CE 4729 Foundations Exam 1

Test Pits

Best method of obtaining detailed information. High quality sampling. Limited by depth, water level, roc

Drill Equipment - Continuous Flight Auger

usually used when hole can stand open without support. Hand or power driven

Drill Equipment - Hollow Stem Auger

used where hole needs support to stay open. Power driven

Hand Auger Boring

Fast and economical. Very suitable for soft to stiff cohesive soils not suitable for very hard or cemented soils or very soft soils and also can be used to determine ground water table. Soil removed by this is disturbed but it is better than wash boring, percussion, or rotary drilling.

Wash Boring

Chopping bit is raised and dropped. Water is pumped down the drill rod and the cuttings are removed by water. Inexpensive and portable. Completely disturbed sample. Not suitable for soft soils, fine to medium grained sands

Rotary

solid auger, hollow stem auger, chopping bit with hole support. Can be used with all soil types. Depth is unlimited

Power Hollow-Stem Auger Borings

Max depth ~ 100 ft. Sample available quickly. Large volume of waste. Cannot be used in stiff and dense formations. Difficult drilling in extremely fine dry materials. Problems in coarse grained soils

Percussion Drilling

Used in most of the soils and rocks and can drill any material. Hole is done by alternatively lifting and dropping a heavy drilling bit. Material at the bottom of the hole is disturbed by heavy blows and cannot detect thin strata.

Mud Rotary

>1000 ft. Sample available in a short time. Large volume of waste. Very expensive, difficult drilling in soil formations with boulders and cobbles.

Undisturbed Samples

sampling techniques that attempt to limit changes in the soil structure from that of the in-situ (in-place) soil. Only possible with cohesive soils that have apparent cohesion.

Ex: Thin-walled sampler (Shelby tube), Piston Sampler, Hand carved samples

Disturbed samples

soil or rock specimen whose natural structure, density, particle arrangement, or moisture distribution has been altered during the sampling process

Ex: SPT, Heavy wall sampler (Modified California sampler, Dension sampler), bulk samples

Disturbed samples are usually taken for

general information and soil properties not related to soil structure, water content, grain size distribution, plasticity

Undisturbed samples are usually taken to measure

unit weight, strength, and deformation. They are taken in the final subsurface exploration phase

Block sampling

limited by clays, silty clays, depth, water levels

Grab samples

used for water contents, grain size, atterburg limits

Split Spoon Sampler Standard Penetration Test (SPT)

soil sample is disturbed because of thick sampler wall and dynamic force due to driving. Retrieved by splitting the spoon in half by taking the shoe off. Not performed continuously

Shelby Tube (thin-walled sampler)

Cohesive soils. undisturbed sampling, good for clays, silty clays, clayey silts.

Piston sampler

undisturbed sampling, cohesive soils

Continuous Sampling

Provides more reliable and detail information on soil condition. Takes samples continuously.

Rock sampling (Coring)

used for sampling weathered or fractured rock. continuous coring after bedrock is encountered

SPT parameters

relative density of cohesionless soils, unit weight, internal angle of friction, undrained compressive strength, Young’s Modulus

drawbacks of SPT testing

sample is disturbed, impact of gravel and larger particles, hammer energy is highly variable

Cone Penetration Test (CPT) parameters

cone resistance (qc), sleeve friction (fs), total pore water pressure

Drawbacks to CPT

equipment not universally available, not suited for gravel or stiff/hard cohesive soils, depth limited by reaction support, no samples or cuttings obtained, correlations are indirect

Characteristics of SPT

simple, easy, gives point values, suitable for all soils except gravels, universally available and used routinely, limited for potential future development

Characteristics of CPT

complex, easy, continuous values, suitable for all soils except gravels, generally available and used routinely, great for potential for future development

Characteristics of Pressuremeter Test

complex, gives point values, suitable for all soils, difficult to located and used on special projects, great for potential for future development

Characteristics of Dilatometer Test

complex, easy, point values, suitable for all soils except gravel, difficult to locate and used on special projects, great for potential for future development

Characteristics of Beck Penetration Test

simple, easy, gives continuous values, suitable for sands through boulders, difficult to locate and used on special projects, uncertain for potential for future developments

Lab advantages

engineering parameters are easily defined, boundary conditions controlled, rate of loading controlled, direction of loading controlled, drainage controlled, can induce a known stress history

Lab disadvantages

specimen/sample may not be representative, sample removed from natural environment, test only “good” materials, expensive and time consuming, soil disturbance during sampling

In-situ advantages

engineering parameters are measured in natural state: stress and pore water, can investigate natural variability in site (vertical & horizontal), inexpensive, quick, sometimes immediate results, can test “poor” material not easily sampled

In-situ disadvantages

unknown boundary conditions, unknown drainage (measure response only), most of the interpretation is empirical backed up with theoretical and experimental, variability of soils affect results, cannot see soils being tested

Geophysical Methods

evaluation of subsurface between borings using measurements of indicative soil properties such as particle wave velocity, electrical resistance, magnetic fields. No direct measurement of soil properties such as unit weight, strength, deformability. Relatively fast and low cost.

