What is Bi-directional static load testing?
The Bi-Directional Static Load Test (BDSLT) method is described in ASTM D8169/D8169M-18. The method was perfected over the last two decades by a company which at the time employed LTC’s founders. BDSLT utilizes one or more sacrificial high precision hydraulic jacks configured into a load test assembly (LTA), pre-installed into the designated test foundation element. The cylinders are designed to have an extremely linear relationship between pressure and load, even under tilting conditions which can occur during the test. LTC designs and fabricates our own made-in-the-USA precision hydraulic jacks that we calibrate ourselves to full scale, with pressure vs. load linearity R-squared values of 0.999 or greater.
In the field, the LTA is mechanically connected to a reinforced steel cage or beam to lock it in place within the foundation. Once the foundation element concrete or grout attains sufficient strength (usually after a week and to around 3,000 to 4,000 psi) the load test is performed.
The foundation element is separated into two sections once the LTA is pressurized. The concrete or grout fractures on the plane of jack opening. Movement is resisted in both the upward and downward directions by upper side shear above and by the combined side shear and end bearing below the LTA, respectively.
In the USA, the test is performed almost exclusively using “Procedure A” of ASTM D8169 (and in general compliance with ASTM D1143). This involves about four hours to set up and four hours to test. The load is increased in regular increments until the ultimate capacity either above or below the LTA is reached – or both. In some cases the maximum capacity of the hydraulic jacks is reached first. Most engineers specify strain gages at various depths to assess the shear resistance of various soil strata.
LTC’s Scope of BDSLT Services
On a typical project, our scope of services includes:
- Interpret project specifications and soil borings, assist client in designing a value-based load test program, balance the test (locate LTA elevation to achieve equal resistance above and below).
- Prepare Method Statement documents including all drawings and proposed plans, means and methods.
- Source all instrumentation and other miscellaneous consumables specific to the test, or advise the client on sourcing these (LTC does not mark up items that we ourselves do not fabricate or produce).
- Design the load test assembly (LTA), ship all components including jacks, bearing plates and gusset plates to the site. If the client requests, we can assemble the LTA off-site and ship as a completed unit for seamless in-situ integration into the contractor’s cage or frame.
- Participate in pre-construction meetings to review lifting, installation and concreting plans with the drilled shaft contractor to advise on potential pitfalls.
- Provide a QA inspection of all components of the test element assembly and instrumentation prior to installation in excavation.
- Set up and conduct the test per ASTM D8169 Procedure A or per project-specific procedures.