This document is written as a resource for participants in a short course covering the subject of construction and design of bored shaft foundations for bridges and other structures. The introductory chapters cover an overview of the characteristics of bored shafts, site investigations for bored shafts (to gather information for both construction and design) and details of bored shaft construction. The Federal Highway Administration has produced two educational publications (in 1977 and 1988) on the construction and design of bored shaft foundations.
INTRODUCTION TYPES OF DEEP FOUNDATIONS TYPES OF DEEP FOUNDATIONS
The drilled shaft alternative required the replacement of horizontal pile sets and piles with three large diameter drilled shafts. Use cases for drilled shafts: (a) bearing in stiff clay, @) skin friction design, (c) socket in rock, (d) installation in expansive clay (continued). The construction of shafts drilled through contaminated soils is problematic due to the costs incurred in disposing of the spoil.
SITE CHARACTERIZATION PURPOSE OF SITE CHARACTERIZATION PURPOSE OF SITE CHARACTERIZATION
1 During Of
The locations of the boreholes are carefully selected based on the planned locations of the drilled shafts. Em = Young's n$odulus of IGM rock or core (can be approximated from q if not measured) (units of F/L2) and. The position of the piezometric surface (or surfaces -- there may be separate piezometric surfaces in each layer) must be determined by subsurface investigation using piezometers or observation wells.
As much information as possible must also be obtained to guide the construction of the drilled shafts. Torque and crowd (downward drilling force applied to the drill string) of the drilling machine being used. First, the successful completion of the design and construction of a drilled shaft foundation is highly dependent on obtaining accurate information about the subsurface materials.
One of the major tasks that the drilled shaft designer must perform is the selection of a safety factor or a resistance factor. The lateral abutments will cause a temporary decrease in the water content of the soil at the pile wall and an increase in the shear strength. Nevertheless, the purpose of the subsurface investigation must be to determine as well as possible the properties of the in-situ soil.
One of the important considerations for drilled shafts with rock sleeves is the condition of the side of the borehole. In argillaceous (clay-based) rocks such as shale, mudstone and shale, the presence of free water in the borehole during drilling (for example due to a small influx of water from a small aquifer close to the surface or due to the deliberate introduction of water by the contractor to assist in the excavation of cuttings) can cause the surface of the rock to become completely soft, or. The subsurface investigation and subsequent laboratory testing performed for the design of drilled shafts must take into account the soil and rock properties to be used.
An element, shown in Figure 2.8a, is taken along the length of the drilled shaft at depth z below the ground surface.
Average unit side shear resistance, f (MPa)
GENERAL CONSTRUCTION METHODS INTRODUCTION INTRODUCTION
Recently, significant progress has been made in understanding the use of synthetic. Conical bell tools (described in more detail in Chapter 4) have hinged arms that are pushed outward by a downward force on the kelly (drill rod) so that rotation of the tool in the borehole will cause soil to be cut away. As shown in Figure 3.2 b, the concrete was allowed to fall freely without hitting the sides of the hole.
At this point, before placing the casing, it is good practice to check the concentration of sand in the slurry near the bottom of the pit. A bell tool can be placed in the case, as shown in Figure 3.3 f, and the base of the drilled shaft can be enlarged. During this operation, sludge is trapped in the annular space between the outer part of the casing and.
Excavation is carried out to the full depth of the hole, with the slurry in place. Any fluid flow near the hole would thus be from the excavation outward, rather than the other way around. A sliding plug (or "pig") is inserted into the top of the tremie line before the concrete is poured.
This loss of resistance will have a negative effect on the performance of the second drilled shaft.
METHODS OF EXCAVATION
The drive unit, turntable and kelly can be obtained separately and mounted on the crane at the contractor's option. Some drill buckets are designed to be used to pump water out of the hole. The hardness of the teeth of cone drill bits can be varied to drill into different types of rock.
