Retaining Walls and Deep Foundations - Day 2 - Answers

I marked these pages:

Piles - Section 3 - Page 38-3

Today's answers:

Retaining Walls and Deep Foundations - Day 2 - Questions

1. A 10.75 in diameter steel pipe is driven 70 ft into stiff insensitive clay. The clay has an un-drained shear strength of 1250 lbf/ft^2. The clay's specific weight is 120 lbf/ft^3. Water table at ground surface. Entire pile length is effective.
a) Find the end bearing capacity.
b) Find the friction capacity.
c) Find the allowable bearing capacity.


2. Test show that a single pile by itself would have an uplift capacity of 140 tons. And a compressive capacity of 600 tons. 36 piles are installed on a grid with a spacing of 3.5 ft. The pile group is capped and connected at the top by a thick steel-reinforced concrete slab adding it's own axial load of 600 tons. What is the maximum moment that the pile group can take in the x and y directions assuming axial load is applied to permit the moment.


3. A 25 ft deep, 35 ft square excavation in sand is being designed. The sand is dewatered before excavation. The angle of internal friction is 40° and the specific weight of the sand is 120 lbf/ft^3. Bracing is horizontal lagging, supported by 8 in soldier piles separated horizontally by 7 ft.
a) What is the shape of the pressure diagram on the lagging?
b) What is the bending moment on the lagging?

Retaining Walls and Deep Foundations - Day 2

Chapters 38 and 39 in Lindeburg.



These are the chapters in the practice problems book:



Chapter 38 - Piles and Deep Foundations - 2 problems

Chapter 39 - Temporary Excavations - 2 problems

Retaining Walls - Answers

I marked these pages:

Retaining Wall Analysis - Section 11 - Page 37-6

And today's answer's are:

Retaining Walls - Questions

1. A retaining wall is designed for free-draining granular backfill. After several years of operation the weep holes become plugged and the water table rises to within 8 feet of the top of the wall.

a) What is the resultant force for the drained case?

b) What is the point of application of the drained case?

c) What is the resultant force for the plugged case?

d) What is the point of application of the resultant force for the plugged case?

2. Retaining Wall Design:
A reinforced concrete retaining wall is used to support a 15 ft cut in sandy soil. The backfill is level, but a surcharge of 450 lbf/ft^2 is present for a considerable distance behind the wall. Factor of safety of 1.4 against sliding and overturning are required. Customary and reasonable assumptions can be made. Passive pressure is disregarded. The need for a key must be established.

Soil drained specific weight = 125 lbf/ft^3
Angle of internal friction = 35°
Coefficient of friction against concrete = 0.45
Allowable soil pressure = 4800 lbf/ft^2
Frost line = 3.5 ft below grade

a) What is the minimum stem height?

For b-j, assume a base length B of 12 ft and a base thickness d = 2 ft. A stem thickness at base of 2 ft. And a stem thickness at top of 1.5 ft. A stem height above base of 19 ft. And a heel extension (past back of stem) = 7 ft.

b) What thickness of backfill is equivalent to the surcharge?
c) What is the horizontal reaction due to the surcharge?
d) What is the active soil resultant?
e) What is the total overturning moment, taken about the toe, per foot of wall?
f) What is the factor of safety against overturning?
g) What is the maximum vertical pressure at the toe?
h) What is the minimum vertical pressure at the heel?
i) What is the factor of safety against sliding without a key?
j) What is the factor of safety against sliding if a key of 2 ft wide and 1 ft deep is used?

Retaining Walls and Deep Foundations - Day 1

Chapter 37 in Lindeburg.

These are the chapters in the practice problems book:
Chapter 37 – Rigid Retaining Walls - 3 problems

Soils, Foundations, and Settlements - Part 2 - Answers

I marked these pages:

Bearing Capacity - Sections 4 (and on) - Starts of Page 36-3

Taylor Slope Stability - Figure 40.6 - Page 40-7



And today's answer's are:

Soils, Foundations, and Settlements - Part 2 - Questions

1. A mat foundation is used to support a building with dimensions 75 ft by 45 ft and a total load of 5400 tons. The mat is at a depth of 7.5 ft below ground surface. The soil beneath the mat has a density of 115 lbf/ft^3 and an average SPT N-value of 18.
a) Find the allowable bearing capacity.
b) Find the factor of safety against bearing capacity.


