RISKS AND FAULTS IN GEOTECHNICS.
A MINI CASE-STUDY OF THE LEANING TOWER OF PISA
1. Beyond computational models
The material from previous lectures strongly focused on computational aspects in geoengineering, because numbers make an indispensable tool for structural design and assessing structural safety.
At the end of this lecture, some issues about risk, faults and construction failures should be analyzed in a broader context - with reference also to many unmeasurable factors. Just like in the car industry - roads in the USA are considered safe, cars are well designed and generally safe, but every day 100 Americans die on the roads in car accidents; no improvements of design calculation methods can change this situation.
Of course, the lecture must end up with probably the most famous European construction - at least for geotechnical engineers - the leaning tower of Pisa. The study of this case, a combination of different circumstances, has many educational aspects.
2. Risk classification
The best information about hazards – not only virtual ones - is provided by the analysis of geotechni- cal failures, and more generally - the analysis of geotechnical misinterpretations and faults. Some well-known publications1,2 and many others are very useful for this purpose; although these analyzes do not cover the latest foundation technologies, new trends in foundation engineering, and modern methodology for soil testing, this does not significantly affect the universal classification of causes of hazards and certain proportions between them. Another current supplements are specialized confe- rences devoted to geotechnical case-studies, separate geotechnical sessions or detailed panel discussions on specific spectacular geotechnical failures3.
The main elements in the classification of geotechnical hazards are as follows:
1) irrelevant investor/architect/designer concept that is not consulted by a geotechnical engineer, 2) insufficient or faulty identification of the subsoil genesis, structure and loading history,
3) incorrect calculation models, irrelevant design situations,
4) incorrect selection of parameter values for the calculation model,
5) faulty forecast of subsoil behaviour and response, unforeseen changes in conditions, 6) human errors in design and construction process,
7) lack of independent control or its poor quality at the design and construction stages, 8) incorrect actions in emergency situations.
1 Rosiński, B. (1978). Błędy w rozwiązaniach geotechnicznych. Warszawa: Wyd. Geol.
2 Wysokiński, L. (2007). Błędy systematyczne w rozpoznaniu geotechnicznym i ich wpływ na projektowanie budowlane.
Mater. III Konf. Naukowo-Techniczna „Awarie budowlane”, Szczecin-Międzyzdroje, 527-540, Szczecin: ZUT.
3 Leonards, G.A., ed., (1987). Dam Failures. Proc.Inter.Workshop on Dam Failures, held at Purdue Univ., West Lafayette, Ind., U.S.A., August 6-8, 1985: Elsevier.
Challenges and real possibilities in the area of risk reduction are not always based on the use of the latest research equipment or sophisticated numerical methods. Assessing the problem from a distance, improving reliability in geotechnics can be achieved the fastest and the easiest by raising the general level of awareness and geotechnical knowledge, by a good system of codes and by – first of all - eliminating the possibility of "gross errors" "(a wide supervision and monitoring systems).
A few reflections from the above points are as follows.
ad 1)
If the office building with 3 under-ground floors and 5 above-ground floors in a place A proved to be a construction success, it does not follow that a similar construction will be equally good in a nearby place B, but in "worse" ground and water conditions. Usually, a flexible approach is recommended when considering the concept, maybe 2 under-ground floors and 6 above-ground floors designed instead will be more beneficial, if acceptable by local architectural development plans. The difference in the excavation depth of 3m and water pressure of 30kPa can be very significant, like for a light underground car parking (UPL stability), etc. An ill, misdefined or chaotic concept can no longer be
"rescued", and in any case these are not simple and cheap correction measures. It may also be risky to use new geoengineering technologies without a profound verification of this technology on a natural scale - including a sufficiently long time scale and in conditions not deviating from the design situation.
Gradual evolution works better in geoengineering solutions than revolution.
ad 2)
Almost everything has been said and written about "investor’s savings" on ground recognition that absorbs only a negligible part of the cost of the entire investment. It is not a common case that during the design stage, additional detailed tests are ordered to supplement/refine the ground data in the face of doubts (this situation should not be confused with additional tests performed by large design and construction companies before submitting a commercial offer in the “Design & Construct”
system). If the problem of poor data-quality is noticed at all, large data scattering and technical doubts will probably result in an increase of the safety margin and a total increase in the cost of investment;
about these circumstances, the "economical" investor may even not know at all; an additional research could, in balance, significantly reduce costs, e.g. by subsoil improvements or weak soil exchange instead of expensive pile foundations.
