CE 8220 Final Exam
What does the FS depend on?
- SOil Type - Site characterization Data - Soil variability - Importance of the structure - Liklihood of the design load ever actually occuring
What are the reasons why the true FS might be much higher than the design FS?
- shear strength data are normally interpreted conservatively - service loads are probably less than design loads = settlement, not bearing capacity, controls the design - spread footings are commonly built somewhat larger than the plan dimensions.
What is the friction angle for an undrained clay?
0
What are the limitations to the Osterburg Load Test?
1) Advance installation of the O-Cell is required 2) Balanced components requirement 3) Sacrificial O-Cell 4) Equivalent Top Load 5) Not suitable for all types of piles
What are the two checks performed when designing a shallow foundation?
1) Bearing capacity check 2) Settlement check
What are the methods for calculating consolidation settlement?
1) Classical Method 2) Skempton and Bjerrum method
What are the testing methods for determining the ultimate bearing capacity of deep foundations?
1) Conventional Static-Load Test 2) Instrumented Static-Load Test 3) Ostenburg Test (O-Cell)
What is the process of limit equilibrium analysis?
1) Define failure surface 2) Evaluate the stresses along that surface
What are the changes in clays caused by a driven pile?
1) Distortion 2) Compression and excess pore water pressures 3) Loss of contact between the foundation and the soil
What are the typical deep foundation types?
1) Driven Piles 2) Drill Shafts 3) Augercast Piles 4) Geo-jet piles 5) Micro-piles
What are the advantages to the Osterburg Load Test?
1) Economic (cheap) 2) High test load capacity 3) Shear/Bearing components (separates the side shear and end-bearing components) 4) Improved Safety 5) Rock socket 6) Reduced work area 7) Applied to over-water and battered piles 8) Static creep effects
What are the two categories of soil settlement?
1) Elastic or IMMEDIATE settlement 2) Consolidation or TIME-DEPENDENT settlement
What are some remedying techniques for differential settlement?
1) Enlarge all of the footings until the differential settlements are acceptable 2) Connect the footings with grade beams 3) Replace the spread footing with a system of deep foundations
What are the pile conditions seen when performing lateral capacity analysis?
1) Free-head condition 2) Fixed-head condition 3) Pure moment condition
What are the three types of bearing capacity failure?
1) General Shear Failure 2) Punching Shear Failure 3) Local Shear Failure
Proper application of analytical methods of deep foundations requires an understanding of what?
1) How side friction and toe bearing resistances develop in different deep foundations 2) What kinds of changes in the soil occur during the installation of different deep foundations
What advantages does a pile group provide?
1) Increased rotational stiffness 2) Increased footprint
What are the failure modes for pile groups?
1) Individual Failure 2) Group/Block Failure
How does ground water affect shear strength?
1) It increases the pore water pressure (decreasing shear strength) 2) It reduces the apparent cohesion (from unsaturated soil conditions)
When is a mat foundation selected over a spread footing foundation?
1) Loading is very high and the 2) Soil conditions are very poor 3) Uplift loads are larger than spread footings can accommodate 4) When water proofing is needed
What are some benefits to shallow foundations?
1) Low Cost 2) Easy Construction
Full-scale static load tests are most likely to be cost effective when one or more of the following conditions are present:
1) Many foundations are to be installed 2) Soil conditions are erratic or unusual, and thus difficult to assess with the analytical methods 3) Pile is supported in soils that are prone to dramatic failures 4) Structure is especially important or especially sensitive to settleents 5) Engineer has little or no experience in the area 6) Foundations must resist uplift loads
What are the limitations of Terzaghi's Bearing Capacity Formula?
1) Only applicable to DF <= B (the depth of the foundation must be smaller than the base) 2) Concentric vertical load only 3) Soil must be homogenous and no consolidation occurs
What does pile group efficiency depend on?
1) Pile group parameters (number, length, arrangement, etc, of piles) 2) Load-Transfer Mode 3) Contruction procedures used to install piles 4) Sequence of installation of the piles 5) Soil type 6) Elapsed time since piles were driven.
What are the changes in sands caused by a driven pile?
1) Positive excess pore water pressure develops in loose sand, resulting in temporary loss of shear strength (easy to drive) 2) Negative excess pore water pressure can develop 3) Densification and crushing of sand particles
What are the requirements that a foundation must satisfy?
1) Stress criteria 2) Settlement criteria 3) Constructability criteria 4) Economic criteria
What are the methods of determining ultimate bearing capacity of deep foundations?
1) Tests 2) Analytical methods
What does the distribution of bearing pressure depend on?
