S. Marcos’ Road, Vila Franca de Xira - Slope StabilizationSolutions

Teresa Margarida Duarte Lopesteresamdlopes@ist.utl.pt

Instituto Superior Técnico, Lisboa, Portugal

October 2014

Abstract

Every year, in Portugal, specially during the rainy periods, numerous landslides occur,many with high social and economic cost. There are a lot of solutions for slope stabilizationapplied in road embankments, so it is intended to describe the main causes and the appro-priate solutions to the various types of slope instability. In particular, the present case study,São Marcos’ road, in Calhandriz, allows to exemplify the procedures inherent to the designand implementation of a stabilization project. Included in this analysis there are performingstabilization work, excavation, deep foundations, embankment and drainage. The adoptedsolutions are diverse, including piles, micropiles, retaining walls, and an extensive network ofdrainage devices. In addition to description of the involved processes are, also, presentedthe theoretical fundamentals for understanding the used techniques, performing a qualitativeand comparative assessment of the various solutions adopted and designed. The solutionimplemented in about 100 m of the restored road are divided into four type sections, whichtells us a lot about the specificity and complexity of the intervention, as well as the extremeimportance of the development and successful implementation of a instrumentation andobservation plan, both during the stabilization works and during their operation. Finally, it ispresented a numerical modeling of the solution as well as the design of two other alternatives.

Keywords: landslide, earth retaining structures, slope stabilization, instrumentation

1. Introduction

S. Marcos’ road was completed in 2007. Thisroad made the connection between EN (Na-tional Road) 10-6 and the Lugar da Igreja inCalhandriz. In February 2010, a geotechni-cal accident results in the partial destructionof the road platform between 0 + 325 Km and0 + 400 Km, so as can be seen at figure 1.

In 2011, with the aim of containing the insta-bility effects, mitigating measures were imple-mented, such as a new survey and geotechni-cal campaign, earthworks for the removal ofthe remain road embankments, shallow and

deep drainage as well as the installation of in-strumentation (inclinometers). Due the dead-line, it was adopted a stabilization solution us-ing bored piles, prefabricated tray and rein-forced embankment.

2. Slope Stability

The main causes of slope instability can be di-vided into three types, External, Internal andIntermediate causes.

• External causes - prolonged and/or in-tense rainfall, earthquake, excavation ofslope or its toe, loading of slope or its

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(a) Perspective 1. (b) Perspective 2.

Figure 1: Geotechnical accident in 2010.

crest, such as placing earth fill at the topof a slope and artificial vibration such aspile driving, explosions, or other strongground vibrations.

• Internal causes - Weak, susceptible,weathered and sheared materials, jointedor fissured materials, adversely orientedmass and stuctural discontinuity, con-trast in permeability and/or stiffness (stiff,dense material over plastic materials) andincreases in pore water pressure.

• Intermediate causes - Subterranean ero-sion (piping),rapid changes in the ground-water level and liquefaction.

In the following table shows a schematic land-slide classification adopting the classificationof Varnes (1978), which are briefly describebelow [8].

Figure 2: Classification of Landslides sug-gested by Varnes (1978). [10]

2.1. Fall

A fall begins with the detachment of soil orrock, or both, from a steep slope along a sur-face on which little or no shear displacementhas occurred. The material subsequently de-scends mainly by falling, bouncing, or rolling.Falls are abrupt, downward movements of rock

or earth, or both, that detach from steep slopesor cliffs. The falling material usually strikes thelower slope at angles less than the angle offall, causing bouncing. The falling mass maybreak on impact, may begin rolling on steeperslopes, and may continue until the terrain flat-tens.

2.2. Topple

A topple is recognized as the forward rota-tion out of a slope of a mass of soil or rockaround a point or axis below the center of grav-ity of the displaced mass. Toppling is some-times driven by gravity exerted by the weightof material up slope from the displaced mass.Sometimes toppling is due to water or ice incracks in the mass. Topples can consist ofrock, debris (coarse material), or earth mate-rials (fine grained material). Topples can becomplex and composite.

