1. ICGI Wollongong 2012 (30 October – 2 November, 2012)
GEOTECHNICAL ASPECTS OF THE EARTHFILL AND SOIL IMPROVEMENT TECHNIQUES IMPLEMENTED IN AN AREA OF AN INDUSTRIAL PLANT NEAR COIMBRA (PORTUGAL)
Cláudia Santos1 and Alexandre Santos-Ferreira2
1Faculdade de Ciências e Tecnologia (FCT), Universidade Nova de Lisboa, Portugal,
2IPTM, IP – Instituto Portuário e dos Transportes Marítimos, Lisboa, Portugal,
INTRODUCTION
In an industrial plant near Coimbra (Portugal), the project for “Zona A” area included a pit and a compartment structure whose main platform was executed by cut and fill.
To execute the pit and its support structures, an excavation 10.7m deep was done, with lateral slopes at 4(V)/1(H) or 1(V)/1.5(H), according to the geotechnical formations intercepted. The filling and compaction of the area around the pit was executed using materials from the cuttings in site, that included marls and clayey marls.
After the completion of the foundations and the building’s main structure, and after a heavy rain period, increasing settlements were observed in the earthfill around the pit. These settlements lead to significant displacements and cracking in the built concrete elements, mainly the floor beams.
To identify the cause of the inadequate behavior of the earthfill and to propose the remedial solutions, the topographical survey results were analyzed, as well as the compaction control reports of the earthfill, and a new geotechnical prospection survey was done (TECNASOL, 2011).
Figure 1. Site location, geological map and industrial plant layout with the studied area enhanced, “Zona A” area.
GEOTECHNICAL CHARACTERIZATION
The studied area is located in the Cretaceous formations, namely “Arenitos do Carrascal” (C1A), consisting of clayey sand with scattered small pebbles. This formation overlies the marly limestones and the marly clays’ units dated from Jurassic, namely “Calcários margosos e margas” (J) (Figura 1). Superficial deposits of clayey silts and scattered pebbles are also observed at the site location.
After the compacted earthfill’s completion, a geotechnical survey was executed including two percussion borings (S1 and S3) with systematic Standard Penetration Tests (SPT) and soil sampling.
Both borings intercept a top layer constituted by filling materials namely silty clay, grayish and brown, with scattered gray marly clays/marls. This earthfill is followed in depth by “in situ” gray marly clays/marls, from the Jurassic formations. According to the Unified Soil Classification and of the AASHTO Soil Classification the tested soil samples are SC-Clayey Sand and A–6 (2).
It was noticed that the SPT results vary significantly along the borings and, particularly, in S3 boring the lower SPT value was registered at the higher depth. The SPT results variation with depth suggests an inadequate execution and control of the earthfill.
Figure 2. Cross-section (NE-SW alignment, C-C’ in Figure 1) showing the natural terrain surface, the boring logs of the design geotechnical survey, the level of the platform, the excavation for the execution of the pit,
its slopes and area to refill and compact.
Table 1. Site location’s geotechnical parameters.
EARTHFILL BEHAVIOUR. SETTLEMENTS
Two months after the completion of the earthfill and the main structure, a rainy weather period caused a significant settlement of the earthfill and, in consequence, a displacement of the concrete structure.
A topographic survey system was implemented and survey marks were fixed in the foundation level elements of the concrete structure. The results showed the maximum structure displacement, due to the structure rigidity, was much smaller than the observed earthfill settlement. As a consequence some elements were submitted to excessive stress and, as the ultimate resistance was reached, some of them broke, namely the linking of the foundation beams to the footings.
Figure 3. Earthfill settlement and displacements and cracking in the beams.
REMEDIAL SOLUTIONS
Several solutions were studied, namely micropiles, reinforced concrete piles, and, at the limit, demolishing the complete structure, redesigning and rebuilding it.
The selected soil improvement technique considered the structural forces acting in the structure and included the execution of 96 jet grout columns with 600mm of diameter under the built footings. So, four borings were done in each of the footings, as represented both in plant and profile, in Figure 5.This solution allowed a quick completion at a reasonable cost and with an adequate reliability.
Three test columns were made to validate the chosen jet parameters and mix, and to guarantee that the strength and deformability parameters defined in design were reached. These test columns were carried out in the same area, so all the variables are similar. Considering that the compression tests were carried out at 14 days, it is reasonable to accept that, at 28 days, the limit values specified in project are attained.
Besides the remedial solutions, the structural elements were repaired in two ways: mostly, the cracking was repaired with epoxy resin; two of the foundation beams had to be demolished and rebuilt.
Figure 4. Earthfill’s settlement in alignments A1 and A11.
CONCLUSIONS
The execution of the jet grout columns, four for each footing, solved the problem of the structure itself, proving to be an efficient, quick and reasonably low cost solution for the remedial solution in this case.
After the remedial measures, the topographic survey results showed that the structural elements had recovered their previous deformation, that is, the structure displacement is now between 0 and +1mm.
An adequate control of compaction works by a geotechnical team would avoid these remedial works, and, perhaps more important, would have prevented the disturbance to the construction plan.
Figure 5. Earthfill’s settlement in alignments A1 and A11
Table 2. Compression tests results of the jet grout column test samples, at 14days