1) The eccentricity of the applied load 2) The magnitude of the applied moment 3) The structural rigidity of the foundation 4) The stress-strain properties of the soil 5) The roughness of the bottom of the foundation
** A deep foundation is preferred over a shallow foundation in some situations, such as:
1) The loading is very high 2) There is a high uplift 3) The structure is settlement sensitive 4) The near surface soils are weak
Bearing capacity depends on what?
1) The properties of the soil 2) The size of the footing
** A shallow foundation is established some distance below the ground surface for the following reasons:
1) To get below the frost line 2) To eliminate scour 3) To get past near-surface soils 4) To avoid expensive clays or other problematic soils
Why do we typically place shallow foundations underground?
1) To get past the near-surface soils 2) To get below the frost line 3) to eliminate expensive clays or other problematic soils 4) To eliminate scour
List three approaches used for calculating the ultimate bearing capacity of shallow foundation of layered soil:
1) Use the smallest values for c', phi', and gamma for all the soil layers 2) Use a weighted average for c', phi', and gamma relative to the thickness of the soil layer 3) Define failure surfaces and calculate the stresses along those surfaces.
What causes differential settlement?
1) Variations in soil profile 2) Variations in structural loads 3) Design controlled by bearing capacity 4) Construction tolerances 5) Rigidity of the structures.
What three zones in the soil are created byt Terzaghi's model?
1) Wedge Zone 2) Radial Shear Zone 3) Linear Shear Zone
What is the iportance of the thickness of the foundation?
1) selected for ease of construction 2) Provides shear capacity of the concrete section
*** What is the unit weight of concrete (density) in lb/ft^3 and kN/m^3>
150 lb ft^3 and 23.6 kN / m^3
What is the typical settlement required to fully mobilize side friction?
15mm
How long must a pile sit in a sand before it reaches full axial capacity?
2 days
How much time is needed to dissipate the pore water pressure in a static low tests for sands and clays, repsectively?
2 days for sand 30 days for clay
How many methods are there for calculating bearing capacity in layered soils?
3
How many types of bearing capacity failures are there?
3
How long must a pile sit in a clay before it has reached its full axial capacity?
30 days
What is the distance away, in a conventional static-load test, that the reference beam needs to be away from the test pile?
5 diamters of the pile (at least 2.5 m) to prevent changing the soil stresses
What is a physical sign that general shear failure has occurred?
A clearly formed bulge appears on the ground surface adjacent to the foundation.
What is pile relaxation and what causes it?
A decrease in pile axial capacity caused by the development of negative pore water pressure (dilation - pulling in tension) during pile installation and dissipation thereafter
What type of pile has a well-defined soil-foundation contact?
A driven pile or drilled shaft with a closed-section foundation.
What is the definition of a shallow foundation?
A foundation that bears at a depth less than about two times its width
What is a hybrid foundation?
A foundation that consists of both a soil-supported mat and piles.
What is the product of a static or instrumented load test?
A load-displacement curve
What is net bearing pressure?
A measure of the increase of vertical effective stress at the bottom of a footing.
Who must supervise a conventional static load test?
A professional engineer
Are instrumented full-scale static load tests common?
Absolutely not
Do you add or subtract the weight of the foundation when calculating the upward capacity?
Add
What does ASD stand for?
Allowable stress design
Where is the O-Cell placed in an Osterburg Load Test?
At or near the bottom of the deep foundation
What is a typical deep foundation with an enlarged base?
Belled drilled shaft
What can be calculated through the Osterburg Load Test?
Both side friction vs axial movement and toe-bearing vs axial movement
If we are creating a footing on a silt, what is the conservative approach to the shear strength?
Compute the bearing capacity analysis using undrained shear strength
What is the method to determine the ultimate capacity from a load-settlement curve of a deep foundation?
Davisson's method
What is the lower bound on the zone of influence?
Df + B
What type of deep foundation can be an open-section foundation?
Driven pile
What type of deep foundation can be a closed-section foundation?
Driven pile or drilled shaft
Which have larger toe resistances, driven piles or drilled shafts?
Driven piles
Stiff deep foundations are bad for what kind of loads?
Dynamic
Battered piles are bad for what kind of loads?
Dynamic loads
What are telltale rods also known as?
Extensometers
** T/F: During the installation of auger cast pile, the reinforcement gauge is set in place in the drilled hole and then concrete (or grout) is poured and compacted to remove air bubbles trapped in the concrete.
F
** T/F: In the allowable bearing pressure method, the most heavily loaded footing is used for the bearing capacity analysis and the lightly loaded footing is used for settlement analysis.
