Bulletin of the Transilvania University of BraşovCIBv 2015 • Vol. 8 (57) Special Issue No. 1 - 2015

FOUNDATION SOLUTIONS OF A BUILDINGON SATURATED GROUND, COMPRESSIBLE

IN BRAŞOV CITY

M. MĂNTULESCU1 I. TUNS 1

Abstract: Construction of a building with 5 floors on saturated, verycompressible ground, without the possibility of water drainage, requiredfurther study and monitoring of its behavior in time. Avoiding differencialsettelments required cement injection in a rectangular grid with a side of1.50 m. It was found that injection consumed 60% more solution than thevolume of the borehole, which to a diameter of 30 centimeters, with anaverage porosity of 35%, highlighted a radius of 46 centimeters influential.This article aims to review the phases of research, execution and monitoringthe behavior of the construction.

Key words: soil investigation, soil improvement, uniforming the foundationsoil

1 Transilvania University, Faculty of Civil Engineering, Department of Civil Engineering

1. Introduction

The need of reusing built land in urbanareas often leads to the emergence ofdifficult situations from a geotechnicalconditions' point of view. Most of the timethere are, especially on the site of oldindustrial plants, infested land, networks,buried construction and so on.

The terrain in discussion is situated inBrasov and it is relatively flat, but it shouldbe noted that the current layout is the resultof urban improvements made over theyears. Until recently, bearing masonryconstruction up to four floors, some withbasement, has existed on site. The easternside of the terrain is bordered by the canal,currently covered with tiles. On theopposite side there is a canal throughwhich Timisul Sec river was conducted,and which has obvious flaws; the canal ismade by prefabricated slabs and protected

from exterior with masonry.This is the case of a proposed project onthe site of a former mill in Brasov. A flatbuilding with a height of B + GF + 6F willbe build there.

The construction will have the structurestrength made of reinforced concrete withbrick infill on isolated foundations underthe posts and continueunder the walls.

2. Geological ContextThe site in this case belongs, in view of

geomorphological punch, to the plainalluvial of Brasov.

The study area lies between Mill Hill andthe Timis canal, which is currentlycovered.

In terms of lithology, we find cohesivematerials (silt, silty sand) and noncohesivematerials such as gravel and sand (ofQuaternary age, which dominate the wholeBrasov depression).

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Inhomogeneous stratification is notretrieved as identical in any of the threepoints of investigation. The inhomogeneityof the soil has two causes: natural: the soil being made up of

accumulated proluvial at the downhill withcross-bedding laminate the soil was affected over time by

various municipal facilities or due toindustrial buildings.

On the ground surface we met a fillingblanket unorganized, unconsolidated, witha thickness of approximately 4 meters,consisting generally of topsoil, debris – ontop gravel, dust, etc.; we assume that thisfilling resulted from the demolition ofbuildings from the site and from urbanworks which were carried out in the area inthe past.

3. Direct Field Investigations

To identify the stratifications weexecuted two penetrations doubled byboreholes (see fig. 1), according to NP074-2014 [1]. The boreholes wereexecuted in depths up to 12.00 meters(positioned on the situation plane).

Fig. 1. Geotechnical surveys LocationPlan

Penetrations were performed with superheavy dynamic penetration facility SCPT73/75 - Deep Drill. The penetrometer is ofsuperheavy type, with the hammerweighing 73 kg. and a drop height of 75cm; the cone has a diameter of 5 cm and anangle of 60 °.

The penetrations results were processedwith the "WIN-DIN" application, providedby the instrument manufacturer.

Boreholes were studied and other workwas also performed in the area.The boreholes were performed by Borros8659 facility, with continuous coring, 112mm diameter. From the cohesive material,undisturbed samples were collected intoShelby tubes and standard dynamicpenetration tests (SPT) were executed inthe ground with coarse fragments.

The stratification found in boreholes asilustrated in figure 2 is as follows:• 0.00 to 4.30 m filling blackish color(topsoil, debris - the top, gravel, silt etc.);• 4.30 to 5.00 m gravel with reddish claysand filling appearance;• 5.00 to 6.00 m gravel with grey sand• 6.00 to 12.00 m red sandy clay plasticconsistent, wet. Between depths of 10.00to 10.40 m there is a dry yellow sand lens.

