Studies, investigations, and time monitoring regarding the stabilization of a structure located close to the top of an unstable slope

The paper presents studies carried out to restore safety to a metal pillar belonging to a high-voltage electric line that supplies an important city. The studies were necessary considering the position of the pillar near the top of a slippery slope that could affect the construction by the loss of slope stability. The area on which the pillar is located is very narrow. Also, the pillar does not have all the foundations on the property, and the neighbors did not allow the partial execution of the intervention works on their land. Geotechnical investigations and stability analyses were carried out and multiple perimeter stabilization solutions were proposed for the pillar location, but in the end, a stabilization solution was applied with drilled piles on two strings anchored by tie rods to a pilled massive raft foundation only on the property ground. During monitoring, measurements indicated that the stabilization works carried out led to securing the safety of the perimeter location of the pillar.

INTRODUCTION

1. Stability of natural slopes

Slope stability assessment is an important, interesting, and challenging aspect of geotechnical engineering. Despite the progress made, the assessment of slope stability remains a challenge (SACHPAZIS 2016). Water plays a role in many of the processes that reduce soil resistance; water is also involved in many types of slope loading cases that increase the shear stress along the failure surface. Theoretically, any slope with a factor of safety above 1.0 should be stable. In practice, however, the level of stability is rarely considered acceptable unless the safety factor is significantly greater than 1.0 (SACHPAZIS 2016).

Landslides are often caused by processes that increase the shear stress and/or reduce the shear strength of the ground along the failure zone; an accurate evaluation of both shear stress and shear strength provides the necessary data to efficiently design an appropriate stabilization work (ABRASON et al. 2002, SACHPAZIS 2013).

2. Expansion of inhabited areas

Urban development in the metropolitan cities area has been accelerated by population growth and high demand for housing conditions. In some situations, the city expansion took place along the natural slopes, hence the necessity to cautiously plan the location of new buildings in the crest area of the slope.

When the loading from the structure is very high or the point of action is too close to the crest, the shear stress along the potential failure surface will become greater than the shear strength of the soil, resulting in reduced slope stability, potentially failing (ZHIHAI et al. 2019).

3. Methods of stabilization

In the vast majority of cases of complex interventions to stabilize the ground and/or constructions, combined traditional stabilization solutions are used, such as bored piles with anchors into the ground, and bored piles with geosynthetics as in terra-mesh systems.

In this paper, the use of a support system made of bored piles anchored by a stable bulb with the role of a spur also made of bored piles is presented and discussed using 3D models based on Finite Element Method (FEM), intended to obtain the smallest possible displacements.

CONSTRUCTION AND SITE DESCRIPTION

1. Historical data

According to the information received from the beneficiary, the metal pole was made around the 1970s.

A stability loss of the pole’s ground area of influence could have resulted in the collapse of the pole itself and put the power line out of service, with dramatic consequences on the electricity supply to consumers and causing serious human accidents.

Comparing the topographic surveys carried out on the site at the time of the start of the safety works and the topographic survey carried out in 1983, it can be seen that, over the years, poor and unprofessional fillings were made near the pillar in order to expand the available construction areas.

By overlaying the topographic elevations (fig. 1), it is found that the pillar was located in 1983 at 20 m from the crest of the slope, and in 2017 at 25 m from the crest, after a soil movement of about 5.00 m in 2016.

Figure 1: Overlay of the 2017 topographic survey with the 1983 topographic survey

2. Description of the construction

The existing construction is a latticed metal pole with a leg spacing of 7.75 m to 5.60 m, and a height of 40.00 m. Following the manual excavations to uncover the foundations of the pillar (photo 1) it was found that the foundations are isolated (fig. 2), the footing has the dimensions of 4.35 m x 4.35 m, a height of 4.55 m, and a foundation depth of 4.00 m compared to the elevation of the natural terrain (fig. 2).

As two of the isolated foundations of the pole are on private land, interventions only for the accessible two foundations were considered.

Photo 1: Foundation excavation and pillar

Figure 2: Foundation layout plan

3. Evolution of landslides over time

By analyzing fig. 3, it can be seen that the slide is progressive and it advances towards the pillar during periods of precipitation, whenever water loss from the water supply system occurs, in the event of an earthquake, with detachment steps parallel to the current top of the slope and to the line of top of slope from 1983.

