of a mineral determines its particle density.

not affected by pore space; not related to particle size or to the arrangement of particles (soil structure).

arable mineral surface soils (1%–5% organic matter), assume = 2.65 Mg/m3

large amounts of high-density minerals, assume = 3.0 Mg/m3

organic matter (particle density of only 0.9–1.4 Mg/m3)

Question 2

Bulk density

Answer 2

weight of the solid particles in a given volume of dry soil (which includes both solids plus pore space occupied by air).

mass of a unit volume of dry soil (including solids + pores)

determined by drying soil (remove water) + weigh dry mass. using “undisturbed soil core”

strongly influenced by porosity, o.m., depth

–> knowledge of the bulk density of various soils is useful in estimating the weight of (dry!) soil to be moved for construction/landscapers, etc.

Question 3

1 Mg = 1 million grams
1 m3 = 1 million cubic centimeters
1 Mg/m3 = …

Answer 3

1 g/cm3.

Question 4

Effect of soil texture on bulk density

Answer 4

fine-textured soils (silt loams, clays, clay loams) = lower bulk densities
–> solid particles organized in porous granules
–> pores exist between and within the granules = ensures high total pore space

sandy soils = higher bulk densities
–> low organic matter content
–> solid particles are less likely to be aggregated
–> only few fine/within the ped pores = less total porosity (even if similar amount of large pores as fine-textured)
–> denser = mixture of sand sizes (well-graded sand) and a tight packing arrangement.
–> less dense = sand particles are mostly of one size class (i.e., well-sorted sand)

Question 5

Effect of porosity on bulk density

Answer 5

high proportion of pore space to solids = lower bulk densities

more compact, less pore space
= higher bulk densities

–>any factor that influences soil pore space will affect bulk density.

Question 6

Effect of depth in soil profile on bulk density

Answer 6

Deeper in the soil profile, bulk densities are generally higher. Result of :
- compaction by the weight of the overlying layers
- lower organic matter contents
- less aggregation
- fewer biopores.

–> Many soils formed from glacial till have extremely dense subsoils (past compaction by glaciers)

Question 7

Increases in bulk density usually indicate a poorer environment for …. + ….and undesirable changes in hydrologic function, such as reduced water infiltration. Tree growth and forest ecosystem function are particularly sensitive to increases in …

Answer 7

root growth
reduced aeration
bulk density.

Question 8

Forested lands and bulk densities

Answer 8

forested soils’ surface horizons =
low bulk densities

Conventional timber harvest compacts 20%–40% of the forest floor

Intensive recreational and transport use of soils in forests and other areas with natural vegetation can also lead to increased bulk densities. (access roads, trails, and campsites are found)

increased bulk density
= diminished capacity of the soil to take in water
= increased losses by surface runoff.

Answer 9

An expensive, but effective, means of moving logs while minimizing compactive degradation of forest lands is the use of cables strung between towers or hung from large balloons.

Damage from hikers can be minimized by restricting foot traffic to well-designed, established trails that may include a thick layer of wood chips, or even a raised boardwalk in the case of heavily traveled paths over very fragile soils, such as in wetlands.

Question 10

Urban soils and bulk density: trees planted for landscaping purposes =severely compacted soils.

5 ways to make it better

Answer 10

1. large planting hole = loose soil zone for early root growth
2. Mulch spread to enhance root growth
3. Rigid + porous cover (iron grill) to protect against compaction
4. Narrow trenches radiating out from the planting hole and backfilled with loose, enriched soil = paths for root expansion
5. “artificial soil” of coarse angular gravel to provide strength and stability, and a mixture of loam- textured topsoil and organic matter to provide nutrient- and water-holding capacities.

Question 11

Green roofs and bulk densities

Answer 11

Soil bulk density is critical in the design of rooftop gardens. The mass of soil involved must be minimized in order to design a cost-effective structure of sufficient strength to carry the soil load.

relatively thin layer of soil (say, 15 cm) could be used, keeping the total mass of soil from being too great.

reduce the cost of construction by selecting a natural soil having a relatively low bulk density, such as some well-aggregated loams or peat soils.