2. A 2.5 m by 2.5 m square spread footing is designed to support an axial column load of 650 kN and a moment of 155 kN*m. The footing is place 1m into a sandy soil with a density of 2050 kg/m^3 and an angle of internal friction of 30°.
a) Find the eccentricity of the resultant load.
b) Find the equivalent width that should be used for the footing design.
c) Use the Meyerhoff theory, what is the value of the bearing capacity factor Ny to be used in calculating bearing capacity?
d) Use the Meyerhoff theory, what is the value of the bearing capacity factor Nq to be used in calculating bearing capacity?
e) Use the Meyerhoff theory, what is the value of the shape factor for Ny to be used in calculating bearing capacity?
f) What is the value of the net bearing capacity?
g) What is the value of the allowable bearing capacity?
h) What is the total allowable load?
i) What is the maximum soil pressure on the bottom of the footing?
j) What is the factor of safety against bearing capacity?


3. A 12 ft x 12 ft footing exerts a pressure of 4000 lbf/ft^2 on the soil below it. What is the increase in vertical soil stress 8 ft below and 6 feet from center of the footing?


4. A 2:1 (horizontal :vertical) sloped cut is made in homogeneous saturated clay. The clay's specific weight is 110 lbf/ft^3 and ultimate shear strength is 1050 lbf/ft^2. The cut is 45 ft deep. The clay extends 12 ft below the toe of the cut to the rock layer. What is the cohesive factor of safety of this cut?

Soils, Foundations, and Settlements - Part 2

Chapters 36 and 40 in Lindeburg.

These are the chapters in the practice problems book:
Chapter 36 – Shallow Foundations - 4 problems
Chapter 40 – Special Soil Topics - 9 problems

Soils, Foundations, and Settlements - Answers

These are the marked pages:

AASHTO Soil Classification - Table 35.4 - Page 35-5

Sieve Sizes - Table 35.2 - Page 35-3

Angle of Failure - Equation 35.41 - Page 35-26

Soil Indexing Formulas - Table 35.7 - Page 35-9

While other stuff in the chapter is useful - I think that marking it would be more hindering than helpful during the test.


Today's answers:

Soils, Foundations, and Settlements - Questions

1. A sample moist soil has the following attributes:

Volume = 0.52 ft^3 (as sampled)
Mass = 56.75 lbm (as sampled)
= 48.7 lbm (after oven drying)
SG of solids = 2.7

After drying the soil was run through a set of sieves.

Sieve ---------- % finer by mass
½ in ----------- 52
No 4 ---------- 37
No 10 ---------- 32
No 20 ---------- 23
No 40 ---------- 11
No 60 ---------- 7
No 100 ---------- 4

a) what is the density of the in situ soil?
b) What is the unit weight of the in situ soil?
c) What is the void ratio of the in situ soil?
d) What is the porosity of the in situ soil?
e) What is the degree of saturation of the in situ soil?
f) What is the uniformity coefficient?
g) What is the coefficient of curvature?
h) What is the USCS classification?
i) What is the AASHTO classification?


2. A triaxial shear test is performed on well drained sand. At failure the normal stress was 6300 lbf/ft^2 and shear stress was 4200 lbf/ft^2.
a) what’s the internal angle of friction?
b) What is the maximum principal stress?

3. A sample of sand has a relative density of 39% with solids SG = 2.7. The minimum void raito is 0.46 and the maximum is 0.96
a) what is the specific weight of the sand in saturated condition?
b) If the sand is compacted to a relative density of 70%, what will be the decrease of thickness of a layer 4 ft thick.

Soils, Foundations, and Settlements

This is our first 2 day topic.

I am going to break it into 2 sections

Chapter 35 in Lindeburg.

These are the chapters in the practice problems book:
Chapter 35 – Soil Properties and Testing - 14 problems