Section 1812A Earth Retaining Shoring
The requirements of this section shall apply to temporary and permanent earth retaining shoring using soldier piles and lagging with or without tie-back anchors in soil or rock, only when existing or new facilities are affected. Shoring used as construction means and methods only, which does not affect existing or new facilities, are not regulated by this section and shall satisfy the requirements of the authorities having jurisdiction.
Shoring shall be considered temporary when elements of the shoring will be exposed to site conditions for a period of less than one (1) year, and shall be considered permanent otherwise. Permanent shoring shall account for the increase in lateral soil pressure due to earthquake. At the end of the construction period, the existing and new structures shall not rely on the temporary shoring for support in any way. Wood components shall not be used for permanent shoring lasting more than two (2) years. Wood components of the temporary shoring that may affect the performance of permanent structure shall be removed after the shoring is no longer required.
All components of the shoring shall have corrosion protection or preservative treatment for their expected duration. Wood components of the temporary shoring that will not be removed shall be treated in accordance with AWPA U1 (Commodity Specification A, Use Category 4B and Section 5.2), and shall be identified in accordance with Section 2303.1.9.1.
Surcharge pressure due to footings, traffic or other sources shall be considered in design. If the footing surcharge is located within the semicircular distribution or bulb of earth pressure (when shoring is located close to a footings), lagging shall be designed for lateral earth pressure due to footing surcharge. Soil arching effects may be considered in the design of lagging. Underpinning of the footing may be used in lieu of designing the shoring and lagging for surcharge pressure. Alternatively, continuously contacting drilled pier shafts near the footings shall be permitted. The lateral surcharge design pressure shall be derived using Boussinesq equations modified for the distribution of stresses in an elastic medium due to a uniform, concentrated or line surface load as appropriate and soil arching effects.
Except for the modifications as set forth in Sections 1812A.4.1 through 1812A.4.3 below, all Prestressed Rock and Soil Tie-back Anchors shall comply with PTI Recommendations for Prestressed Rock and Soil Anchors (PTI-2004).
The geotechnical report for the earth retaining shoring shall address the following:
- Minimum diameter and minimum spacing for the anchors including consideration of group effects.
- Maximum unbonded length and minimum bonded length of the tie-back anchors.
- Maximum recommended anchor tension capacity based upon the soil or rock strength/grout bond and anchor depth/spacing.
- Allowable bond stress at the ground/grout interface and applicable factor of safety for ultimate bond stress for the anchor. For permanent anchors, a minimum factor of safety of 2.0 shall be applied to ground soil interface as required by PTI-2004 Section 6.6.
- Minimum grout pressure for installation and post-grout pressure for the anchor. The presumptive post grout pressure of 300 psi may be used for all soil type.
- Class I corrosion protection is required for all permanent anchors. The geotechnical report shall specify the corrosion protection recommendations for temporary anchors.
- Performance test for the anchors shall be at a minimum of two (2) times the design loads and shall not exceed 80 percent of the specified minimum tensile strength of the anchor rod. A creep test is required for all prestressed anchors that are performance tested. All production anchors shall be tested at 150 percent of design loads and shall not be greater than 70 percent of the specified minimum tensile strength of the anchor rod.
- Earth pressure, surcharge pressure and the seismic increment of earth pressure loading, when applicable.
- Maximum recommended lateral deformation at the top of the soldier pile, at the tie-back anchor locations, and the drilled pier concrete shafts at the lowest grade level.
- Allowable vertical soil bearing pressure friction resistance, and lateral passive soil resistance for the drilled pier concrete shafts and associated factors of safety for these allowable capacities.
- Soil-pier shaft/pile interaction assumptions and lateral soil stiffness to be used in design for drilled pier concrete shaft or pile lateral loads.
- Acceptable drilling methods.
- Geotechnical observation and monitoring recommendations.
- Tendons shall be thread-bar anchors conforming to ASTM A722.
- Anchor design loads shall be based upon the load combinations in Section 1605A.3.1 and shall not exceed 60 percent of the specified minimum tensile strength of the tendons.
- The anchor shall be designed to fail in grout bond to the soil or rock before pullout of the soil wedge.
- Design of shoring system shall account for as-built locations of soil anchors considering all specified construction tolerances in Section 1812A.8.
- Design of shoring system shall account for both short and long-term deformation.
- The geotechnical engineer shall keep a record at job site of all test loads, total anchor movement, and report their accuracy.
- The contractor shall determine the cause of failure — variations of the soil conditions, installation methods, materials, etc.
- The contractor shall propose a solution to remedy the problem. The proposed solution will need to be reviewed and approved by the geotechnical engineer, shoring design engineer and building official.
- After a satisfactory test, each anchor shall be locked-off in accordance with Section 8.4 of PTI 2004.
- The shoring design engineer shall specify design loads for each anchor.
The construction procedure shall address the following:
- Holes drilled for piles/tie-back anchors shall be done without detrimental loss of ground, sloughing or caving of materials and without endangering previously installed shoring members or existing foundations.
