Chapter 12 Full Displacement Auger Piles Produce Little Spoil

Chap. 12 Deep Foundation Systems and Construction Methods

12.1 What is meant by a low displacement versus high displacement pile? Why is it important to

distinguish between the two?

Solution

Low displacement piles are those with relatively small cross–sectional areas such as H–piles or

open ended pipe piles. As these piles are driven into the ground, they displace a relatively small

Chap. 12 Deep Foundation Systems and Construction Methods

12.2 Discuss some of the primary advantages and disadvantages of the following types of driven piles

and suggest a potential application for each:

• Timber

• Steel

• Prestressed concrete

Solution

Timber piles

• Advantages: Inexpensive, relatively easy to drive, easy to cut off, resists ship impacts

making the advantageous in use for piers and wharves. Durable when placed under the

water table.

Chap. 12 Deep Foundation Systems and Construction Methods

12.3 What is predrilling and when might it be used? What might happen if the predrill diameter or

length was excessive?

Solution

Predrilling is boring a pilot hole near the surface to start the pile installation. It’s often done

Chap. 12 Deep Foundation Systems and Construction Methods

12.4 What type or types of piles would be appropriate to support a heavy structure on an undulating

bedrock surface located 25 to 40 m below the ground surface? Assume the side friction in the

overlying soils provides less than 20 percent of the total axial load capacity. Explain the reasons

for your choice.

Solution

This is an ideal situation for steel H–piles. End bearing piles on the undulating bedrock means

that the pile lengths will vary across the site and the specific length of each pile may be uncertain.

Chap. 12 Deep Foundation Systems and Construction Methods

12.5 Pressure–injected footings are best suited for sandy or gravelly soils with less than about 15

percent fines. Why would this construction method be less effective in a stiff saturated clay?

Solution

Pressure–injected footings (PIF) gain much of their capacity by densifying the soil when they’re

Chap. 12 Deep Foundation Systems and Construction Methods

12.6 Why are most driven concrete piles prestressed instead of being conventionally reinforced?

Solution

Concrete is strong in compression and weak in tension. Prestressing puts all of the concrete in

Chap. 12 Deep Foundation Systems and Construction Methods

12.7 In the context of pile construction, what are cushions, when are they used, and what is their

purpose?

Solution

There are two types of cushions used in pile driving: Hammer cushions (also called capblocks)

12.8 Pile foundations that support buildings usually have at least three piles for each column. Why?

Solution

This provide redundancy. Should a single pile have a reduced capacity because of damage

Chap. 12 Deep Foundation Systems and Construction Methods

12.9 Pile driving in loose sands without predrilling tends to densify these soils. What effect does this

densification have on the load bearing capacity of such piles?

Solution

Chap. 12 Deep Foundation Systems and Construction Methods

12.10 Describe the difference in construction between drilled shafts and auger piles. Compare the

advantages and disadvantages of each.

Solution

Drilled Shafts

• Construction: Drill shafts construction first opens a hole in the ground and then the hole

is filled with concrete.

Auger piles

• Construction: In auger piles, no hole is ever opened in the ground. The auger is screwed

into the ground without removing any soil and the shaft is formed by pumping concrete

Chap. 12 Deep Foundation Systems and Construction Methods

12.11 Explain the differences in construction between auger cast in place piles and drilled displacement

piles. Give examples of soil conditions where each type would be best suited.

Solution

Auger cast in place (ACIP) piles remove all the soil from the pile volume when the grout is

Chap. 12 Deep Foundation Systems and Construction Methods

12.12 Both driven piles and auger piles can be good foundations in loose to medium sands, but drilled

shafts are generally a less viable choice in these conditions. Explain why.

Solution

Neither driven piles nor auger piles require maintaining an open hole during installation. This

Chap. 12 Deep Foundation Systems and Construction Methods

12.13 In what circumstances would you expect caving or squeezing conditions to be a problem? What

construction methods could a contractor use to overcome these problems?

Solution

Caving will be a problem in loose cohesioless soils, particularly below the water table.

Chap. 12 Deep Foundation Systems and Construction Methods

12.14 The dry method of drilled shaft construction is most suitable for which types of soil conditions?

Solution

Medium to stiff cohesive soils, particularly above the water table. This method requires a soil

Chap. 12 Deep Foundation Systems and Construction Methods

12.15 There are many ways to build midstream foundations for bridges that cross rivers and other

bodies of water. One of them is to use a caisson, as described in this chapter. Another is to drive

piles from a barge. Suggest some advantages and disadvantages of these two methods.

Solution

Caissons are large structures and require a relatively long construction time and a large amount

of construction equipment. They may be constructed off–site and barged to the final location.

Chap. 12 Deep Foundation Systems and Construction Methods

12.16 Suggest some critical items that a construction inspector should watch for during the

construction of auger piles.

Solution

Some of the most critical items are:

• Consistency and quality of the grout.

Chap. 12 Deep Foundation Systems and Construction Methods

12.17 Create a table with five columns labeled “foundation type,” “typical applications,” “best soil

conditions,” “advantages,” and “disadvantages.” Under the column “foundation type” label the

rows “driven piles,” “auger piles”, “drilled shafts,” and “caissons.” Fill in each cell of this table

with the appropriate information for each foundation type.

Solution

Foundation

Type

Typical

Applications

Best Soil

Conditions

Advantages

Disadvantages

Driven piles Building

foundations, bridge

abutments & piers

Soft to medium

clays, loose to

medium dense

sands.

Can densify loose materials,

driving process provides

information on pile capacity,

high production rates possible,

easily driven below water

Equipment noisy,

difficulty to install in

very dense soils.

Chap. 12 Deep Foundation Systems and Construction Methods

12.18 A proposed ten-story office building is to be supported on a series of deep foundations embedded

60 ft below the ground surface. The soils at this site are loose to medium dense well–graded

sands (SW) and silty sands (SM), and the groundwater table is at a depth of 12 ft. What type or

types of deep foundations would be most appropriate for this project? What type or types would

probably not be appropriate? Explain the reasons for your selections.

Solution

The two major concerns at this site are the dense cohesionless soils and the height of the water

table being well above the expected foundation level. Presumably, there will be dewatering to

Chap. 12 Deep Foundation Systems and Construction Methods

12.19 A new reinforced concrete pier is to be built in a major harbor area. This pier will service ocean–

going cargo ships. The underlying soils are primarily low plasticity silts (ML) and clays (CL),

with some interbedded sand layers. What type or types of deep foundations would be most

appropriate for this project? What type or types would probably not be appropriate? Explain the

reasons for your selections.

Solution

This is a site where driven piles are well suited. Large displacement prestressed concrete piles

would be preferred to low displacement piles because the large displacement piles should