Earth Structures

Question 1

Purpose of Ground Improvement

Answer 1

Densify Soil
Increase soil bearing capacity
Reduce foundation settlement
Reduce liquefaciton potential
Mitigate/Reduce Seepage

Question 2

How to calculate apparent opening size?

Answer 2

Calculate O95 then correlate that size with US sieve size

Question 3

Slope Stability in Dry Cohesionless Soils

Question 4

Effects of water on Slope Stability in: fills on clay; cuts on clay; slaking

Answer 4

Excess pwp created when fill placed on clay from applied load –>as pwp dissipates the shear strength increases with time so FS increases with time

Cuts in clay: effective stress is reduced which allows clay to expand & absorb water which leads to DECREASE in clay strength with time

Sudden moisture increase in weak rock produces pwp in air pores leading to strength decrease –> slaking occurs

Water table in cohesionless soils decreaes effective stress which decreases frictional strength

Seasonal gw fluctuations do not influence cohesive soils

Question 5

Order of pavement layers from bottom to top

Answer 5

Subgrade
-constructed on natural soil
-provide enough compaction so no deflection

Subbase
-lower quality than base
-typically granular
-can be treated to increase strength
-not always included in RIGID pavements
-prevents fine grained material into base layer
-minimize frost in subgrade
-provide drainage

Base
-provides significant portion of structural capacity in flexible pavements
-improves foundation stiffness for rigid pavements
-high quality aggregates like crushed stone
-uniform and stable support for surface

Surface
-resist skidding, traffic abrasion and climate effects

Question 6

Rigid Pavements

Answer 6

Surface is portland cement which supports most of load.
Sublayer quality can be compromised
25-30 year lifetime with little maintenance

Question 7

Flexible Pavements

Answer 7

Asphaltic surface layer with NO cement slabs
Either high quality hot mix asphalt or lower strength and stiffness
Rely heavily on strength and stiffness of sub layers
Every 4-5 years need repair

Question 8

Rankine vs Coulomb

Answer 8

Rankin does not take friction into account, so Ka is overestimated slightly, and Kp is underestimated, therefore making rankine conservative

Coulomb takes wall friction into account, but results can be unreliable for PASSIVE

Question 9

Is bearing pressure greater at toe or heel?

Answer 9

At toe (side without/less soil) is always greater than heel

Question 10

How to increase resistance against sliding?

Answer 10

Add a key below foundation - useful for walls supported by rock, granular soils, or stiff clays

Question 11

Resultant Active Force

Answer 11

Pa = 0.5KaUnit wt*H^2

Question 12

Tension Cracks

Answer 12

Extend to a depth where lateral earth pressure = 0
Depth of tension crack = (2*c) / unit wt

Question 13

Rankine Theory Assumptions

Answer 13

• Soil is homogenous and isotropic
• Wall is infinitely LONG (not deep)
• Wall moves sufficiently to develop active or passive conditions
• REsultant of normal and shear forces that act on back of wall is inclined at angle equal to friction angle bw wall and retained soil

Question 14

FS Against Sliding (shortcut

Answer 14

FS = tan (phi) / tan(beta)