Ground penetrating radar (geophysics test)

locating anomalies in subsurface, changes in material properties, and locate voids and cracks

Magnetic resonance (geophysics test)

subsurface soil water content, hydraulic conductivity, locating metal like utility lines and buried metal drums

Did these buildings fail during the earthquake? A retail structure and a hospital constructed in a seismically active area and both are designed considering the maximum likely earthquake for the area. After several years, design earthquake occurs. Neither building collapses and no lives are lost. Both buildings suffer significant structural damage and must be demolished

Retail structure: No. Category II, not collapse during earthquake, allow occupants to escape, not expected to survive earthquake

Hospital: Yes. Category IV, required to survive earthquake and perform their designed function immediately after earthquake

Did both buildings fail? An industrial warehouse and an expensive home were both built on compressible soils, after several years undergo some settlement and exhibit interior cracks up to 3 mm wide. There is no threat to human life from the performance of either structure

Industrial warehouse: No. Acceptable, crack might not visible

Expensive Home: Yes. Likely generate expensive lawsuits

Ultimate Limit State (ULS)

Failure modes that involve fracture of structural materials or the soil, and thus result in serious structural damage or collapse (Bearing Capacity Failure - Catastrophic)

Service Limit State (SLS)

Failure moder that adversely affects the functionality of the structure, but do not threaten the basic structural integrity (Deformation/settlement failure)

Allowable Stress design (ASD)

a global factor of safety is applied on the resistance side of the design equation to account for the possibilities that these elements are overloaded during their service life and the materials providing resistance to the load are not as strong as expected

Ensure that the nominal applied loads from the structure is

less than the load that will rupture the soil

ASD ensures the supporting design elements are sufficient to carry potential overloads but the approach does not supply

total capacity of the supporting element and mode of failure

Factor of safety

working or allowable stress method is currently used in the most geotechnical practice (usually 2.0-4.0)

Dead loads

D - structure weight and permanent equipment

Live loads

L - people, furniture, inventory, moveable equipment, vehicles, etc.

Snow and Rain Loads

S and R

Seismic Earthquake Loads

E - result of accelerations from earthquakes

Earth Pressure Loads

H - weight and lateral pressures from soil and rock such as behind a retaining wall

Fluid Loads

F - water in storage tank

Wind Loads

W - imparted by wind onto the structure

Self-Straining Loads

T - caused by temperature changes, shrinkage, moisture changes creep, differential settlement

Impact Load

I - result of vibratory, dynamic, and impact efforts. Impact loads from vessels are especially important in some bridge and port facilities

Stream Flow and Ice Loads

SF and ICE - caused by the action of water and ice in bodies of water, and are especially important in bridges, offshore drilling platforms and port facilities

Centrifugal and Braking Loads

CF and BF - caused by the motion of vehicles moving on the structure. Centrifugal forces occur when the vehicle is turning, such as on a curved bridge, while braking forces are those transmitted to the structure when a vehicle brakes

Types and Sources of Loads

Normal Loads (P), Shear Loads (V), Moment Loads (M), Torsion Loads (T)

Serviceability loads for cohesive soils consist of

all of the dead loads (DL) plus the sustained live loads (LL)

Serviceability loads for cohesionless soils consist of

all of the dead loads (DL) plus the maximum live loads (LL) that could possibly occur

International Building Code (IBC)

Has been adopted in the United States. Represent minimum requirements that often have to be exceeded. Codes are guides and are not a substitute for engineering knowledge, judgment, or common sense

Spread footings (footer/footing) - shallow foundations

an enlargement of column or bearing wall that spreads over a large soil area q=P/A

the most common foundations due to low cost and ease of construction are

column footing and continuous (wall) footing

Mat (Raft) Foundation

very large spread footing that encompasses the entire footprint of the structure

Column Footings

carries the load of a single column. Most often used in small to medium size structures on sites with moderate to good soil conditions. Can be used for large structures if underlying soils exceptionally good or with shallow badrock

Strap Footing

consists of two or more column footings connected by a concrete beam. It is used to help distribute the weight of either heavily or eccentrically loaded column footings to adjacent footings

Equivalent footing

a footing that carries the same applied load as the original footing but has different dimensions such that the resulting bearing pressure is uniform. Accomplished by decreasing B, to an equivalent width, B’, and L to an equivalent length, L’

Allowable bearing pressure

a pressure that, if exceeded, may present an unsafe condition as related to either ultimate or serviceability limit states

Presumptive allowable bearing pressure

allowable bearing pressures obtained directly from the soil classification without any further analysis

Case 1: A two-story residential building is proposed on a site with dense sand. The building has moderate column loads, and the soil conditions are relatively uniform.

