Drilled Shafts in Rock

Analysis and Design

Lianyang Zhang

• Publisher: CRC Press
• Year: 2004
• Print ISBN:
  9789058096500
• eBook ISBN:
  9781135290160
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

Drilled shafts in rock are widely used as foundations of heavy structures such as highway bridges and tall buildings. Although much has been learned about the analysis and design of drilled shafts in rock, all the major findings are published in the form of reports and articles in technical journals and conference proceedings. This book i

1
Introduction

1.1 DEFINITION OF DRILLED SHAFTS

A drilled shaft is a deep foundation that is constructed by placing concrete in an excavated hole. Reinforcing steel can be installed in the excavation, if desired, prior to placing the concrete. A schematic example of a typical drilled shaft socketed into rock is shown in Figure 1.1. The drilled shaft can carry both axial and lateral loads. Drilled shafts are sometimes called bored piles, piers, drilled piers, caissons, drilled caissons, or cast-in-place piles.

To increase the bearing capacity, drilled shafts are commonly socketed into rock. The portion of the shaft drilled into rock is referred to as a rock socket. In many cases where there is no overburden soil, drilled shafts entirely embedded in rock are also used. Because of the high bearing capacity of rock sockets, the analysis and design of them are extremely important.

This book discusses both rock sockets and drilled shafts entirely embedded in rock.

1.2 HISTORICAL DEVELOPMENT OF DRILLED SHAFTS

The ancient “well foundation” can be considered the earliest version of drilled shafts. Such foundations were stone masonry pedestals built in hand-excavated holes, long before hydraulic cements came into common use.

During the late nineteenth and early twentieth centuries when taller and heavier buildings began to appear, high-capacity foundations became necessary in large cities such as Chicago, Cleveland and Detroit. These cities are underlain by relatively thick deposits of medium to soft clays overlying deep glacial till or bedrock. Because traditional spread foot foundations settled excessively under the heavier building load, engineers began to use shafts such as the hand-dug “Chicago” and “Gow” caissons. The shafts were constructed by making the excavation and by placing sections of permanent liners (wooden lagging or steel rings) to retain the soil by hand.

Hand excavation methods were slow and tedious, so machine-drilled shafts soon superseded the hand-dug caissons. A few examples of horse and engine-driven augers appeared between 1900 and 1930, but they had limited capabilities. By the late 1920s, manufacturers were building practical truck-mounted engine-driven augers, thus bringing drilled shaft construction into its maturity.

Fig. 1.1 A drilled shaft socketed into rock.

During the next three decades, manufacturers and contractors developed larger and more powerful drilling equipment, which allowed more economical and faster construction of drilled shafts. In the late 1940s and early 1950s, drilling contractors introduced techniques for drilling in rock. By introducing casing and drilling mud into boreholes, a process long established by the oil industry, boreholes could be drilled through difficult soils economically. By the 1960s, drilled shafts had become a strong competitor to driven piles.

In the past decade, the use of drilled shafts has increased dramatically. In 1997, the value of drilled shaft construction in the United States reached more than one billion US dollars (O’Neill, 1998). Today, drilled shafts support different structures including one-story wood frame buildings to the largest skyscrapers, highway bridges, and retaining structures.

1.3 USE OF DRILLED SHAFTS

Compared to other types of deep foundations, drilled shafts have the following major advantages:

1. The costs of mobilizing and demobilizing a drill rig are much less than those for a pile driver.
2. The construction process generates less vibration and noise, making drilled shafts appropriate for urban construction.
3. The quality of the bearing material can be inspected visually and tests can be run to determine its physical properties. For end-bearing designs, the soil/rock beneath the base can be probed for cavities or weak layers if desirable.
4. The diameter or length of the drilled shaft can be easily changed during construction to compensate for unanticipated soil/rock conditions.
5. The drilled shafts can penetrate through soils with cobbles or boulders. They can also be socketed into rock.
6. It is usually possible to support very large loads with one large drilled shaft instead of several piles, thus eliminating the need for a pile cap.
7. Large-diameter drilled shafts are particularly well-suited as foundations for structures that must resist extreme events that produce large lateral loads (e.g. earthquake and vessel impact loading) because of the very large moments of inertia.

Drilled shafts also have the following major disadvantages:

1. The quality and performance of drilled shafts is very dependent on the contractor’s skills. Poor workmanship can produce weak foundations that may not be able to support the design load.
2. Since shaft construction removes soil/rock from the ground, it may decrease the competency of the bearing stratum.
3. The construction of drilled shafts through contaminated soils/rocks is problematic because of the expenses associated with disposing of the spoil.

Because of the above advantages, drilled shafts have become an appropriate and economical foundation system for heavily loaded structures. When deep foundations are required, drilled shafts should always be considered as an option.

An application example of drilled shafts in rock

O’Neill and Reese (1999) presented an application example of drilled shafts in rock. It clearly shows the advantages of drilled shafts over pile-footings for the foundations of the interior bents of a river bridge in the United States.

The bridge is a two-lane bridge with four spans. Siltstone near the surface at one end dips to a depth of about 6.1 m (20 ft) near the other end of the bridge. Mixed fine sediments exist above the siltstone. Two alternate foundation designs were considered by the design agency before the project bid. The first one called for the construction of one spread footing and two capped groups of steel H-piles for the three interior bents that were required to be placed in the river. Both the spread footing and driven piles (with pile caps) were to be constructed within cofferdams because of the need to construct footings/caps. The second one called for the replacement of the spread footing and driven pile groups by three large-diameter drilled shafts. The drilled shafts could be drilled during low water using a crane-mounted drill rig positioned on timber mats within the river and pouring the concrete for the shafts to an elevation above the water level, eliminating the need for cofferdams. Comparison of the pile-footing alternate with the drilled shaft alternate is shown in Table 1.1. The cost savings realized by using drilled shafts were $422,000 (50%).