F
** T/F: The factor of safety for footings in sand is higher than that in clay.
F
T/F: There is a clear procedure available for analyzing the capacity of pile groups
F
Are mat foundations rigid or flexible?
Flexible
What must we do when thinking about cost of construction and deep foundation testing?
Full scale load tests are much more precise than any analytical method or other testing method, but they cost a lot. If we perform a full scale load test we can lower the factor of safety, which reduces construction costs to up to 50%. We compare the savings with the cost of the test.
What test provides the most precise assessment of ultimate load capacity?
Full-Scale Load Tests
What type of bearing capacity failure is the most common?
General Shear Failure
What type of shear failure is most likely to occur in soils that are relatively incompressible?
General Shear Failure
What type of failure is most likely to occur in soils that are normally consolidated clays that are loaded rapidly enough that the undrained condition prevails?
General shear failure
What must occur if settlement is to be found in the laboratory?
High quality soil samples must be obtained
What is the rule of thumb used to determine whether a spread footing or mat footing should be used?
If the spread footing covers more than 50% of the building footprint, a mat or deep foundation is more economical.
If a footing is on a saturated clay, when is failure most likely to occur?
Immediately after the load is applied
In practice, how are sister bars and rebar strainmeters typically installed? Why?
In pairs on either side of the neutral axis of the pile to tell the bending moments from the axial loads.
What are the two types of static-load tests?
Instrumented and conventional, but they also have stress controlled and strain-controlled
What is a physical sign that punching shear failure has occurred?
It causes large settlement and poorly defined vertical shear surfaces.
What does the pile group efficiency value do in clays as time increases?
It increases
What effect does having an enlarged base on a drilled shaft have?
It increases the upward capacity of the deep foundation
What, regarding saturated soils and unsaturated soils, is a conservative approach when considering bearing capacity analysis?
It is a conservative approach to assume the soil is fully saturated
How do you calculate ud when computing bearing pressure?
It is the hight up the foundation the water table is times 9.81
What does predrilling or jetting do to a pile group efficiency factor?
It lowers it
What are the changes in soils caused by a drill shaft?
It may or may not change the proeprties of the soils, but if it does, in the case of clay, it can remold the wall.
What must happen to normally consolidated clay for it to experience general shear failure?
It must be loaded rapidly enough that the general undrained condition prevails.
What is a con to a pile group?
It requires the construction of a pile raft
In dense sands, the pile efficiency has what bounds under what conditions?
It's greater than 1.0 as long as the pile was installed without predrilling or jetting.
What does LRFD stand for?
Load and Resistance Factor Design
What type of failure do deep foundations experience?
Local shear failure or punching shear failure
What does ML mean, with regards to static-load testings?
Maintained load -- indicates that it is a stress (load) controlled test, where you apply the load and you measure the settlement.
What are the methods for calculating bearing capacity in soils?
Method 1) Use the lowest values of c', phi', and gamma in the zone between the bottom of the foundation and a depth B below the bottom. This method is conservative. Method 2) Use the weighted average values of c', phi', and gamma based on the relative thickness of each stratum. This method is conservative or unconservative. Method 3) Consider a series of trial failure surfaces beneath the footing and evaluate the stresses on each of those footings. This is complicated and hard; don't do it.
The downward load applied by the O-cell in the Osterburg Load Test does what?
Mobilizes end-bearing resistance
The upward load applied by the O-cell in the Osterburg Load Test does what?
Mobilizes shear resistance along the sides of the shaft or pile
For plugging of an open-ended deep foundation, does it need to go deeper in clays or sands?
Much deeper for sands. 10-20 m for clay, 25-35 m for sand.
Are load factors always greater than one?
No
Can you tell the side friction from the toe resistance in a conventional static load test?
No
Does bulging occur in both general shear failure and punching shear failure?
No, only general shear failure
When is equilibrium analysis performed?
On shallow foundations
What type of foundation has a poorly defined foundation-soil contact plane?
Open-ended foundations
Failure develops ____ in punching shear failure.
Over time -- gradually.
What, in Terzaghi's model, is the soil between the ground surface and the bottom of the footing treated as?
Overburden pressure
What is Q?
P + Wf
What is the decrease in pile capacity over time known as?
Pile relaxation
What is the increase in pile axial capacity over time referred to as?
Pile setup
What is the required spacing for pile groups in sands and clays?
Piles should be installed at center-to-center spacing of at least 3 diameters.
What type of failure occurs in very loose sand?
Punching shear failure
What type of failure occurs in weak clays loaded under a slow drained condition?