Spread footing

Case 2: A 20 story commercial building is proposed on a site with weak, compressible soil. The building has heavy structural loads. Which type of shallow foundation would you recommend? Justify your selection.

Mat or Raft

Case 3: A building has a column located close to the property boundary. The foundation must remain entirely within the property. What foundation arrangement would you recommend for this column? Explain your choice.

Strap footing

Case 4: Two columns in a commercial building are located close to each other and carry different vertical loads. What type of foundation would you recommend for these columns? Explain your reasoning.

Combined footing

Case 5: A commercial building has numerous columns distributed throughout its footprint. The soil has low bearing capacity, and settlement must be carefully controlled. Which type of shallow foundation would you recommend? Explain your reasoning

Mat or Raft footing

Small two-story house on dense sand

spreading footings or continuous strip footings

multi-story building on moderately weak soil

Mat (raft) foundation

High-rise building on thick, soft clay

Pile foundation

You want laboratory estimates of effective friction angle and cohesion for soil subjected to controlled drainage conditions.

Triaxial shear tests b/c they can determine effective-stress shear strength parameters

A structure has many closely spaced columns, and individual spread footings would cover most of the building footprint.

Mat foundation b/c it is practical when isolated footings become large or overlap, while spreading loads across the footprint

An engineer needs the liquid limit and plastic limit to classify a fine-grained soil.

Atterberg limits b/c measures consistency characteristics of fine-grained soils

A light, one-story building will be built on shallow dense granular soil with adequate allowable bearing pressure.

Spread footings b/c they are economical when near-surface soil can support loads without excessive settlement

You want to estimate soil resistance and stratigraphy continuously as a probe is pushed into the ground.

CPT b/c it produces a near-continuous resistance profile, useful for identifying soil layers and estimating properties

A building foundation is planned on saturated clay. The engineer is concerned with long-term settlement.

Consolidation test b/c it estimates compressibility, consolidation settlement and its rate

A 15-story building will be constructed on a thick deposit of soft, compressible clay, with competent soil much deeper.

Deep pile foundations b/c piles can transfer loads to deeper competent material or mobilize adequate shaft resistance, while limiting settlement

A contractor needs to determine the optimum moisture content and maximum dry density for a soil fill.

Proctor compaction test b/c it provides the moisture-density relationship needed for soil compaction

What is the difference between ASD and LRFD?

ASD compares service loads to allowable resistance, while LRFD uses factored loads and factored resistance

What is the difference between ultimate limit state (ULS) and serviceability limit state (SLS)?

ULS focuses on preventing failure or collapse, while SLS focuses on acceptable performance such as limiting settlement, deflection, and tilt

Why does AASHTO separate structural dead loads from wearing surface dead loads?

Because pavement thickness and weight can change when a bridge is resurfaced, while structural dead loads generally remain constant

What is differential settlement?

The difference in vertical settlement between two points of a structure

Why is differential settlement often more damaging than uniform settlement?

It can cause distortion, cracking, and damage to structural elements and finishes

What is the LRFD resistance equation?

U < 0/N, where U is the factored load, 0/ is the resistance factor, and N is nominal capacity

A seven-story steel-frame office building has columns spaced 7 m apart and typical interior finishes. What allowable angular distortion should be used?

1/500

A two-story art museum contains delicate tile murals and sensitive wall finishes. Would you allow more or less differential settlement than in a typical warehouse?

Less. The museum's sensitive finishes require stricter settlement limits, such as an angular distortion of 1/1000.

A one-story steel warehouse has corrugated steel walls and no sensitive interior finishes. What allowable angular distortion is used in the document?

1/250

A 100-story building is 300 m tall and leans 1 m horizontally. Is this excessive based on the document's aesthetic limit?

Yes. Its tilt is 1/300, which exceeds the allowable tilt of 1/500.

A pedestrian bridge connects a plaza to office buildings. What load combination should be used to check deck deflection?

D + L (dead load plus full live load).

For the same pedestrian bridge, what load combination should be used to check settlement?

D + 0.5L, because settlement is associated with average loading over time

A sandy soil contains 0.03% sulfates. Is sulfate attack on concrete a major concern?

No. According to the document, the risk is negligible and no special preventive measures are necessary.

A clayey soil contains 0.30% sulfates. What concrete precautions are recommended?

Type V cement and a maximum water-cement ratio of 0.45 because the document classifies the sulfate exposure as severe.

Steel piles will extend 50 ft into sandy soil, and groundwater is located 35 ft below the surface. Is corrosion automatically expected?

No. The document does not anticipate corrosion based on these conditions alone, but additional testing is recommended.

What soil or groundwater properties should be tested to evaluate potential steel pile corrosion?

Electrical resistivity, redox potential, organic material, chemical content, pH, and anaerobic microbes.

A foundation has a nominal capacity of 1,800 kN. Using ASD with a factor of safety of 3, what is its maximum allowable load?

Pallow = 1800/3 = 600 kN