Often an artificial growing medium is created from such lightweight ma- terials as perlite and peat. However, such very low-density materials may require a surface netting system to prevent wind from blowing them off the roof, and this type of media will not perform the anchorage function of soils as a plant medium (see Section 1.2) for tall plants such as trees.

Question 12

Agricultural lands + bulk density

Answer 12

heavy machines used to pull
= yield-limiting soil compaction.

tillage implements:
moldboard plow+disk harrow: compact the soil below their working depth. can form plow pans or traffic pans, dense zones im- mediately below the plowed layer.

chisel plow + spring-tooth harrow: do not press down upon the soil beneath them, and so are useful in breaking up plow pans and stirring the soil with a minimum of compaction. Large chisel-type plows can be used in subsoiling to fracture soils to considerable depth and break up dense subsoil layers, thereby enhancing deep root penetration. These implements should loosen the soil without producing horizontal compacted lay- ers, and cut through crop residues while causing little disturbance of the soil surface.

However, in most soils the effects of subsoiling are quite temporary and any tillage tends to reduce soil strength, thus making the soil less resistant to subsequent compaction. In moist fine-textured soils, subsoiling can cause planes of compaction as the implements slide through the soil.

Question 13

In cold climates, repeated cycles of freezing and thawing during winter can break up …. However, even where it is cold enough to freeze the soil down to 50 or 100 cm, the repeated freeze–thaw cycles occur only on the upper 10–20 cm. Thus, if heavy equipment driven on wet soils have cause …., one cannot rely on cold winters to alleviate the problem.

Answer 13

compacted soil near the surface.

deep compaction

Question 14

Minimize soil compaction (alleviate deep subsoil compaction)

Answer 14

cover crop plants:
penetrate compacted layers during moist periods and leave deep root channels for subsequent crops’ roots to follow

hand hoes or animal-drawn implements to stir the soil: their weight is applied to the soil in a relatively small area (foot- or hoof-print), and so can also cause considerable compaction.

number of tillage operations and heavy equipment trips over the field should be minimized and timed to avoid periods when the soil is wet.
-prevent compaction
-prevent yield reductions
-prevent loss of profits

–>Traffic is particularly damaging on wet soil. Generally, with heavier loads and on wetter soils, compactive effects are more pronounced and penetrate more deeply into the profile. can be unavoidable in humid temperate regions in spring and fall.

restrict all wheel traffic to spe- cific lanes, leaving the rest of the field (usually 90% or more of the area) free from compaction. controlled traffic systems, establishing permanent footpaths between planting beds.

wide tires are fitted to heavy equipment so as to spread the weight over more soil surface, thus reducing the force applied per unit area. Wider tires do lessen the com- pactive effect, but they also increase the percentage of the soil surface that is impacted.

Question 15

Influence of Bulk Density on Soil Strength and Root Growth

Answer 15

Root growth is inhibited by excessively dense soils due to:

1. soil’s resistance to penetration
2. poor aer- nation
3. slow movement of nutrients and water
4. buildup of toxic gases and root exudates.

Roots penetrate the soil by pushing their way into pores. If a pore is too small to ac- commodate the root cap, the root must push the soil particles aside and enlarge the pore.

density restricts root growth, as the roots encounter fewer and smaller pores.

root penetration is also limited by: soil strength

Compaction generally increases both bulk density and soil strength.

Question 16

2 factors (both related to soil strength) must be considered to determine the effect of bulk density on the ability of roots to penetrate soil.

Answer 16

1. effect of soil water content:

soil water content and bulk density affect soil strength
–> Soil strength increased if :
compacted to a higher bulk density, dry/hard finer-textured soils

effect of bulk density on root growth is most pronounced if those soils are dry, a higher bulk density being necessary to prevent root penetration when the soils are moist.