Table 1.1 Comparison of the pile-footing alternate with the drilled shaft alternate—Queens River Bridge, Olympic Peninsula, Washington, USA (after O’Neill & Reese, 1999).

1.4 CHARACTERISTICS OF DRILLED SHAFTS IN ROCK

The characteristics of drilled shafts in rock are closely related to the special properties of rock masses. The following briefly describes some of the special rock mass properties that will affect the performance of drilled shafts.

1.4.1 Effect of discontinuities

The primary difference between drilled shafts in rock and those in soil is that rock masses contain discontinuities. The intact rock may have a high strength but the presence of discontinuities in the rock may result in very low strength of the rock mass. Wedges or blocks formed by sets of unfavorably orientated discontinuities may fail by sliding or toppling, causing excessive movement or failure of drilled shaft foundations.

Figure 1.2 shows the drilled shaft foundations of a bridge across a river. The rock at this site consists of two sets of discontinuities with about the same dip angles; but set A is discontinuous and more widely spaced than set B. At the East side, the drilled shaft foundation would be stable because the discontinuities approximately parallel to the rock slope face are not continuous. In contrast, at the West side, the discontinuities approximately parallel to and dipping out of the slope face are continuous and movement of the entire foundation along these discontinuities is possible.

Fig. 1.2 Effect of discontinuities on the stability of drilled shafts.

1.4.2 Effect of groundwater

Groundwater may affect the performance of drilled shafts in the following ways:

1. The most obvious is through the operation of the effective stress law. Water under pressure in the discontinuities defining rock blocks reduces the normal effective stress between the rock surfaces, and thus reduces the potential shear resistance which can be mobilized by friction. In porous rocks, such as sandstones, the effective stress law is obeyed as in granular soils. In both cases, the effect of fissure or pore water pressure is to reduce the ultimate strength of the rock mass, and thus decrease the bearing capacity of the drilled shaft foundation.
2. Groundwater affects rock mechanical properties due to the deleterious action of water on particular rocks and minerals. For example, clay seams may soften in the presence of groundwater, reducing the strength and increasing the deformability of the rock mass. Argillaceous rocks, such as shales and argillitic sandstones, also demonstrate marked reductions in material strength following infusion with water. According to Hoek and Brown (1997), strength losses of 30–100% may occur in many rocks as a result of chemical deterioration of the cement or clay binder.
3. Groundwater flow into the excavation of a drilled shaft can make cleaning and inspection of bearing surfaces difficult and result in decreased bearing capacity for the drilled shaft.

1.4.3 Effect of karstic formations

A number of problems may arise when drilled shafts are built in karstic formations (Brown, 1990; Goodman, 1993; Sowers, 1996):

1. An existing cavity may underlie the base of the drilled shaft and collapse when the building is under construction or in service. The collapse of the cavity may be caused by excessive construction loading or erosion by acid groundwater [Fig. 1.3(a)].
2. The tip of the shaft may slide along a steeply inclined rock instead of penetrating into bedrock, especially when part of the tip is located on top of a pinnacle with existing joints or cracks [Fig. 1.3(b)].
3. The drilled shaft is placed on a cantilever rock over cavities or soft clay, so that excessive loads or continuing water erosion may cause rock collapse [Fig. 1.3(c)].
4. A shifting rock slab or rock block floating in the residual soil may lead people to mistakenly believe that the bedrock has been reached and the bearing stratum has been located (d)].

Chapter 11 will discuss the performance of drilled shafts in karstic formations in more detail.

1.5 CONSIDERATIONS IN THE DESIGN OF DRILLED SHAFTS

As for the design of any foundations, the design of drilled shafts must satisfy criteria related to strength, deformation and durability. For the strength, criteria are applied to both the structural strength of the shaft itself and the geotechnical strength, i.e., the load carrying capacity of the soil/rock. The structural and geotechnical strength criteria depend on the basis of the design method. The traditional working stress design method, sometimes referred to as the allowable stress design (ASD) method, relies on an overall safety factor against ultimate failure and the corresponding design criteria can be expressed as

(1.1)

where Q_u is the ultimate load bearing capacity; FS is the global factor of safety; and Q is the allowable working load or the allowable design load.

Equation (1.1) applies to both axial and lateral loadings. Typical factors of safety for the geotechnical strength of drilled shafts range between two and three, depending on the method of capacity calculation, the extent of the designer’s experience and knowledge of the site and the geotechnical conditions, and the likely consequences of failure. In cases where there is extensive experience of the site and field shaft load tests have been carried out, values of safety factor as low as 1.5 may be appropriate. On the other hand, where knowledge of the site is limited, and the consequences of failure may be extreme, safety factors of three or higher may be appropriate.

Fig. 1.3 Failure modes of drilled shafts in karstic formations (after ...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Preface
• 1 Introduction
• 2 Intact rock and rock mass
• 3 Characterization of discontinuities in rock
• 4 Deformability and strength of rock
• 5 Site investigation and rock testing
• 6 Axial load capacity of drilled shafts in rock
• 7 Axial deformation of drilled shafts in rock
• 8 Lateral load capacity of drilled shafts in rock
• 9 Lateral deformation of drilled shafts in rock
• 10 Stability of drilled shaft foundations in rock
• 11 Drilled shafts in karstic formations
• 12 Loading test of drilled shafts in rock
• References