2.3. Slide

A slide is a downslope movement of a soil orrock mass occurring on surfaces of ruptureor on relatively thin zones of intense shearstrain. Movement does not initially occur si-multaneously over the whole of what eventu-ally becomes the surface of rupture; the vol-ume of displacing material enlarges from anarea of local failure. A rotacional landslide hasa curved upward surface of rupture(spoon-shaped) and the this slide movement is moreor less rotational about an axis that is paral-lel to the contour of the slope. The mass ina translational landslide moves out, or downand outward, along a relatively planar surfacewith little rotational movement or backward tilt-ing. This type of slide may progress over con-siderable distances if the surface of ruptureis sufficiently inclined, in contrast to rotationalslides, which tend to restore the slide equilib-rium. Translational slides commonly fail alonggeologic discontinuities such as faults, joints,bedding surfaces, or the contact between rockand soil.

2.4. Spreads

An extension of a cohesive soil or rock masscombined with the general subsidence of thefractured mass of cohesive material into softerunderlying material. Spreads may result from

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liquefaction or flow (and extrusion) of the softerunderlying material. Types of spreads includeblock spreads, liquefaction spreads, and lat-eral spreads.

2.5. Flows

A flow is a spatially continuous movement inwhich the surfaces of shear are short-lived,closely spaced, and usually not preserved.The component velocities in the displacingmass of a flow resemble those in a viscous liq-uid. Often, there is a gradation of change fromslides to flows, depending on the water con-tent, mobility, and evolution of the movement.

Flow A form of rapid mass movement inwhich loose soil, rock and sometimes organicmatter combine with water to form a slurry thatflows downslope. They have been informallyand inappropriately called “mudslides” due tothe large quantity of fine material that may bepresent in the flow. Occasionally, as a rota-tional or translational slide gains velocity andthe internal mass loses cohesion or gains wa-ter, it may evolve into a debris flow.

Debris Avalanche Essentially a large, ex-tremely rapid, often open-slope flows formedwhen an unstable slope collapses and theresulting fragmented debris is rapidly trans-ported away from the slope.

Earth Flows This can occur on gentle tomoderate slopes, generally in fine-grained soil,commonly clay or silt, but also in very weath-ered, clay-bearing bedrock. The mass in anearthflow moves as a plastic or viscous flowwith strong internal deformation.

Creep It is the informal name for a slowearthflow and consists of the imperceptiblyslow, steady downward movement of slope-forming soil or rock. Movement is caused byinternal shear stress sufficient to cause defor-mation but insufficient to cause failure.

2.6. Slope stabilization solutions

Usually, slope stabilization techniques can bedivided into five groups:

• Slope stabilization by changing the ge-ometry - Removal of soil from the headof a slide, reducing the height of theslope, backfilling with lightweight material,benches and flattening or reducing slopeangle.

• Slope stabilization using inclusions - An-chors, networks of micropiles, piles, nail-ing and shortcrete, geogrids, cellularfaces and jet grouting.

• Slope stabilization using retaining walls -Gabion walls, reinforced earth wall, tun-nels, flexible retaining walls and multi-anchored retaining walls.

• Slope stabilization using drainage - Shal-low and deep drainage, rock-fill but-tresses.

• Slope stabilization using vegetation - Hy-droseeding and plant cover.

3. Case Study

Calhandriz is a place with a long history ofgeotechnical accidents and widely studied.Were identified in the parish of Calhandriz,with an area of 11.3 km2, in 2002, 144 land-slides and total unstable area of 1 100 000 m2,ie, 9.9% the total area of the parish. Thedensity of landslides is 12.7 per km2. The 144movements identified 67% were classified asbeing active. [13]

Then it is enumerated and described the mostrelevant geotechnical accidents in Calhandrizzone.

• November 1968 - This period is indicatedby the local population, to define the pe-riod of activation of translational landslidein the area-sample Calhandriz. [12]

• March 1978 - In the month of March isreported by the population a translationallandslide on the Silveira riverside, nearAdanaia (area-sample Calhandriz), whichreached the river channel and destroyedthe road linking the Alverca to Arruda dosVinhos. [12]

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• February 1979 - After a period of intenserainfall, which led to soil degradation, trig-gered a large number of slope move-ments. The most significant occurredin the Calhandriz, Adanaia, Quebradas(Fanhões) and Albogas. [12]

The main cause of these landslides, andextensively studied and described in [12] ,has been the rainfall. Then are some graphsillustrating the rainfall in the period precedingthe accidents. In all of them there are incommon a period of low rainfall in the monthbefore the movements then followed by aperiod of heavy rainfall in a short time. Seefigures 3, 4, 5 and 6.

Figure 3: Daily rainfall in S.Julião Tojal be-tween September 1968 and December 1968.[1].

