Geotechnical monitoring is used more and more in engineering works. Successful monitoring requires, on one side, to understand which are the limit states conditions and principal variables to be measured, as well as instrumentation, data and comprehensive processing techniques to analyze and transform sensed data into diagnostics for decision-making.
The aim of this paper is present the settlements analysis based on geotechnical monitoring instrumentation for the preloads during construction of the reclamation for the new Container Terminal, in execution at Moin, Costa Rica.
The current phase of the project, comprises the construction of a 600 meters wharf, the dredging of 10 million cubic meters in the waterways and the reclamation of a 30 hectares island required for the operations areas, including storage yards, buildings and other facilities and utilities. Due to fat clay layer on the soil profile up to 45 m depth, if no geotechnical improvement was done, high settlements on surface could be expected.
Prefabricated vertical drains (PVDs) were installed all over the reclamation, which was then preloaded with several surcharges using the same dredged material. This was done to produce most of the expected total settlement, before the construction of the yard and structures, reducing settlements during the operation phase, maintaining them within tolerable service limits for the foundations. The preloads are sand fills of approximately 10 m high, which was calculated originally on the expected service loads during operations, but incremented to that height to accelerate treatment time.
The instrumentation used on site were surcharge plates and piezometers, placed on the initial filling of the land reclamation area at the work fill level of the port, and prior to the preload to measure the consolidation of the deeper clayey soils. For settlement control, the project was divided into 18 work areas, which were preloaded in a sequence using the same material, but moved from one area to another. In consequence, the treatment time at each area was critical for the overall construction schedule of the project.
Settlement auscultation plates were placed on the initial filling of the port area without preloads, to then survey elevations in the same plate to obtain the settlements produced by the preloads. The information was taken in weekly measurements. The measured settlements were used as a basis for model calibrations and predictions.
So, the main objectives of the settlement data analysis were: 1) to verify the settlements monitored on the settlement plates and 2) to predict the remaining holding times necessary to achieve the target consolidation. Based on the information from the contractor, the designer and inspector of the project decides when was adequate to withdrawn the surcharges at each area, for efficiency of the soil improvement and release of the areas for other activities on the construction schedule.
Also, CPT tests were executed before and after, to verify measures not only the consolidation degree of the soils, but also the resistance parameters for slope stability in the slope embankment below the pier.
The analysis included in this paper was realized as the government supervision, to revise the designer and inspector calculation, during the execution of the preloads in each area, at the Container Terminal Port of Moín. Consolidation was revised through the Asaoka Method (1978), which is a method based on the plates settlement measurements, to calculate future settlement.
As part of the geotechnical verifications, a computational analysis of settlements for the final scenario was performed in a critical area of the project, simulating the scenario after the construction of the project, and compared with analytical method of Asaoka (1978). This was a 3D model based on finite differences. The model includes stratigraphy and parameters for each layer, taken from the soil investigations, and representative boundary conditions.
The analysis evaluated if the required performance settlement would be achieved from the method, according with the technical project specifications. The relation between instrumentation and diagnostics is described as consecutive processes that are intrinsically related for effectiveness, according to that it´s shown that is necessary to keep the instrumentation points (plates points and survey points) permanently verify the predictions at least any 6 months and not just in the construction stages.