Legend of slope top evolvement:

1. Orange – Slope top before the last slide (2016)
2. Red – Actual slope top (since 2017)
3. Blue – Previous slope top (1983)
4. Magenta – Possible future steps of failure

Figure 3: Possible evolution of sliding steps

4. The geotechnical context

The fieldwork consisted of three 350 mm diameter mechanized geotechnical boreholes to a maximum depth of 25.00 m, as well as 2 DPSH-B super-heavy dynamic penetrations between the pillar and top of the slope, to determine the thickness, the limits and the extent of existing fillings according to fig. 4.

Figure 4: Plan view with the location of the geotechnical investigations

Groundwater was intercepted during investigations at 12.60 m in borehole F01. At the bottom of the slope, the water is located at the level of the ground surface. Water does not have an ascending character.

Analyzing the geotechnical investigations, it is found that the thickness of the fillings layering over time towards the top of the slope is quite large and it varies from 1.00 m in the F1 borehole performed near the pillar, up to 10.00 m in DHSH-B and in the F borehole (+159.00 m ) made at the top of the slope.

On average, near the pillar, from -0.20 m to -1.50 m, the soil contains unconsolidated fillings: from -1.50 m to -8.00 m, a silty clay mixed with clayey silt ‒ plastic hard, from -8.00 m to -11.50 m, a clayey sandy silt mixed with silty sand, from -11.50 m to -12.80 m, a clay with sand intercalations, from -12.80 m to -16.80 m, a clayey silty sand mixed with sand and beyond this depth the ground is composed of hard marly clay.

4.1. Characteristic and design values of the geotechnical parameters

The determination of the characteristic and design values of the geotechnical parameters was performed according to SR EN 1997:1-2004 (SR EN 1997:1, 2004) and Romanian Design Norm (NP 122, 2010). The design values are determined based on Eurocode SR-EN 1997:1-2004 in accordance with the Design Approaches concept. Following the calculations made, the Design Approach 1 ‒ Combination 2 presents the most unfavorable results, consequently only the results of the analyses performed for this approach will be presented and discussed.

Considering the uneven stratification of the soil layers intercepted in the boreholes, statistical processing of the geotechnical parameters values was performed. The values used for the partial factors on Design Approach 1 – Combination 2 for permanent and variable unfavorable actions (A2), γ_G,unfav = 1.00, γ_Q,unfav = 1.30, and for soil parameters (M2), γ_φ = 1.25 for (the tangent of the) internal friction angle, γ_c = 1.25 for cohesion and γ_γ = 1.00 for volumetric weight, are presented in Table 1.

Table 1: Design values for geotechnical parameters

STABILITY ANALYSES

Based on the topographic survey carried out on site in the zone of interest and the geotechnical design values from the previous chapter, multiple stability analyses were performed to evaluate the slope stability at present and in various design scenarios. In the stability calculations, considering the static and seismic loading conditions, the following situations were analyzed:

• Pillar foundation on the slope, without any stabilization works;
• Pillar foundation on the slope, considering 2 rows of stabilizing piles;
• Pillar foundation on the slope, considering 2 rows of stabilizing piles, also anchored by tie rods to a massive pilled spur raft.

The software uses the design approaches according to SR-EN 1997:1-2004, thus a stability factor higher than 1.00 indicates stability.

1. Pillar foundation on the slope, without any stabilization works

The stability analyses carried out in the existing situation (fig. 5) confirm the existing loss of stability in both static and seismic loading conditions, with ground displacements downstream of more than 40 cm, as presented in Table 2.

Figure 5: 3D modeling of the pillar foundation on the slope, without any stabilization works

2. Pillar foundation on the slope, considering 2 rows of stabilizing piles

In the stability analyses carried out in the proposed possible situation study with 2 rows of support piles, positioned at 12.00 m from the pillar (fig. 6), the values of the stability factors are higher than 1.00, in the situation without earthquake and, at the limit, in the situation with earthquake, according to Table 2, with lower downstream earth displacements of 12 cm.

Figure 6: 3D modeling of the pillar foundation on the slope, considering 2 rows of stabilizing piles

3. Pillar foundation on the slope, considering 2 rows of stabilizing piles, also anchored by tie rods to a massive pilled spur raft

In the stability analyes carried out in the proposed stabilization with 2 rows of piles positioned at 12.00 m from the pillar, anchored by tie rods to a massive pilled spur raft, positioned at 5.00 m from the pillar (fig. 7), the values of the stability factors are higher than 1.00, both in the situation without earthquake and in the condition of an earthquake, according to Table 2, with lower downstream ground displacements of 2 cm.

Figure 7: 3D modeling of the pillar foundation on the slope, considering 2 rows of stabilizing piles, also anchored by tie rods to a massive pilled spur raft

4. Results of the stability analyses

Following the stability analyses presented previously, the Table presents the values of the stability factors and ground displacements.

