The Leaning Tower of PisaThe Leaning Tower of Pisa

Items that will be discussed:Items that will be discussed:

• Geology under PisaGeology under Pisa• Issues as a result of the geological conditionsIssues as a result of the geological conditions• Characteristics of the towerCharacteristics of the tower• Construction of the towerConstruction of the tower• Inclination of the towerInclination of the tower• Monitoring InclinationMonitoring Inclination• Seismic activitySeismic activity• Management techniques adoptedManagement techniques adopted• The possible future optionsThe possible future options• SummarySummary

Where is the tower?Where is the tower?

Where is Pisa?Where is Pisa?

Tuscany

The River ArnoThe River Arno

What does the tower stand on?What does the tower stand on?

A

•Sediments covering bedrockSediments covering bedrock

•Layer A: Layer A: ≈ 10m thick≈ 10m thick

Fluvium from River ArnoFluvium from River Arno

•What is fluvium?What is fluvium?

•Soft, estuarine (river) depositsSoft, estuarine (river) deposits

What does the tower stand on?What does the tower stand on?

B

•Layer B: Layer B: ≈ 20m thick≈ 20m thick

Pancone marine clayPancone marine clay

flat, jumbled, loose flat, jumbled, loose particles particles

•Very soft and plasticVery soft and plastic

•Easily deformableEasily deformable

What does the tower stand on?What does the tower stand on?

Layer C: Thick layer of dense Layer C: Thick layer of dense silty sand silty sand

C

What are the problems with What are the problems with building on these conditions?building on these conditions?

•Built on soft clay, not rockBuilt on soft clay, not rock

•Dish shaped basin under towerDish shaped basin under tower

•2.5 to 3m settlement!2.5 to 3m settlement!

•Lean caused by silt on South Lean caused by silt on South side being compressed more side being compressed more than on North side.than on North side.

•South side silt has Pancone clay South side silt has Pancone clay mixed in.mixed in.

•Result = differential settlement = Result = differential settlement = badbad

What are the problems with What are the problems with building on these conditions?building on these conditions?

•Built on floodplainBuilt on floodplain

•High groundwaterHigh groundwater

•Ground water rises in rainy Ground water rises in rainy seasonseason

•Greater rise on North sideGreater rise on North side

Characteristics of The TowerCharacteristics of The Tower

• Height from foundations to belfry 60mHeight from foundations to belfry 60m

• Ring shaped foundation has external Ring shaped foundation has external diameter of 19.6mdiameter of 19.6m

• Tower weighs 14,500 tonnesTower weighs 14,500 tonnes

• The inclination is 5½ degrees towards the The inclination is 5½ degrees towards the south; this means that the seventh cornice south; this means that the seventh cornice protrudes about 4.5m over the first protrudes about 4.5m over the first cornice.cornice.

Construction HistoryConstruction History

• The Tower was constructed in the course of The Tower was constructed in the course of about 200 years.about 200 years.

• Work on the foundation began on 9 August Work on the foundation began on 9 August 1173.1173.

• The construction of the belfry (8th order) began The construction of the belfry (8th order) began around 1360 and was completed c. 10 years around 1360 and was completed c. 10 years later.later.

Materials Used in ConstructionMaterials Used in Construction

The following are the stone types used which The following are the stone types used which come from areas adjacent to Pisa: the Monti come from areas adjacent to Pisa: the Monti Pisani and the Monti d'Oltre Serchio:Pisani and the Monti d'Oltre Serchio:

•Agnano BrecciaAgnano Breccia

-internal wall facing-internal wall facing

•"Bench" calcarenite"Bench" calcarenite

-vaults and arches-vaults and arches

•Filettole black limestoneFilettole black limestone

-ornamental bands-ornamental bands

•"Flinty" limestone"Flinty" limestone

-concrete filling-concrete filling

Materials Used in ConstructionMaterials Used in Construction

• "Verrucano" quartzite"Verrucano" quartzite-used in foundation concrete-used in foundation concrete

• GranodioriteGranodiorite-half base columns-half base columns

• San Giuliano marbleSan Giuliano marble -most used incl. external walls-most used incl. external walls

Re-emphasis of SubsidenceRe-emphasis of Subsidence

• Tower subsided vertically by 2.8mTower subsided vertically by 2.8m

• The differential subsidence is 1.89m; this The differential subsidence is 1.89m; this means that the extreme north and south means that the extreme north and south have subsided by 1.86m and 3.75m have subsided by 1.86m and 3.75m respectively.respectively.

