Monitoring of Pore Water Pressure in Permafrost

Permafrost and Mountain Glaciers.

Permafrost refers to soil: for example, rock or loose sediments, the temperature of which is constantly maintained at 0°C or below.

During the summer period, thawing occurs in near-surface areas - the active layer. Permafrost covers about four percent of Switzerland's territory and is predominantly found at an altitude of 2500 m above sea level. In mountainous regions, there are two main types of permafrost:

• ice-poor permafrost in rock formations, where ice is found only in cracks, crevices, fissures, and pores of the frozen soil.
• ice-rich permafrost at the foot of steep slopes, where deposits from mass movements (avalanches and rockfalls) accumulate and form a substrate oversaturated with ice and containing more ice than rock.

In this study, we focus on rock glaciers (Fig. 1a).

Figure 1.

a) Drone shot of the Schafberg-Ursina rock glacier, Pontresina, Switzerland (photo: A. Bast). The tongue-shaped structure with a coarse-blocky surface, steep frontal lobes with strongly inclined edges, pronounced ridges, and depressions is clearly visible. The white arrow indicates the location of the borehole with piezometers.

b) Drilling of a borehole on the rock glacier in August 2020 (photo: N. Bühler). c) Piezometer KELLER PAA-36XiW with textile protection (photo: M. Phillips).

In some cases, considering soil characteristics and installation conditions, a pressure transducer with a conical sintered steel filter tip is used for "pore pressure".

They are a characteristic feature of ice-rich mountain permafrost. They represent tongue-shaped landforms consisting of layers of ice and rock that move downward at speeds ranging from a few centimeters to several meters per year. They transport loose rock material downslope like conveyor belts, and their steep frontal lobes are potential source zones for rockfalls and debris flows.

Movements of Rock Glaciers and Water.

In the Alps, acceleration of rock glacier movement has been recorded, increasing the likelihood of mass movements such as debris flows originating from the frontal lobes on steep terrain. The acceleration can be explained by climate change and the associated warming of permafrost, which is accompanied by an increase in water content in ice-rich permafrost.

 Ice and water content in mountain glaciers have been modeled to study future water availability. To study glacier movement, hydrology is examined, aerial photographs, GNSS (Global Navigation Satellite System) data, meteorological data, and snow cover timing are analyzed. Additionally, water runoff from rock glaciers was determined and their potential moisture capacity assessed.

Until 2020, no direct measurements of water in permafrost had been conducted. However, such direct information on the hydrology of mountain glaciers, changes in ice and water content, and the formation of unfrozen zones, taliks, is necessary for a better understanding of mountain glaciers and their movement. Depending on soil properties, salinity, and pressure, a significant portion of unfrozen water can exist at temperatures below 0°C, as shown, for example, by ground-penetrating radar measurements conducted by Muzil (2006) on the Muragl rock glacier, Oberengadin, Switzerland.

New Measurement Method for Monitoring Mountain Glacier Hydrology.

 Rock glaciers are near the ice melting point, and temperature data in boreholes alone do not allow distinguishing ice from water, as both substances can coexist at around 0°C. This means that relative changes in ice and water content must be tracked using other methods, such as geophysical methods and/or piezometric measurements. This is important because water content in permafrost partly determines how fast ice-rich permafrost moves.

In summer 2020, we drilled three boreholes in the Schafberg Ursina rock glacier, north of Pontresina, Oberengadin in the Eastern Swiss Alps (Fig. 1 a and b). One of the boreholes is equipped with ten KELLER PAA-36iW piezometers at depths from 2 to 8.5 m (Fig. 1c).

In the other two boreholes, multi-core cables were installed for electrical resistivity tomography (ERT) in the transverse direction. Based on ERT data, soil resistivity models can be created, providing information on relative changes in water and ice content and complementing piezometric data.

Piezometer data show the development of effective pressure measured on the sensor membrane (measured relative to vacuum; pressure range 60-230 kPa, accuracy ±11.5 kPa). The sensors were combined with ten PT 1000 temperature sensors (accuracy ±0.1°C). Before installation, the sensors were greased with petroleum jelly and wrapped in thin material (face masks) for protection. The sensors are connected to two KELLER ARC-1 boxes with 4G data loggers, which also contain a barometer. Data are collected hourly and transmitted daily to a cloud data platform via the mobile phone network.

Results.

Pore water pressure sensors have never before been used in ice-rich glaciers. The first results are promising. Clear signs of water presence in the ice-rich glacier are evident, confirmed by the stratigraphy of boreholes recorded during drilling in August 2020.

Figure 2:

b) Borehole stratigraphy in August 2020 and position of the KELLER piezometer in the borehole (blue dots; GOF: ground surface).

From 2021 to 2023, water content decreased due to low precipitation and soil cooling as a result of two low-snow winters (Fig. 2a and b). Lower temperatures and dry conditions allowed the mountain glacier to cool and freeze, which also slowed the glacier's movement rate. At some depths, water content increases during snowmelt or after heavy rains, indicating lateral water flow in the rock glacier. Piezometric data are consistent with ERT measurement results.

 Laboratory experiments.

Since piezometers have not previously been used in mountain permafrost boreholes, the obtained data should be interpreted with caution. If soil temperature drops below 0°C, ice formation can strongly affect the pressure measured inside the sensor housing and thus may not reflect the prevailing pressure at that depth.

Harris and Davis (1998) encountered similar problems in their laboratory experiments. Currently, other experiments are being conducted under controlled conditions in the SLF cold laboratory to determine the behavior of KELLER PAA-36XiW sensors at varying soil ice and water contents. However, piezometers on the ice-rich Schafberg glacier provide meaningful data and indicate the presence of air, water, and/or ice, as well as seasonal pressure fluctuations in wet layers. The data presented here highlight the heterogeneous and seasonally variable nature of the substrate, confirmed by contrasting borehole stratigraphies.

 Sensors in challenging conditions.

The following technical difficulties arose during sensor installation:

Collapse of borehole walls between drill bit removal and sensor installation, requiring the use of a stabilizing PVC pipe in the upper 4 m of soil.

Establishing effective contact between sensors and borehole walls.

The material used to fill the borehole differs from the original rock glacier substrate. It is unknown whether the fill material subsequently settled, whether sensors were effectively sealed during filling, and whether voids existed. Rock glaciers are dynamic landforms. Processes such as settling, creep, or changes in ice and water content may pose challenges for the long-term preservation of sensors in uncased boreholes.

 Conclusion.

Initial analyses show daily, as well as seasonal and long-term changes in water content in rock glaciers.

This information will help fill the gap in direct quantitative determination of water content in rock glaciers and achieve a better understanding of the causes of rock glacier movement.

Piezometric data provide valuable information about local substrate characteristics of rock glaciers. These data contribute to our understanding of factors controlling the kinematic acceleration of rock glaciers and the future water supply of these landforms.

KELLER PAA-36XiW Transducer

Autonomous Transmitter with Data Logger ARC1

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