SECTION 1006 — DRILLED CAISSONS


1006.1 DESCRIPTION — This work is construction of reinforced cement concrete drilled caisson foundations consisting of shaft sections with or without casings left in place, as indicated or directed, and with or without rock sockets or belled footings, all formed within drilled excavations.
The following definitions apply:

(a) End Bearing Drilled Caisson. Cast-in-place foundation element consisting of shaft section with or without enlarged bearing area at its base, a minimum of 30 inches in diameter, and deriving the majority of its compression load capacity through load transfer to the shaft base.

(b) Bell Footing. Enlargement at base of shaft.

(c) Bearing Strata. Layer(s) of soil and/or rock providing principal support at base of shaft.

(d) Rock Socket. Length of excavation extending through rock which cannot be drilled with conventional earth augers and/or underreaming tools and requires the use of special rock augers, core barrels, air tools, blasting, and/or other methods of hand excavation. Rock is defined as a geomaterial having an unconfined compressive strength equal to or greater than 250 psi.

(e) Permanent Casing. Steel pipe, typically of cylindrical shape, installed by drilling, driving, or vibrating, which, when filled with concrete, becomes a permanent part of the drilled caisson.

(f) Temporary Casing. Protective steel pipe, typically of cylindrical shape, installed by drilling, driving, or vibrating, which provides lateral earth support during shaft excavation, cleaning, and inspection; controls groundwater infiltration; and is removed as part of the concrete placement operation.

(g) Obstruction. Cobble, boulder, or any other natural or manmade object above designated rock socket elevation which cannot be drilled with conventional earth augers and/or underreaming tools, and which requires the use of special rock augers, core barrels, air tools, blasting, and/or other methods of hand excavation.

1006.2 MATERIALS —

(a) Casing. Smooth, clean, rust-free metal casing, of sufficient strength to withstand handling stresses; the pressure of concrete, water, and the surrounding earth; and installation stresses, and to prevent water seepage.

(b) Class A Cement Concrete. Section 704 and meet the additional requirements of Section 1001.2(j) for Tremie Cement Concrete. Provide concrete having a slump of 6 inches 1inch for the free fall method of placement.

(c) Reinforcement Bars. Section 1002.2. Use deformed bars.

(d) Bentonite Slurry. A mixture of fully hydrated bentonite and clean fresh water, of adequate unit weight. Submit specifications, type, and properties of the slurry to the Engineer for approval.

(e) Mortar. Section 705.7(a)1.; except, use three parts fine aggregate.

(f) Welding Material. Section 1105.02(t)

1006.3 CONSTRUCTION —

(a) Excavation. Excavate to the dimensions and elevations indicated, or as required by the site conditions and as directed by the Engineer. Unless otherwise indicated, bore excavations for vertical caissons plumb to within a tolerance equal to 2% of the shaft length; and for battered caissons, as indicated, to within a tolerance equal to 5% of the shaft length but not more than 1 foot. If holes are more than 6 inches out of plumb, redesign the footing which is to be supported by the caissons. Make all needed corrections to the structure at no additional cost to the Department. In addition, do not place the tope of a caisson out of the indicated position by more than 1/24th of the shaft diameter or 3 inches, whichever is less. When belled footings are required, excavate to form a bearing area of the size and shape indicated. Blasting may be used, if permitted. Submit blasting plan to the Engineer for review and approval 3 weeks prior to the start of blasting operations. Do not disturb formations below or outside the limits of the caisson under construction or any previously constructed caissons adjacent to the excavation.
Do not excavate within 3 diameters of caissons with new concrete until 24 hours after concrete placement.
If satisfactory foundation materials are encountered at other than the indicated elevations, adjust drilling depths as directed by the Engineer.
If a caving condition or excess groundwater is encountered, discontinue the drilling operation and employ a construction method which satisfactorily prevents caving and groundwater infiltration, such as the installation of casings. As an alternate, drilling in a bentonite slurry with removal of cuttings or dewatering, or other construction methods which control the size of the excavation, will be permitted provided the Contractor can demonstrate the ability to perform such work to the satisfaction of the Engineer.
When bentonite slurry is used to support the caisson excavation, provide slurry mix design for review and approval prior to construction. As a minimum, provide mix design criteria for density, viscosity, shear strength, and pH and procedures for mixing, quality control, maintaining level in excavation, cleaning, reuse, and disposal.
Remove caked material from the sidewalls and loose cuttings from the bottom of the excavation, as directed, so that such material will not cause unanticipated settlement, reduce caisson capacity, or affect concrete strength.
Provide casing through soil and broken or unstable rock at all times during hand cleaning and inspection of the excavation. When joining two or more sections of casing to obtain the required length, weld sections together in accordance with Section 1105.03(m) so as to develop the full tensile strength of each section.

(b) Test Holes. For caissons founded on rock, drill standard NX size core borings at each caisson location, to a minimum depth of 10 feet below the bottom of the excavation (caisson or rock socket), unless otherwise specified or directed. Drill, log, store, and ship the samples in accordance with the Department's Standard Specification for Subsurface Boring, Sampling, and Testing, Publication No. 222. The samples will be used to determine whether there is material of sufficient strength and thickness to support the required load and for proper founding of caissons.

(c) Probe Holes. Drill 2-inch diameter unsampled holes at the specified locations, to the specified depth below the bottom of the excavation (caisson or rock socket), or as directed. The rate of drilling of the holes will be used by the Engineer to determine whether there is satisfactory material or rock of sufficient thickness and type to support the required load, and/or to locate the presence of open joints, voids, soft rock, or other deleterious material which may be inadequate for support of the required load.

(d) Inspection. Do not place concrete until the foundation excavation has been inspected and accepted. If access to the bottom of the excavation for visual inspection cannot be provided because of inability to dewater the excavation using acceptable pumping methods, provide drilling logs and core samples, or other acceptable data, for determining the condition of the excavation and the quality of the foundation material. As a minimum, inspect each shaft which cannot be dewatered with an underwater video camera.
Follow safety practices as specified in Section 107.08 and include, as a minimum, the following items specifically required for inspection of drilled caisson excavations:
No Supplement

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