Drilled Shafts.
a rebar cage, and then filling the excavation with concrete. A typical drilling rig for constructing drilled shafts is shown in Figure 5.4.
The drilling machine may be mounted on one of three types of carriers: a truck, a crane, or a crawler. The smaller machines are truck-mounted and may move readily along a public road. The soil-filled auger is visible on the truckmounted machine in Figure 5.4, and spinning will dislodge dry soil. Exca- vations for light loads may be made with diameters as small as 12 to 18 in. and to depths of a few feet. Excavations for massive loads may be made with diameters of 15 ft or more and to depths of 200 ft. The drilled-shaft industry
provides technical information and training through the International Association of Foundation Drilling (ADSC) in Dallas, Texas.
The engineer develops an appropriate design by sizing the drilled shaft for a particular application, but significant effort is necessary in preparing of specifications for construction. The engineer who made the design and prepared the specifications should manage the field inspection. Drilled shafts are a popular type of deep foundation, but as with other deep foundations, special care must be taken by the engineer to ensure proper construction.
Figure 5.3 Impact hammer atop a driven pile.
Drilling machines are fitted with powerful engines to drive a rotary table and kelly. A variety of drilling tools are available. The appropriate drilling tool operating with downward force from cables or with a weighted drill string can be used for drilling into rock. Rock sockets are common to accommodate design requirements, giving the drilled shaft an advantage over some other types of deep foundations.
The following sections describe three types of constructions in common use, but the details of each may vary with the contractor. The specifications for construction prepared by the engineer must be prescriptive in some in-
Figure 5.4 Construction of a drilled shaft with a truck-mounted unit.
stances (e.g., giving the required slump of concrete for the particular job) but should lean toward the performance desired from the drilled shaft. Prescribing a particular construction method is usually unwise because anomalies in the soil profile could lead to the use of a wet method even though the dry method
appeared feasible at the outset.
Dry Method of Construction The dry method of construction may be employed in soils that will not cave, slump excessively, or deflect inward when the hole is drilled to the full depth. A type of soil that meets these requirements is stiff clay. The water table may be located in the stratum of clay, and above the water table the clay may be saturated by capillarity. A problem occurs if the clay below the water table contains fractures that allow water to flow into the excavation. If such fractures were not observed in the soil in- vestigation, the dry method of construction may have been specified and then may have become impossible to achieve.
The steps in employing the dry method of construction are shown in Figure 5.5. A crane-mounted drilling machine is positioned as shown in Figure 5.5a.
The location has been surveyed and staked to give the contractor precise information on location and on the final position of the top of the shaft.
Specifications will inform the contractor about tolerance in placement and in deviation from the vertical as the shaft is advanced. A temporary surface
appeared feasible at the outset.
Dry Method of Construction The dry method of construction may be employed in soils that will not cave, slump excessively, or deflect inward when the hole is drilled to the full depth. A type of soil that meets these requirements is stiff clay. The water table may be located in the stratum of clay, and above the water table the clay may be saturated by capillarity. A problem occurs if the clay below the water table contains fractures that allow water to flow into the excavation. If such fractures were not observed in the soil in- vestigation, the dry method of construction may have been specified and then may have become impossible to achieve.
The steps in employing the dry method of construction are shown in Figure 5.5. A crane-mounted drilling machine is positioned as shown in Figure 5.5a.
The location has been surveyed and staked to give the contractor precise information on location and on the final position of the top of the shaft.
Specifications will inform the contractor about tolerance in placement and in deviation from the vertical as the shaft is advanced. A temporary surface
casing is frequently placed after the excavation is advanced a few feet. The surface casing prevents raveling of the soil at the surface and provides a positive guide for inserting the auger as drilling proceeds.
Figure. 5.5b shows concrete being placed in the bottom of the excavation, where the computed stresses in the drilled shaft show that no reinforcing steel is needed. Specifications almost always state that the concrete must be poured without striking the sides of the excavation or any obstruction to prevent segregation during placement, and the drop chute serves to guide the concrete in free fall. Research has shown that concrete may fall great distances without segregation if no obstruction is encountered during falling.
