Geotechnical investigations on the Zagrad location in Rijeka, Croatia

Ž.ArbanasCivil Engineering Institute of Croatia, Department of Rijeka, Croatia, Faculty of Civil Engineering, University in Rijeka, Croatia

M.S.Kova eviFaculty of Civil Engineering, University in Zagreb, Croatia

B.JardasCivil Engineering Institute of Croatia, Department of Rijeka, Croatia

Keywords: site investigations, geophysics, excavation, open pit, rock support, active design

ABSTRACT: On the Zagrad location, in the very vicinity of the Rijeka city center, the garage-accommodation-business complex is under construction, layout dimensions 90 x 60 m. The geotechnical con-ditions on site are very complex: on the lateral sides limestone rock-mass protrudes to the surface while in the central part of the location is a sinkhole with thick clay deposits. Regarding the location morphology, the con-struction pit being 2 m deep on the northern end of the location to the 5 m depth on the southern end - adja-cent to the existing buildings and a traffic line-was designed. For the designing of the excavation, the supportsystem and the building foundation design, a complex geotechnical investigation was performed. These inves-tigation works included borehole drilling, geophysical measurements, laboratory testing of rock and soil specimens. In the northern end of the location, having the highest cut in the carbonate and flysch rock mass,investigation boreholes and seismic refraction were carried out, along the future excavation pit contours. Theanalysis of the refraction data was made by the inverse modeling (Delta T-V method). The results obtained by these methods were used to determine the geotechnical profile, the contacts between flysch deposits and car-bonate rock mass. On the same basis the rock mass parameters for the walls of the future excavation pit wereevaluated and used for the rock support design. On the lowest, southern, part of the site the main problem en-countered was determining the location of the bedrock on which the foundation of the building was foreseen.On that part of the site investigation boreholes were drilled and ground probing radar scanning performed.This was allowed to determine cover deposits, weathered zones and basic rock masses in the soil. On the basis of the data collected, pit excavation methodology, rock support and foundation design options for that part ofthe site are taken. Due to the great variations in the cover thickness, the building foundation was designed as acombination of shallow foundations on easy-to-reach carbonate rock base, and deep foundations on top-loaded drilled piles. During the construction of the pit and building foundations, the design assumptions wereadapted according to the in situ situation.

INTRODUCTION

The City of Rijeka is the second town in Croatia and the biggest port in the Adriatic Sea. On the Zagrad location, in the very vicinity of the Rijeka city cen-ter, a garage-accommodation-business complex is under construction. The first phase of construction, the six store underground garage, is already fin-ished.

The construction location is closely surrounded by the existing accommodation buildings, having up to seven stories, and a traffic line in the north and a railway in the south. The site geotechnical condi-tions are very complex: laterally limestone rock-mass protrudes to the surface while, in the central part of the location there is a sinkhole with thick clay deposits. The northern side of the site contacts with limestone and flysch deposits. The complexity

of the geotechnical conditions explains the fact that the majority of the site, situated almost at the center of the city, was not used for construction purposes.

Investigated location has an inclined sinkhole shape, running in the SW-NE direction. The natural terrain surface was significantly changed by several centuries of construction in Rijeka. Due to cuttings, and especially filling, the terrain has a cascade-like shape. The elevation above the sea level in the sink-hole ranges from 6.0 to 7.5 m, and the elevation of the traffic line above the construction site is 24 m a.s.l. Very high underground water level exists on the site due to the great inflow from the mainland towards the sea level.

The construction of the garage-accommodation-business complex (layout dimensions 90 x 60 m) was designed for that location. Regarding the loca-

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Proceedings ISC-2 on Geotechnical and Geophysical Site Characterization, Viana da Fonseca & Mayne (eds.)© 2004 Millpress, Rotterdam, ISBN 90 5966 009 9

tion morphology, the construction pit was predicted to 2 m deep on the northern end of the location to 5 m deep on the southern end, adjacent to the existing buildings and a traffic line. Due to the complex ground conditions and unaccepted cover thickness variations in the southern part of the location, the structure of the building had to be adjusted accord-ing to these conditions and inherently the solution for foundations. The high water table conditioned the ground level elevation of the building.

