Engineering Structures 32 (2010) 1821–1829

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Engineering Structures

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Deep trench, landslide and effects on the foundations of a residential building:A case studyA. Brencich ∗DICAT - Department of Civil, Environmental and Architectural Engineering., University of Genoa, via Montallegro 1 – 16145 Genova, Italy

a r t i c l e i n f o

Article history:Available online 12 March 2010

Keywords:ExcavationCohesionLandslideFoundations

a b s t r a c t

The case study discussed in this paper deals with the excavation of an excessively deep and long trenchin a clay slope made at one edge of an urban connecting road. The subsequent failure of the top portion ofthe slope did not cause any human casualty, but did severe damage to a neighbouring retaining wall andto an adjacent building. The works aimed at the stabilization of the slope are suspected to have increasedthe damage to the foundation system of the building.The aim of this paper is to show that all these events could be foreseen just by relying on the basic

concepts of mechanics and on proper engineering practice. The causes for the landslide, and subsequentproblems, are to be found in an excessive confidence in soil cohesion and in the application of standardengineering procedures in a non-standard case.

© 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Systematic studies on the stability of soil slopes [1, amongothers] and the research carried out in the last 30 years [2–15]has gathered relevant information on slope and excavation sta-bility such that this issue is now substantially understood fromthe theoretical point of view. Several manuals [16,17, among oth-ers] provide clear suggestions on the limits with unsupportedslopes for safety excavations to be performed. Nevertheless, inthe recent years several failures of trenches and slopes have beenrecorded [18], showing that scientific research has not yet beencompletely transferred into common engineering practice. Fromthis point of view, a large part of the job is for the academics andtheir teaching efforts.In this paper a case study is discussed showing how the fail-

ure of the top side of a deep trench may involve a large part ofthe neighbouring area including, in the specific case, a residen-tial building, due to complex interaction between the soil and thestructure causing alarming cracks in the building. The strengthen-ing works are also described.Excessive confidence in soil cohesion and in common engineer-

ing procedures are shown to be the causes of the landslide, i.e. theapplication of standard solutions to non-standard cases. It is shownthat the landslide could be foreseen just by recalling the basic con-cepts of mechanics and proper engineering practice, and that fur-ther scientific research, in this case as in many other cases, is notneeded to explain the events.

∗ Tel.: +39 0103532512; fax: +39 0103532534.E-mail address: brencich@dicat.unige.it.

0141-0296/$ – see front matter© 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.engstruct.2010.02.019

2. The landslide and the neighbouring area

In 1984 the Municipality decided to build a connecting road,Fig. 1, to relieve one of the most difficult traffic nodes in the area(a) to a major road (b), Fig. 1. The total length of the road was 250m, the first 120 m requiring some embankment of reduced height,the second 130 m requiring deep trench excavations, which areillustrated in Figs. 2 and 3.The first part of the road (upper part, left-hand side, approxi-

mately up to drilling D1 of Fig. 1) was built in 1984, while the sec-ond part was completed in 1990. The second part of the road, Fig. 2,required the excavation of a 7-to-9 m deep trench.The walls of the trench were massive structures built in front

of a double series of φ 220 mm piles at a 280 mm net distanceone from the other, Figs. 3 and 4; the piles were also intended asprovisional support of the soil. According to the original design, theexcavations and the related retaining walls were expected to beextended by steps no longer than 5m in order to prevent landslidesof the slope. In reality, however, the excavations were left withoutany provisional support, so that on the 5th of October 1990, after2-day long heavy rain, the top portion of the trench collapsed,involving a large part of the area close to the trench, Figs. 2 and4–7, including a residential building dating back to 1962–1964 anda retaining wall holding the embankment in front of the building.In the first days after the landslide, the pavement of the

embankment behind the old retaining wall showed severe cracksthat were undoubtedly caused by the movement of the oldretaining wall and the subsequent settlement of the embankment,Fig. 7; the crackswere sealedwith bitumen. The increasing openingof the cracks requiring re-sealing every two-three days, and theleaning outwards of the vertical rods of the railings (Fig. 7) showed

1822 A. Brencich / Engineering Structures 32 (2010) 1821–1829

Fig. 1. Aerial view of the area and of the connecting urban road between area (a) and road (b). Dashed polygon: Area of interest for the case study, Fig. 2. D1-to-D4 locatethe drillings for pre-design geological investigations. Bold dotted line indicates the extent of the landslide, see also Fig. 2.

