Bulletin of the Transilvania University of Braşov CIBv 2015 • Vol. 8 (57) Special Issue No. 1 - 2015 LAND SLIDE STABILIZATION FOR A WORKING PLATFORM WHICH OCCURRED DURING EXCAVATION IN CLUJ-NAPOCA I. ABRUDAN 1 C. PLESCAN 2 Abstract: The present study contains the technical data and design method, with details, regarding the stabilization of a landslide using drilled piles on a site from Faget area in Cluj-Napoca where a family home is to be constructed. A landslide occurred during the excavations on a slope through which drainage was oriented from the area, without former measures to avoid such phenomena. In order to regain the slope's stability and resuming the project, it was proposed a consolidation solution to prevent any further landslides and a safe way for the building process. In this paper it is presented the slope analysis method and design method for the landslide stabilization with some aspects regarding the final technical solution. Key words: landslide stabilization, drainage, slope, piles. 1 Faculty of Civil Engineering, Technical University of Cluj-Napoca. 2 Civil Engineering Department, Transilvania University of Braşov. 1. Introduction A home with ground floor and one floor up will be constructed on the site from Cluj-Napoca Faget, 34C D.D. Rosca Street. After the foundation excavation for the building was made, a landslide took place which affected the respective place and the neighboring parcels from upstream. In order to stabilize the landslide was proposed a pile wall upstream of the excavation to support the earth and a building located nearby and to prevent the extension of the landslide. A new geotechnical survey was necessary for the design of the stabilization solution. The landslide stabilization design method for combines slope stability analysis with the piled wall design using the subgrade of reaction modulus method. 2. Site Description The site in question is located in Cluj- Napoca in Faget area in downstream slope of the road Cluj-Napoca - Ciurila, behind the former Faget camping. The building to be made is a residential home (Figure 1). From a geomorphological point of view the site is part of the Transylvanian Depression and the central-eastern side of the Someșan Plateau. The landscape is predominantly hilly. The whole slope is
Transcript
Bulletin of the Transilvania University of BraşovCIBv 2015 • Vol. 8 (57) Special Issue No. 1 - 2015
LAND SLIDE STABILIZATION FOR AWORKING PLATFORM WHICH
OCCURRED DURING EXCAVATION INCLUJ-NAPOCA
I. ABRUDAN1 C. PLESCAN2
Abstract: The present study contains the technical data and design method,with details, regarding the stabilization of a landslide using drilled piles on asite from Faget area in Cluj-Napoca where a family home is to beconstructed. A landslide occurred during the excavations on a slope throughwhich drainage was oriented from the area, without former measures toavoid such phenomena. In order to regain the slope's stability and resumingthe project, it was proposed a consolidation solution to prevent any furtherlandslides and a safe way for the building process. In this paper it ispresented the slope analysis method and design method for the landslidestabilization with some aspects regarding the final technical solution.
1 Faculty of Civil Engineering, Technical University of Cluj-Napoca.2 Civil Engineering Department, Transilvania University of Braşov.
1. Introduction
A home with ground floor and one floorup will be constructed on the site fromCluj-Napoca Faget, 34C D.D. RoscaStreet. After the foundation excavation forthe building was made, a landslide tookplace which affected the respective placeand the neighboring parcels fromupstream.
In order to stabilize the landslide wasproposed a pile wall upstream of theexcavation to support the earth and abuilding located nearby and to prevent theextension of the landslide. A newgeotechnical survey was necessary for thedesign of the stabilization solution.
The landslide stabilization designmethod for combines slope stabilityanalysis with the piled wall design usingthe subgrade of reaction modulus method.