For linear objects, these hazards are particularly significant. The attached cross-section in Fig.1 shows the image of the actual geological structure along a small section in Warsaw (construction of Trasa Łazienkowska, post factum mapping)4. If a geological subsoil model of the strata was made based on four 15-meter boreholes marked in blue and an analogous model was made based on four 15-meter boreholes, but shifted by about 10m (in green), it would probably be difficult to guess that they relate to the same design situation.
The lack of permanent cooperation between a geologist and a construction engineer - primarily at the early stages of investment planning, but not only, when carrying out excavations and foundation elements - should also be considered as a potential risk to the quality of ground recognition.
4 W.Brząkała, Referat Generalny w Sesji nr 3, XVIII Krajowa Konferencja Mechaniki Gruntów i Inżynierii Geotechnicznej, Warszawa, wrzesień 2018 (prezentacja na podstawie pracy Godlewski T., Niemyjska M., Ryzyko geotechniczne w projektowaniu i realizacji głębokich wykopów. Acta Sci. Pol.Architectura 17 (3) 2018, 27–36).
Fig.1. Complicated geological image of a real subsoil.
ad 3)
In the overall assessment of design reliability, it is difficult to overestimate the role of good, short and clearly composed technical codes that regulate matters including procedures and computational models - based on experience, well proven in practice. Editing a good legal act, not only a CE design code, is a great art and the Eurocode EC7 is not the best example here, because it turns to a
textbook or encyclopedia. It is not only about the scope of the merits, but equally about the form, the degree of perception, avoidance of ambiguity and logical gaps, interpretation misunderstandings, collision of symbols, etc. When creating a technical code or other legal acts, a cooperation with a (sub)team of "non-specialists" is often needed, because specialists do not see the "traps" that they leave in the created legal regulations5.
ad 4)
The selection of parameter values used in the calculation model directly influences the quality of the design, though the popular saying "Garbage In - Garbage Out" is probably too strong; the most (in)famous Warsaw landslide from 20 years ago - the fracture of the diaphragm wall at Puławska St. / Chocimska St. - is a good example, how strength parameters derived by various teams of experts can differ. The parameters of the calculation model should also include safety factors, but it is difficult to find examples that their underestimated values cause failure; just derived and characteristic values of subsoil parameters are less reliable than the design ones.
One can get an impression that building foundations "tolerate" individual local geotechnical faults that could be found in almost every construction process - but does not “tolerate" overlapping of many faults of the similar nature and located in the same place; and above all - "gross errors" have to be avoided.
ad 5)
Unforeseen changes in conditions should be distinguished from unpredictable changes; the latter, generally cannot be predicted, such as the failure of a building A as a result of improper construction and failure of a neighbouring object B (though proper supervision and monitoring can be very helpful).
Anticipating changes in the ground-water situation do not always require a lot of specialistic knowledge - it is known what can happen with a very intensive pumping from wells, if water is dirty and flushes out fine silty fractions, that rheological phenomena will occur in permanent anchors, what can be the impact of peat layers to pile bearing capacity, etc. Prediction of the load-increase plays also a special role, first of all in relation to the filtration capacity of the subsoil.
5 Ratajczak, Z. (1988). Niezawodność człowieka w pracy. Studium psychologiczne. Warszawa: PWN.
This group of unpredictable hazards does not include well-recognized phenomena, such as landslide areas in the Carpathian Flysch, which has a characteristic micro-stratified structure (there are over 3000 potential landslides in the inhabited zone in Podkarpacie, S-E Poland, up to 30÷40% of area in some communes). Lack of active protection measures, inefficient drainage, unregulated water flow - especially from torrential rains, allowing for stepping, deforestation and biodegradation of land cover can cause much more significant consequences than oversimplified calculation models.
ad 6)
Human errors are the dominant cause of failures, especially in CE - various sources indicate from a dozen percent to over 65% of their share. Such huge discrepancies result from unclear, fragmentary, and sometimes falsified documentation of accidents and from the lack of precision of the term "human error"; this can be confusing of the "peak" and the residual soil strength, the lack of necessary
drainage of the subsoil or incorrect inclinometer calibration (initial or “zero” reading), but not always interpreting the position of the interface between different soil layers. Better documented data on
“human errors” is provided by aviation accidents, usually very carefully investigated in the interests of various parties – different estimates indicate at least 75% contribution of human errors; in the group of car accidents this is an even larger percentage.