2. effect of soil texture:

The more clay present in a soil
=smaller pore size
= greater resistance to penetration

roots more easily penetrate a moist sandy soil than a moist clayey one (if same bulk densities)

3. effect of land use/mgmt:
   row- crop agriculture, pasture, rangeland, forestry, or off-road trafficking = affect soil bulk density and strength, restrict or enhance root growth and water movement.

Question 17

soil strength

Answer 17

property of the soil that causes it to resist deformation.
quantify soil strength ; measure the force needed to push a standard shaped rod (a penetrometer) into the soil

Question 18

4. soil porosity

Answer 18

total porosity varies widely

Bulk density values help us predict only total porosity.

soil pores occur in a wide variety of sizes and shapes that largely determine what role the pore can play in the soil.

Pores can be grouped by size into macropores, mesopores, micropores,

macropores (larger than about 0.08 mm)

micropores (smaller than about 0.08 mm).

Question 19

One of the main reasons for measuring soil bulk density is that this value can be used to calculate ….

Answer 19

pore space

Question 20

For soils with the same particle density, the lower the bulk density, the …. the percent pore space (total porosity)

Answer 20

higher

Question 21

cultivation tends to .. the total pore space compared to that of un- cultivated soils. This reduction usually is associated with a decrease in ….and a consequent lowering of granulation.

Answer 21

lower
organic matter content

Question 22

Macropores

Answer 22

allow the ready movement of air and the drainage of water.

accommodate plant roots and the wide range of tiny animals

spaces between individual sand grains in coarse- textured soils.

Interped pores:
well-structured soils, the macropores are generally found between peds. as spaces between loosely packed granules or as the planar cracks between tight-fitting blocky and prismatic peds.

Biopores:
Macropores created by roots, earthworms, and other organisms usually tubular in shape.
–>Perennial plants = channels as conduits for roots

soil structure and texture influence the balance between macropores and micropores in a soil.

Question 23

even though a sandy soil has relatively low total porosity, the movement of air and water through such a soil is surprisingly …because of the dominance of the ….

Answer 23

rapid
macropores

Question 24

decrease in organic matter and increase in clay that occur with depth in many profiles are associated with a shift from … to …

Answer 24

macropores to micropores.

Question 25

In some clayey soils, biopores are the principal form of …, greatly facilitating the growth of

Answer 25

macropores
plant roots

Question 26

Micropores

Answer 26

usually filled with water in field soils.

Even when not water-filled, they are too small to permit much air movement.

Water movementin micropores is slow, and much of the water retained (not available to plants)

Fine-textured soils, especially those without a stable granular structure, may have a preponderance of micropores, thus allowing relatively slow gas and water movement, despite the relatively large volume of total pore space.

Aeration, especially in the subsoil, may be inadequate for satisfactory root development and desirable microbial activity.

larger micropores accommodate plant root hairs and microorganisms

smaller micropores (nanopores) too small to permit the entrance of even the smallest bacteria or some decay-stimulating enzymes produced by the bacteria. act as hiding places for some adsorbed organic compounds (both naturally occurring and pollutants), thereby protecting them from breakdown for long periods of time,

Question 27

… (3) of soil pores (rather than their combined volume) are of greatest importance in determining soil ..(2)., and other such processes.

Answer 27

size, shape, and interconnection

drainage, aeration

Question 28

Continuous cropping, particularly of soils originally high in organic matter, often results in a reduction of ….spaces. Such cropping drastically reduces the amount of macropore space that is so critical for ready …

Answer 28

macropore

air movement.

Question 29

In recent years, conservation tillage practices, minimize plowing and associated soil manipulations,

Because of increased accumulation of …near the soil surface and the development of a long-lived network of … (especially biopores), some conservation tillage systems lead to greater … on the … layers.