2. Site Description
The site in question is located in Cluj-Napoca in Faget area in downstream slopeof the road Cluj-Napoca - Ciurila, behindthe former Faget camping. The building tobe made is a residential home (Figure 1).From a geomorphological point of viewthe site is part of the TransylvanianDepression and the central-eastern side ofthe Someșan Plateau. The landscape ispredominantly hilly. The whole slope is
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covered with new buildings. The site islocated on the middle of the slope. Theinclination of the slope is approx. 10°. Atthe upstream limit of the site there is asteeper slope, with an approximated heightof 1.50 m, formed artificially by earthdepositing. The general inclination of theslope is about 12°. It is important to noticethat despite the inclination of the slopefrom south-east to north-west, thesurrounding terrains have an inclinationtowards the site in question, which makesthe meteoric water to line up on this terrainnot only from upstream but also from theside lanes [5]. Based on previousobservations of the terrain in upstream ofthe new construction it has been noted thatsince 3 years ago the terrain has alwaysbeen saturated with water resulted from theexcavations and drainages from theterrains upstream. In the upper right cornerupstream there is a home with two pipes
for draining water. There is also anevacuation pipe from this place but thehome shows signs of damage so the waterfalls onto the terrain. Furthermore, thevegetation here corresponds to a swampyone and the terrain is always soaked. Fromfurther observations it was noticed that 2-3years ago the drainages from upstreamwere continued on the investigated terrainand the water was eliminated. Besidesthese facts from the geotechnical surveywere not intercepted any water levels. Onthe left side a drainage tube has truly beenintercepted, draining the water downstreamfrom the terrain.
The foundation system is a raft beneaththe building with a thickness of 35cm. Inorder to build the raft a general excavationwas made, with an average height of 3meters and peaks about 4.5 meters withslopes about 70 to 80 degrees.
Fig. 1. Building and consolidation plan
I. ABRUDAN & C. PLESCAN: Land Slide Stabilization for a Working Platform ... 181
In short time after the excavationbegan the earth started to move from theright upstream of the excavation. The earthmovement was caused by the bigexcavation height, but the area of thelandslide was much bigger than the slidecaused by excavating using big slopes.Upstream from the new building locationwas a small swamp that was probably theremains of a former landslide in theneighbor area. In the nearby region of theexcavation the wooden floor from theground level of a coop was showing signsof recent movements, the fence betweenneighboring properties were showing signsof movements and some cracks started toappear in the soil. The landslide area wasvery close to some buildings in the area,but none of these were showing signs ofmovement or cracks.
The length of the landslide affectedarea is about 20m, from which 10m on theupstream terrain. The width of the affectedarea is about 35 m from which 8m are onthe right side neighbouring terrain. Afterthe landslide was produced, the constructorfilled the excavation with earth. After thisfill the landslide did not evolvesignificantly.
According to the geotechnical surveydata, two types of soil layers have beenintercepted: from the terrain level up toabout 3.5m is a silty brown-yellowconsistent clay, from 3.5m depth up to thetoe of the drilling a silty brown-grey firmclay was found. After the landslide
occurred five more dynamic penetrationswere made. The penetration diagrams wereindicating a weak soil layer for about 3min depth, which led to the conclusion thatthis could be the possible depth of thelandslide. The average layer depths andsoil characteristics are shown in Table 1.
3. Slope Stability Analysis
The usual pattern followed toconsolidate a slope with potential of alandslide is to analyze the slope stability inorder to find the sliding surface with thebiggest potential [4]. This potential isusually expressed by the safety factor (Fs)which represents the ratio between theresistive forces/ rotating moments anddestabilizing forces/rotating moments,depending on the method of analysis. Ifthis safety factor is showing values under1.0 it’s a clear sign of slope instability butin current practice this value is limited to aminimum of 1.4÷1.5 in order tocompensate the lack of knowledge or somedestabilizing matters
that somehow could be overlooked orneed further complex investigations whichare time and cost ineffective.
For a slope that could be affected by alandslide it is necessary to know mainlythe shear strength of the soil, expressedthrough shear strength parameters: theinternal friction angle Φ and cohesion c.
The average layer depths and soil characteristics from geotechnical survey Table 1
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If the stability analysis is made for aslope on which the landslide did not takeplace, the shear strength parameter valuesused are those determined by the attempts,namely medium or peak values. In the caseof a landslide that already took place theslope analysis is made by means of socalled residual values for the Φ and cparameters which are of course lower thanthe peak or medium ones. Generally thevalues could be introduced as Φr=(1/2÷1)Φ and cr= (0÷1/2)c. Since thelandslide occurred on the site, the slopestability analysis was made using theresidual values as shown in Table 2.