Figure 4: Daily rainfall in S.Julião Tojal be-tween January 1978 and April 1978. [1].

Figure 5: Daily rainfall in S.Julião Tojal be-tween September 1978 and March 1979. [12]

Figure 6: Daily rainfall in S.Julião Tojal be-tween January 2010 and March 2010. [1].

3.1. Geologic and Geotechnical Scenario

Based on the analysis and interpretation of theresults obtained in several geotechincal inves-tigations campaigns, and with the objective ofobtaining the information needed to study thestability of the landslide area, allowed to definethree geotechnical zones, ZG3, ZG2 and ZG1.Geotechnical Zone 3 - The ZG3, compris-ing landfill deposits and thicknesses between0.90 m and 13.50 m. For the SPT tests resultsare between 5 and 60, with the most frequentvalues between 13 and 23.Geotechnical Zone 2 - This geotechnical areacorresponds mostly to clay layers belongingto the formation of Margas da Abadia. TheZG2 presents thicknesses between 1.50 mand 7.35 m. The values for STP test lies be-tween 8 and 53.Geotechnical Zone 1 - This geotechnical areahas the better geotechnical characteristics.Corresponds to the formation Margas da Aba-dia with SPT values always higher than 40.

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3.2. Vicinity Constrains

On the right side of S. Marcos’s Road there aretwo small houses and on the left side there is anational road (EN10-6) that connects Alvercato Arruda dos Vinhos. See figure 7.

Figure 7: Vicinity constrains.

3.3. Adopted Solution

The executed solution is divided into 4 sub so-lutions, ST1, ST2, ST4 and ST4.The ST1 (Section Type 1) is located in thenorth, near the intersection with EN 10-6 andhave not been directly affected by the insta-bility. This solution consists of 800 mm boredpiles, 5 m spaced, ensuring a minimum of 5 membedding in competent and geologically sta-ble layer (ZG1). Then was executed a longi-tudinal beam that connects every piles and,over the beam is a reinforced concrete re-taining wall. Inside the beam were executedmicropiles with outer joints, in order to en-sure stabilization and nailing. The length ofthese should be such that it ensures a sealinglength of 6 m inside ZG1. Finally, a lightweightlandfill reinforced with a geotextile and ge-ogrid. To ensure a correct drainage, were ex-ecuted deep drainage trenches placed abovethe lightweight landfill.The ST2 (Section Type 2) is located next toST1 and the zone where the instability phe-nomena had greater amplitude. This solutionconsists on frames, each 5 m, supported bytwo piles of 600 mm, one at each end, anda central pile of 800 mm equidistant from the600 mm piles. Then was executed a longi-tudinal beam that connects every piles and,over the beam is a reinforced concrete re-

Figure 8: ST1 solution.

taining wall. A transverse beam connectsthe three piles for each alignment. Finally, alightweight landfill reinforced with a geotextileand geogrid. To ensure a correct drainage,deep drainage trenches were placed abovethe lightweight landfill.

Figure 9: ST2 solution.

The ST3 (Section Type 3) is located on thesouth side, next to the small building and wasnot directly affected by the instability of theslope. The ST3 solution is very similar to theST1 but piles have 600 mm and are spaced2.5 m. To ensure the slope stability near to thesmall building a gabion wall was executed.

The ST4 (Section Type 4) is located betweenST2 and ST3, where the phenomena of insta-bility had a higher amplitude. The solution forthis section was the constrution of viaduct with60 m in length, as can be seen in figure 11,supported by 800 mm bored piles.

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Figure 10: ST3 solution.

Figure 11: ST4 solution.

3.4. Monitoring and Survey Plan

The monitoring and survey plan is an impor-tant tool for prevention and risk management,aiming to ensure the constrution in conditionsof safety and economy of all works, as wellas the analysis of the behavior of the slopeand the structures and neighboring infrastruc-ture during and after the execution of the work.This plan was determined based on the anal-ysis of the main constraints. The analysisof these, as well as its likelihood allowed thequantification of the major risks associatedwith the execution of the work.During the execution of the work it was pro-posed to monitoring of the following quantities:

(a) Horizontal and vertical displacements ofthe structures stabilization elements.

(b) Vertical displacements of the ground nextto the stabilization structures.

(c) Horizontal ground displacements next tothe stabilization structures.