The placement of the rebar cage is shown in Figure 5.5c, and the final concrete is placed by use of a tremie or by pumping. Guides are placed on the rebar cage to ensure centering. A service crane is required to hold the tremie or the pump line, as shown in the figure. Alternatively, the crane with the drilling machine could do the work, but further drilling would be delayed.
Specifications frequently require that the concrete be placed the same day the excavation is completed to prevent time-dependent movement of the soil around the excavation. The completed shaft is shown in Figure 5.5d. Casing Method of Construction The casing method of construction may be employed where caving soils are encountered, as shown in Figure 5.6a.
Slurry, either from bentonite or polymer, is introduced when the caving soil is encountered and when drilling proceeds through the caving layer and into cohesive soil below. The casing is placed, and the bottom is sealed into the cohesive soil. Prior to placing the casing, the slurry is treated to remove excessive amounts of inclusions and to ensure that specifications are met for properties of slurry before placing concrete. The contractor twists and pushes the casing to make a seal that prevents the slurry from entering the excavation
below the casing. Figure 5.6b shows a crane-mounted drilling unit inserting a drill through the casing and drilling into the cohesive soil below. The excavation below the casing is smaller than that used in the initial drilling, and the difference in size, not as great as indicated in the figure, must be taken into account in computing the geotechnical capacity of the shaft. Figure 5.6c shows that an underream has been excavated at the base of the excavation and that the casing is in the process of being retracted. The fluidity of the concrete and the retained slurry are very important. The concrete must be sufficiently fluid that the excavation will be completely filled and the slurry will be ejectedfrom the excavation. The cleaned slurry must be free of inclusions and easily displaced from the excavation by the fluid concrete. Specifications address the desirable slump of concrete and the characteristics of the slurry.
Figure 5.6c shows the slurry at the ground surface. Preferably the slurry is directed to a sump, where a pump sends the slurry to a tank for cleaning and reprocessing. The disposal of the slurry must meet environmental standards. Slurry from polymers is usually much more easily disposed of than slurry from bentonite. The completed shaft is shown in Figure 5.6d.
Figure. 5.5b shows concrete being placed in the bottom of the excavation, where the computed stresses in the drilled shaft show that no reinforcing steel is needed. Specifications almost always state that the concrete must be poured without striking the sides of the excavation or any obstruction to prevent segregation during placement, and the drop chute serves to guide the concrete in free fall. Research has shown that concrete may fall great distances without segregation if no obstruction is encountered during falling.
The placement of the rebar cage is shown in Figure 5.5c, and the final concrete is placed by use of a tremie or by pumping. Guides are placed on the rebar cage to ensure centering. A service crane is required to hold the tremie or the pump line, as shown in the figure. Alternatively, the crane with the drilling machine could do the work, but further drilling would be delayed.
Specifications frequently require that the concrete be placed the same day the excavation is completed to prevent time-dependent movement of the soil around the excavation. The completed shaft is shown in Figure 5.5d. Casing Method of Construction The casing method of construction may be employed where caving soils are encountered, as shown in Figure 5.6a.
Slurry, either from bentonite or polymer, is introduced when the caving soil is encountered and when drilling proceeds through the caving layer and into cohesive soil below. The casing is placed, and the bottom is sealed into the cohesive soil. Prior to placing the casing, the slurry is treated to remove excessive amounts of inclusions and to ensure that specifications are met for properties of slurry before placing concrete. The contractor twists and pushes the casing to make a seal that prevents the slurry from entering the excavation
below the casing. Figure 5.6b shows a crane-mounted drilling unit inserting a drill through the casing and drilling into the cohesive soil below. The excavation below the casing is smaller than that used in the initial drilling, and the difference in size, not as great as indicated in the figure, must be taken into account in computing the geotechnical capacity of the shaft. Figure 5.6c shows that an underream has been excavated at the base of the excavation and that the casing is in the process of being retracted. The fluidity of the concrete and the retained slurry are very important. The concrete must be sufficiently fluid that the excavation will be completely filled and the slurry will be ejectedfrom the excavation. The cleaned slurry must be free of inclusions and easily displaced from the excavation by the fluid concrete. Specifications address the desirable slump of concrete and the characteristics of the slurry.