Fig. 2. Final part of the connecting road at which the layout required a 7-to-9 m deep trench. Shaded area: Extension of the landslide. Sections A–A and B–B in Fig. 4.

that the settlement process was still in progress several weeksafter the landslide. Rough measurements showed that the oldretaining wall underwent an approximately 9–10 cm horizontaldisplacement and a 4–5 cm vertical displacement.Somemonths after the landslide, cracks were discovered on the

longitudinal beam of the first floor of the building, which consistedin a open space (garage) at ground level, Fig. 8. A detailed surveyon the r.c. frame of the building showed that the other beams ofthe lower level (ground floor) were also damaged in a similar wayand that lesser damage could also be found in the beams at higherlevels.

3. Deficiencies in the residential building and in the geologicalinvestigations for the design of the road

The four storey residential building was built in 1962–64. Itsoriginal structural design could not be found, but a detailed surveyshowed the structure of the building to be an r.c. frame with

high beams (with low percentage of steel bars), deep columnsand independent foundations. The depth of the foundations, Fig. 4,have been deduced from subsequent investigations by meansof direct drilling through them. The downstream columns hadvery deep foundations to reach the first rock layer, while theupstream columns could be directly laid on the rock layer, and theintermediate ones were laid on a concrete well which relied on therock layer.Due to the excessive length of the downstream columns, having

foundations more than 10 m distant from the first floor, anintermediate layer of r.c. beams was introduced in the framein order to prevent buckling. These beams were just below thepavement of the ground floor of the building and were discoveredonly during the retrofitting works.When building a house on a slope, the retaining walls are the

first structures to be built in order to create the embankmentaround the building from the waste of the construction. Accord-ing to the over-simplified design and building procedures of fifty

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Fig. 3. Detail of the landslide area and of the road layout. Sections A–A and B–B in Fig. 4.

a

b

Fig. 4. (a) Section A–A; (b) section B–B of Fig. 3.

1824 A. Brencich / Engineering Structures 32 (2010) 1821–1829

Fig. 5. View of the landslide from below, 10 days after the occurrence. A large partof the soil had already been removed to make space for the wagon drill (in the fardistance).

Fig. 6. Photo of the landslide taken from the large road (b) of Fig. 1 some daysafter the landslide. A part of the collapsed soil has been already removed to allowthe wagon drill to reach the proper position. In the background: The underminedfoundation of the buttress.

years ago, these walls were built without any design, leaving thestructure to the experience of the bricklayers.In this case, no exact information on the original shape of the

wall could be derived. The only information was that, as usual, theretaining wall had been built before the building and that, duringthe construction of the residential building, i.e. approximately in1962, a period of strong rainfall caused a mudflow to fill a largepart of the space behind the newly built wall; by that time, theembankment was not yet to its final height. As a consequence ofthe mudflow, the buttresses of Figs. 4 and 6 were built allowingsome kind of equilibrium to be reached until the events of this casestudy. It is clear that the safety margins of the retaining wall werefar from the modern standards.According to the local building code, in the middle of the 80’s a

geological survey of the area had been performed before the designphase of the connection road. Points D1-to-D4 of Fig. 1 show thepositions of the drillings, 18 to 25 m deep, used to investigate thelayers of soil and rock.

Table 1Main geotechnical data assumed for the design of the retaining walls.

Soil depth(m)

Internal frictionangle

Cohesion(N/mm2)

Density(kN/m3)

0–7 27°–28° 0.020–0.025 1.9>7 35° 0 2.0–2.1

The investigations were done with rotary-percussion drillsunable to give reliable results. The different types of soil and rockhad to be identified on the basis of a couple of rough parameters:(i) the speed of the drilling hammer related to the actual powerof the wagon drill; (ii) the colour of the material coming outfrom the drillings. Thus, identification of the soils and rocks wassubstantially provided by the experience and skill of the peopledriving the wagon drill, and by some other drillings performedsome years before in a neighbouring area, as stated by the geologistin charge for the geological study. In addition, the four drillingswere all performed on the sides of the designed road, i.e. almostalong a line, which made it impossible to reconstruct a detailedgeological section of the area.The design of the retaining structures required a large number

of piles, a double vertical row and an inclined row, with very highsafety factors. The designer asked for the piles to be done first,also as a provisional device to protect the workers from furtherlandslides, then to dig the trench. But the contractor changed theprocedure (see Table 1).After the landslide, several drillings were done in the area,

allowing the geological reconstruction of Fig. 4. It was discoveredthat the first rock layer (rock type 1) consisted of severely fracturedlimestone layers (10-to-50 cm thick) alternated with clay layers(5-to-10 cm thick). The inclination of the bedrock proved to beimportant.