(d) Furthermore, it was predicted the adop-tion of instrumentation key profiles, count-ing on, whenever possible, with the de-vices already installed in place, if theywere operational.

3.5. Alert and Alarm Criteria

The alert and alarm criteria (Table 1) were es-tablished based on the type of solution for theimplemented interventions as well as the geol-ogy of the site and the model calculations re-sults.

Table 1: Alert and alarm criteria. [2]

Alert Alarm

Vertical Displ. (mm) 30 50

Horizontal Displ. (mm) 30 50

The interpretation of the observed valuesshould be compared with the values obtainedin previous readings, therefore, beyond the in-formation given by absolute values, it is impor-tant to analyze the trends of its evolution.According to the nature of the works it wasestablished, at the design stage, that the fre-quency of readings should be at least once aweek.Upon completion of the work, it was proposedto conduct quarterly readings for a minimumperiod of three years.In 2011 there were installed four inclinometers,which were measured until August 2014, andwhose results confirmed the possibility to exe-cute bored piles, properly embedded in ZG1.In inclinometer IC2 (figure 12) it is observed atrend of deformation at 7.5 m of depth, that issignificantly accentuated March to May 2013,around 20 mm accumulated. The inclinome-ter IC3/4 (figure 13) shows a strong tendencyof deformation around the 5.5 m of depth, of30 mm and may be associated with a possiblefailure surface, since these shifts occur next toZG2/ZG1 interface.As noted, during the first year of operation ofthe new road, the inclinometers continued toregister accentuated displacements which in-dicates that the slope, as a whole, is not stabi-lized. Regarding the road itself, since the piles

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Figure 12: Results of inclinometer IC2.

are properly embedded in competent layer andtaking into account their geometry and me-chanical characteristics, can not be expectedthat it suffers displacements of the same orderof magnitude as the ones read on inclinome-ters.

Observations During the monitoring of thework was not possible to observe the instal-lation of the targets and topographical marks.There were also no available readings of thesedevices up to the date.On the other hand, the two inclinometers thatwere installed during the execution of the workwere not possible to get any readings, as oneof them was inaccessible under the pavementand the other was installed with an inclination.Given these constraints, a new monitoringplan was elaboreted and consists on ten cam-paigns planned to include the reading of exist-ing inclinometers, 4 installed in 2011 and theconsequent clearing and reading inclinometerI1, installed during the work, excluding fromthese campaigns the inclinometer which is in-clined. Up to the present date this new planhas not been actively implemented.Taking into account the measured displace-ments and the impossibility of reading theequipment installed during the works, as wellas the missing instalation of the other equip-

Figure 13: Results of inclinometer IC3-4.

ments contemplated in the project, the instal-lation of new devices of instrumentation suchas inclinometers and targets is urgent.It was also found that there was an interval ofmore than one year (May 2013 and August2014) between readings of inclinometers in-stalled in 2011, i.e., there was no type of mea-surement during the construction phase andthis interval is not complete compatible withthe type of intervention in question.

3.6. Solution Modeling

As mentioned before it was used a finite ele-ment software: Plaxis 2D, to model the ST1,ST2 and ST3 sections. According to [4], theresults obtained through the software are agood approximation to the real slope behav-ior. This happens because it is possible to re-spect the model geometry as well as the struc-ture or the geologic settings, and the samefor the characterization of the mechanical pa-rameters. This modeling aimed to comparethe values of displacements obtained by a nu-merical calculation program, with the monitor-ing displacements. The model parameters arepresented on table 2 (AL stands for lightweightlandfill).The total displacement on ST1 was 3 mm, asit can be seen in figure 14 To ST2 2.9 cm, (fig-ure 15) and ST3 a total displacement of 1.2 cm

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Table 2: Model parameters.

Hardening soil AL ZG3 ZG2 ZG1

γunsat[kN/m3] 5 16 19 22

γsat[kN/m3] 9 19 21 24

Eref50 [MPa] 10.0 18.0 27.0 60.0

Erefoed [MPa] 10.0 18.0 27.0 60.0

Erefur [MPa] 30.0 54.0 81.0 180.0

c′[kN/m2] 0 0 4 37

φ′[◦] 35 23 25 35

υ[−] 0.2 0.2 0.2 0.2

(figure 16).

Figure 14: Total displacements of ST1 - 3mm.Scaled 500 times.

