What is Bi-directional static load testing?
ASTM first published ASTM D-1143 in 1950. The standard covered static load testing of deep foundations. Top-load, or compressive load testing as it is often called, involves building multiple reaction shafts (typically four) with a test shaft at the center. A large reaction frame is then constructed to connect all the elements together. The reaction shafts resist or oppose the forces created when the test shaft is pushed down. As test loads surpass 500 tons or so these reaction systems can become cumbersome, expensive and potentially unsafe. More problematic, larger drilled shafts and other foundation elements such as barrettes or LBEs (slurry walls) were growing in capacity as engineers pushed design and efficiency to the limit. These shafts were difficult or impossible to test. The practical limit of a top-down load test is still debated. But 10,000 to 16,000 kip load tests have been performed. They are rare and expensive. They are also somewhat recent.
Realizing this, in the late 1980s Jorg Osterberg developed and then patented the bi-directional load test which became known as the O-cell test1. With a company he helped found, the method was developed and eventually became ubiquitous around the world. The Bi-Directional Static Load Test (BDSLT) method is described in ASTM D8169/D8169M-18. LTC’s founders were a large part of the team that perfected the method over the past quarter century. BDSLT utilizes one or more sacrificial high precision hydraulic jacks configured into a load test assembly (LTA), installed into the designated test shaft, barrette (LBE) or ACIP. The jacks 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 precision hydraulic jacks in the USA. We calibrate them ourselves above 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 (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. The strain gages can also help isolate the end bearing if the LTA is significantly above the shaft tip. If the end bearing load-deflection behavior is difficult to predict and very important to test directly a multi-level test may be appropriate. In this configuration, a second LTA is placed at or near the shaft bottom.
LTC’s Scope of BDSLT Services:
- Interpret project specifications and soil borings
- Assist our 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, carrying beams (as needed), bearing plates and gusset plates to the site (Typically we can assemble the LTA off-site and ship as a completed unit for seamless in-situ integration into the contractor’s cage or our provided beam)
- 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