Of course, “gross” construction errors can always destroy the efforts associated with reliable soil investigations, modelling and reliable foundation design.
ad 7)
The Polish Building Law requires that any building design-project must be checked by a professional, an authorized person, but in case of more specialized executive design projects - there is no such legal obligation; this is difficult to explain. The role of supervision inspector in the construction
process, including geotechnical supervision, is very important; there are specialized companies which deal with this task professionally. It is not uncommon for investors to hire a commission of indepen- dent experienced specialists to review the project contents, its individual solutions or concepts – and this happens also at very early stages of investment. It is worrying, however, that public authorities do this the least frequently, though the use of the advice/consulting from research institutes in the region should be a standard. Nevertheless, a significant progress should be noted in this area after the political transformation in Poland.
Monitoring of the construction site and its surroundings (including post-built) is gaining due rank which significantly reduces hazards.
ad 8)
In the case of geotechnical failures, it is necessary to carefully (but quickly) consider the effects of undertaken rescue operations, as in case of a medical first aid. If ignoring some circumstances, it is easy to make worse the emergency condition. Some old attempts to stabilize the leaning tower of Pisa are a good example of this.
3. Summary of geotechnical hazards
1. Safety in geotechnics depends on many different factors:
from the geological history, initial design stage and identification of the subsoil model, to changes in soil and water conditions caused by a construction and a post-built monitoring.
2. Geotechnical construction failures - dangerous exceeding of the ultimate limit states with geotechnical reasons in the background - are not frequent, as opposed to violation of the
serviceability limit states with troublesome consequences. It is rare for a building failure to have only one cause, but overlapping of many causes is always dangerous, i.e. a series of negligence,
violations of procedures and ordinary human errors. Elemental errors dominate among the most significant ones, at the level of the textbook knowledge - most often associated with groundwater and lack of monitoring.
3. Eurocode EC7, with its "textbook style" of formulating rules and its informative character, is not able to replace a professional analysis of the situation by an experienced geotechnical engineer.
On the other hand, even just pointing out fundamental problems as a "checklist" plays a preventive role, although this has a serious antagonist in the form routine.
4. Leaning Tower of Pisa – a mini case-study6
Brief characteristic
Fig.2. Cathedral and bell-tower of Pisa7
To be honest, it is just the cathedral that is a pearl of the Romanesque architecture in the northern part of Italy and it is a place of religious worship - not a subsequent addition to it (the belfry). For Italians from this part of the country, the rank of the cathedral (with the most valuable neighbouring baptistery) is special; not all foreign tourists are aware of this, because the cathedral was not lucky enough to ... tilt. These three outstanding monuments of architecture, and the adjacent slightly younger cemetery, form the Square of Miracles (Piazza dei Miracoli)8, located on a flat area, a few kilometers from the sea-shore.
This is a first important information from the environmental investigation. Due to significant sea-level fluctuations in the last several millennia and some shoreline changes in this region, loose hetero- geneous fine-grained soils with a contents of organic parts (mud, silt, limestone minerals) can be expected as well as full saturation and possible pore pressure fluctuations.
6 This study is based mainly on:
Constanzo D., Jamiołkowski M., Lancellotta R., Pepe M.C., Leaning tower of Pisa. Description of the behavior.
ASCE Specialty Conference on Vertical and Horizontal Deformations of Foundations and Embankments.
College Station, Texas, June 17th, 1994.
Short Polish version:
Jamiołkowski M., Lancellotta R., Pepe M.C., Krzywa Wieża w Pizie. Inżynieria Morska i Geotechnika, nr 1/1994.
7https://toskania.org.pl/krzywa-wieza-w-pizie/
8 https://albumromanski.pl/album/piza-campo-dei-miracoli-z-xixii-wieku
The object is considered as “a classical geotechni- cal failure”, but this is not true, because only few constructions erected 850 years ago have survived in such condition to this day; the tower of Pisa - what one would say – did survive. Moreover, it has survived four major earthquakes in its history.
Of course, as every historical object it needs some special service and maintenance.
The tower is an 8-storey bell-tower (called belfry or campanile), in form of a hollow masonry cylinder, built several dozen years after the cathedral shown in Fig.2; note that the beginnings of the construction of the cathedral itself date back to 1063/64.
The basic question is:
is it possible to straighten the tower today, in the coming era of group flights to Mars?
