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Soil Mechanics Final Exam

Terms in this set (52)

T/F: The sheepsfoot roller is most often used to compact plastic clay soils because the small peg like feet apply high stresses needed to shear through the clay and manipulate the clay soils into dean set condition.
True
If sand has a soil void ratio of 0.30 then what term best describes its density?
Very dense
Symbol and equation for porosity
n =
Symbol and equation for void ratio
e = volume(water)/volume(solids)
T/F: In working with a "performance" specification also known as "end-product" specification only the required percent compaction is specified but often the allowable range of moisture content is also specified.
True
What soil fabric results from compacting clay soil that is dry of optimum water content?
flocculated
-higher strength
-slower permeability
Symbol and equation for Degree of Saturation
S = volume(water)/volume(voids) * 100
Symbol and equation for Water Content
wc = weight(water)/weight(solids) * 100
Symbol and equation for Specific Gravity
sg or G(s) = weight(soilds)/volume(total)/unit weight(water)
T/F: To achieve good degree of compaction throughout the lift thickness of a clayey soil, the thickness of loose placed fill before compaction should be 8-10 inches
True
T/F: Clay soils will almost always have a void ratio that is greater then 1.0
True
The Hammer used in performing a Modified Proctor Compaction Test weighs 10.5 lbs and is dropped from 24 inches.
False, drop from 18 inches
What should be the water content of clay soil being compacted for a landfill liner or as core of and earth fill dam, both of which need to be very permeable?
Wet of Optimum
Standard Proctor Test
-hammer weight, drop height, number of soil layers, drops per layer
-5.5 lb hammer
-12" drops
-3 layers of soil
-25 drops per layer
T/F: When clean, well-graded sand and gravel soils is being compacted using a vibratory smooth drum roller, the density test should be conducted at a depth of at least five inches below the compacted surface to avoid the loosening that occurs due to the intense vibrations of the roller.
True
What type of compaction equipment should be used for a non-plastic silty sand without any gravel?
pneumatic rubber-tire roller with water-filled ballast tank
What type of compaction equipment should be used for a low plastic sandy clay with 25% silt?
Self-propelled pad foot-type sheepsfoot roller
What type of compaction equipment should be used for a well-graded coarse to fine sand with very little silt?
smooth steel drum vibratory roller
What is the effect of decreased compactive effort on the same soil? (Standard v Modified Proctor Test)
decreased maximum dry unit weight
increased optimum water content
When the nuclear density test apparatus is used to determine density of compacted soil in the field, what are two types of soils that it is most likely to experience greatest difficulties due to interfere with gamma ray and neutron transmissions?
gravely glacial till
micaceous silty sand
What is the ASTM Number for Modified Proctor Test?
...
What term is commonly used on the earthwork construction site to describe that a sheepsfoot roller has completed its wok of compacting the clay soil layer?
Walked On
What sieve number is used to separate coarse sand from medium sand?
#10
What grain size diameter (in mm) is the dividing line between sand particles and fines?
0.075 mm
The plastic limit test involves rolling out a soil thread until it just breaks into small pieces when the thread is what diameter?
1/8 inch
T/F: A clay soil is considered to be at its liquid limit when the slot closes from 1/2 inch of its length at 35 drops of the brass cups.
False, 25 drops
T/F: All clay soils exhibit the same behavior and shear strength when at their Liquid Limit, even if one clay has LL=130% and another clay LL=25%.
True
What clay particle structure results from deposition of the clay in a fresh water lake environment?
Dispersed
What is the equation for Cc?
D30^2/(D10*D60)
List the three most common clay minerals that were discussed in class.
illite
kaolinite
montmorillonite
Which clay mineral has the most potential for shrink and swell when water content changes?
montmorillonite
What is the relationship between Plastic Index, Plastic Limit, and Liquid Limit?
PI = LL - PL
A soil in its plastic state is more readily usable for building a compacted fill embankment?
one in its plastic state
What are the names of the two different crystal mineral sheets that stack together to form the various types of clay minerals?
silica tetrahedron
alunina octahedron
What lab test is used to determine the grain size distribution of the particles of fine soil, that pass through the No. 200 sieve?
Hydrometer Test
What is the best soil type to be used as an impervious cap over a landfill that must provide impermeability, resistance to shrink swell, and relative ease of compaction?
CH
What term is given to a clay soil in which the existing vertical effective stress on the clay today is less than the maximum past pressure?
Over-consolidated
What physically happens in normally consolidated clay soil that would cause several inches of settlement due to compression of the soft clay to occur when there is a substantial lowering of the groundwater level? Clay is saturated and remains submerged below the groundwater level.
As groundwater level decreases, the effective stress on the soil below increases. Larger stress = larger rate of consolidation.
T/F: OCR = sigma'(c)/sigma'(i)
True
T/F: When determining the value of Cr parameter from the strain versus log pressure graph, Cr = (log p2 - log p1) / (epsilon1 - epsilon2)
False, Cr = (epsilon1 - epsilon2) / (log p2 - log p1)
T/F: If a clay soil deposit currently has an effective vertical stress sigma'(z) that is less than the sigma'(c) found in the consolidation test, then the OCR value < 1.0.
False
T/F: The one physical aspect of clay soil that causes the amount of consolidation compression of clay soils to be much greater than compression of sand soil, even under the same magnitude of loading, is the fact that clay has a much larger initial void ratio than does sand.
True
What thickness of clay layers would you use to make calculation of Primary Compression settlement with a 12x12 footing.
Layers from top: 6', 6', 12', 12'...
T/F: After a new large wide area fill has been placed over a soft clay soil layer that has sand above and below, the excess pore water pressure in the clay caused by the fill placement dissipates quickly at the top and bottom of the clay stratum, but very slowly in the middle of the clay stratum.
True
T/F: The rate at which clay consolidates in Virgin Compression is much slower than the rate that clay compresses in recompression.
False
T/F: When load is first added to a saturated soft clay soil, such as by placement of big highway embankments, the added pressure is initially carried by an increase in the soil effective stress.
False, initially carried by pore water pressure
What two factors make plastic wick drains greatly speed up the time for clay to consolidate?
-shorter Hdr for water to travel
-Ch>Cv by 2-3 times, can more easily travel horizontally
In sand soil, what is the fundamental physical process by which sand develops its shear strength?
friction
What is a typical vale of sand's phi for loose, uniform sand with sub-rounded particles?
...
When using Mohr's circle to determine the shear strength of a sand soil tested in triaxial compression, the value of the minor principle stress is the confining pressure and the major principle stress is the axial stress.
True
What fundamental property of water causes moist sand to exhibit "apparent cohesion"?
surface tension
Why does the dense sand curve in a graph of shear stress v horizontal displacement reach a peak then eventually come back down to meet the loose sand curve?
Dense sand particles dilate when loaded and gain strength, then move around enough so the voids in the originally dense sand are the same as the voids in the loose sand and show the same shear stress, loose sand particles just move and slip around when loaded
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