Standard Test Methods for Deep Foundation Elements Under Static Axial Tensile Load

Importancia y uso:

5.1 Field tests provide the most reliable relationship between the axial load applied to a deep foundation and the resulting axial movement. Test results may also provide information used to assess the distribution of side shear resistance along the element and the long-term load-deflection behavior. The foundation engineer may evaluate the test results to determine if, after applying appropriate factors of safety, the element or group of elements has a static capacity, load response and deflection at service load satisfactory to support the foundation. When performed as part of a multiple-element test program, the foundation engineer may also use the results to assess the viability of different sizes and types of foundation elements and the variability of the test site.

5.2 If feasible and without exceeding the safe structural load on the element or element cap (hereinafter unless otherwise indicated, “element” and “element group” are interchangeable as appropriate), the maximum load applied should reach a failure load from which the foundation engineer may determine the axial static tensile load capacity of the element. Tests that achieve a failure load may help the foundation engineer improve the efficiency of the foundation design by reducing the foundation element length, quantity, and/or size.

5.3 If deemed impractical to apply axial test loads to an inclined element, the foundation engineer may elect to use axial test results from a nearby vertical element to evaluate the axial capacity of the inclined element. The foundation engineer may also elect to use a bi-directional axial test on an inclined element (D8169/D8169M).

5.4 Different loading test procedures may result in different load-displacement curves. The Quick Test (10.1.2) and Constant Rate of Uplift Test (10.1.4) typically can be completed in a few hours. Both are simple in concept, loading the element relatively quickly as load is increased. The Maintained Test (10.1.3) loads the element in larger increments and for longer intervals, which could cause the test duration to be significantly longer. Because of the larger load increments the determination of the failure load can be less precise, but the Maintained Test is thought to give more information on creep displacement. Although control of the Constant Rate of Uplift Test is somewhat more complicated (and uncommon for large diameter or capacity elements), the test may produce the best possible definition of capacity. The foundation engineer must weigh the complexity of the procedure and other limitations against any perceived benefit.

5.5 The scope of this standard does not include analysis for foundation capacity in tension, but in order to analyze the test data appropriately it is important that information on factors that affect the derived mobilized static axial tensile capacity are properly documented. These factors may include, but are not limited to, the following:

5.5.1 Potential residual loads in the element which could influence the interpreted distribution of load along the element shaft.

5.5.2 Possible interaction of friction loads from test element with downward friction transferred to the soil from reaction elements obtaining part or all of their support in soil at levels above the tip level of the test element.

5.5.3 Changes in pore water pressure in the soil caused by element driving, construction fill, and other construction operations which may influence the test results for frictional support in relatively impervious soils such as clay and silt.

5.5.4 Differences between conditions at time of testing and after final construction such as changes in grade or groundwater level.

5.5.5 Potential loss of soil supporting the test element from such activities as excavation and scour.

5.5.6 Possible differences in the performance of an element in a group or of an element group from that of a single isolated element.

5.5.7 Effect on long-term element performance of factors such as creep, environmental effects on element material, negative friction loads not previously accounted for, and strength losses.

5.5.8 Type of structure to be supported, including sensitivity of structure to settlements and relation between live and dead loads.

5.5.9 Special testing procedures which may be required for the application of certain acceptance criteria or methods of interpretation.

5.5.10 Requirement that non-tested element(s) have essentially identical conditions to those for tested element(s) including, but not limited to, subsurface conditions, element type, length, size and stiffness, and element installation methods and equipment, so that application or extrapolation of the test results to such other elements is valid. For concrete elements, it is sometimes necessary to use higher amounts of reinforcement in the test elements in order to safely conduct the test to the predetermined required test load. In such cases, the foundation engineer shall account for the difference in stiffness between the test elements and non-tested elements.

5.5.11 Tension tests are sometimes used to validate element compression capacity in addition to tension capacity. When subjected to tension loads, elements may have different stiffness and structural capacity compared to elements subjected to compression loads.

Note 1: The quality of the result produced by these test methods is 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 these test methods 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:

D7383-19, D3966_D3966M-25, D8169_D8169M-18

Volúmen:

04.08

Número ICS:

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

Palabras clave:

axial load test ; deep foundation capacity; field testing; jack; load cell; loading procedure; reference beam; static load test;