Geotechnical Design Exam Review
UPDATED ACTUAL Questions and
CORRECT Answers
What are indicator piles. and how do they benefit the foundation design process? -
CORRECT ANSWER✔✔- Indicator piles are prototype piles that are constructed before the
production piles. These prototype piles
are typically evaluated using pile driving analyzers, load tests, or other means to evaluate
their load
capacity. The results of these evaluations are then used to guide the design and construction
of the
production piles. In many cases, the indicator piles also are used to support the structure.
Many engineers believe the only disadvantage to over designing pile foundations is the
additional cost of
the pile materials. However, there is another important consequence of over design which can
significantly
affect construction cost and may even make the foundation impossible to build. What is this
consequence and how can it be avoided? - CORRECT ANSWER✔✔- In addition to
increasing materials costs, over designed piles are more difficult to install, thus increasing
labor and equipment costs. In some cases, the additional labor and equipment costs, as well as
the
associated delays in the construction schedule, can be dis proportionally large compared to
the additional
capacity gained. These unnecessary costs and delays can be avoided by more precise designs
and by the used of the various techniques of verification and redesign during construction.
These measures help assure the as built foundation has sufficient capacity and performance,
without being excessively over designed.
Under what circumstances would you most likely require integrity testing of newly-
constructed drilled
shaft foundations? - CORRECT ANSWER✔✔- Integrity testing is especially important
when using the slurry method of construction.
,Explain the difference between the active, at-rest, and the passive earth pressure conditions. -
CORRECT ANSWER✔✔- The at-rest pressure is that which acts on a wall that has not
experienced any lateral displacement or
rotation. In other words, it reflects the horizontal stresses that were present in the undisturbed
ground. The active pressure is that which acts on a wall that has moved a sufficient distance
away from the back fill to
fully mobilize the shear strength of the soil. The passive pressure is that which acts on a wall
that has
moved a sufficient distance into the back fill to fully mobilize the shear strength of the soil.
Which of the three earth pressure conditions should be used to design a rigid basement wall?
Why? - CORRECT ANSWER✔✔- A rigid basement wall should be designed using at-rest
pressure because it has not moved enough to
develop the active condition.
A pre-stressed concrete pile is being driven into a saturated cohesive soil with a hammer blow
count of 17 blows per foot of pile penetration. Unfortunately, the pile driving rig breaks down
before the pile reaches
the required depth of penetration, resulting in a 15-hour delay. Once the rig is repaired, pile
driving
resumes but the blow count is now 25 blows per foot. Explain the primary reason for this
change in blow
count. - CORRECT ANSWER✔✔- Driving of the pile caused large shear stresses in the soil.
Since the soil was a saturated cohesive soil,
positive pore pressures were generated during driving and therefore decreased the effective
stress and
both the end bearing and side friction compared to static loading. During the delay, the soil
had time to
dissipate some of the excess pore pressure generated during driving and the affected soil was
able to
consolidate. The decrease in pore pressure and the consolidation increased the soil strength
and thereby
the pile capacity. This effect is known in practice as ground "setup" or ground "freeze".
Sometimes pile driving contractors use predrilling when installing piles. This method consists
of drilling a
, vertical hole that has a smaller diameter than the pile, then driving the pile into this hole.
Could
predrilling affect the side-friction resistance? Why? Is the diameter of the predrill hole
important? Why? - CORRECT ANSWER✔✔- Predrilling can definitely affect the side-
friction resistance by loosening the soil and by reducing the
lateral earth pressure. Both the size of the predrill hole and its depth are important. The
danger in
predrilling is that it can reduce pile capacity if the predrilling is too long or the diameter of
the hole is
much larger than the pile. Ideally, the hole diameter should be smaller than the pile diameter
and the
length should be kept to a minimum.
Why is it important for drilled shaft contractors to place the concrete soon after drilling the
shaft? What
detrimental effects can occur if the contractor waits too long before placing the concrete? -
CORRECT ANSWER✔✔- There are two potential detrimental effects which can be caused
by delaying concrete placement. The first
is related to reduction in lateral earth pressure. The drilled shaft construction process will
always decrease
the lateral earth pressure around the shaft. The longer time shaft is open, the greater the
reduction, up to
some limit (e.g., the active failure state of the soil around the shaft). The second potential
detrimental
effect is caving and squeezing of soils. The longer the shaft is open, the greater the possibility
of soils
entering the shaft through squeezing or caving. Both of these effects can reduce the capacity.
