Normally a strain gauge installed in a test pile is used to determine how does a applied load be transfered from top to bottom. The principle is based on elatic shortening theory of a single pile. Meanwhile there should be have an another set of tell-tale (or extensometer)to measure the displacement profile along the pile shaft. The purpose of tell-tale is to determine the elastic compression of pile and subsequently a t-z relationship for soils may be established. Somehow I found there is inconsistent for the elastic shortening of pile using the strain gauge and tell-tale. Anyone has the same experience before? What would be the possible reasons behind ?
It is quite common to have a measurement of elastic shortening of pile under either compression or tension. I have similar experience in both cast-in-situ bored piles and precast concrete piles. This is mainly due to the pile sectional properties, such as pile sectional area, uniformity of elastic modulus and error in the applied axial load by using the pressure gauges on the hydraulic jack.
Sometimes, if single strain gauge is used to measure the axial strain at certain pile section, there could be a component of flexural stress being captured by the strain gauge attached off centre from the pile. In such case, the strain gauge may be picking up both the axial strain and the bending component (either compressive or tensile strain) due to eccentric imposed load. This can be avoided by having more strain gauges attached at equal distance from the centriod of the same section and taking the average measured strain for the axial strain. The flexural component can be interpreted from the tilting of the strain profile/plane of the section by the strain gauges.