4. Investigation on the causes of the landslide and its effects

The investigations after the landslide showed that the contrac-tor dug a deep trench along the road layout, but they could notidentify the exact depth of the trench, which was estimated to be5 m deep on average at a distance of 5-to-10 m from the old re-taining wall, Fig. 2. The top portion of the trench did not collapseimmediately but after some five days, the last two of which wereaffected by heavy rainfalls. Fig. 6 shows that the landslide reachedthe foundations of the buttresses in section B–B, but not in sec-tion A–A; this is probably due to the reduced depth of the trenchclose to section A–A and the higher depth close to section B–B. Therock layer shown in Fig. 4 suggests that the sliding surface couldbe activated by the bedrock; but this is only a conjecture since theposition of the rock layer n. 1 is not known exactly. Some data fromthe drillings suggests that there should be an increase in the slopejust below and close to the trench.In order to understand the basic phenomena that led to the

landslide we do not need very detailed analyses. In the followingwe assume that the trench depth was 5m. The first parameter thatwe can try to estimate is the apparent cohesion obtained, assumingthat 5 m is critical depth of tension cracks into the soil [19,20]:

Hcr =3.8c ′

γ= 5 m, (1)

from which, assuming γ = 19 kN/m3, we obtain: c ′ = 25 kN/m2= 25 kPa, which is quite a standard value for the area. Sinceno precise information on the exact dimension of the trench isavailable, we assume the simplified scheme of Fig. 9, where theslip surface is assumed circular even though it is cycloidal [21]. Weassume an internal friction angle of 27° and a cohesion of 25 kPafor the soil. With these assumptions the safety factor [22] of the

A. Brencich / Engineering Structures 32 (2010) 1821–1829 1825

Fig. 7. Pavement of the embankment behind the old retaining wall one year after the landslide. Cracks are a clear sign of movement of the old retaining wall and relaxationof the embankment itself.

top part of the slope of Fig. 9 would be 0.77. During the first daysafter the landslide, cracks parallel to the excavation slope wereobserved; they were probably tension cracks close to the slidingsurface, Fig. 9, which are quite typical of this kind of landslide.The heavy rainfall filled the cracks with water, expediting theactivation of the landslide.The scheme of Fig. 9 is intended to represent an average con-

dition of the top side of the trench dug by the contractor; calcula-tions would lead to different results if the value for cohesion weredifferent, if the slope above the trench were assumed horizontalinstead of inclined, or if the trench side were inclined instead ofvertical. Whatever the choice of the basic parameters, the safetyfactor of the schemeof Fig. 9 shows that failure of the top side of thetrench was unavoidable. The circumstance of its failure five daysafter the trench had been dug and after two days of heavy rain-fall suggests that the cause of failure was the reduction of effectivecohesion due to saturation of the soil, which caused failure of oneside of the trench because the safety factor was already too closeto unity. Actually, it is well known [3,23,24, among others] that theshort term (or effective stress analysis) and the long term (or to-tal stress analysis) conditions are rather different. In the first case,the effective cohesion may attain significant values, thus allowingsignificant excavations; in the latter case, cohesion is strongly re-duced, almost vanishing in some cases. The case study here dis-cussed is related to the changes in short term conditions: theymaysubstantially change the effective stress analysis, turning the situ-ation from stable to unstable.The old retainingwall, never designed but just built, clearly con-

sisted of poor concrete. Nevertheless, the wall behaved as a rigidblock, apart some secondary cracks, showing a clear movementdue to a translation downwards and a rotation. For this reason, thequality of its materials become a secondary issue.Such a collapse mechanism is coherent with the long term be-

haviour of thewall. After the new retainingwall had been built andthe soil replaced in front of the buttress foundation, the wall wentonmoving for approximately two years with decreasing speed un-til, probably, the frontal passive pressure and the vertical actionson the foundations could be reactivated by soil compaction. Thisdemonstrates that the quality of materials in retaining structures

maynot be the cause of their collapse. Equilibrium is lost because ofinadequate dimensions of the foundations, either of thewall and ofits buttresses, that lead to proportionally inadequate safety factors.The landslide was therefore unavoidable. This can be clearly