What is "block failure" in a group of piles and how does it differ from individual failure? -
CORRECT ANSWER✔✔- Block failure is the mode of failure where the pile group and the
soil between the files moves as a unit,
and the side friction resistance is developed along the perimeter of this unit. In contrast,
individual failure
is the mode of failure where each pile fails individually and the side friction occurs along the
side of each
UPDATED ACTUAL Questions and
CORRECT Answers
What are indicator piles. and how do they benefit the foundation design process? -
CORRECT ANSWER✔✔- Indicator piles are prototype piles that are constructed before the
production piles. These prototype piles
are typically evaluated using pile driving analyzers, load tests, or other means to evaluate
their load
capacity. The results of these evaluations are then used to guide the design and construction
of the
production piles. In many cases, the indicator piles also are used to support the structure.
Many engineers believe the only disadvantage to over designing pile foundations is the
additional cost of
the pile materials. However, there is another important consequence of over design which can
significantly
affect construction cost and may even make the foundation impossible to build. What is this
consequence and how can it be avoided? - CORRECT ANSWER✔✔- In addition to
increasing materials costs, over designed piles are more difficult to install, thus increasing
labor and equipment costs. In some cases, the additional labor and equipment costs, as well as
the
associated delays in the construction schedule, can be dis proportionally large compared to
the additional
capacity gained. These unnecessary costs and delays can be avoided by more precise designs
and by the used of the various techniques of verification and redesign during construction.
These measures help assure the as built foundation has sufficient capacity and performance,
without being excessively over designed.
Under what circumstances would you most likely require integrity testing of newly-
constructed drilled
shaft foundations? - CORRECT ANSWER✔✔- Integrity testing is especially important
when using the slurry method of construction.
,Explain the difference between the active, at-rest, and the passive earth pressure conditions. -
CORRECT ANSWER✔✔- The at-rest pressure is that which acts on a wall that has not
experienced any lateral displacement or
rotation. In other words, it reflects the horizontal stresses that were present in the undisturbed
ground. The active pressure is that which acts on a wall that has moved a sufficient distance
away from the back fill to
fully mobilize the shear strength of the soil. The passive pressure is that which acts on a wall
that has
moved a sufficient distance into the back fill to fully mobilize the shear strength of the soil.
Which of the three earth pressure conditions should be used to design a rigid basement wall?
Why? - CORRECT ANSWER✔✔- A rigid basement wall should be designed using at-rest
pressure because it has not moved enough to
develop the active condition.
A pre-stressed concrete pile is being driven into a saturated cohesive soil with a hammer blow
count of 17 blows per foot of pile penetration. Unfortunately, the pile driving rig breaks down
before the pile reaches
the required depth of penetration, resulting in a 15-hour delay. Once the rig is repaired, pile
driving
resumes but the blow count is now 25 blows per foot. Explain the primary reason for this
change in blow
count. - CORRECT ANSWER✔✔- Driving of the pile caused large shear stresses in the soil.
Since the soil was a saturated cohesive soil,
positive pore pressures were generated during driving and therefore decreased the effective
stress and
both the end bearing and side friction compared to static loading. During the delay, the soil
had time to
dissipate some of the excess pore pressure generated during driving and the affected soil was
able to
consolidate. The decrease in pore pressure and the consolidation increased the soil strength
and thereby
the pile capacity. This effect is known in practice as ground "setup" or ground "freeze".
Sometimes pile driving contractors use predrilling when installing piles. This method consists
of drilling a
, vertical hole that has a smaller diameter than the pile, then driving the pile into this hole.
Could
predrilling affect the side-friction resistance? Why? Is the diameter of the predrill hole
important? Why? - CORRECT ANSWER✔✔- Predrilling can definitely affect the side-
friction resistance by loosening the soil and by reducing the
lateral earth pressure. Both the size of the predrill hole and its depth are important. The
danger in
predrilling is that it can reduce pile capacity if the predrilling is too long or the diameter of
the hole is
much larger than the pile. Ideally, the hole diameter should be smaller than the pile diameter
and the
length should be kept to a minimum.
Why is it important for drilled shaft contractors to place the concrete soon after drilling the
shaft? What
detrimental effects can occur if the contractor waits too long before placing the concrete? -
CORRECT ANSWER✔✔- There are two potential detrimental effects which can be caused
by delaying concrete placement. The first
is related to reduction in lateral earth pressure. The drilled shaft construction process will
always decrease
the lateral earth pressure around the shaft. The longer time shaft is open, the greater the
reduction, up to
some limit (e.g., the active failure state of the soil around the shaft). The second potential
detrimental
effect is caving and squeezing of soils. The longer the shaft is open, the greater the possibility
of soils
entering the shaft through squeezing or caving. Both of these effects can reduce the capacity.
What is "block failure" in a group of piles and how does it differ from individual failure? -
CORRECT ANSWER✔✔- Block failure is the mode of failure where the pile group and the
soil between the files moves as a unit,
and the side friction resistance is developed along the perimeter of this unit. In contrast,
individual failure
is the mode of failure where each pile fails individually and the side friction occurs along the
side of each
