# PE Exam Preparation for Geotechnical Engineers

This blog intends to help anyone preparing for PE exam or practicing in the area of geotechnical engineering. You are welcome to post your questions related to soil mechanics and I'll try to solve them. I am a senior-level professional geotechnical engineer currently based in Houston, TX. I started this blog in order to help anyone who is preparing for PE and/or has questions related to geotechnical engineering. I took my PE exam on April 2009 (and passed!).

## Thursday, February 12, 2015

## Monday, June 23, 2014

## Wednesday, October 21, 2009

### Lateral Earth Pressure

My inability to be able to steal some time out of my schedule leaves me no option but to provide you with some links that might help you. I'll, however, post my take on lateral earth pressure in near future which may be too late for the candidates taking their exam day after tomorrow but may help those who are taking their exams next year or later. I really apologize for the inconvenience and wish all the examinees the very best!

The following link provides a pretty good (and brief) lesson on lateral earth pressure.

http://www.pdhonline.org/courses/c155/c155content.pdf

For any questions, feel free to ask me.

The following link provides a pretty good (and brief) lesson on lateral earth pressure.

http://www.pdhonline.org/courses/c155/c155content.pdf

For any questions, feel free to ask me.

## Wednesday, October 14, 2009

### A question by a visitor

Anonymous said...

I'm confused in calculating the effective vertical stress in a sandy gravel soil layer below the graound water level and it is between a sand layer on the top and a clay layer on the bottom.

What unit weight i have to consider the sand unit waight or I need the porosity(n)of the sandy gravel to get the unit waight?

Thanks

What unit weight i have to consider the sand unit waight or I need the porosity(n)of the sandy gravel to get the unit waight?

Thanks

geotechie said...

suppose we have a situation like the following:

_____________________________________________

Sand wet unit weight = Ys,w

layer thickness = d1

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

water table @ depth dw, unit weight of water = Yw

Sand saturated unit weight = Ys,sat

______________________________________________

Sandy Gravel

saturated unit weight = Ysg

layer thickness = d2

______________________________________________

Clay

saturated unit weight = Yc

layer thickness = d3

______________________________________________

Effective Stress = Total Stress - Pore Water Pressure

Total Stress at the bottom of clay layer = (Ys,w)(dw) + (Ys,sat)(d1-dw)+ (Ysg)(d2) + (Yc)(d3)

Pore water pressure = (Yw)(d1-dw) + (Yw)(d2) + (Yw)(d3)

Therefore, Effective Stress =

(Ys,w)(dw) + (Ys,sat)(d1-dw)+ (Ysg)(d2) + (Yc)(d3) - (Yw)(d1-dw) - (Yw)(d2) - (Yw)(d3)

= (Ys,w)(dw) + (Ys,sat-Yw)(d1-Yw) + (Ysg-Yw)(d2) + (Yc-Yw)(d3)

Let me know if it helps (or if it does not).

Sand wet unit weight = Ys,w

layer thickness = d1

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

water table @ depth dw, unit weight of water = Yw

Sand saturated unit weight = Ys,sat

______________________________________________

Sandy Gravel

saturated unit weight = Ysg

layer thickness = d2

______________________________________________

Clay

saturated unit weight = Yc

layer thickness = d3

______________________________________________

Effective Stress = Total Stress - Pore Water Pressure

Total Stress at the bottom of clay layer = (Ys,w)(dw) + (Ys,sat)(d1-dw)+ (Ysg)(d2) + (Yc)(d3)

Pore water pressure = (Yw)(d1-dw) + (Yw)(d2) + (Yw)(d3)

Therefore, Effective Stress =

(Ys,w)(dw) + (Ys,sat)(d1-dw)+ (Ysg)(d2) + (Yc)(d3) - (Yw)(d1-dw) - (Yw)(d2) - (Yw)(d3)

= (Ys,w)(dw) + (Ys,sat-Yw)(d1-Yw) + (Ysg-Yw)(d2) + (Yc-Yw)(d3)

Let me know if it helps (or if it does not).

### Sorry guys...

I am extremely sorry guys...its been a very busy several weeks...I'll try to post something on retaining walls before the PE exam.

## Saturday, October 3, 2009

### Shear Strength - Problem #3

A Consolidated Undrained (CU) triaxial test is conducted on silt. The effective cell pressure is 4.32 ksf. The deviator stress is 3.45 ksf. The total pore pressure for this test is 2.88 ksf. Draw Mohr's circle of the initial and final condition. Also, show circles that represent total and effective stresses.

## Thursday, October 1, 2009

### Shear Strength-Triaxial Test

Advantages:

1. Suitable for cohesive soils as well

2. Samples can be saturated

3. Principal planes do not rotate

4. Failure will seek the weakest plane

Three permissible drainage cases:

UU or Unconsolidated Undrained

CD or Consolidated Drained

CU or Consolidated Undrained

Think of practical situations where these tests would be appropriate.

1. Suitable for cohesive soils as well

2. Samples can be saturated

3. Principal planes do not rotate

4. Failure will seek the weakest plane

Three permissible drainage cases:

UU or Unconsolidated Undrained

CD or Consolidated Drained

CU or Consolidated Undrained

Think of practical situations where these tests would be appropriate.

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