POSS-Lab

LONATE POZZOLO LABORATORY

 

The territorial laboratory of Lonate Pozzolo is aimed at studying the desiccation and the desaturation (i.e. the progressively reduction in soil water content, or soil moisture) phenomena in fine cohesive soils (silts and silty clays), resulting from the washing process of granular materials excavated from sand quarries. Wide pools (approximately 2000 m2, 6-7 meters high) are filled (flooded) with mud progressively depositing on the bottom under its own weight. During the evolution of time, a thick under-consolidated fine soil stratum is formed. A desiccation process progressively takes place, whereas in the upper part of the stratum a dessicated “crust” characterised by macroscopic superficial fissuring pattern appears.

An in situ campaign has been performed with the aim at describing both the desiccation/desaturation phenomenon and the consolidation of the soil stratum during its life cycle.
As a result of the campaign it has been possible to point out that the resulting soil stratum is homogeneous along depth but heterogeneous in the horizontal plane. This is due to a progressive decrease in grain size at increasing distance from the inlet pipe. During the evolution of time the material consolidation evolves progressively from the upper and lower boundaries and, because of the lack of homogeneity of the material, the phenomenon evolves non uniformly in the horizontal plane. The presence of lens of saturated materials is therefore quite dangerous when the settling basin is excavated. From a mechanical point of view, this process thus induces the formation of weak lenses between more resistant soil layers. These weak lenses are likely to induce dramatic failures (Figure 1) during the excavation process of the basin (taking place every 5-6 years, before a new filling cycle starts), sometimes realised by the “vertical cut” technique. The aim of this laboratory consisted in defining some practical guidelines to be followed for a secure managing of the excavation phase.

 

Figure 1: Failed front of excavation


The main results of this laboratory consisted:

  1. in a single day conference supported by Provincia di Varese during June 2005;
  2. in a small textbook, available in a pdf version by clicking here

textbook

 

The following activities have been performed:

  1. field monitoring system and data acquisition,
  2. laboratory characterization of the mechanical properties of the soil,
  3. modelling of the consolidation/desiccation and the failure processes.

A further action has been recently activated in collaboration with the Strathclyde University of Glasgow (http://www.strath.ac.uk/).

 

 

This action is granted by ICE (Institute of Civil Engineers), the Scottish Government and the East Ayrshire Council, to Dr. Philippe Sentenac and Dr Stefano Util, Strathclyde University of Glasgow (U.K.). The goals are

  1. to investigate the rate of desiccation for a newly constructed flood embankment made of Glacial Till;
  2. to develop new robust methodologies for monitoring the onset and extent of desiccation fissuring and differential settlements using non-intrusive geophysical techniques based on electrical resistivity for later use in condition inspection surveys where the risk of desiccation fissuring is known to be critical;
  3. to detect and map the desiccation fissuring network within a newly constructed embankment using environmental tracers

The cosen site is a flood embankment in Galston (40 km far from Glasgow; Figure 2), where several instruments have been installed in two monitored sections (sections A and B, Figure 3 and Figure 4) order to measure the water content and the suction within the soil, the air and soil temperatures, the rainfall and the air relative humidity.

 

Figure 2: Plan view of the monitored site

 

Figure 3: Monitoring devices in section A



Figure 4: Monitoring devices in section B

The data concerning the first two months of acquisition are shown in Figure 5..

Figure 5: Evolution of monitored data from September 20th to November 16th 2008

 

Research activities


1) Design of field monitoring systems and data acquisition

 

The processes taking place within the fine soil strata have been investigated in Lonate Quarries (Varese, Italy) in two different artificial basins: (i) a newly constructed basin (Figure 6a) in which was possible to investigate both the consolidation and the desiccation processes and (ii) an already closed basin (Figure 6b) in which mainly desiccation process was analysed.


Figure 6: (a) new pond; (b) old pond.

