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Hydrotechnika Journal. 2022. No. 4. pp. 58–61.
Anton Vladimirovich Legontsev, Sergey Nikolaevich Dolgikh (Yakutniproalmaz Institute, ALROSA PJSC, Mirny, Russia)
Construction conditions in permafrost soils require particularly careful selection of technologies to ensure reliable and safe operation of structures from both technical and environmental perspectives. Experience in the design and construction of mining complex facilities in the permafrost zone demonstrates the effectiveness of geosynthetic materials. They were successfully used at the deposits of the Nakyn Kimberlite Field (including the «Maiskoye» deposit), located in the Sakha Republic, in the basin along the middle reaches of the Markha River and between its left-bank tributaries, the Khannya and Nakyn Rivers.
Climate characteristics of the area
The area under consideration is part of Nyurba District. It is located 205 km northwest of Nyurba, 320 km northeast of Mirny, on the left bank of the Markha River, between the Khannya and Nakyn Rivers.
The area has a sharply continental climate with a cold, long winter and a short, warm summer, a wide range of temperature fluctuations, and short transitional periods. The average annual air temperature over the long-term period is –12.1 °C (Eyk weather station data). The coldest month is January, with an average monthly temperature of –37.3 °C. The warmest month is July, with an average monthly temperature of 14.6 °C. The average annual precipitation in the area is 246 mm. From April to October, 218 mm of precipitation falls, while 28 mm falls from November to March. According to data from the Nyurba weather station, most precipitation (63%) falls as rain, while one-third of the total (30%) falls as snow.
Protection of the open pit from water inflows
The open pit at the «Maiskoye» deposit is located in the floodplain of the Dyakhtar-Yuryage Creek. To protect it from water inflows, the project provides for pit dewatering, construction of a diversion channel on the northwestern side of the pit, a flow-directing embankment and protective earth embankments.
Diversion channel
The channel is intended to divert the Dyakhtar-Yuryage Creek beyond the mining area and intercept surface runoff from the catchment area. The maximum surface-water flow rate with a 5% exceedance probability for the channel is 24.38 m3/s. At the design flow rate, the maximum water depth in the channel will be 1.94 m, and the maximum flow velocity will be 1.27 m/s.
Excavated soils are stockpiled along the pit-side bank of the channel and are also used as fill for the flow-directing and protective embankments at the inlet and outlet of the diversion channel.
The main parameters of the diversion channel are: route length, 1570.0 m; average gradient, 0.001; bottom width, 6 m; side slopes, 1:2; maximum depth, 8.23 m; average depth, 4.41 m; excavation volume, 283.7 thousand m3.
The diversion channel passes through frozen ice-rich soils consisting of sands and loams. The following channel reinforcement options were considered:
1) Reinforcement of the channel with a geocell filled with crushed stone (ТУ 224600256910-145-2011).
2) Construction of the channel using corrugated metal structures.
3) Reinforcement of the channel with a flexible concrete mat (СТО 56910145-025-2017).
Based on a technical and economic comparison of the options and an assessment of the risk of possible pit flooding, reinforcement of the channel with a geocell (ТУ 2246–002–56910145–2011) 150 mm high and filled with 20–70 mm crushed stone was selected.
To protect the permafrost soils beneath the channel reinforcement structure from thawing, the project provides for a 1.2 m-thick sand insulation layer; the layer thickness was determined by thermal engineering calculations. To ensure maximum water tightness and reliability under challenging construction conditions, it was decided to install waterproofing over the sand layer using bentonite mats (СТО 30478650–006–2014). Excavated channel soils are used as fill for the flow-directing and protective embankments; excess volumes are stockpiled in windrows along the pit-side channel at a distance of 10 m from the excavation edge. To prevent premature hydration of the material, a 0.3 m-thick protective sand layer with a compaction coefficient of 0.95 m is placed over the bentonite mat.
To redirect water from the Dyakhtar-Yuryage Creek into the diversion channel and protect the «Maisky» pit from flooding, the project provides for a flow-directing embankment. A road is constructed along its crest from the «Maisky pit — Botuobinsky pit» road to the diversion channel. The main parameters of the flow-directing embankment are: crest width, 40 m; side slopes, 1:2; average fill height in the channel, 2.4 m; fill volume, 35.5 thousand m3.
To prevent flooding of the «Maisky» pit when water returns from the diversion channel to the natural channel, a protective embankment is provided. A road is constructed along the embankment crest from the «Maisky pit — potentially fertile soil storage area (ППГ)» road to the diversion channel. The main parameters of the protective embankment are: crest width, 40 m; side slopes, 1:2; average fill height in the channel, 2.3 m; fill volume, 112.8 thousand m3.
The channel slopes and bottom are reinforced with a geotechnical geocell (ТУ 2246–002–56910145–2011) filled with crushed stone and installed over an underlying layer consisting of two layers of nonwoven «Kanvalan MF 30» fabric (СТО 8397–007–69093357–2013). In accordance with ОДМ 218.3.032–2013, the geocell is fixed with 1.2 m-long anchors at a spacing of 0.75.0.75 m to prevent uplift and failure along the entire channel and in the scour hole. Due to the specific design of the channel foundation, the anchors are installed by puncturing the waterproofing layer of bentonite mats. The ability of bentonite mats to self-heal minor damage makes it possible to implement these design solutions without compromising the waterproofing.
To dissipate the water energy at the channel outlet, a gradually widening section is constructed. Its bottom width increases from 6 to 19 m over a distance of 50 m and transitions into a 4 m-deep scour hole. The reinforcement design for the outlet section and scour hole is the same as that used on the main channel section.
Access roads are provided along the channel for inspections, preparation for flood passage and repairs. The road surfacing consists of a 0.2 m-thick crushed-stone layer. The crushed stone is placed over a layer of RD-100 road geocell (СТО 30478650–001–2012), made of low-density polyethylene and carbon spring steel wire. This made it possible to reduce the pavement thickness without reducing the strength and reliability of the access roads. The RD-100 geocell is installed over an underlying layer consisting of two layers of nonwoven «Kanvalan MF 30» fabric and 0.3 m of sand.
Conclusions
Modern geosynthetic materials address complex challenges arising in the design of structures in permafrost conditions.
According to calculations, the design used makes it possible to preserve permafrost beneath the channel throughout the pit life cycle, substantially increases the reliability of the structure and reduces flooding risks.
Reinforcement of the channel with a crushed-stone-filled geocell provides reliable protection against scour caused by high-velocity water flow.
The channel waterproofing and reinforcement design allows work to be carried out in winter at negative temperatures down to minus 40 °C.