The answer is simple: you cannot do this, because ... nobody will agree!
1) First of all, it would not be a good solution for the local community, because hardly anyone from abroad will come to see the "straighten tower of Pisa"; note that recently, the entry ticket is up to 20 euros with several million tourists a year.
2) It is also impossible from a technical reason, because the tower, apart from the tilt, is bent and has a banana-shape (Fig.3).
The tower construction was distributed in time for over 200 years and the tower had already been tilted during construction; probably, the possibility of its demolition was considered, but eventually its shape was corrected around the IV-th floor. As an adjustment, the IV-th floor was made horizontally in form of a wedge, it has variable thickness, Fig.3, which is easy to check; cornices of various
thicknesses are also visible. Therefore, the next floor was erected vertically, followed by further tilting of the tower and the cycle repeated in next years.
Building process of the tower
Although in the old past centuries this region of Tuscany did not belong to peaceful ones, nevertheless, very detailed documentation from the construction period has been preserved:
1173-1178r. – beginning stage, interrupted at the IV-th floor (approx. 50% of dead loads), 1272-1278r. - resumption of construction, floors IV-VII; tilt was small, but already noticeable, 1360-1370r. - completion of the last VIII-th floor; suspension of the bell.
Documents, measurements of the tilt, the curvature of the tower, the thickness of floors and cornices indicate that at the end of the construction the tilt was already about 1.5°, so 1:40. The construction period was over 200 years long, but this was not so long due to technical reasons, but rather political and probably financial ones, because in the XIII/XIV-th century Pisa was already losing its over- regional importance (apparently, in the original plans from prosperous times, there was about 70- meter tower, so 12m more than at present). Such a delay was unintentional, construction in those times could undoubtedly be completed within a few years, up to a dozen years. Note that in the light of today's knowledge it is almost certain that the tower would overturn quickly, if being built efficiently and without such long breaks. The reason is a very high level of stresses under foundation and first of all the occurrence of hardly drained soils with low filtration capacity, i.e. conditions close to unfavor- able undrained condition; long-term raising of the structure enabled gradual consolidation of the subsoil and a release of excess pore pressure due to the Terzaghi law τ = (σ-u)⋅tgϕ’+ c’. Such a load splitting into stages is being used quite often nowadays, but always with measures supporting the consolidation of the soil, i.e. drains, mainly vertical ones.
Fig.3.
Tilt and curvature of the Leaning Tower of Pisa.
Geometrical parameters. The current situation.
• Tower height: 58.36m (of which almost 4m under ground level, also due to large settlements),
• Own weight: 144.53MN (over 14,000 Tons, about 50% more than the 324m high Eiffel Tower),
• External radius of the ring foundation: 9.79m,
• Internal radius of the ring foundation (a circular hole in the slab): 2.25m,
• Width of the foundation ring B = 9.79-2.25 = 7.54m,
• Founding level: currently very variable due to the tilt, approx. 4m under ground level, i.e. approx. 1m below the average sea level,
• Wall thickness in the base: 4.1m (hidden spiral staircase in masonry walls, 1m wide),
• Eccentricity of the tower weight: e = 2.3m,
• Average load under the foundation: qav = 500kPa.
Due to the large eccentricity, the subsoil response is not uniform, currently under the edges can be roughly estimated between qmax = +950kPa (!) and qmin = +50kPa; this caused and still causes the uneven settlement of the foundation.
At the end of the XX-th century, the tower tilt was already 5.5o (about 10%, horizontally 5.7m at the tower head), this was the maximum value in the history of the tower and - worst of all – the tilt was constantly increasing; it was estimated that there were still stability reserves for the tower overturning, but other dangerous symptoms appeared, which indirectly resulted from the tilt of the tower. This was a redistribution of forces in the tower walls and, as a result, local loosening of the marble-cladding bonds on the wall outer surfaces (additional compressive forces, facing buckling and crushing).
This led in 1989 to a critical decision: it was necessary to close the tower for tourists and to fence the dangerous area around the tower.
The wall structure was reinforced with clamps and prestressed cables, which is a separate issue.
The stabilization measures undertaken at the beginning of this century allowed to decrease the tilt to the current level of approx. 5.0o, which was considered as an acceptable value, close to the tilt over 100 years ago.