Fig. 2. The stratification identified ingeotechnical borehole

Groundwater under strong seepage wasmet between the depths of 5.20-6.00meters. The assumption that the presenceof this type of water is due to the existenceof the canal, which may leak (although it isconcreted in principle), is proved realduring some heavy rain that occurredduring the execution of the generalexcavation.

MANTULESCU, M., et al.: Foundation Solutions Of A Building On Saturated Ground… 115

Fig. 3. Image from the execution afterheavy rain

4. Investigation Results

As a consequence, several foundationsolutions were proposed see [4], [6]:

indirect foundation, on a thicknesscushion of ballast from 1.20 to 1.50 m,with continuous foundations;

direct foundation on a slab at adepth of 5.00 meters from ground level,partially on a layer of reddish sandy clay,partially on gravel in sandy clay matrix.

Throughout the execution, anincompressible rock (cretaciousconglomerate) was found in a corner of thesite on a relatively small area, at the depthof the foundation.

Therefore we need to avoid thedifferential settlements that exceedallowable limits imposed by the structure.These differential settlements can occurdue to great difference between thecompressibility of the rocky area and theproluvial deposits.

Several remedial actions for thissituation have been proposed:

- Execution of the construction in a slowrhythm, so that the primary settlementswould be consumed before the higherlevels are built;

- The excavation of the rocky area andits replacement with compressible

material from the rest of the foundationarea. This was proved difficult due theirproximity to an existing historicalmonument, especially due to the presenceof water.

- Improving the compressible groundwith cement injections, this last one beingagreed on by the customer.

5. Improvement soil works

The work of improving focused on thelayer of cohesive material with themaximum depth of 6,00 m. This layer wastargeted because it has a high permeabilitywhich makes the penetration of theinjected agent easier. On the other hand,there is a red clay with better mechanicalcharacteristics under this layer.

Fig. 4. Diagram of superheavy penetrationDIN2

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Fig.5. Description of borehole F1

Cement injection in a rectangular grid witha side of 1.50 m. was ordered.

The maximum depth that the injectionswere performed at exceeded the layer of

clay or rock by at least one meter.

Fig. 6. Cement injection plan

It was found that injection consumed60% more solution than the volume of theborehole, which to a diameter of 30centimeters, with an average porosity of35%, highlighted a radius of 46centimeters influential.

5. Conclusions

Estimated settlements of foundations,

Fig. 7. Geotechnical situation - sketch

MANTULESCU, M., et al.: Foundation Solutions Of A Building On Saturated Ground… 117

Fig 8. Image of the general excavation

considering the actual pressurecommunicated by the designer, led todifferential settlements of about 0.0025

between the eastern and western sides.

Fig 9. Settelments’distributions before andafter creating the cementblanket (principle sketch)

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The option for improving the foundationsoil that we have proposed, among otherpossible ones, was the fastest and thecheapest that we have found. Byimproving the soil and reducing itscompressibility by up to 66%, the relativesettlement is reduced by at least 0.0008.

By monitoring the building, we believethat we will find that the entire load wastransmitted to the stiff clay.

From our point of view, to interpret thedata mechanically from direct or indirectexploration, including laboratory tests, isquite superficial. In general, directinvestigations are made on a volume thatdoes not surpass a millionth of the quantityof foundation soil considered. Therefore,in addition to the works that help toestablish the foundation solution, we addedthe insight (understood as a synthetic resultof the accumulation of knowledge) of thegeotechnician or geologist. Extrapolationof the data obtained directly in the field orin the laboratory must be completed withthe aspect of intuition.

References

1. *** Preparation and verification ofGeotechnical ConstructionsDocumetation, NP 074-2014 –Part I

2. *** Eurocode 7: Geotechnical design.Part 1:General Rules, SR EN 1997-1:2006:

3. Stanciu A. şi Lungu I., (2006),Foundations-vol I, Ed. Tehnică,Bucureşti, 1620 p.

4. Păunescu M., Viţă I., Scordaliu I.,(1990), Mechanisation of works forImproving Foundations Soil, Ed.Tehnică

5. Măntulescu M., Tuns I., A real estatefoundation on improved ground in aresidential area in Braşov,Proceedings of the InternationalConference DEDUCON - SustainableDevelopment in Civil Engineering, Iasi2011

6. Lambert S., Rocher-Lacoste F., LeKouby A.(2012), Soil-cement columns,an alternative soil improvementmethod, ISSMGE - TC 211International Symposium on GroundImprovement IS-GI Brussels 31 May &1 June 2012.

Fig. 10. Image during the building of theinitial levels