The Leaning Tower of Pisa 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 Pisa? Tuscany

The River Arno

What does the tower stand on? A Sediments covering bedrockSediments covering bedrock Layer A: ≈ 10m thickLayer A: ≈ 10m thick Fluvium from River Arno Fluvium from River Arno What is fluvium?What is fluvium? Soft, estuarine (river) depositsSoft, estuarine (river) deposits

What does the tower stand on? B Layer B: ≈ 20m thickLayer B: ≈ 20m thick Pancone marine clay Pancone marine clay flat, jumbled, loose particles flat, jumbled, loose particles Very soft and plasticVery soft and plastic Easily deformableEasily deformable

What does the tower stand on? Layer C: Thick layer of dense silty sand C

What are the problems with 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 side being compressed more than on North side.Lean caused by silt on South side being compressed more than on North side. South side silt has Pancone clay mixed in.South side silt has Pancone clay mixed in. Result = differential settlement = badResult = differential settlement = bad

What are the problems with building on these conditions? Built on floodplainBuilt on floodplain High groundwaterHigh groundwater Ground water rises in rainy seasonGround water rises in rainy season Greater rise on North sideGreater rise on North side

Characteristics of The Tower Height from foundations to belfry 60mHeight from foundations to belfry 60m Ring shaped foundation has external diameter of 19.6mRing shaped foundation has external diameter of 19.6m Tower weighs 14,500 tonnesTower weighs 14,500 tonnes The inclination is 5½ degrees towards the south; this means that the seventh cornice protrudes about 4.5m over the first cornice.The inclination is 5½ degrees towards the south; this means that the seventh cornice protrudes about 4.5m over the first cornice.

Construction History The Tower was constructed in the course of about 200 years.The Tower was constructed in the course of about 200 years. Work on the foundation began on 9 August 1173.Work on the foundation began on 9 August The construction of the belfry (8th order) began around 1360 and was completed c. 10 years later.The construction of the belfry (8th order) began around 1360 and was completed c. 10 years later.

Materials Used in Construction The following are the stone types used which come from areas adjacent to Pisa: the Monti Pisani and the Monti d'Oltre Serchio: Agnano BrecciaAgnano Breccia -internal wall facing "Bench" calcarenite"Bench" calcarenite -vaults and arches Filettole black limestoneFilettole black limestone -ornamental bands "Flinty" limestone"Flinty" limestone -concrete filling

Materials Used in Construction "Verrucano" quartzite"Verrucano" quartzite -used in foundation concrete GranodioriteGranodiorite -half base columns San Giuliano marbleSan Giuliano marble -most used incl. external walls

Re-emphasis of Subsidence Tower subsided vertically by 2.8mTower subsided vertically by 2.8m The differential subsidence is 1.89m; this means that the extreme north and south have subsided by 1.86m and 3.75m respectively.The differential subsidence is 1.89m; this means that the extreme north and south have subsided by 1.86m and 3.75m respectively.

Inclination of the Tower Progressive increase in inclination can be attributed to the instability of the equilibrium + the viscous plastic response from the ground.Progressive increase in inclination can be attributed to the instability of the equilibrium + the viscous plastic response from the ground. Increase in rotation induces increase in tendency to capsize; in turn, increases stresses + deformations and therefore a further increase in rotation.Increase in rotation induces increase in tendency to capsize; in turn, increases stresses + deformations and therefore a further increase in rotation – 1974 intense pumping of water from the subsoil led to an increase in the rate of rotation of the tower.1968 – 1974 intense pumping of water from the subsoil led to an increase in the rate of rotation of the tower.

Monitoring the Movement of the Tower 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 measurement of: - automatic system with sensors for the measurement of: 1.Wind 2.Air temperature 3.Seismic actions 4.Inclinations 5.Horizontal point movement 6.Width of the fissures 7.Levels within the stratum

Seismic Activity Area around Pisa has been subject to strong seismic activity.Area around Pisa has been subject to strong seismic activity. Studies estimate a grade 6 earthquake (Mercalli scale) has probability of occurrence every 130yrs.Studies estimate a grade 6 earthquake (Mercalli scale) has probability of occurrence every 130yrs. Grade 7 probability of occurrence every 520yrs and grade 8 once over 3000yrs.Grade 7 probability of occurrence every 520yrs and grade 8 once over 3000yrs. Analyses of response of towers show - limit would be reached in a grade 6 earthquake.Analyses of response of towers show - limit would be reached in a grade 6 earthquake. It would not survive a grade 7 earthquake undamaged.It would not survive a grade 7 earthquake undamaged.

Management of the Tower Professor John BurlandProfessor John Burland Currently responsible for the status of the tower.Currently responsible for the status of the tower.

Management Solutions Adopted 600 tonne lead ingots applied in 1993 – modest reduction of the inclination and arrested progressive rotation of tower.600 tonne lead ingots applied in 1993 – modest reduction of the inclination and arrested progressive rotation of tower.

Management Solutions Adopted Safety stays – correct unexpected small movements (temporary safeguard during works).Safety stays – correct unexpected small movements (temporary safeguard during works).

Management Solutions Adopted Soil extractionSoil extraction Involves placing tubes on the North side of the tower to remove soil.Involves placing tubes on the North side of the tower to remove soil. Modelling and trials to determine the exact placement of the tubes.Modelling and trials to determine the exact placement of the tubes. Special drill designed to avoid disturbance to surrounding ground.Special drill designed to avoid disturbance to surrounding ground.

Management Solutions Adopted Preliminary soil extraction over limited width in February 1999.Preliminary soil extraction over limited width in February Full soil extraction takes place over the full width of the foundations in February 2000.Full soil extraction takes place over the full width of the foundations in February Soil extraction finished in June 2001.Soil extraction finished in June 2001.

Management Solutions to be Adopted in the Near future Anchorage – medium intervention intended to substitute the weights, reducing the unsightliness of counter-weighting.Anchorage – medium intervention intended to substitute the weights, reducing the unsightliness of counter-weighting.

Possible Future Management Techniques Electro-osmosis – clay contracts upon absorbing electrical current under north of tower causes subsidence of base.Electro-osmosis – clay contracts upon absorbing electrical current under north of tower causes subsidence of base. Has been tested in a full-scale experimental site.Has been tested in a full-scale experimental site.

Summary Lean is caused by differential settlement of silt under tower.Lean is caused by differential settlement of silt under tower. Lean is also caused by uneven rise of groundwater.Lean is also caused by uneven rise of groundwater. Many attempts to correct lean.Many attempts to correct lean. The most recent and successful being soil extraction.The most recent and successful being soil extraction.