Geological structure of the subsoil
Although dozens of people have probably dealt with geological investigations, and for over 100 years, the data set is still not complete. In order to protect the tower, it is recently forbidden to perform invasive investigations (borings, sampling, and even CPTU testing) at a distance less than 10m from tower's foundations. These are not exaggerated fears - in 1985, a record number of geological bore- holes were made in the vicinity of the tower and sensitive inclinometers quickly noted the acceleration of the tower tilting. Non-invasive geophysical surveys (geo-radar scanning) are a valuable and safe supplement in this respect.
For this reason, the technical condition of the foundation slab is also not exactly known, but studies of similar foundations from the end of the XII-th century reveal stone blocks and rubble bonded by a lime mortar, locally eroded. It is obvious that not so much the geological strata in the subsoil, but
documented soil characteristics and physical-mechanical parameters are different at a distance of 10÷30m from the tower, because the soil directly under the foundation had been subjected to heavy loads for centuries; significant increase of OCR-values can be expected.
In terms of geotechnical categories, there is of course a third geotechnical category - complicated ground conditions, apart from the unique monumental type of the tower.
The area is flat, located about 3m above the average sea level.
Under the founding level, three generalized layers (packages) are distinguished, called Formations.
Formation A - is about 10m thick saturated silt, sandy and slightly clayey, with interbedded lenses and layers of clay and uniform sand; water table in its highest piezometric level is found at the depth of 1.0÷1.5m above the average sea level, therefore almost the entire foundation with a height of 1.60÷2.97m is permanently submerged.
Formation B - bedding the Formation A, it consists of a horizontal about 30m thick, heterogeneous, laminated soft clays called Pisa Clay, layered with lenses and laminations of saturated fine/silty sands (with water under pressure), possible admixtures of organic parts; despite the similarity, Formation B causes more geotechnical hazards than Formation A, because of significantly increased plasticity index of these soils (especially in the Upper Pisa Clay B1
which is 10m thick).
Formation C - bedding Formation B - consists of very thick saturated silty sands to a depth of over 70m under the ground level; this deep layer only seemingly has no effect on the behaviour of the tower - large seasonal fluctuations of pore pressure (±1m) were revealed, and in the years 1970–1974 a lowering of the piezometric level by 4 meters was associated with deep wells in this region (intensive watering of crops during the growing season); as a consequence, the yearly tilting of the tower clearly accelerated; after intervention and reduction of pumping, this annual speed decreased (after about 2 years); at present, the prohibition of deep pumping applies in the 1000-meter protection zone around the Pisa tower.
Some interfaces of the layers are found as not exactly horizontal, which may, to some extent, promote uneven settlement; the average piezometric levels of groundwater in Formation A and B are approxi- mately constant in time. Some local heterogeneities of the subsoil are revealed by fluctuations in the cone resistance value qc in CPTU tests (at a distance of 20÷25m from the tower) - generally qc is about 1÷2MPa, but locally up to 6÷7MPa (close to the bottom of the Formation A); therefore, the subsoil should be classified as horizontally heterogeneous.
In the field of seismic engineering, the lack of strong soils in the geological profile is considered as very beneficial due to the reduction (damping) of earthquake impacts; when founded on a solid rock, the tower would not resist the violent horizontal inertia forces and would collapse several centuries ago. On the other hand, the model of the subsoil indicates a likelihood of dynamic soil-liquefaction (quasi periodic increase of pore pressure u) in some sublayers of silts and fine sand, but this aspect is not raised by experts.
Activities to secure tower stability
The starting point for corrective and protective measures is to determine the causes and mechanism of the tower tilting. There is no doubt that the basic reason is the high unit load q [kPa] under the foundation of the tower, getting more and more uneven, as well as the geological structure of the subsoil under the foundation slab. One could suspect a loss of bearing capacity of weak soils from Formation B1 (ULS, GEO bearing capacity for heave next to the overloaded edge of the foundation).
But studies have not confirmed this hypothesis - the reason of the tower tilt is just uneven settlement in compression, i.e. SLS, not ULS. The criterion here is the shape of displacements of the soil near the foundation – upward ground heave is a typical symptom of loss of bearing capacity. In this case, geophysical research was used, which showed that the originally horizontal interface between both Formation A and B settled downwards under the tower (over 2.5m), creating a regular bowl-shaped depression, like for elastic half space, a little more at the overloaded edge. Soil displacements
upwards of the Formation B was estimated at only 0.4 m, which may also be the result of a high value of the Poisson ratio. Rheological effects (secondary settlements) are also identified.