The consolidation process has been investigated by means of an electrical pressure transducer and a vibrating wire piezometer. The water pressure data collected (Figure 7) allowed to validate the FEM code used to simulate the consolidation process.



Figure 7:  time vs water pressure (red data) and temperature (green data)

The desaturation of the soil layers has been studied by considering the soil water content, the suction and the temperature. Both Time and Frequency Domain Reflectometry technologies (TDR or FDR) probes where used to investigate in time (Figure 8a) and in space (Figure 8b) the water content profiles, respectively. The water content has been observed to be strongly affected by the weather conditions up to 80 cm depth (Figure 8a), while no desiccation has been observed for deeper layers. A partial seasonal re-saturation of the superficial layers occurred during the fall/winter months (Figure 8b).



Figure 8: (a) evolution with time of the water content spatial profile; (b) water content variation in time at three different depths.

2) Laboratory characterization of the soil mechanical properties

This activity dealt with (i) the standard soil characterization and (ii) the definition of the partially saturated behavior of soil samples. The laboratory activities have been carried out both at Politecnico di Milano-Structural Eng. Department and at Università degli Studi di Milano Bicocca-Geology and Geotechnology Dept.

  • Standard laboratory tests on dry or saturated samples:
    A large amount of grain size distribution test have been carried out on different samples, collected along the entire ponds surface. These tests allowed to point out that the slope instability occurred only in silty layers. Atterberg limits, triaxial, oedometric and standard shear tests on saturated samples were also carried out on silty and clay samples. All these tests were carried out to define the mechanical/geotechnical properties of the soil layers investigated (e.g., unit weights, porosity, shear strength parameters f’, c’, etc)



Figure 9 :
triaxial test on clay sample,. (a) during and (b) after the test

  • Laboratory tests on partially saturated samples:
    In order to define a “constitutive law” for the partially saturated “Lonate silty clay” a large number of pressure plate extractors tests have been performed. The results obtained allowed to graph both a characteristic curve (Figure 10a) and a conductivity-suction relationship (Figure 10b) for the soil here studied.


Figure 10: (a) sandy and clayey soil characteristic curve and (b) clayey soil conductivity vs suction curve

 

The mechanical behavior has been studied by means of simple shear tests on non saturated clayey samples (Figure 11a-b).

Figure 11: (a) simple shear test apparatus, (b) increase on apparent cohesion

 

3) Modeling of consolidation/desiccation and failure processes

The three processes taking place in a basin (i) sedimentation-consolidation, (ii) desiccation and (iii) failure, have been simulated by means of three FEM numerical codes.

    • The sedimentation-consolidation process has been studied by means of Abaqus FEM code. The code has been validated by means of the water pressure values logged by the electric transducer. This approach allowed to define the right boundary conditions (e.g., a lower draining surface) at the basin bottom (Figure 12a). The code was used to simulate the pressure profiles up to five years after the basin closure (Figure 12b)

    Figure 12: (a) local water pressure evolution during the consolidation process and (b) water pressure profiles evolution after five years

    • For the sake of simplicity, the desiccation process has been simulated by means of SEEP/W code. This code allows to study both the desaturation induced by evapo-transpiration and the change in conductivity induced by suction in the deeper soil strata.

    Figure 13: variation in time saturation spatial profile

    The ABAQUS and SEEP/W numerical simulation allowed to define the consolidation and saturation conditions before excavation both of the sandy and clayey soil layers.

      1. The non-uniform distribution of pore pressure and saturation described before induces locally a different mechanical response of the material. The influence of saturation conditions on the excavation front stability during the basin dismantling operations has been studied with the GEO-Slope SLOPE/W code. Also superficial fissuring has been evaluated. The flow slide occurrence was studied by taking into consideration the planar distribution of saturated zones.

    Figure 14 : (a) failure mechanism, (b) influence of front side thickness in limit state conditions

     



    Figure 2 : Decrease in water content (desaturation) of the upper 80 cm layer

     

    Location

    Lonate Pozzolo (VA)

     

    Location