Punching shear failure
Failure deveops ____ in general shear failure.
Quickly -- suddenly.
Are spread footings rigid or flexible?
Rigid
What is a soil that is nearly impossible to collect a high quality sample of?
Sand
In what types of soils does pile setup occur?
Saturated clays and silts
What is the method that involves the graphs that really sucks?
Schmertmann's method
What is the most common settlement analysis method based on in-situ tests?
Schmertmann's method
What method is most often used with CPT and SPT results to obtain settlement analysis data from in-situ testing?
Schmertmann's method
Which criteria satisfies the settlement criterion?
Serviceability criteria
What usually controls the design of spread footings?
Settlement
What are the principal foundation types?
Shallow, deep, and hybrid
What types of piles can the Osterberg Load Test not be applied to?
Sheet piles, steel H-Piles, wood piles, and some tapered piles.
Which mobilizes first, side friction or toe bearing?
Side friction
What are the "bounds" for the force vs. depth curve given by the instrumented static load test?
Side friction only and toe-bearing only.
What soils are easy to get high quality samples of?
Silts and clays
What type of strain gauge is used for an instrumented static load test?
Sister bar strain gauge and rebar strainmeters
What type of soil has a well-defined failure from load-settlement curve?
Soft clay
What soil types does general shear failure occur in?
Soils that are RELATIVELY INCOMPRESSABLE and REASONABLY STRONG. Stiff sand, stiff cohesive soil and NORMALLY CONSOLIDATED CLAY that are LOADED RAPLIDLY ENOUGH that the undrained condition prevails.
What are common shallow foundation types?
Square, rectangular, circular, ring, continulous, mat.
Stiff deep foundations are good for what kind of loads?
Static load
Battered piles form a stiff or flexible foundation?
Stiff
What type of soil has a poorly-defined failure from a load-settlement curve?
Stiff clay and sand
What additional instrumentation is required for an instrumented static load test, as compared to a conventional static load test?
Strain gauges, telltale rods, load cells
What about structural loads causes the increase of the likelihood of differential settlement?
Structural loads are erratic
Do you add or subtract the weight of the foundation when calculating the downward capacity?
Subtract
Sufficient side friction resistance should be available to perform what function? This results in the toe-bearing to function as what?
Sufficient side friction resistance should be able to resist all service loads, causing the toe-bearing to serve as a factor of safety.
** T/F: In general, shallow foundation design is governed by settlement rather than bearing capacity.
T
** T/F: Shallow foundation failure is most likely to occur immediately after the load is applied in clay soil.
T
** T/F: The analytical methods of calculating ultimate bearing capacity of shallow foundations are reasonably accurate.
T
What do we know about cyclic capacity of deep foundations?
The calculation of cyclic capacity is complex and cyclic loading reduces the capacity if the loading is above a particular threshold
What are Osterberg Load Tests widely used to measure?
The capacity of drilled shafts which are designed to resist very high loads
What is one of the biggest pros to the instrumented static load test?
The data from the gauges and telltale rods can be used to define the toe resistance and the skin resistance, allowing for the design of other foundations with different lengths and diameters.
** What is the major issue in the plate load test result for designing shallow foundations?
The depth that the plate load test reaches is far less than the actual foundation.
If we are creating a footing on saturated sand and gravel, what do we use to conduct the bearing capacity analysis?
The effective shear strength
Allowable lateral load is typically the smaller of what?
The failure (soil or pile) over the FS The allowable lateral load that gives acceptable lateral deflection
What fails first when lateral forces are applied to a long deep foundation?
The foundation fails first
What happens to pile group efficiency in loose sand as the number of piles in the group increases?
The group efficiency increases
What is pile setup? What causes it?
The increase in pile capacity due to the positive pore water pressure (compression) during pile installation and dissipation thereafter
In what zone created by Terzaghi's model ignores shear strength of the soil?
The linear shear zone
** Although there are three bearing capacity failure modes observed in shallow foundations, not all of them are used for developing ultimate bearing capacity equations. Which one of the three is used for developing bearing capacity equations and how are the others taken into the calculation?
The modes are general shear failure, punching shear failure, and local shear failure. The one used for developing the bearing capacity equations is the general shear. The others are taken in through settlement analysis.
What happens if piles in a pile group are too far spaced?
The pile cap becomes too large and may not be economical
What in-situ test involves limit equilibrium analysis?
The plate load test.
What happens if piles in a pile group are too close?
The pressure bulbs overlap and impact axial capacity. It also causes difficulties in aligning and positioning the piles.
What fails first when lateral forces are applied to a short deep foundation?