Answer 29

organic matter
macropores
macroporosity of the surface layers.

Question 30

8. engineering uses

Answer 30

A. Field Rating of Soil Consistence and Consistency

B. Soil Strength and Sudden Failure

(A+B; guide decisions about loading and manipulating soils)

C. Settlement—Gradual Compression

D. Expansive Soils

E. Atterberg Limits

Question 31

Soil consistence

Answer 31

ease with which a soil can be reshaped or ruptured.
amount of force needed to crush the clod and on the manner in which the soil responds to the force.

The degree of cementation of the soil by such materials as silica, calcite, or iron is also considered in iden- tifying soil consistence.

Moisture content greatly influences how a soil responds to stress; hence, moist and dry soils are given separate consistence ratings.
–>A dry, clayey soil that cannot be crushed between the thumb and forefinger but can be crushed easily underfoot would be designated as very hard. The same soil, when wet, would exhibit much less resistance to deformation, and would be termed plastic.

The degrees of stickiness and plasticity (malleability) of soil in the wet condition are often included in describing soil consistence

Moist clod that crumbles with only light pressure is said to be friable. Friable soils are easily excavated or tilled.

Question 32

Soil consistency

Answer 32

how a soil resists penetration by an object, while the soil scientist’s consistence describes resistance to rupture.

attempts to penetrate it

simple field estimation of soil strength or penetration resistance

Question 33

Soil bearing strength

Answer 33

capacity of a soil mass to withstand stresses with- out rupturing or becoming deformed.

Failure of a soil to withstand stress can result in a building toppling over as its weight exceeds the soil’s bearing strength.

Question 34

Cohesive soils

Answer 34

soils with a clay content of more than about 15%

1. the inherent electrostatic attractive forces between clay platelets and between clay surfaces and the water in very fine pores
2. frictional resistance to movement between soil particles of all sizes.

The strength of clines dramatically if the material is very wet and the pores are filled with water. Then the particles are forced apart so that neither the cohesive nor the frictional component is very strong, making the soil prone to failure, (mudslides, soil creep)

On the other hand, if cohe- sive soils become more compacted or dry down, their strength increases as particles are forced into closer contact with one another—a result that has implications for plant root growth as well as for engineering

Question 35

Non cohesive soils

Answer 35

The strength of dry, noncohesive soil materials such as loose sand de- pends entirely on frictional forces, including the interlocking of rough particle surfaces. interparticle friction

angle of repose, the steepest angle to which a material can be piled without slumping.

Smooth, rounded sand grains cannot be piled as steeply as can rough, interlocking sands.

If a small amount of water bridges the gaps between particles, electrostatic attraction of the water for the mineral surfaces will increase the soil strength Interparticle water bridges

Question 36

collapsible soils

Answer 36

considerable strength at low in situ water con- tents lose their strength suddenly if they become wet. Such soils may collapse without warn- ing under a roadway or building foundation.

thixotropy, the sudden liquification of a wet soil mass when subjected to vibrations, such as those ac- companying earthquakes and blasting.

Most collapsible soils are noncohesive materials in which loosely packed sand grains are cemented at their contact points by small amounts of gypsum, clay, or water under ten- sion.
–>arid and semiarid regions, where such cementing agents are relatively stable.

Many collapsible soils have derived their open particle arrangement from the process of sedimentation beneath past or present bodies of water. When these soils are wetted, excess water may dissolve cements such as gypsum or disperse clays that form bridges between particles, causing a sudden loss of strength. In some cases, similar behavior is exhibited by highly weathered Oxisols in humid tropical regions.

Question 37

most foundation problems result from slow, often uneven, vertical subsidence or settlement of the soil.

Answer 37

A. compaction control
Some soil particles, such as certain silicate clays and micas, can be compressed when a load is placed upon them. If that load is removed, these particles tend to regain their orig- inal shape, in effect reversing their compression. As a result, soils rich in these particles are not easily compacted into a stable base for roads and foundations.