Figure 5.6c shows the slurry at the ground surface. Preferably the slurry is directed to a sump, where a pump sends the slurry to a tank for cleaning and reprocessing. The disposal of the slurry must meet environmental standards. Slurry from polymers is usually much more easily disposed of than slurry from bentonite. The completed shaft is shown in Figure 5.6d.
Wet Method of Construction The wet method of construction permits the rebar cage to be placed into a drilled hole filled with fluid. As with the casing method, the drilling fluid is slurry, as shown in Figure 5.7a. The excavation is made to the full depth with slurry, with the contractor exercising care to keep the height of the column of fluid in the excavation above the water table. The slurry acts to create a membrane at the wall of the excavation in the caving soil, usually a granular material, and any flow of fluid will be from the excavation into the natural soil.
The rebar cage may be placed directly into the fluid column, as shown in Figure 5.7b. Prior to placing the cage, samples of the slurry are taken from the excavation, with most samples taken from the bottom, where suspended particles collect. Not shown in the figure is the system for pumping the slurry from the excavation, directing the pumped fluid to a container, usually a tank, where the slurry can be cleaned with screens and centrifuges if necessary.
Specifications are available from the owner of the project or from standard specifications regarding such characteristics as the sand content of the slurry, the pH, and the viscosity. The aim of the specifications is to ensure that no debris will collect at the bottom of the excavation to interfere with load trans- fer in end bearing and that no slurry remains along the sides of the excavation to interfere with load transfer in skin friction.
The placing of the concrete is shown in Figure 5.7c using a tremie and a concrete bucket. The bottom of the tremie is sealed with a plate that detaches when the tremie is charged fully with concrete. Alternatively, a plug is placed in the tremie to separate the concrete for the slurry and moves downward as the concrete is placed in the tremie. The plug, perhaps made of foam rubber, is compressed and remains in the concrete or floats upward in the column of fluid concrete. The completed shaft is shown in Figure 5.7d.
Plain water can sometimes be employed as the drilling fluid. One of the authors worked at a site in Puerto Rico where the founding stratum was a soft rock with joints and cracks. The water table was high, and drilling dry was impossible. Water was employed as the drilling fluid, and the level of water in the excavation was kept above the water table to prevent inward flow and possible weakening of the founding stratum.
Specifications are available from the owner of the project or from standard specifications regarding such characteristics as the sand content of the slurry, the pH, and the viscosity. The aim of the specifications is to ensure that no debris will collect at the bottom of the excavation to interfere with load trans- fer in end bearing and that no slurry remains along the sides of the excavation to interfere with load transfer in skin friction.
The placing of the concrete is shown in Figure 5.7c using a tremie and a concrete bucket. The bottom of the tremie is sealed with a plate that detaches when the tremie is charged fully with concrete. Alternatively, a plug is placed in the tremie to separate the concrete for the slurry and moves downward as the concrete is placed in the tremie. The plug, perhaps made of foam rubber, is compressed and remains in the concrete or floats upward in the column of fluid concrete. The completed shaft is shown in Figure 5.7d.
Plain water can sometimes be employed as the drilling fluid. One of the authors worked at a site in Puerto Rico where the founding stratum was a soft rock with joints and cracks. The water table was high, and drilling dry was impossible. Water was employed as the drilling fluid, and the level of water in the excavation was kept above the water table to prevent inward flow and possible weakening of the founding stratum.
Categories:
Soil
0 comments:
Post a Comment