The rod, which can be called a kelly, is substantial enough to allow easy positioning of the tool. As before, a mechanism must be provided for opening and closing the bucket jaws. The transverse dimension of the tool must be in accordance with the shape of the guides used.
The debris can be picked up using air (ie . debris is blown from the borehole) if the hole is dry. Hammer grips can also be used to construct non-circular barrettes by varying the length of the long side. The version of the machine that is effective in excavating soil is shown in Figure 4.20.
Explosives must be handled by experts and should only be used with the permission of the governing authorities.
CHAPTER 5: CASINGS AND LINERS
An example of the use of a permanent casing is when a drilled shaft is to be installed through water and the projecting portion of the casing is used as formwork. One of the most important factors will be the lateral tension to which the permanent enclosure will be subjected prior to placing the concrete. Not only is the housing expensive, but the skin friction along the sides of the drilled shaft could be significantly reduced.
The engineer-in-charge must use judgment in the evaluation of the loss (or gain) of axial capacity of a drilled shaft that results from inadvertently leaving temporary casing in the borehole or intentionally using permanent casing. The two drilled shafts constructed with the permanent casings had virtually no skin friction load transfer in the area of the oversized excavation as might have been expected. A second drilled shaft was constructed following the same procedure, but in the second case the contractor was careful before the concrete was placed to ensure that the casing could be lifted.
Both drilled shafts were load tested and the one with permanent casing was able to carry much less load than the conventionally constructed one. If the rigid sheath is to be sealed to shear, the bottom of the sheath must be specially prepared. One of the procedures is to weld hardened teeth to the base, as shown in Figure 5.4.
The setting of the teeth at the base of a casing can be varied for different types of rock.
DRILLING SLURRY INTRODUCTION
These are just four examples of the use of drilling mud in the construction of drilling shafts. This natural process requires treatment of the slurry to restore its properties if it is to be reused. The main concern is that the slurry has the right properties during the excavation of the borehole.
The procedure for removing slurry from the bottom of the well is to use an air lift. In both methods, the fluid flow rate must lift all sediments in the slurry out of the wellbore. However, it is still desirable to agitate the slurry (as with auger) to ensure that the particles remain in suspension.
The sand content at the bottom of the pit will stabilize to a small value (typically less than 1 percent by volume) after the slurry column is allowed to stand undisturbed for a period of time [for example, about 30 minutes to 2 hours in pits less than 20 m (66 ft) deep]. For this reason the contractor must be very diligent to keep cement out of the slurry. A mud balance (lever-arm scale) is commonly used to measure the density, or unit weight, of slurry.
The density of the slurry is read directly from a scale on the beam in various forms [unit weight (lbtcubic foot, lbtgallon), specific gravity].
Torsional
REBAR CAGES INTRODUCTION INTRODUCTION
The design of the reinforcing or "reinforcement" cage for a drilled shaft is a necessary step in the engineering process. In this manual, reinforcement cages are examined from two perspectives: (1) the geometry of the steel required to withstand these stresses. When placing reinforcing steel, the stresses that may occur are taken into account, and the correct load factors are used in the calculations.
The specifications in the table do not apply to the welding of M 3 1 or M 42 steels, as these bars should not be welded in normal practice. Where welding of the reinforcement cage is desirable, a weldable steel, ASTM A 706, can be specified, but availability is often limited. The main role of longitudinal reinforcing steel is to resist stresses due to bending and tension.
However, in the casing method of construction, the cage must be able to stand alone on the bottom of the borehole during the placement of the concrete; so some of the longitudinal bars must extend over the h l l length of the shaft. It is conceptually possible to change the spacing of the longitudinal bars and to orient the cage in a specific direction in the case where the main forces causing bending have a preferred direction. A view of the longitudinal steel in a bar cage being assembled on a job site is shown in Figure 7.1.
Various authorities recommend that the minimum clear space between bars be three to five times the largest coarse aggregate in the concrete mix.
TRANSVERSE REINFORCING
Preferred)