Category:701 Drilled Shafts
Substructure foundations may be designed to transmit loads to foundation strata by concrete columns cast in drilled holes. See EPG 751.37 Drilled Shafts for design guidance and additional information.
This type of foundation is identified in Sec 701 of the Standard Specifications as Drilled Shafts. A drilled shaft is generally considered a deep foundation. Drilled shafts are to be constructed with or without a casing. When casing is used it can be either a temporary or permanent steel casing. Requirements for plan reporting of steel casing are given in EPG 7126.96.36.199 Casing.
The shaft portion of a drilled shaft is usually founded on rock (limestone, dolomite or other suitable material with qu ≥ 100 ksf) or weak rock (shale or other suitable material with 5 ksf ≤ qu ≤ 100 ksf) with a smaller diameter rock socket drilled into same. The inspector should carefully study all general specifications and special provisions pertaining to drilled shafts and become familiar with the designer's intent.
The integrity of the rock socket shall be verified by a foundation inspection hole. This is usually performed after the shaft is drilled. Setting up over a drilled hole can be difficult. The contractor can perform the inspection hole in advance if they submit a procedure that assures the correct location is cored. If the integrity of the cores are questionable the Bridge Division should be contacted to see if the rock socket length should be extended.
Most problems with drilled shafts occur during the concrete pour. The concrete placement requirements in Sec 701 should be reviewed carefully.
An anomaly may be detected on a Cross Hole Sonic log test. If, on further investigation, there is a confirmed defect what are some of the steps needed to remediate the defect?
- 1. The contractor is responsible for submitting a remediation plan for the repair.
- 2. The plan should include as a minimum the following:
- a) The area of deficient material must be clearly defined using coring or other means.
- b) The clean-out process is typically accomplished by flushing the weak material. The access holes needed, water pressure used, and disposal of the soils should be addressed.
- c) Confirmation of the deficient material removal must be made. This can be accomplished by camera inspection, CSL, or by other means acceptable to the engineer.
- d) The grouting plan should include: grouting type, grout mix design including w/c ratio, complete pressure grouting timeline. The grouting timeline should include placement times, pressure, volume, refusal criteria.
- 3. A final confirmation of the effectiveness of the grouting should be made. This is typically accomplished by coring. The number of cores required, and depth shall be submitted to the engineer for approval prior to coring. If all the CSL tubes are still usable, a final CSL can be made for acceptance. The engineer of record for the design should be consulted for final acceptance.
Question: Per Sec 701.4.17.2.1 Installation of Pipes, “The pipes shall be filled with water and plugged or capped before shaft concrete is poured.” Why is this necessary?
The water in the tube helps to regulate the temperature of the CSL tube. Without the water, the tube will heat up from the hydrating concrete and cause de-bonding. This de-bonding from the concrete will cause erroneous CSL readings and show up as an anomaly. Typically, de-bonding is more prevalent in the upper 6 ft. of the tube. The water also serves a second purpose: it helps the energy transmission from the wall of the tube to the probes and vice versa.