The secondary role of the ULS is also supported by values of the strength parameters of the weakest layer in the B Formation, derived as ϕ’= 22o, c′ = 16kPa which are not very low. What's more, this weak layer occurs at a depth of about 7m below the founding level - the role of the stabilizing soil
which is about 10m thick above the discussed sublayer is significant (see q’ in GEO for bearing capacity).
As early as in the XIX-th century, it was correctly concluded that a significant reason for the growing tilt of the tower could be changes in water relations in the subsoil and more specifically the intense water drawing from deep horizons. This is in line with today's knowledge, because dewatering is accompanied by ground settling – in heavily loaded places, first of all.
In the mid-30s of the XX-th century, an attempt was made to seal and strengthen the subsoil under the most settling part using a grouting. The injection was made in silts with sandy laminations in the Formation A. The concept is questionable, because standard grouting with cement suspension is not very effective (often impossible) in soils of such small pores, which are comparable to cement
particles. What's more, pressing the cement suspension to laminate soils, in the region of heavily loaded part of the foundation, may cause hydraulic breakdown. This means that the suspension
"escapes" into more permeable soils and/or under the less loaded part of the foundation; next, pres- sing from the bottom against the foundation can happen, which will increase the tilt. It is hard to expect that these actions will prove effective; indeed, as the effect, the tilt of the tower suddenly increased. Another sudden increase in the tilt of the tower was also observed in 1953-1955, immediately after 15 test boreholes were drilled directly under the tower; the same thing happened when additional borings were made in 1985, though this time in some distance from the tower.
Averaged trend lines indicated that the tower was tilting at a speed of:
in 1940-1950 about 3.0 arc seconds/year = about 1/1200o/year,
in 1960-1970 it was about 4.5 arc seconds/year = about 1/800o/year and in 1980-1990 about 6.0 arc seconds/year = about 1/600o/year.
In the entire period of the years 1911-1990, when systematic and very precise measurements of tower tilting were carried out, an average value of 5.5 sec/year was obtained, but each of the
mentioned operations: geological investigation boreholes, grouting, intense pumping from deep wells gave much higher speed increase in next year, up to 7.5 sec/year and even 10.0 sec/year (pumping).
Stability index for overturning was estimated as low as 1.05÷1.15; strongly non-linear soil behaviour and random subsoil inhomogeneities are the reasons that such numerical estimations are difficult - very sophisticated constitutive models and effective FEM codes must be used.
The post-WWII years are the era of modern soil mechanics and the "festival of ideas" - literally, because several competitions were announced for the concept of "straightening the leaning tower of Pisa", which attracted dozens of contributions from around the world, including several from Poland.
There are generally two groups of methods:
a) lifting the lower part of the foundation (in South, Fig.3), b) making the upper part settles down (in North, Fig.3).
The mixed method c) could be also mentioned, i.e. a part of a) plus a part of b).
The analogy with methods of straightening tilted buildings in mining areas is obvious, which was the subject of a brief analysis in Lecture 4. Here, the variant a) is unrealistic for three reasons:
1) the weight of the tower is huge, even lifting only 40% of this weight is a big problem,
2) the force from point 1) should be applied as a more or less concentrated reaction to the subsoil, which is composed of weak, highly deformable soils; probably a pile foundation would be necessary as a support,
3) such a lifting operation would require a very resistant foundation slab itself, but just the opposite is true.
Difficulties from p.2) were proposed to be solved in the following way: digging a vertical shaft more than 10m from the foundation, say 6m deep, then drilling a horizontal tunnel towards the tower
foundation, construction of a well-supported chamber under the foundation, drilling of piles over 30m long embedded in soils of Formation C, construction of a pile cap as a support for hydraulic jacks lifting part of the tower. The scenario looks very unrealistic, both effectiveness and safety of such a method are questionable, and the obstacle presented in p.3) remains still unsolved. Using of standard Jet Grouting columns cannot be considered neither, because of a short-term phase when the subsoil is weakened by jets and additional settlements can appear.
Very unconventional methods have also been proposed, like freezing of soils under a part of the foundation, causing strengthening and swelling of frozen silt, which would create a lifting force over a large area of the foundation9. Calculations were also presented that the required lifting force could be provided by the use of several huge helium balloons, de facto the zeppelins.
