Design and technology concepts in providing auxiliary track roadbed stability on permafrost

. The main reason for the loss of sustaining capacity in structures on permafrost is connected with settlements that take place in thawing soils of the roadbed in its enclosed spaces in the subgrade. The technogenic impact of the linear structures produces a thermal effect on the roadbed soils as the structures play an obstacle on the way of ground waters because the thermal conductivity of the structures’ soils is higher than that of the subgrade natural soils. Besides, the vibro-dynamic impact of traffic also leads to the thermal settlement of the roadbed first, and after 5-7 years it results in the plastic soil extrusion of high plasticity loose ground in a water-bearing subgrade (moss, turf, silt, ect.). Thus, the roadbed becomes stable after 16-19 years.


Introduction
The BAM Vostochny Polygon (a part of railroad network possessing unified technical and technological features that determine its operation) is located mostly on the areas with high temperature permafrost.One of the main reasons for a great number of deformations in the embankment is the disturbance in water and heat balance.It is a result of a warming thermal impact of man-made and natural factors on weak thawing soils in the subgrade support [1,2].
Just after the roadbed of an auxiliary track on permafrost is filled to the embankment that has been in operation for more than 19 years, the secondary disturbance in water and heat balance takes place in the "roadbed-subgrade" geotechnical system.The thermal impact of soils of the additionally filled roadbed on the state of soils in the thawing basin under the current roadbed disturbs its natural equilibrium of stability.Thus, the thermal behavior of soils that has already been formed is disturbed, especially if the roadbed filling of the auxiliary track is made on high side.It leads to the embankment deformations in vertical and horizontal directions, while the track structure experience deformations in plans and profiles.When the traffic is growing, the vibro-dynamic impact from the rolling load is also increasing that makes thixotropic processes in soils greater.It leads to the extrusion of uncompacted yielding soils, and consequently to and additional subsidence of the structure.Thus, line structures experience increased settlements, and the settlement deformations in the roadbed go on for 11-13 years of its operation after being refilled [3,4].
It is a common knowledge that the embankment stability is provided due to the roadbed irrigation engineering measures.However, the roadbed soils irrigation in endorheic areas is problematic, so one of the ways to eliminate the settlement of such roadbed soils is to create reinforcement and drainage structures, such as counter dams of changing sectional areas made of rocks and drainage soils, counterforting, rock cartridges of nonwoven synthetic materials and rock soils, elastic ramps of nonwoven synthetic materials, drain ditches, etc [5,6,7].

Results and discussion
A well-known way of constructing the roadbed for an auxiliary track on weak thawing soils is the following.On the side where the secondary auxiliary track is filled, the face slope of the existed embankment is cut for at least two benches to make a soil adhesion between the main and secondary tracks.Next, the roadbed of the auxiliary track is filled in layers, each one being consolidated, and slopes are planned to the specified position [8].
The filled embankment transmits the static to the supporting subsoil of roadbed from the embankment and advancing (traffic) load.First, the benches on slopes of the main embankment slow down the creeping of embankment thawed soils on its sliding surface due to the soil adhesion between the main embankment and a thawing embankment of the secondary track.Then, when the thawed soils in the secondary track subgrade are consolidated under the static and advancing load, the thawing basin under the supporting subsoil is formed.Meanwhile, the previously formed thawing basin in the supporting subsoil of the main track is enlarging.The auxiliary track embankment is settled as a result of subsidence, the angular soil patterns are lost and the adhesion forces become weaker as a result of decreasing adhesion.
Another roadbed sustaining method is aimed at the roadbed stability in the conditions of long-term exploitation on thawing permafrost when water-bearing weak ground of the subgrade gets frozen [Patent #1567741 RF].It is very close solution to the previous on according to its essential features and achieved results for the roadbed construction on weak thawing soils.
The method of constructing the roadbed on a weak thawing subgrade includes the filling of rock material prisms onto the roadbed slopes and the soil reinforcement before the filling.It includes trenching to the depth of active soil, laying a synthetic nonwoven material around the perimeter of the trenches done and their fixing on slopes followed by rock soil and prisms filling (Figure 1).The subgrade soil reinforcement goes as following.At the beginning of the winter period, the water from weak soils is squeezed and released into the roadbed slope foot areas because moisture is bound to the freezing front, that is to the filled rock materials, where some moisture passes to the trenches, while the other immediately turns into ice.Subsequently, the permafrost line is rising gradually in the trenches, and thawed soils get frozen there almost completely during one season.Due to an additional thermal regulation, the body of rock material filling and slopes receive additional freezing in the subgrade thawed areas that increases the roadbed stability.In the warm period, the reverse process takes place: the ice in the trenches melts, and water then flows along them to outlets.