4. Alternative Solution

The alternative solution consists in Jet Grout-ing columns reinforced with HEB profiles withthe embedding made in ZG1 with micropiles.This achieves, on one hand, an improvementin the characteristics of the ground and, onthe other, a stabilization of the slope by reach-ing the failure surface on the interface (ZG2 /ZG1).This solution was predesigned only for thesection where was larger the instability phe-nomena , i.e., in ST2. It was assumed a heightof two meters landfill, in order to does notchange the road project geometry.Thus, the solution is materialized by a framewith three columns of reinforced jet groutingand embedded in the competent layer with theaid of micropiles and connected at the top by

Figure 15: Total displacements of ST2 -2.9 cm. Scaled 100 times.

Figure 16: Total displacements of ST3 -1.2 cm. Scaled 50 times.

a reinforced concrete beam. For practical pur-poses, all drainage systems and geotextilesprovided by the original solution will remain.

This solution compared with a piles solution,presents certain advantages, such as, easeof drilling, and increased yield, greater versa-tility in terms of geometry and sections anda greater ability to transmit lateral loads byfriction. Regarding the use of micropiles, forthe transmission of loads to competent layer,instead of piles, presents some advantages,such as, ease of drilling and higher efficiency(similar to Jet Grounting), with a smaller soildisturbance, greater versatility of equipment,limiting the terrain-induced vibrations and agreater ability to transmit lateral loads by fric-tion. However, both the micropiles and the JetGrounting have a common disadvantage com-pared to the piles, which is the lowest bearingcapacity. In total this solution would allow usto use less size and versatility equipment. Seefigure 17.

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Figure 17: Alternative solution.

5. Main Conclusions

As the great majority of geotechnical works,adaptation of the initial designed solutions iscritical, leading to the necessety of the mon-itoring a survey works. Thus it is consideredessential to confirm the geotechnical zoningand geomechanical characteristics of crossedmaterials through ongoing analysis of theircharacteristics. During all excavation anddrilling works, it’s crucial to ensure all lengthsof embedded and sealing, both the piles andthe micropiles, in the competent layer and con-firming all the estimated parameters when de-signing and implementing through the properimplementation of a monitoring plan (instru-mentation and observation). This is the morecapable tool for managing geotechnical risk ,allowing the analysis of the behavior of thework and, consequently, timely validation ofthe assumptions considered.One of the major objective of this dissertationwas not fully achieved. The plan defined in in-strumentation and observation plan, and fromwhich it was intended to draw conclusions asto the points mentioned in the previous para-graph, was not entirely implemented. The onlymonitoring instruments that are installed, tothis date, are unreadable and it is not possibleto make a back analysis based on displace-ment results.It should also be pointed out that during theinitial construction of the S. Marcos’ Road, in

2007, were not placed any monitoring devicesof the slope, which does not contributed to theantecipation of the geotechnical accident. Theimplemented solution ensures local stability ofthe slope and the safety of road platform, how-ever, and having a history of landslides, shouldbe rethink one intervention with a more globalnature, in order to ensure the global slope sta-bilization.As for the numeric modeling, one of the maindifficulties encountered was the faithful repro-duction of the local geometry, partly due to lim-itations of the Software and on the other handthe type sections were based on interpretativegeological cross section between the variousbored holes.Finally, for the alternative solution presentedwas not possible to make a comparison interms of displacements measured during thework with any of these numerical modeling.The alternative solution tried to keep all thegeometry and non-structural elements of thecurrent solution, but uses a different construc-tion technologies and has relevant advantageswhen compared with the implemented solu-tion. Should also be noted that this work, inaddition to the technical constraints of this na-ture (location, constructive solution, construc-tive phasing, among others) was strongly in-fluenced by social and political factors, since itis a public project contracting, and in this caseended up having as much or more weight thanthe other constraints involved.As future developments is suggest a com-prehensive approach to the entire landslidemeaning the creation of a monitoring and sta-bilization plan for the entire landslide that ac-commodates both S. Marcos’ Road and EN10-6, since through the obtained results of in-strumentation is inferred that need.For the work done in the framework of the dis-sertation, it is proposed a future 3D model-ing, using a finite element program in order toconfirm both the assumptions and the resultsobtained. As mentioned before, one of thegoals was not reached. To this date was notbeen possible to obtain readings of the moni-toring equipment installed at the beginning ofthe work. So it is not possible a complete com-parison between the results obtained by mod-

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eling and by monitoring.

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