Friday, November 23, 2012

Drilled Shafts - Performance Control.

Continuous monitoring is essential to ensure that the boreholes are properly prepared to minimize loss of soil friction and end-bearing capacity and that the concrete mix is placed to achieve a continuous adequate shaft.  Complete details of a drilled shaft const ruction control and an example of quality control forms  may be found in FHWA-HI-88-042, “Drilled Shafts: Construction Procedures and Design Methods” and ADSC
(1989)  report, “Drilled Shaft Inspector’s Manual.” Construction and quality control include the following:

(1)  Borehole excavation.  Soil classification provided by all available boring logs in the construction area should be correlated with the visual description of soil or rock removed from  the excavation.  Any observed groundwater levels should also be recorded.  Characteristics to be observed and determined  include determined include location of the various strata, location and nature of the bearing stratum, and any seepage.  The observer should also determine if the soil profile is substantially different from the one assumed for the design based on knowledge of the plans, specifications, and previous geotechnical analysis.  The design engineer should be at the construction site during boring of the first holes to verify assumptions regarding the subsurface soil profile and periodically  thereafter to check on requirements for any design modifications.

(a)  Excavation details such as changes in the advance rate  of the boring tool and changes in the soil cutting,
groundwater  observations, and bottom heave should be recorded.  These details can be used to modify excavation procedure and improve efficiency in the event of problems as well as to provide a complete record for later reference.

Other important data include type of excavation (e.g., dry, cased, or slurry), time of initiation and completion of the boring, estimates of location of changes in the soil strata, and description of each soil stratum. 

Determine any evidence of pervious  lenses and groundwater, problems encountered during excavating (e.g., caving, squeezing, seepage, cobbles, or  boulders), and the location of the bearing stratum.  A small diameter test boring from the excavation bottom can be made and an undisturbed sample recovered to test the bearing soil.

(b)  The excavation should be checked for proper length, diameter, and underream dimensions.  Any lateral deviations from the plan location and unintentional inclination or batter should be noted on the report and checked to be within the required tolerance.  Provided that all safety precautions have been satisified, the underream diameter can be checked by placing the underream tool at the bottom of the excavation and comparing the travel of the kelly when the underreamer is extended to the travel when it is retracted in the barrel of the underream tool.  Electronic calipers may be used if the excavation was made with slurry or the hole cannot be entered for visual inspection.  Extreme safety precautions must be taken if an inspector enters an excavation to ensure no  fall-in of material, and he should be provided with adequate  air supply, communications and lifeline, and hoisting equipment.  In the event of entry, a liner or casing should be in place to protect against fall-in.  Fresh air may be pumped through hoses extending to the bottom.  Minimum diameter of casing for personal inspection is 2 feet.  An alternative to downhole inspection is to utilize ADSC drilled shaft inspectors manuals.

(c)   Slurry used during excavation should be tested for compliance with mix specifications after the slurry is mixed and  prior to placing in the excavation.  These tests are described  in Table 6-3 and should be performed by the Government and reported to construction management and the designer.

(d)  The bottom of the excavation should be checked before placement of the reinforcement cage and concrete to ensure that all loose soil is removed, water has not collected on the bottom of open boreholes, and the soil is in the correct bearing stratum.  Depth of water in an open borehole should be less than 2 inches.  Casing should be clean, smooth, and undeformed.

(2)  Placement of reinforcement.  The reinforcement cage should be assembled prior to placement in the excavation with the specified grade, size, and number of bars.  The cage  should be supported with the specified horizontal stirrups or spirals either tied or welded in place as required to hold bars in  place and prevent misalignment during concrete placement and removal of casing.  The minimum spacing between bars should be checked to ensure compliance with specifications  for adequate flow of concrete through the
cage.   The cage should be checked for placement in the specified  position and adequately restrained from lateral movement during concrete placement.

(3)  Concrete placement.  The properties of the concrete mix and placement method must be closely monitored to avoid defects in the shaft.  A record of the type of cement, mix proportions, admixtures, quantities, and time loaded on the truck should be provided on the delivery ticket issued by the concrete supplier.  The lapse of time since excavation of the borehole and method of concrete placement, including details of the tremie used to place the concrete, should be recorded.  Concrete slump should be greater than 6 inches and the amount of concrete placed in the excavation for each truck should be recorded.  A plot of the expected quantity calculated from the excavation dimensions and the actual  quantity  should be prepared to indicate the amount and location of the concrete overrun or underrun.  Excessive overruns  or any underruns observed during concrete placement will require an investigation of the cause.  Any unusual  occurrence that affects shaft integrity should be described.

 Table 6-3 Specifications for Bentonite Slurry

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