Synthetic nonwoven material
Thus, in its exploitation, the roadbed stability on a weak thawing subgrade is provided by the elimination of the subgrade soil settlement through a drainage trench arrangement that supplies a reinforcement of foot zones and free water and moisture discharge from the soils.The drained ground consolidates the roadbed.The drainage of soils takes place in both vertical and horizontal directions in the subgrade that stabilizes the roadbed and increases its operation life.
Another method of constructing the roadbed for an auxiliary track on a weak thawing subgrade supposes the following.The soils on a high shoulder are reinforced before filling.This is achieved through a construction of a water-squeezing berm made of a clay and a rock material prisms.The top of the clay prism is located on the embankment slope, while its width covers the subgrade of the auxiliary track roadbed being filled.Also, a synthetic nonwoven material is laid on the water-squeezing berm and fixed on the roadbed slope.In case an auxiliary track is constructed on a low side, the subgrade and the rock prisms are reinforced on both main road side and the auxiliary track side.
The pressure is increasing in weak soils downstream to the outlet under their increasing weight.Meanwhile, rock soils of the prism go lower down into the weak soils to a correspondingly increasing level because of the proportionally increasing weight in longitudinal direction.All these processes form a slope (gradient) in the longitudinal direction along the whole length of the prism to the field making the underground run-off possible top down to the outlet area with the help of the drain.
Thus, the subgrade lateral reinforcement in the filling roadbed takes place thanks to the reinforcement of the water-squeezing prism made of clay in the auxiliary track subgrade, nonwoven synthetic material and the drain in weak soils.It provides the uniformed structure stability.
Drainage discharge reinforcement with the drain to the outlet area increases the life of the structure many-fold as it provides continuous and free run-off of moisture from the weak soil underground in longitudinal direction, thus dewatering the soils and preventing the permafrost degradation.
The structure works in the following way.After the enlarging rocky prisms have been filled along the structure length on its high shoulder, the subgrade soils experience an increasing pressure in the lateral and longitudinal directions of the structure from the static weight of the filled embankment and a clay prism.As the weight is increasing, the rock prism soils are going down uniformly over the length, thus creating a longitudinal discharge in the drain and reinforcing the subgrade of an additionally filled roadbed.
At the beginning of the winter period, the water from weak soils is squeezed and released into the roadbed slope foot areas because moisture is bound to the freezing front.Initially, the moisture in the water-squeezing clay prism proceeds into the drain and is discharged through it in the subgrade of the rock prism to the outlet.When the temperature goes under zero, the moisture freezes.It subsequently uplifts the permafrost line in the rock prism slope area as thawed soils get frozen there.Due to an additional thermal regulation in the drain and prism slopes the thawed area of the subgrade support gets additional freezing that improves the roadbed sustainability.The moisture from the subgrade support is collected in the area of longitudinal drainage bed and is discharged into a low relief as moisture appears.It means that the subgrade support soils are dewatered and the roadbed becomes stable.
In the course of the roadbed operation, clay prisms move down in the weak soils of the subgrade support under the pressure of rock prisms, thus preventing overwatering and strength loss of the subgrade support due to "a clay wall" in the soils (Figure 2).Besides, cell moisture is discharged on the synthetic nonwoven material into the drain that provides isolation of the subgrade support soils from moisture penetration along the whole length of the structure in the lateral direction keeping the soil strength high which is consequently proves the stability and sustainability of the earth structure [9,10].

Conclusion
The "roadbed-subgrade" system that is located on Vostochny Polygon of the Far Eastern Railroad Company is experiencing the stage of hypothetically restored thermodynamic equilibrium.Its filling with double-track inserted sections and a second line produces a negative influence on the changes in heat transfer and the temperature-moisture regime of supporting subsoil of the roadbed.As a result, we have:  a disruption in a surface and in-ground drainage;  concentration of warming lentic water in the fill-slope toe collecting when the water flow is not provided;  terrain disturbance in the embankment adjacent area during construction or exploitation.The external evidences of cryogenic processes in the subgrade are abrupt alternating strains in the track (heaves, frost mounds, pockets, sinkholes, etc.), a ballast prism of excessive height, berm settlements, cracks and settlements along the berms and embankment slopes.
Unavailability of drainage facilities or their unworthiness due to the exceeded term of their exploitation results in water stagnation, developments of erosion in slopes and subgrade supports, formation of water flow strips and lateral filtering through the embankment.
Unavailability of small man-made facilities in ravines and low relief forms leads to overwetting and thixotropic softening of ground in the embankment that provokes its dramatic settlements and suffusion-icing features.
Thus, the stability measures should include some steps in providing the roadbed drainage as well as the steps in keeping the thermal regime in subgrade soils.
When designing anti-deformation structures for the roadbed in the Northern Latitudinal Railway:  in normal conditions it is possible to use standard design solutions with a local georeference  in difficult a complex solutions should be used  in especially difficult conditions only individual solutions justified by necessary calculations should be used.For instance, the solution on the brevets d' invention #2706152 can be adapted.In the bottom part of the berm, some large structural gravel is filled in portions on the non-woven material and rolled up to make a drain of 0.3-0.7 m in diameter along the drain outlet.After that, the berm is filled to make the roadbed for an auxiliary track and the prism of rock material with its top being above the drain.The berm of clay and the prism of rock material are filled along the track widening them gradually to the outlet side.