Inclination of the TowerInclination of the Tower

• Progressive increase in inclination can be Progressive increase in inclination can be attributed to the instability of the equilibrium + attributed to the instability of the equilibrium + the viscous plastic response from the ground.the viscous plastic response from the ground.

• Increase in rotation induces increase in Increase in rotation induces increase in tendency to capsize; in turn, increases stresses tendency to capsize; in turn, increases stresses + deformations and therefore a further increase + deformations and therefore a further increase in rotation.in rotation.

• 1968 – 1974 intense pumping of water from the 1968 – 1974 intense pumping of water from the subsoil led to an increase in the rate of rotation subsoil led to an increase in the rate of rotation of the tower.of the tower.

Monitoring the Movement of the Monitoring the Movement of the TowerTower

• Since 1913 – geodetic measurementsSince 1913 – geodetic measurements• Since 1934 – Girometti-Bonechi pendulumSince 1934 – Girometti-Bonechi pendulum

- civil engineers spirit level- civil engineers spirit level• Since 1991 – accurate internal optical systemSince 1991 – accurate internal optical system

- automatic system with sensors for the - automatic system with sensors for the measurement measurement of: of:

1.1. WindWind2.2. Air temperatureAir temperature3.3. Seismic actionsSeismic actions4.4. InclinationsInclinations5.5. Horizontal point movementHorizontal point movement6.6. Width of the fissuresWidth of the fissures7.7. Levels within the stratumLevels within the stratum

Seismic ActivitySeismic Activity

• Area around Pisa has been subject to strong Area around Pisa has been subject to strong seismic activity.seismic activity.

• Studies estimate a grade 6 earthquake (Mercalli Studies estimate a grade 6 earthquake (Mercalli scale) has probability of occurrence every scale) has probability of occurrence every 130yrs.130yrs.

• Grade 7 probability of occurrence every 520yrs Grade 7 probability of occurrence every 520yrs and grade 8 once over 3000yrs.and grade 8 once over 3000yrs.

• Analyses of response of towers show - limit Analyses of response of towers show - limit would be reached in a grade 6 earthquake.would be reached in a grade 6 earthquake.

• It would not survive a grade 7 earthquake It would not survive a grade 7 earthquake undamaged.undamaged.

Management of the TowerManagement of the Tower

•Professor John BurlandProfessor John Burland

•Currently responsible for the status Currently responsible for the status of the tower.of the tower.

Management Solutions AdoptedManagement Solutions Adopted

• 600 tonne 600 tonne lead lead ingotsingots applied in applied in 1993 – modest 1993 – modest reduction of the reduction of the inclination and inclination and arrested progressive arrested progressive rotation of tower.rotation of tower.

Management Solutions AdoptedManagement Solutions Adopted

• Safety staysSafety stays – correct unexpected small – correct unexpected small movements (temporary safeguard during movements (temporary safeguard during works).works).

Management Solutions AdoptedManagement Solutions Adopted

•Soil extractionSoil extraction

•Involves placing tubes on the North side of the Involves placing tubes on the North side of the tower to remove soil.tower to remove soil.

•Modelling and trials to determine the exact Modelling and trials to determine the exact placement of the tubes.placement of the tubes.•Special drill designed Special drill designed to avoid disturbance to to avoid disturbance to surrounding ground. surrounding ground.

Management Solutions AdoptedManagement Solutions Adopted

• Preliminary soil extraction over limited width in Preliminary soil extraction over limited width in February 1999.February 1999.

•Full soil extraction takes Full soil extraction takes place over the full width of place over the full width of the foundations in the foundations in February 2000.February 2000.

•Soil extraction finished in Soil extraction finished in June 2001.June 2001.

Management Solutions to be Management Solutions to be Adopted in the Near futureAdopted in the Near future

• AnchorageAnchorage – medium – medium intervention intended intervention intended to substitute the to substitute the weights, reducing the weights, reducing the unsightliness of unsightliness of counter-weighting.counter-weighting.

Possible Future Management Possible Future Management TechniquesTechniques

• Electro-osmosisElectro-osmosis – clay contracts upon – clay contracts upon absorbing electrical current under north of tower absorbing electrical current under north of tower causes subsidence of base.causes subsidence of base.

• Has been tested in a full-scale experimental site.Has been tested in a full-scale experimental site.

SummarySummary

• Lean is caused by differential settlement of Lean is caused by differential settlement of silt under tower.silt under tower.

• Lean is also caused by uneven rise of Lean is also caused by uneven rise of groundwater.groundwater.

• Many attempts to correct lean.Many attempts to correct lean.

• The most recent and successful being soil The most recent and successful being soil extraction.extraction.