The Proctor test is used to guide efforts at compacting soil materials before con- struction. Proctor curve indicates the maximum bulk density achievable and the soil water content that maximizes com- pactability. spray water to bring the soil water content to the determined optimum level before heavy equipment compacts the soil to the desired density.

B.Compressibility:

A consolidation test may be conducted on a soil specimen to deter- mine its compressibility—how much its volume will be reduced by a given applied force.

Because of the relatively low porosity and equidimensional shape of the individual mineral grains, very sandy soils resist compression once the particles have settled into a tight pack- ing arrangement. They make excellent soils for foundations.

The high porosity of clay floc- cules and the flakelike shape of clay particles give clayey soils much greater compressibility.

Soils consisting mainly of organic matter (peats) have the highest compressibilities and gen- erally are unsuitable for foundations.

compression can occur only as fast as water can escape from the soil pores—which for the fine pores in clayey materials is not very fast.

Question 38

expansive soils

Answer 38

smectites swell when wet and shrink when dry. Expansive soils are rich in these types of clay.
The electrostatic charges on clay surfaces attract water molecules from larger pores into the micropores within clay domains. Also, the adsorbed cations associated with the clay surfaces tend to hydrate, drawing in additional water. The water pushes apart the layers of clay, causing the mass of soil to swell in volume.

The reverse of these processes occurs when the soil dries and water is withdrawn from packets of clay plate- lets, causing shrinkage and cracking.

The swelling and shrinkage cause sufficient movement of the soil to crack building foundations, burst pipelines, and buckle pavements.

Relatively pure, mined smectite clay especially when saturated with Na+ ions, can have far greater potential for swelling and plasticity than the impure clays in soil. Smectite clays generally have high liquid limits and plasticity indices, especially if satu- rated with sodium. Kaolinite and other nonexpansive clays have low liquid limit values. The tendency of expansive clay soils to literally flow down steep slopes when the liquid limit is exceeded, producing mass wasting and landslides

Question 39

Atterberg Limits

Answer 39

As a dry, clayey soil takes on increasing amounts of water, it undergoes dramatic and distinct changes in behavior and consistency. A hard, rigid solid in the dry state, it becomes a crumbly (friable) semisolid when a certain moisture content (termed the shrinkage limit) is reached. If it contains expansive clays, the soil also begins to swell in volume as this moisture content is exceeded. Increasing the water content beyond the plastic limit will transform the soil into a malleable, plastic mass and cause additional swelling. The soil will remain in this plastic state until its liquid limit is exceeded, causing it to transform into a viscous liquid that will flow when jarred. These critical water contents (measured in units of percent) are termed the Atterberg limits.

1. Plasticity Index
2. COLE

2 measures of soil expansiveness (much higher for smectitic soils).

Question 40

Plasticity Index

Answer 40

PI = LL - PL

difference between the plastic limit (PL) and liquid limit (LL)

indicates the water-content range over which the soil has plastic properties

Soils with a plasticity index greater than about 25 are usually expansive clays that make poor roadbeds
or foundations.

Question 41

COLE:

Answer 41

Coefficient of Linear Extensibility

COLE = (Lm - Ld)/Lm *100

The expansiveness of a soil (and therefore the hazard of its destroying foundations and pavements) can be quantified as the COLE.

The COLE is the percent reduction in length of the soil bar upon shrinking

Question 42

Particle size, moisture content, and plasticity of the colloidal fraction all help determine the …

Answer 42

stability of soil

Question 43

Well-sorted, loose packing

Answer 43

higher pore volume

Question 44

well-graded, loose packing

Answer 44

lower pore volume

Question 45

well sorted, tight packing = lower pore volume

Question 46

short-term: cultivation/tillage reduces… and creates surface roughness (clogs on surface)

long-term: cultivation/tillage …. bulk density and … pore space

Answer 46

bulk density

increases
pore space