2 GEOTECHNICAL INVESTIGATION WORKS

For the design of the excavation, the support system and the building foundation system, a complex geo-technical investigation works were performed. These works were carried out in two phases: for a prelimi-nary design and for the final execution solution. Geotechnical investigation works included borehole

drilling, geophysical measurements, laboratory test-ing of rock and soil specimens, engineering-geological and geotechnical works. The data ob-tained from the former geotechnical investigations database for this location was also used. The follow-ing was stated: the location soil-profile consists of the cover layer (fill and clay) and carbonatic rock base, represented by the dolomite limestone from the Upper Cretaceous and a completely weathered flysch-sandstones from the Paleogene. The base rock protrusions were visible only on several places.

In the northern end of the location, having the highest cut in the carbonate and flysch rock mass, investigation boreholes and seismic refraction were carried out, along the future excavation pit contours, Figure . The analysis of the refraction data was made by the inverse modeling (Delta T-V method).

Figure . Engineering geological map of Zagrad location ( . Level m a.s.l.; 2. Borehole location; 3. Limestone visible on terrain; 4. Flysch deposits; 5. Cover deposits; 6. Geological border between deposits; 7. Seizmic refraction profile; 8. Ground probing radar profile; 9. Structure of open pit; 0. Cross-section on cut “North”)

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The results acquired by these methods were used to determine the geotechnical profile, the contacts between flysch deposits and carbonate rock mass, Figure 2. On the same basis the rock mass parame-ters for the walls of the future excavation pit were evaluated and used for the rock support design.

On the lowest, southern, part of the site the main problem encountered was to determine the location of the bedrock on which the foundation of the build-ing was foreseen. On that part of the site, investiga-tion boreholes were drilled and ground probing radar scanning performed, Figure . Ground probing radar scanning has allowed to determine cover deposits, weathered zones and basic rock masses in the soil. On the basis of the data collected, pit excavation de-sign, rock support design and foundation design for that part of the site were developed. Due to the great variations in the cover thickness, the building foun-dation was designed as a combination of shallow foundations on easy-to-reach carbonate rock base, and deep foundations on top-loaded drilled piles.

3 CONSTRUCTION

On the basis of the results of geotechnical investiga-tion works, design of open pit and foundation as well as an construction of open pit started. The main

problems during design and construction were rock support in north part and foundation in south part of open pit where deep clay cover deposits were lo-cated.

In northern part of location, a hill of existing traf-fic line on Pomerio St. is on +24.00 m a.s.l. The south part of traffic line is secured by existing stone retaining wall up to 7.00 m height founded on a ter-race on 7.00 m a.s.l. In this part of location excava-tion on +3.20 m a.s. level was predicted. Left and right near prediction excavation, seven stories build-ing were founded on the + 7.00 m a.s.l. On the basis of results of performed stability analysis and stress-strain analysis the main design for rock support sys-tem was performed and then started the open pit ex-cavation, Figure 3.

In the main design of open pit support systems during the excavation and insuring a stability of open pit walls an active design procedure was pre-dicted trough designer’s supervision of executed works (Arbanas, 2002). The active design procedure was predicted on the basis of observational methods in geotechnics (Kova evi , 2003).

Figure 2. Refraction seismic profile RF-

1417Proceedings ISCʼ2 on Geotechnical and Geophysical Site Characterization, Viana da Fonseca & Mayne (eds.)

Construction on cut ”North”, as the most compli-cated geotechnical construction of the open pit, were performed by excavation in phases, in longitudinal stories of 2.0 m height and a successive construction of a grid support system reinforced by a self-drilling rock bolts from top to bottom of the excavation, Figure 4 (Arbanas, 2003). A measuring, observing and monitoring system for the substructure system behavior was established, Figure 4. The monitoring and observing system included observations of a geodetic marks mesh, which were set successively with works execution, on totally eight geodetic con-trol profiles as well as measurements of displace-ments on two vertical inclinometers-extensometers (deformeters) and three horizontal extensometers (deformeters) which were set on locations that en-able observation of cuts during the works execution. Geodetic observations as well as measurements of displacements on inclinometers and extensometers were performed by phases, according to calculated phases of excavations. A testing of rock strength was performed (ISRM, 979; ISRM, 98 ) on samples, which were taken from the excavated material. After any phase of excavation was performed an engineer-ing-geological mapping of rock mass in cut.

