Long term piezometric data analysis to validate landslide models

M.G. Angeli C.N.R. Institute, Perugia, Italy

P. Gasparetto IQT Consulting S.r.l., Rovigo, Italy

F. Pontoni Geoequipe S.T.A., Tolentino, Italy

ABSTRACT: During the study of landslides dynamics, one fundamental moment is the definition of the model in describing the failure mechanism. In this context, the hydrological process knowledge is one of the most important element to validate the hypothesized model. The long term monitoring system provides us a powerful tool. In fact, annual or multi-annual piezometric series, together with displacement and other data, played a fundamental role on the landslide understanding. The analysis of different types of active landslide, stresses the importance of the knowledge of the hydrological conditions of the slopes before and after the re-medial works, respectively to define and validate the landslide model and to asses the role of the remedial works on the stabilization of the landslide.

1 INTRODUCTION

The paper deals with two very interesting situations where the long term monitoring systems helped on the landslide understanding, showing its fundamen-tal importance on the validation of the landslide model and of the effectiveness of the remedial works.

The first described case is a huge rock landslide involving the Montelparo medieval village in the Marche Region. The remedial works, consisting on deep drainage system, played a positive effects on the landslide stabilization.

The second huge landslide is located in Assisi, in the Umbria Region. The phenomenon is a roto-translational slide formed by bodies of slope-waste deposits on a marl-clays bedrock.

2 CASE STUDIES

2.1 The Montelparo landslide The village of Montelparo is affected by a large translational landslide that develops from 580 m ASL to 340 m ASL, with a length varying between 700 and 1100 m and an average width of 600 m (Fig.1). The landslide body is part of a monocline mostly formed of well stratified sandstones, posi-tioned on a deep-seated clay bedrock. A system of direct faults have favored the detachment of the sandstone slab (up to 65-70 m thick) sliding on a clayey sloping slip surface and the formation of a

well marked graben structure (Angeli 1981; Angeli et al. 1996; Angeli & Pontoni 2004).

Figure 1. Map of Montelparo landslide remedial works: (1) Microtunnel drainage system; (2) Tubular drains and drainage trenches; (3) Shallow hydraulic control measures; (4) Land-slide area; (5) Cross-section trace.

-95-

Figure 2. View of the Montelparo landslide area (from the East).

Figure 3. The Montelparo landslide model (cross-section A-B in Fig.1).

Figure 4. Graben structure (the horizontal arrow indicates the moving part of the village).

Due to the erosion operated by the stream flowing

at the toe of the slope, the lowermost part of the landslide phenomena are characterized by progres-sive disruption in blocks of the sliding body, which is there long and thin. These blocks are then divided by cracks up to 3-5 m deep and 2-3 m wide.

In other words, the sandstone hill on which Mon-telparo village is founded is splitted into two parts: the uphill part that is stable, whereas the downhill part is moving (Figs. 3 - 4). In the short term the critical hydraulic conditions are similar to the ones occurring in rockslides, where a triangular water pressure diagram operates inside the graben area (on the subvertical face of the moving mass) and a rectangular one acts on the sloping clayey slip surface. The monitoring system allowed us to detect the water table critical values beyond which movements take place.

According to the mechanism invoked, any in-crease in water level makes the water thrusts (on the rear of the landslide body and at its base) increase exponentially inducing movements of the landslide whenever it reaches the detected critical values. Hence the importance of maintaining the water lev-els permanently low, well below the critical values.

The paper besides discussing on the landslide mechanism introduces the main control works de-signed in order to prevent any acceleration of the movement and to stabilize the landslide. They be-long to the category of the deep drainage, useful to increase the shear strength on the slip surface.

-96-

According to the peculiar geomorphology of the site and to the deep seated location of the slip surface two main subcategories of remedial works were de-signed (Figs. 1, 3), apart from the more superficial control works.

Along the slope a system of long sub-horizontal tubular drains radially running from different levels intercepts the slip surface with the aim to reduce the water pressures inducing movements in the area ex-tending from the toe of the slope to the graben area.

In the graben area and specifically in the down-hill part a compounded drainage system removes the hydraulic causes of the movement, collecting all the water well outside from the core of the landslide.

Figure 5. Cumulative precipitations, piezometric levels and in-clinometric displacements in the upper part of the Montelparo landslide.