shown by also referring to [17], where the vertical part of a trenchis required to be no more than 1.00m in the bottom part of thetrench and the slope of the excavations to be no more than 53°.In this case, the inclination of the trench sides was between 70°-to-90°, showing that the excavations were completely outside anysafety standard.This case shows that clay-type soilsmay show significant values

of apparent cohesion which, actually, may be taken into accountfor the provisional phases of excavation. The limits providedin [16,17] are a reasonable balance between safety and economy.A vanishing cohesion would be an overconservative assumptionin the intermediate phases of excavations, but is justified forlong term safety calculations since it refers to the conditions ofsaturated soils, which is typical of intense rainfall periods.Since the origin of the landslide is rather clear, a large part of

the investigations were related to the damages to the r.c. frame ofthe residential building; several causes have been suggested, mostof which are not mechanically justified. A list of the main causes isbriefly discussed here.One of the causes claimed for the damage to the longitudinal

beam (and the similar ones on the external sides) of Fig. 8 involvesboth shrinkage and creep. The first should be responsible for thecracks that are almost normal to the beam axis. Since the groundfloor has no interior wall, the creep of the upper floors of thebuilding could have overloaded the beams of the first floor, causingsubsequent cracking. These assumptions do not take into accountthe age of the building: 26 years by the time the landslide occurred.Even though both shrinkage and creep are known phenomenanot completely understood and still under research, it is wellknown, and also stated by several codes, that shrinkage lastssome 3 months, while creep lasts not more than 4 years, if nosignificant change occurred to the distribution of the loads in thestructure [25]. The aforementioned hypothesis assumes that thecracks on the longitudinal beam were present since, say, 1968and that nobody noticed them for the following 22 years, neither

1826 A. Brencich / Engineering Structures 32 (2010) 1821–1829

a

b

Fig. 8. (a) Overall view of the lower part of the r.c. frame of the residential building and possible displacements (sliding on the bedrock) of the foundations. (b) Crack patternin the longitudinal beam of the first floor shown in (a).

the people living in the building nor the technicians that studiedthe building’s safety immediately after the landslide. In addition,this explanation would somehow justify the cracks in the centrallongitudinal beam of Fig. 8 but not in the similar longitudinalbeams of the ground level along the perimeter of the building,which were not free but embedded into the veneer walls of thebuilding. For all these reasons, this cause can been consideredunrealistic.Another cause claimed for the damage to the longitudinal beam

was the dynamic effect of the down-hole drilling hammer usedfor the piles on the r.c. frame, essentially resonance. The standardfrequency of drilling hammers is nowadays in-between 1800 and2400 Hz, with a lower limit that can reach 800 Hz for some typesof hammer and was not lower than 30 Hz in the 80’s. Even thoughno specific information is available for the r.c. building of thiscase study, we can assume that the first natural frequency of thebuilding is in the range [1.5–3] Hz, which is quite a typical rangefor this kind of structure. The difference between the frequencies ofthe down-hole hammer and of the building, whatever the error inthe estimation of the latter, shows that no resonance is possible.If the damage to the r.c. beams of the ground floor was due toresonance phenomena, also other parts of the building, such asthe internal and outside walls of the building, should have beendamaged; this was not found. Therefore, the dynamic effects of

20°

5.00

Fig. 9. Schematic of the top part of the trench.

drilling the piles seem to be something affecting the psychologyof people, but not the mechanics of vibrations.The most likely cause for the damage to the r.c. beams is the

sliding of the foundations, which raises a fundamental question.It has been seen that the old retaining wall and the embankmentunderwent significant displacements as a consequence of the land-

A. Brencich / Engineering Structures 32 (2010) 1821–1829 1827

Fig. 10. Retrofitting works: r.c. U-shaped solid slab around the building and active 6 tendons/side.

Fig. 11. Retrofitting works: r.c. U-shaped solid slab around the building and active 6 tendons/side. Original drawing. Geological section not fully reliable.

slide: did the embankment also drag the columns and foundationsof the building or did the embankment move through the columnswithout any effect on the load bearing structure? This cause is ac-tually realistic and mechanically sound. At this stage, before theretrofitting works provided further information, no answer couldbe given because: (i) no severe damage on the r.c. frame could beidentified; (ii) no informationwas available on the foundations, noron the quality of the soil they laid on.

