Midterm 1 Geotech

What is the importance of site investigation in civil engineering projects?

1. gather information about the site

2. determine soil profile

3. 3. obtain soil sample for the lab

4. determine Ground Water Table

Unit weight of concrete

150 lb/ft³

25 KN/m3

What are the three aspects of soil characterization?

Site investigation, laboratory testing, and in situ testing.

2 methods of Site- investigation subsurface exploration

• Geophysical methods

• Exploratory Borings

What is Geophysical Method?

Generating wave at one side and retrieve at the other side to determine soil profile based on resistance to current flow.

Explain what is exploratory boring

Site investigation by drilling a hole and retrieving sample

What are the 4 different boring methods?

• Wet Rotary Drilling

• Coring

• Hollow Stem Continuous flight Auger Drilling

• Solid Stem continuous flight auger drilling

Solid Stem continuous flight auger drilling is best for what kind of soil? WHY?

Fine Grain Soil. The soil does not cave in.

Hollow Stem continuous flight auger is best for what kind of soil? Why

Coarse grain soil, so that it does not cave in

Explain wet rotary drilling method? How do they obtain the sample?

What type of soil?

What type of sampler?

In this method, drill bit is used to drill through the soil. Mud and water is discharged at the bottom and pushing soil up to the surface. the mud also prevents the soil from caving in.

To obtain the sample, the drill bit is removed and they attach a sampler.

Any type of soil

Both shelby tube and split spoon sampler, depending on the type you want

What can be determined from disturbed samples?

Disturbed samples - Mechanical properties have been altered, but it can be used to determine other things like soil classification, grain size, mineral content

What can be determined from un-disturbed samples?

Undisturbed samples is unaltered, it can be used to determine soil strength.

2 Different Sampling Methods

1. Shelby Tube Sampler

2. Split Spoon Sampler

What type of sample is received form shelby tube sampler?

Shelby tube sample: Undisturbed sample

What type of sample is received from split spoon sampler?

Split Spoon sample: Disturbed sample

How does shelby tube sample able to get undisturbed sample?

Shelby tube sampler used thin walled sampler and they are pushed into the soil rather than drived minimizing the impact to the soil.

How does split spoon method disturbed the sample?

It disturbs the soil because it is thick walled and it is driven into the ground with heavy hammer causing vibration.

3 types of in situ test

• Standard Penetration Test

• Cone Penetration Test

• Vane Shear Test

What is the Standard Penetration Test (SPT)? What can the N value be used for?

Split spoon sampler is driven into 18 in into the soil, record number of impact every 6 inches. Calculate the N value, the number of blows needed to drive the soil the last 12 inches. The N value can be used to estimate angle of friction or the undrained shear strength

What does the Cone Penetration Test (CPT) measure? What does the data tell you?

A cone shaped probe is pushed into the soil at a constant rate, tip resistance, force needed to push the soil and sleeve friction, friction force acting on the side of the tip is documented.

When is Vane shear test used ? Explain Procedure of a Vane Shear stress?

Used for soft soil/clays, Metal Vane is is pushed inside the soil, rotate it the soil fail in shear, the torque is calculated to find the undrained shear strength.

2 Example of shallow foundation

Spread footing and Mats

2 example of deep foundation

Piles and Caissons

2 limit state to design foundations

Ultimate Limit state and Service Limit state

To deisgn foundation of a sand should we design using short term or long term design parameters?

Does not matter, both are the same drained

For clay what parameter do we use to design foundation?

We use the undrained Shear strength, Su, which is equal to cohesion

Derive the unit weight formula for soil

Formula for specific weight

Specific weight of water

62.4 pcf or 9.81 KN/m3

Is there pore water pressure above GWT?

No

When using bousinesq method to find the induced stress in the corner which q do you use?

You use the original q

Is this chart for corner or middle

corner

Is this chard for middle or corner?

Middle along the line

1 m to ft ?

3.3 ft

Fomula for risk

Probability of * Cost of failure ($)

What is the unit for risk?

$

What is the formula for Reliability

r= 1-Pf

To calculate strength of fine grain soil, what is c’ and what is phi’

c’ is SU and phi’ is 0

Short term behaviour of fine grain soil is undrained, True or False

True

What is the pressure caused by footing to the ground?

Bearing Pressure

3 things that affect bearing capacity

cohesion, surcharge/embedment, friction

Maximum depth dimension that failure happens under the footing

Equals to B

What does higher sleeve friction and low tip resitence in CPT tells you? What kind of soil?

Higher cohesion, fine grain soil, clay

What does lower sleeve friction and high tip resistance in CPT tells you? What kind of soil?

Granular soil, sand

LRFD Pu equation

3 Modes of failure?

1. General Shear Failure

2. Local Shear Failure

3. Punching Shear Failure

When does General Shear Failure occurs

• Dense sand, over consolidated clay

When does Local Shear Failure occur?

Mediumly dense sand, lightly consolidated clay

When does punching shear occur?

Loose sand and weak clays

Which one is the resistance factor, which one is the load factor?

Unit weight of concrete in SI and English

23.6 KN/M3 and 150 LB/ft³

If the footing is embedded,