The slope stability analysis in this casewas made for proposed known surfaces thatcould occur during landslide by means ofblocks method. This method consists individing the sliding terrain in vertical blocksand to set for each block of the active forceand the resisting force. The ratio betweenthe sum of the resisting forces and the sumof the active forces of all the blocks is thecoefficient of safety at sliding, and thepushing force of the sliding terrain is thedifference between these forces.
The results of slope stability for differentproposed sliding surfaces are presented inTable 3.
As shown in Table 3 the minimum safetyfactor was obtained in case 5 – slip surface(Fs=0.68) and it is lower than 1.0 whichshows the clear sign of landslide.
4. Consolidation Solution
The method used for slope stabilityanalysis provides the value for safety factorand the active force that the retainingstructure should support [1, 3, 6]. As aretaining structure was proposed, a pile wallwith piles anchored in the rigid soil layer atthe toe and connected at the pile head with araft/beam. From the stability analysis theearth active force acting on the piles isabout 315 kN/m, and it was obtained fromthe lowest safety factor. The piles are600mm in diameter and 12m long withspacing between them of 1.2m÷2.0m toallow water drainage (Figure 2).
The design method for the pile wall isusing the subgrade of reaction modulus. Byusing this method the forward results arepresented: horizontal displacements of thepile wall, horizontal pressure in thefoundation soil, shearing force and bendingmoments along the pile [1, 3,7].
The average layer depths and residual soil characteristics Table 2
No. Layer description
Depth fromterrain level
[m]
Soil characteristics
γ[kN/m3]
cr’[kN/m2]
ɸr’ [ º
]1 silty clay, consistent,
brown-yellow0.00÷3.50m 19.7 5 7
2 silty clay, soft consistent ,brown-yellow
3.50m÷4.00m
17.9 5 7
3 silty clay, firm, brown-grey
4.00m÷8.00m
20.7 35 15
I. ABRUDAN & C. PLESCAN: Land Slide Stabilization for a Working Platform ... 183
Safety factor for proposed slip surface Table 3
No.
SafetyFactor
(Fs)
Slip Surface
1 0.98
crack1
2
Case 1natural slope
excavation slope
2 0.81
crack
1
Case 2natural slope
excavation slope
3 0.95
crackCase 31
natural slope
excavation slope
4 0.771
2
natural slope
excavation slope
Case 4 crack
5 0.681
2
natural slope
excavation slope
Case 5 crack
6 0.731
2
Case 6natural slope
excavation slope
crack
7 0.95 1
Case 7natural slope
excavation slope
crack
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By using these outputs the reinforcedconcrete elements were designed to sustainthe earth active force.
The consolidation solution for theworking platform also included a drainagesystem (Figure 3) that should collect therain water from neighbor areas and possiblewater flow from underground [2].
The consolidation solution also includes achange of the foundation system for thenew building to prevent any furtherdamages. The new foundation systemconsists of micropiles or piles anchored inthe rigid soil layer.
Fig. 2. Pattern for the consolidation with drilled piles
I. ABRUDAN & C. PLESCAN: Land Slide Stabilization for a Working Platform ... 185
Fig. 3. Drainage system plan
5. Conclusions
The presented landslide occurred duringthe excavations for a new buildingconstruction. The surrounding area wasshowing signs of water accumulation in soiland swamp vegetation. Due to the smallslope on the site the landslide risk analysiswas never a major priority in the designproject.
The main cause of the landslide was theexcavation with a big slope that easilyactivated the instability and the phenomenawas extended to the areas nearby not onlyon a small area near the excavation. Theunderground water current had also a majorimpact in the activation of the landslide.
Some earthworks need more attentionfrom design to build and are not over
conservative to look and overlook at thepossible problems that could occur on thesite.
Due to the landslide activation thebuilding cost over exceeded the initialestimation and now the whole process isstopped.
ACKNOWLEDGEMENT
This paper is supported by SectorialOperational Programme HumanResources DevelopmentPOSTDRU/159/1.5/s/137516 financedfrom the European Social Fund and byRomanian Government.
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