5. Strengthening of the residential building and new outcomes

In 2000, 10 years after the landslide, retrofitting works wereperformed on the building and its old retaining wall. A large U-shaped r.c. solid slab 60 cm thickwas constructed around the build-ing, lying on the embankment and on the top of the old retainingwall, Figs. 10 and 11. The old retaining wall and the related em-

bankment were shortened by 1 m in order to reduce the pressureagainst the old wall. A new r.c. slab was connected to the solidbedrock by 6 tendons/side each one with a 600 kN traction. Theinclination on the tendons was designed so as to compensate thehorizontal thrust of the building, assuming the inclination of thebedrock as deduced from a detailed geological survey exactly be-low the building. Such an approach assumed that the damage to thebuildingwas due to slippage of the foundations, whichwas consid-ered to be very likely since the detailed geological tests proved thatthe first rock layer consisted of alternate layers of 10-to-50 cm offractured limestone and 5-to-10 cm of wet clay.The slab was 60 cm thick, reinforced with 3 layers of steel

web φ 12 mm 20 cm × 20 cm, except for the parts close to thetendons where the thickness was increased to 1.5 m, the layers ofreinforcing steel proportionally increased; specific reinforcementwas designed close to the tendons heads.

1828 A. Brencich / Engineering Structures 32 (2010) 1821–1829

Fig. 12. South-east corner of the building during the excavations to lower theembankment—position A of Fig. 8(a). R.c. beams at ground level to reduce the freeheight of the columns (in that part the distance between the foundations and thefirst slab was 10 m). On the right hand part: Complete fracture between the r.c.beam and the column.

Fig. 13. South-east corner of the building, detail. The few bars connecting thecolumn to the beam were broken. The smooth surface between the column andthe beam indicates that they had not been cast at the same time; the columns werefirst and the beams afterwards.

The retrofitting of the old retainingwall was obtained bymeansof three devices: (i) reducing the pressure against the wall byremoving 1 m of the embankment; (ii) by the slab providingadditional vertical load to thewall; (iii) detailing the slab geometryclose to the free end, Fig. 11, in order to restrain the top of the oldwall.During the excavations to lower the level of the embankment

at the south-east corner of the building, position A of Fig. 8(a), thebeams used to reduce the free height of the columns were found.One of these beamswas detached by 4 cm from the column, Figs. 12and 13, giving evidence to the hypothesis that the damage to thebuildings was caused by sliding of the foundations on the rocklayer, as already discussed. Actually, this large fracture showed thatthe displacement of the foundations should be expected to be aslarge as 8-to-10 cm in the horizontal direction and some 2/3 ofthat value in the vertical direction.Fig. 13 also shows a detail of the r.c. structure: the beams and

the columnswere not cast at the same time, that the columnswerefirst and the beams afterwards could be deduced from the smoothcolumn/beam surface. The bars connecting the beam to the columnwere φ 12 mm bars at each corner of the beam (4 bars) and wereseverely corroded. After the retrofitting works, neither new cracksappeared on the building nor were any gunshots ever heard by thepeople living in the building. The total cost of the retrofittingworkswas equivalent to e 180.000 nowadays.

The outcomes of the retrofitting phase were crucial todemonstrate that the designed works were proportionate to thespecific case. Nevertheless, they did not answer the fundamentalquestion: did the movement of the embankment make thefoundationsmove, or was it some effect related to the piles neededfor the new retaining wall built to make space for the road?Figs. 12 and 13 allowed the technicians to discover a signifi-

cant detail on the r.c. structure of the building: inadequate reinfor-cement and inadequate connection between structural elements,though this was a common building method in the 60’s.The outcomes revealed during the retrofitting works demon-

strated that the foundations of the building underwent vertical andhorizontal displacements, which could be responsible for the ob-served cracks. Actually, most of the cracks are shear ones and someof them could be attributed to torsion transferred to the beams bythe stiff columns.

6. Conclusions

This case study provides some information on the concept ofcohesion and on the possible interactions between landslides andexisting buildings.

1. Cohesion depends on the water content in the soil and itsmeaning is often misunderstood: the assumption of vanishingcohesion in long term calculations takes into account thecondition of complete saturation. During excavations, theapparent cohesion may attain significant values, thus allowingquite deep excavations that remain stable until a modificationof the water content.

2. The high safety margins included in the modern building codesare justified by noting that structures built in ‘50 s and ‘60 sare sometimes severely weak: (i) retaining walls may not bedesigned; (ii) the foundations may be laid on apparently stablerock layers; and (iii) detailing of the r.c. structure may be ratherpoor.

Acknowledgements

The author is grateful to Prof. Piero Villaggio for his suggestionsand to his father, Alfredo Brencich, for allowing the author to workon this case.

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