This last system is made out of a large diameter

microtunnelling (Angeli & Pontoni 2000) crossing transversally the graben, well beneath the slip sur-face, with a system of subvertical tubular drains automatically perforated from inside the tunnel. The tubular drains collect the water from the graben wedge and carry it into the tunnel. Due to the incli-nation of the tunnel the water naturally moves out-side, preventing any thrust on the landslide body.

Figure 6. Cumulative precipitations, piezometric levels and in-clinometric displacements in the middle part of the Montelparo landslide

Some of these remedial works, in particular the

system of long sub-horizontal tubular drains and the shallow hydraulic control measures, was carried out in the period showed in the figures 5-6 (Fig. 7).

The hypothesized mechanism of the landslide and the effectiveness of the control measures done, was confirmed by the observation of the movement ac-celeration in coincidence with significant recorded piezometric peaks (Fig. 5, 6).

Figure 7. Tubular drain drilled in the medium part of the land-slide body (Fig.3).

In the Figures 5-6 the inclinometric displacement is referred to a pipe moving at 41.5 m of depth (Fig. 3) and at 23.5 m of depth (Fig. 4). It is very evident the positive effects on the reduction of the piezometric level and on the induced movements stopping after the remedial works was carried out.

2.2 Assisi landslide

The landslide body consists of slope-waste deposits, coming from the Scaglia Rossa Formation (lime-stone or marly limestones). The length is more than 1500 m, whereas the average width is about 600 m (Figs. 8, 9).

Figure 8. Aerial view of the Assisi landslide.

The depth of the slip surface varies from 0 to 60

metres (Angeli & Pontoni, 2000).

-97-

Where the landslide body is more superficial (mid-dle-lower part) the envelope of different slip sur-faces are an inclined plane (10°-12°). Where the landslide body is deeper (upper part) the slip surface are partly curvilinear and partly sub-horizontal.

The middle-lower part is densely inhabited, whereas the upper part is free of any building or arti-ficial structure. The cross section of the body shows an asymmetric “V” shaped channel, with its bed co-inciding with the clayey bedrock. This main chan-nel, together with some secondary branches, collect all the groundwater released by the karstic massif (located above the landslide area) together with the water directly infiltrated in the catchment area. In the upper part of the landslide body a very high hydraulic head can be found, whereas in the middle-lower part the groundwater table tends to decrease. The very high hydraulic conductivity observed in the upper part of the landslide (Pumping tests in the piezometric tubes), due to the presence inside the stratified slope-waste deposits of several very nar-row sub-vertical cracks connected with the main scarp (Fig. 9), makes this portion of landslide body working as a very high permeability tank. This tank (of water accumulated from the top of the Mt. Sub-asio or directly from the infiltration of rainfall in the landslide catchment basin) works releasing water into the uncracked structure of the downhill slope-waste deposits. The conductivity values were de-tected using in situ permeability tests too (Lefranc tests). The obtained values (10-4-10-7 m.sec-1) shows an high variability due to the lithology of the slope waste deposits.

In addition, taking into account the bedrock gra-dient and the delay between the observed piezomet-

ric peaks during a critical hydrological situation which occurred in December 1998, an equivalent hydraulic conductivity was estimated (about 10-2 m.sec-1) throughout the whole slope.

The clayey bedrock soil can be considered com-pletely impermeable if compared with the above slope-waste deposits.

The presence of only two porous media along the slope with very different permeabilities (the upper slope-waste deposits and the lower clayey materials belonging to the bedrock) made easier the definition of the flow net in such a large slope.

In fact, in the upper medium there is an uncon-fined aquifer free-flowing above the impermeable bedrock. Under steady state flow conditions the slope can be assimilated to a homogeneous infinite slope overlaying an impermeable bottom. The groundwater table never reaches the ground surface, maintaining itself inside the very thick slope-waste deposits, except for the uphill landslide area, where the hydraulic heads always look very high and the impermeable bedrock is almost sub-horizontal. Un-der transient flow conditions the flow net does not change very much.

Only piezometric peaks overlaying the steady state groundwater table move very rapidly along the slope and in a few days they tend to disappear, due to the high permeability of the slope-waste deposits. When these peaks occur a movement also propa-gates from the top to the bottom of the slope.

In fact the movement appears to be impulsive, even though a part of it (especially in limited areas) seems to be almost continuous (Fig. 10).