- Drilling of earth anchor shafts for tie-backs shall occur when the drill bench reaches two to three feet below the level of the tie-back pockets.
- Casing or other methods shall be used where necessary to prevent loss of ground and collapse of the hole.
- The drill cuttings from earth anchor shaft shall be removed prior to anchor installation.
- Unless tremie methods are used, all water and loose materials shall be removed from the holes prior to installing piles/tie-backs.
- Tie-back anchor rods with attached centralizing devices shall be installed into the shaft or through the drill casing. Centralizing device shall not restrict movement of the grout.
- After lagging installation, voids between lagging and soil shall be backfilled immediately to the full height of lagging.
- The soldier piles shall be placed within specified tolerances in the drilled hole and braced against displacement during grouting. Fill shafts with concrete up to top of footing elevation, rest of the shaft can generally be filled with lean concrete. Excavation for lagging shall not be started until concrete has achieved sufficient strength for all anticipated loads as determined by the shoring design engineer.
- Where boulders and/or cobbles have been identified in the geotechnical reports, contractor shall be prepared to address boulders and/or cobbles that may be encountered during the drilling of soldier piles and tie-back anchors.
- The grouting equipment shall produce grout free of lumps and indispensed cement. The grouting equipment shall be sized to enable the grout to be pumped in continuous operation. The mixer shall be capable of continuously agitating the grout.
- The quantity of grout and grout pressure shall be recorded. The grout pressure shall be controlled to prevent excessive heave in soils or fracturing rock formations.
- If post-grouting is required, post-grouting operation shall be performed after initial grout has set for 24 hours in the bond length only. Tie-backs shall be grouted over a sufficient length (anchor bond length) to transfer the maximum anchor force to the anchor grout.
- Testing of anchors may be performed after post-grouting operations, provided grout has reached strength of 3,000 psi as required by PTI-2004 Section 6.11.
- Anchor rods shall be tensioned straight and true. Excavation directly below the anchors shall not continue before those anchors are tested.
- The shoring design engineer or his designee shall make periodic inspections of the job site for the purpose of observing the installation of shoring system, testing of tie-back anchors and monitoring of survey.
- A complete and accurate record of all soldier pile locations, depths, concrete strengths, tie-back locations and lengths, tie-back grout strength, quantity of concrete per pile, quantity of grout per tie-back and applied tie-back loads shall be maintained by the special inspector and geotechnical engineer. The shoring design engineer shall be notified of any unusual conditions encountered during installation.
- Calibration data for each test jack, pressure gauge and master pressure gauge shall be verified by the special inspector and geotechnical engineer. The calibration tests shall be performed by an independent testing laboratory and within 120 calender days of the data submitted.
- Monitoring points shall be established at the top and at the anchor heads of selected soldier piles and at intermediate intervals as considered appropriate by the geotechnical engineer.
- Control points shall be established outside the area of influence of the shoring system to ensure the accuracy of the monitoring readings.
- Intitial monitoring shall be performed prior to any excavation.
- Once excavation has begun, the periodic readings shall be taken weekly until excavation reaches the estimated subgrade elevation and the permanent foundation is complete.
- If performance of the shoring is within established guidelines, shoring design engineer may permit the periodic readings to be bi-weekly. Once initiated, bi-weekly readings shall continue until the building slab at ground floor level is completed and capable of transmitting lateral loads to the permanent structure. Thereafter, readings can be monthly.
- Where the building has been designed to resist lateral earth pressures, the periodic monitoring of the soldier piles and adjacent structure can be discontinued once the ground floor diaphragm and subterranean portion of the structure is capable of resisting lateral soil loads and approved by the shoring design engineer, geo-technical engineer and building official.
- Additional readings shall be taken when requested by the special inspector, shoring design engineer, geotechnical engineer or building official.
- Monitoring reading shall be submitted to the shoring design engineer, engineer in responsible charge, and building official within three working days after they are conducted. Monitoring readings shall be accurate to within 0.01 feet. Results are to be submitted in tabular form showing at least the intial date of monitoring and reading, current monitoring date and reading and difference between the two readings.
- If the total cummulative horizontal or vertical movement (from start of construction) of the existing buildings reaches 1/2 inch or soldier piles reaches 1 inch all excavation activities shall be suspended. The geotechnical and shoring design engineer shall determine the cause of movement, if any, and recommend corrective measures, if necessary, before excavation continues.
- If the total cummulative horizontal or vertical movement (from start of construction) of the existing buildings reaches 3/4 inch or soldier piles reaches 11/2 inches all excavation activities shall be suspended until the causes, if any, can be determined. Supplemental shoring shall be devised to eliminate further movement and the building official shall review and approve the supplemental shoring before excavation continues.
- Load cells shall be installed at the tie-back heads adjacent to buildings at maximum interval of 50 feet, with a minimum of one load cells per wall.
- Load cell readings shall be taken once a day during excavation and once a week during the remainder of construction.
- Load cell readings shall be submitted to the geo-technical engineer, shoring design engineer, engineer in responsible charge and building official.
- Load cell readings can be terminated once the temporary shoring no longer provides support for the buildings.