Variants from the group b) are much more realistic and were applied twice, not immediately with a great success. It is excluded to use a soil overloading landfill next to the tower (Fig. 4, on the left), which on the principle of "neighbour action" could cause favorable rotation of the tower itself. Apart from the objections of the monument protection services and the local community, this concept is not feasible due to the high stress-level in the subsoil under the tower, as well as high pore pressure and poor soil permeability, which means that there is a significant risk of an unstable behaviour. It is also unrealistic to use large pressure plates and pre-stressed anchors in bearing layers. An attempt was made in the early 90s, supported by freezing the saturated soils with liquid nitrogen to seal and stren- gthen the anchors. As a result, during one night on September 7, 1995, the tilt of the tower increased more than over the past year10 (the effect of the obvious volume increase of the frozen soil).
In the years 1993–94 the less settling northern side of the tower was loaded by a counterweight ballast made of lead panels (designed load 6MN = 600 Tons), mounted on a special supporting structure, see schematic drawing in Fig.4. Such a concentrated moment-loading made a redistribution of internal forces and changes in the tower bending; despite a temporary use of tower strengthening, the facing cladding continued loosing bonds and falling pieces were dangerous.
The tower had to be closed, which certainly did not increase the number of enthusiasts of this method. Abandoning the concept was certainly accelerated by critical opinions about cumulating in one place such a large mass of health-harmful lead and disfiguring the construction itself with the whole historic square for many years; the number of tourists had fallen by half.
However, in principle, for rigid constructions the concept of the counterweight put on a cantilever is correct, apart from the long time needed for several centimeters of tilt-compensating settlements.
Note that after a few months, the tower leaned eventually to the desired northern direction, a little bit.
9 It looks not clear, why the swelling would not happen in the horizontal direction, much less loaded;
any later defrosting would be impossible due to irreversible structural changes in soils.
10 http://madridengineering.com/case-study-the-leaning-tower-of-pisa/
Fig.4. Schemes of methods for increasing subsidence of the left (northern) side of the tower:
- the temporary solution (counterweight ballast of lead plates), - the target method (oblique boreholes in soil mass of Formation A).
The finally implemented variant is different – this is soil removing from the less settling side of the tower - a simple and very safe solution, known for over 3000 years11. It is often forgotten that this method was not only proposed in the 1960s by Italian engineers; it was even designed with details in the 90s of the XX-th century by Italians (regardless of reported foreign solutions of this type, also Polish once - based on experience from mining areas); eventually, however, the merits were
attributed to an international team established in the last years of the XX-th century, which promoted this solution and supervised its implementation.
The "undermining" of the less settling side of the tower took place by means of oblique boreholes (Fig.4 and Fig.5) and the gradual extraction of over 70 tons of soil from one side of tower’s subsoil;
the voids created in this way gradually tightened. Within a few years, a difference of 20 cm between left and right settlements was created. Note that this can be increased by additional drillings at any time.
The most important matter here is a due care and patience, removal of soil in small portions, very slowly (period in months and years), constant observation of the "tower response" and adjustment of the next stage to the development of the situation12.
11 It's about the so-called shaft tombs being used over centuries in ancient Egypt, which replaced the expensive, and yet unsuccessfully protecting pyramids: the almost vertical excavation (open-pit) made in a soft rock was more than 10m deep, and outside - the pit was surrounded by several vertical shafts with underground tunnels leading horizontally to the bottom of the pit; the ready outcrop was filled up with rubble and sand to the initial ground level, a heavy stony sarcophagus was placed on the backfill, and next rubble and sand were mined, but this time by pure underground method - through tunnels and vertical shafts; proper controlling of the shafts’ efficiency made it possible to lower down the sarcophagus evenly;
when the sarcophagus was founded on the bottom of the excavation, the whole pit was finally filled up with rubble and sand again, including the mining shafts; the place was masked.
12 This is a good example of the so-called Observational Method, recommended by the Eurocode EC7, which is useful when solving unusual/unique geoengineering problems.
https://archidose.blogspot.com/2000/07/tower-of-pisa.html
As a summary:
the situation is now completely under control and the use of the tower is not restricted.
One disadvantage of this solution, in fact not very significant,
is the increase of the total settlement of the tower, which already settled a lot (average settlement, regardless of the tilt, Fig. 2).
Perhaps, it would be advantageous to better expose the tower by slightly lowering the local ground level and rearranging its configuration in the region adjacent to the tower.
Fig. 5. Drilling oblique boreholes.
The lead counterweight still seen on the facade.