Figure 9. Assisi landslide model with a simplified scheme of the control works: 1) slope-waste deposits (landslide body); 2) clayey marls; 3) calcareous marls; 4) marly limestones; 5) normal fault; 6) slip surface; 7) boreholes; 8) microtunnelling cross section with tubular drains; 9) microtunnelling longitudinal scheme.

-98-

Figure 10. Piezometric levels and inclinometric displacements in the Assisi landslide (the inclinometric tubes are located from the top to the toe of landslide: S202 – S113 – S111 – S103; the piezometric pipe are located in the middle).

An automatic recording system of all parameters

involved in the landslide mass was installed at the end of 1998 (Fig. 10).

Therefore, in trying to capture as much ground-water as possible from the upper part of the slope, the stability conditions of the slope should improve drastically, as numerically checked by inputting into the stability analysis the shear strength obtained from laboratory tests of clay samples collected from the slip surface (Φ’r = 17°, medium value). The Fs values referred to the present conditions (with no operating drainage system) are close to 1. The planned conditions (drainage systems operating from the microtunnel) give a sensible improvement of the Fs, up to 20%.

The real difficulty was to define a drainage sys-tem capable of collecting groundwater deeply in the ground.

The innovative idea adopted of using a large di-ameter microtunnelling technique (Angeli & Pontoni 2000), with a shield that automatically and precisely advances inside the ground, well beneath the slip surface, without any operator inside and simultane-ously lining the hole with large diameter concrete tubes, seemed to be a reasonable and low cost solu-tion, in comparison with traditional tunnelling tech-niques (Fig. 11).

An automatic drilling device (with no operators inside the microtunnels), fits to perform the planned long tubular drains running from the microtunnelling toward the portions of landslide body, completes the innovation.

Figure 8 shows the position of the already drilled tunnels and of the planned tubular drain systems. From the monitoring system data (Fig. 10) it is pos-sible to see that there is no influence of the drilled tunnels on the groundwater table, and, consequently, on the landslide displacements, due to the not per-meable tunnel linings.

Figure 11. Microtunnelling (2000mm diameter) drilled from the toe of the landslide (Fig. 9).

3 CONCLUSION

The treated examples stress the fundamental role played by the groundwater table in the landslide body and the positive effects due to its lowering.

In the described cases, the landslide model defini-tion and its validation by means of the long term monitoring system gave us the keys to study and plan the remedial works consisting on the lowering of the ground water level.

In addition, the very long term monitoring sys-tems allow to assert that the remedial works achieved in the Montelparo landslide the expected good results.

As regard the Assisi case, the stability analysis carried out give us the same good result when the drainage system will operate.

REFERENCES

Angeli M.-G. 1981. Sulle condizioni di stabilità dell'abitato di Montelparo. Annali della Facoltà di Agraria, Università di Perugia, vol. XXXV, 207-222.

Angeli M.-G., Bisci C., Burattini F., Dramis F., Leoperdi S., Pontoni F., Pontoni F. 1996. Two Examples of Large-Scale Landslides Affecting Built-Up Areas in the Marche Region (Central Italy). Quaderni di Geologia Applicata, vol. 1, 131-140.

Angeli M.-G., Leonori S., Leoperdi S., Pontoni F. 2000. From analysis to remedial works: the case of Montelparo Land-

-99-

slide (Central Italy). Proc. of Natural Hazards on Built-up Areas Intensive Course – European Center on Geomor-phological Hazards, Camerino, Italy, Sept. 25th – 30th 2000.

Angeli M.-G. & Pontoni F. 2000. The innovative use of large diameter microtunnelling technique for the deep drainage of a great landslide in an inhabited area: the case of Assisi (Italy). Proc. of VIII ISL, Cardiff, 26-30 June 2000, vol. 1, 1665-1672.

Angeli M.G. & Pontoni F. 2004. Analysis of a large-scale landslide affecting an ancient village in Central Italy. In: Landslides: Evaluation and Stabilization/Glissement de Terrain: Evaluation et Stabilisation, Set of 2 Volumes: Proceedings of the Ninth International Symposium on Landslides, June 28 -July 2, 2004 Rio de Janeiro, Brazil – Ed.: W. Lacerda, M. Erlich, S.A.B. Fontoura, A.S.F. Sayao, Taylor & Francis Group, London UK, pp. 1265 – 1270.

-100-