Discussion of “Critical assessment of pile modulus determination methods” ¹Appears in the Canadian Geotechnical Journal, <b>48</b>(10): 1433–1448 [doi: 10.1139/t11-050].

Lam and Jefferis (2011) have produced a timely, well written, and worthy paper. I would like to add a few comments to the authors’ presentation of the strain measurements. Moreover, the authors’ recommendations regarding the E-modulus to be used for determining the load distribution in a pile subjected to a static loading test from the strain measurements need both to be emphasized and moderated, which is what the following brief compilation of case records aims to do. The authors’ paper presents the changes of strain of up to 100+ μ3 in the pile occurring as a result of the concreting and during the set-up time between pile construction and the static loading test. I think it is worthwhile to emphasize that the changes are caused by three different processes: temperature change (heating and cooling), swelling of the concrete, and reconsolidation by the soil (Fellenius et al. 2004). First, changes of strain are imposed during the hydration process as a consequence of the differences in thermal response between the concrete and the steel (in the sister bar gages). The hydration process causes an increase of temperature, which takes place over several hours, about 24 h rather than the 3 h the authors report. At first, the concrete is fluid, and there is very little strain or stress transferred between the concrete and the sister bars. Second, when the concrete starts to harden, bonding develops between the rebars and the concrete, and a further temperature increase will result in an apparent elongation — tension — of the rebars. During the subsequent cooling, which can take weeks or months, the concrete (usually) reduces more than the rebars, manifested by a shortening of the rebars — apparent compression — co-occurring with tension in the concrete. During the following period, the concrete absorbs water from the soil, which results in a volume increase — swelling — recorded as a rebar elongation — apparent tension — and, conversely, compression in the concrete. Third, at depth, when the soil recovers from the disturbance imposed by the construction, it usually tends to settle, which causes negative direction shear forces to develop, resulting in an increase of load in the pile — noticeable as residual load, which the authors showed to have taken place for the subject pile. Consequently, when the static loading test commences, a state of stress and strain exists in the pile that to some extent will affect the response to the applied load increments, as registered by the strain gages. The evaluation of the strain-gage records, in particular when the secant stiffness method is applied, needs to consider this, as suggested for a few of the following case histories.

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