The soil
What is a major contributor to the settlement of a foundation?
The soil's consolidation
What controls the ultimate lateral capacity in a short deep foundation?
The surrounding soil
What is a fixed-head condition?
The top moves laterally but is restrained from rotation
What is a free-head condition?
The top of the foundation moves laterally and rotates
What is a pure-moment condition?
The top rotates but is restrained from lateral movement
For open-ended deep foundations being driven into bedrock, what do we use for the area in calculations?
The unplugged area (the actual foundation itself)
What does Wf' mean?
The weight of the foundation including the effect of the water table
Why do static load tests take a lot of time?
There needs to be time to dissipate the pore water pressure
How are sister bar strain gauges installed in an instrumented static load test?
They are tied alongside an exisiting length of rebar in the rebar cage
in Schmertmann's method, how is the equivalent modulus of elasticity calculated?
Through the CPT results
True or false: load tests (full scale) show that analytical methods give a reasonably good estimate.
True
What is the typical loading in a static load test?
Typically 200% of the design load, but they are often loaded until failure to determine the ultimate capacity
Where are shallow foundations most commonly used?
Under columns or load-bearing walls
Do we conduct bearing capacity on underained shear strength of clay or drained shear strength?
Undrained
** What kind of analysis (drained or undrained) would you perform for a shallow foundation on saturated clayey soil. Why?
Undrained, this is a more conservative approach because the pore water pressure is high at the end of loading.
What is zone of influence?
Used with Vesic's equation for bearing capacity.
Which is more accurate, Vesic or Terzaghi?
Vesic
Ideally, static load tests are conducted when, in relation to the construction of the production piles? Why?
Well before the construction of the production piles to allow time to finalize the design, develop a contract to build the foundation, etc.
When does settlement become a problem to spread footings?
When B is large
When are deep foundations considered over shallow foundations?
When it is found to be impractical, uneconomical, or unsafe to have shallow foundations. 1) The foundation load is high 2) The foundation is settlement-sensitive 3) There is high lateral or uplift loads (docks, bridges, or towers)
When do uplift loads occur on a shallow foundation?
When the water table is above the bottom of the foundation
Are resistance factors always less than one?
Yes
Can you tell the side friction from the toe resistance in an instrumented static load test?
Yes
Is maintaining a consistent level of service load bearing pressures for all individual footings encouraged? Why or why not?
Yes, because it will tend to reduce differential settlement, which is usually more of a concern than are total settlements.
Can Vesic's equation be applied to deep foundations? How so?
Yes, the depth factors can be.
In clay, what is the settlement like?
You have immediate elastic settlement and then consolidation settlement.
What is the t-z (load transfer) method?
a method of calculating the ultimate downward capacity of a deep foundation, where the pile and soil resistances are represented by springs
In sand, what is settlement like?
all of it is almost immediate.
In vesic's equation, do we use saturated strength or unsaturated strength in the bearing capacity analysis?
always use saturated strength, even if the influence zone is not saturated.
Describe how a conventional static-load test operates:
build a full-size prototype foundation and load using a jack. Measure stress with a load cell and measure displacement with a reference beam.
The design for eccentrically loaded footings follows the approach for concentric loads with one significant addition:
consideration of overturning stability.
What types of soils does pile relaxation occur?
dense fine sand, inorganic silt, stiff fissured clay.
What is the criteria with one-way eccentric loading for there to be full contact along the bottom of the footing?
e <= B/6
What is the formula for eccentricity?
e = M/Q
Most shallow foundation failures are related to _____ rather than _______
excessive settlement, loss of load carrying capacity.
What is the opposite to punching shear failure?
general shear failure
in settlement analysis of layered soil, if it is mostly sands, what method do we use?
in-situ method
in settlement analysis of layered soil, if it is mostly silts and clays, what method do we use?
laboratory method
** Briefly discuss the difference between allowable bearing pressure method and the design chart method for developing generic design criteria for a shallow foundation:
*** FIND ANSWER
What is the symbol for pile group efficiency?
n
What is the opposite to general shear failure?
punching shear failure
What does the position of the water table effect in Vesic's bearing capacity equation?
q and gamma
What is a physical sign that local shear failure has occurred?
that a well-defined failure surface has occurred under the foundation but not at the surface.
Define bearing pressure:
the contact pressure between the bottom of the foundation and the soil
The common design approach is to increase footing thickness as necessary to avoid:
the need for shear reinforcement
What does Pt' mean?
the net toe bearing resistance including the weight of the foundation and the effects of the water table
Using schmertmann's method, where does the peak occur at?
zf/B = 0.5