The displacements specified in Table 2 for situation 1 represent the displacements at the surface of the ground at a distance of 12.00 m from the pillar.

The displacements in Table 2 for situations 2 and 3 represent the displacements at the top of the piles related to the proposed stabilization solution, at 12.00 m from the pillar.

Table 2: Stability factors and ground displacements

THE STABILIZATION SOLUTION

The adopted solution refers to the one with 2 rows of stabilizing piles anchored by tie rods to a massive pilled spur raft structure, which consists of 2 elements (fig. 8):

• bored piles arranged in a „checkerboard” pattern, in two rows, connected by a reinforced concrete raft at the top;
• massive pilled spur raft made of radially arranged bored piles and thick raft at the top; the raft has 3 tie rods that anchor the 2 rows of bored pile to itself; the tie rods are made of steel profile CHS mounted inside reinforced concrete beams.

The reinforced concrete piles have a diameter of D = 800 mm and a depth of 25.00 m measured from the elevation of the ground surface (fig. 8).

The drilling rig was used in such a way that it did not produce any significant vibrations in the terrain, by adopting a low speed of the drilling rig.

Figure 8: Plan view and cross-section of the construction location including the stabilizing solution

MONITORING EQUIPMENT AND RESULTS

During the stabilization works, two bored piles, T03 and T04, were instrumented with inclinometers, as presented in fig. 9. The results of the measurements are presented in fig. 10.

Figure 9: Layout plan of two piles instrumented with inclinometers

Figure 10: Inclinometer measurements for the two bored piles

CONCLUSIONS 

Although the pillar is located on relatively flat ground, the small distance of only 25.00 m from the crest of an unstable slope resulted in the sliding of the pillar putting the power line out of service, with significant negative consequences on the electricity supply of consumers in that specific area and with the occurrence of serious human accidents.

Through the adopted stabilization solution, the aim was to create a stable „island” around the site of the pillar.

Considering that the good foundation soil is located at a high depth (over -16.00 m), the piles should have presented unrealistic depths of over 40.00 m to ensure an embedment length of at least 2/3 of the pile length.

Anchoring the top of the two rows of bored piles by tie rods to a massive pilled spur raft resulted in the reduction of displacements at the top of the piles by 85%.

As a result of the measurements during the monitoring, it appears that the stabilization works led to the safety of the perimeter location of the power station, with displacement values at the upper part of the piles below 7.00 mm.

ACKNOWLEDGEMENTS 

We are grateful to Associate Professor Dorel PLATICA Ph. D (Technical Expert in Geotechnical Engineering) and member of the Romanian Society for Geotechnics and Foundations for his useful comments and for assisting with his knowledge on the case study presented in this paper.

REFERENCES

[1] SACHPAZIS, C.I. (2016), Probabilistic Slope Stability Evaluation for the New Railway Embankment in Ethiopia. Published in Volume 21 – 2013, Bund. 11 of the Electronic Journal of Geotechnical Engineering (E.J.G.E.);

[2] SACHPAZIS, COSTAS I. (2013), Detailed Slope Stability Analysis and Assessment of the Original Carsington Earth Embankment Dam Failure in the UK. Published in Volume 18 – 2013, Bund. Z of the Electronic Journal of Geotechnical Engineering (E.J.G.E.);

[3] ABRAMSON, L.W., LEE, T.S., SHARMA, S., and BOYCE, G.M., 2002. Slope Stability and Stabilization Methods. John Wiley & Sons Inc, pp.712;

[4] ZHIHAI, Z., ZHANGJIAN, X., YANJIE, S., YUNOU, S. (2019), Influence of Building Load on the Stability of Loess Slope. Published in International Conference on Civil Engineering, Materials and Environment (ICCEME 2019), Paper No. 94, 5 pp, DOI: 10.25236/icceme.2019.019;

[5] SR EN 1997-1 (2004). Eurocode 7: Geotechnical design. Part 1: General rules;

[6] NP 122 (2010). Normative regarding determination of characteristic and calculation values of geotechnical parameters.

Authors:

Paul ȚURCANU, Ph.D Student;

Dana-Mădălina POHRIB, Senior lecturer, Ph.D;

Irina LUNGU, Professor, Ph.D ‒ Faculty of Civil Engineering & Services, Gheorghe Asachi Technical University of Iași, Romania

[Proceedings of the 17th Danube European Conference on Geotechnical Engineering (17DECGE), June 7-9, 2023, Bucharest, Romania – https://17decge.ro/]

…citeste articolul integral in Revista Constructiilor nr. 213 – mai 2024, pag. 50-53