Figure 3. A view of open pit Zagrad during excavation

Performing of stress-strain back analysis, based on the measured deformations and performed tests of bearing capacity of rock bolts, enabled observa-tion and prediction of rock mass behavior in cut, in the future phases of excavation. On the basis of per-formed stress-strain back analysis was possible to make changes in the rock mass reinforcement sys-tem during excavation. Designed works on the cut “North”, were performed with the minor interven-tions in secondary and tertiary reinforcements (Win-dsor and Thompson, 996) in the support system within designed measures of rock mass reinforce-ment. Primary reinforcement was doubled in the area of flysch. The engineering-geological map of rock mass in cut “North” is present on Figure 5. There is visible significant difference real state and

prognosis obtained from geophysical measurements, Figure 2, in west part of cut “North”.

Performed measurements indicated some varia-tions of design values of the security of rock cuts re-inforced by rock bolts from real behavior of in situmade constructions. That is primarily referring to a magnitude of real deformations in the limestone rock mass comparing to values calculated on the base of recommended values of deformation characteristics, which are based on a correlation with values from rock mass classification (Bieniawski, 979; Hoek and Brown, 980; Serafim et al., 983; Hoek and Brown, 997). All correlations are performed from geotechnical (RMR) classifications of rock mass, unaxial compressive strength of rock mass and geo-logical strength index (GSI) of rock mass. The de-sign prognosis was obtained from Hoek and Brown ( 997) relation.

Figure 4. Cross-section of the cut “North” (Arbanas et all., 2004)

All proposed calculations gave higher values of Young’s elasticity modulus for rock mass comparing to values obtained by back analysis that is based on a measured displacements during the excavation. Back stress-strain analysis resulted in values of Young’s elasticity modulus for limestone rock mass, that were 20 and more times lower then those ob-tained from correlations with classifications of rock mass (Hoek and Brown, 997). Those differences are most likely the result of inadequate correlations for the lowest values of RMR classification of rock mass because of a small base of measured data in that area. Future development of those correlations

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for fractured rock mass, with low strength, such as limestone, should be based on the large base of data, obtained by a back analysis from in situ measure-ments. (Arbanas et al., 2004).

4 CONCLUSION

On Zagrad location, in the very vicinity of the Ri-jeka city center, a garage-accommodation-business complex is under construction. For the design of the excavation, the support system and the building foundation, a complex geotechnical investigation were conducted. During the construction of the pit and building foundations, the design assumptions were adapted according to the in situ situation. Dur-ing the excavation of open pit it was realised that the real state of geological fabric on open pit cuts is dif-ferent of the predicted by geotechnical investigation works. Due to the great variations in the cover thickness in the southern part of location, the build-ing foundation was designed as a combination of shallow foundations on easy-to-reach limestone bed-rock, and deep foundations on top-loaded drilled piles.

In complex geotechnical conditions, like these complex geotechnical constructions during excava-tion of open pit, geotechnical investigation works should be only a general base for design. Durng the construction, it is necessary to perform second phase of design, so called active design. During excavation and rock mass supporting it is necessary to establish the monitoring system to ensure information data about rock mass behavior.

For weathered and altered limestone, poor rock mass in RMR classification, geotechnical investiga-tion does not provide a data accountable for execut-ing acceptable stress-strain analysis. For performing stress-strain analysis it is necessary to adopt an ob-servational method based on geotechnical monitor-ing. On the basis of results of measuring and observ-ing during executing of works, like as geological mapping and classifications of rock mass in cuts, it is possible through active design to perform back stress-strain analysis and make necessary changes in rock mass support systems.

Figure 5. Engineering geological map of cut “North” (I.Flysch deposit zone GSI=20-35; II.Limestone GSI=35-45; III.Limestone GSI=45-55)

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