Standard Test Methods for Deep Foundations Under Bi-Directional Static Axial Compressive Load

Importancia y uso:

4.1 The bi-directional axial compressive load test provides separate, direct measurements of the pile side shear mobilized above an embedded jack assembly and the pile end bearing plus any side shear mobilized below the jack assembly. The maximum mobilized pile resistance equals two times the maximum load applied by the jack assembly. Test results may also provide information used to assess the distribution of side shear resistance along the pile, the amount of end bearing mobilized at the pile bottom, and the long-term load-displacement behavior.

4.2 The specified maximum test load should be consistent with the engineer’s desired test outcome. For permanent (working) piles, the engineer may require that the magnitude of applied test load be limited in order to measure the pile movement at a predetermined proof load as part of a quality control or quality assurance program. Tests that attempt to fully mobilize the axial compressive resistance of the test pile may allow the engineer to improve the efficiency of the pile design by reducing the piling length, quantity, or size.

4.3 The engineer and other interested parties may analyze the results of a bi-directional axial compressive load test to estimate the load versus movement behavior and the pile capacity that would be measured during axial static compressive or tensile loading applied at the pile top (see Notes 1-3). Factors that may affect the pile response to axial static loading during a static test include, but are not limited to the:

(1) pile installation equipment and procedures,

(2) elapsed time since initial installation,

(3) pile material properties and dimensions,

(4) type, density, strength, stratification, and groundwater conditions both adjacent to and beneath the pile,

(5) test procedure,

(6) prior load cycles.

Note 1: To estimate the load displacement curve for the pile as if it were loaded in compression at the top (as in Test Methods D1143/D1143M), the engineer may use strain and movement compatibility to sum the pile capacity mobilized above and below the embedded jack assembly for a given pile-top movement. This “top-load” curve will be limited by the lesser of the displacement measured above or below the embedded jack assembly. To obtain adequate minimum displacement during the test, the engineer may wish to specify a maximum test load greater than the desired equivalent “top load”.

Note 2: A bi-directional load test applies the test load within the pile, resulting in internal pile stresses and pile displacements that differ from those developed during a load test applied at the pile top. Bi-directional testing will generally not test the structural suitability of a pile to support a load as typically placed at the pile top. Structural defects near the pile top may go undetected unless separate integrity tests are performed prior to or after bi-directional testing (see Note 8). The analysis of bi-directional load test results to estimate the pile-top movement that would be measured by applying a compressive load at the top of the pile should consider strain compatibility and load-displacement behavior. ASTM D1143/D1143M provides a standard test method for the direct measurement of pile top movement during an axial static compressive load applied at the pile top.

Note 3: The analysis of bi-directional load test results to estimate pile displacements that would be measured by applying a tensile (uplift) load at the top of the pile should consider strain and movement compatibility. Users of this standard are cautioned to interpret conservatively the tensile capacity estimated from the analysis of a compressive load. ASTM D3689/D3689M provides a standard test method for the direct measurement of axial static tensile capacity.

4.4 For the purpose of fully mobilizing the axial compressive capacity, the engineer will usually locate the jack assembly at a location within pile where the capacity above the assembly equals the capacity below it. A poorly chosen assembly location may result in excessive movement above or below the jack assembly, limiting the applied load and reducing the usefulness of the test result. Determination of the assembly’s location requires suitable site characterization, consideration of construction methods, and the proper application of engineering principles and judgement (see Note 4). More complex test configurations, using multiple levels of jack assemblies, may provide a higher probability that the full resistance of the pile along its entire length may be determined. Details regarding such complex arrangements are beyond the scope of this standard.

Note 4: The bi-directional load test may not fully mobilize the axial compressive pile resistance in all sections of the pile. Practical, economical, or code considerations may also result in bi-directional load tests that are not intended to fully mobilize the axial resistance in some or all sections of the pile. In these cases, interpretation of the bi-directional test may under-predict the total axial compressive capacity of the pile.

Note 5: The quality of the results produced by this test method are dependent on the competence of the personnel performing it, and the suitability of the equipment and facilities used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective testing/sampling/ inspection/etc. Users of this test method are cautioned that compliance with Practice D3740 does not in itself assure reliable results. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of those factors.

Subcomité:

D18.11

Referida por:

D3966_D3966M-22, D3689_D3689M-22

Volúmen:

04.09

Número ICS:

93.020 (Earthworks. Excavations. Foundation construction. Underground works)

Palabras clave:

bi-directional load test; deep foundation; displacement; embedded jack; end bearing; field testing; loading procedure; movement; shaft resistance; side friction; side shear; static axial compressive pile capacity;