多年冻土区空间通风壳型基础设备技术

Goncharov Y.M.，Petrova Y.M.

(西伯利亚联邦大学 土木工程学院，克拉斯诺亚尔斯克 660041，俄罗斯)

0 Introduction

In the view of essential novelty of the structure of the surface ventilated shell-type foundation and unusual conditions of its work，with presence of permafrost soils，and a lack of information about the technology of its device and temperature condition of soil base formation，the aim was set to develop and investigate:

1)The device technology of intermediate layer from different nonfrost-susceptible soils;

2)The filling technology of internal spaces of shell-type foundation by a low-strength material;

3)The manufacture of prefabricated shell-type foundation elements outside the factory conditions;

4)The manufacturing technology of cast in situ thin-walled structures shell-type foundation at the construction site in the North conditions，in particular in reinforcement，and concreting structures;

5)To investigate the formation of temperature conditions base soils of operated experimental buildings which built at high-temperature and strongly-icy soils.

1 Materials and Methods

The design of the spatial ventilated shell-type foundation implements the concept of load transmission and deformation from foundation，combining the functions of bearing structures and freezing soil of device base，through an intermediate layer，realized from nonfrost-susceptible material，which leads to a significant reduction of intensity of cryogenic processes in thermal and mechanical interaction of buildings with frozen soil.

Possible design schemes of the ground part of building in shell-type foundations are shown in Fig.1.

Fig.1 Structural scheme of the building cap(a)for a building with technical floor;(b)the same with ventilated underground1-fold;2-insert;3-intermediate layer;4-bearing wall;5– floor slabs;6-foundation ventilated space;7-column;8-;9-brick fencing;10-air hole;11-spandrel beam under the socle panel;12-glass holder under the column.

Taking into consideration，qualitative device of intermediate layer fill pad is an essential condition for reliable stability of a building;this issue is to be paid great attention.For this purpose，it is necessary to make layering fill and compact to increase its bearing capacity by mechanical methods using rollers.When you select a method of soil compression and soil compression-type machines，you should consider the soil properties(granulometric composition，moisture，degree of homogeneity，required density)and also amount of work and season.

Work on non-cohesive soils compression(sand，gravel and sand，etc.)necessary to produce at their moisture level closing to the optimum，so that the greatest compression effect is achieved.

The above mentioned，the device of intermediate layer is based on the experience of building in Igarka and Norilsk.

Before，experimental works on developing the technology of intermediate layer device from coarse sand were conducted before experimental housing construction in Igarka.Fill was made layer by layer by 20～25 cm.

To obtain the required density of fill with void ratio of e=0.5～0.55，each layer was sprinkled with water(the watering machine)to a moisture content of W=7%～9%.Pneumatic roller brought about rolling layers.

The rolling density using a Dorn densitometer was pre-determined after 5～6 trips in one track.The rolling density was considered to be achieved for each layer，if the dive striker occurred at 8～10 dropping weights with mass of 3 kg densitometer.

Before the fill of the next layer with the help of rings，samples were selected and un laboratory conditions humidity and average density were determined to calculate the void ratio.Sampling was carried out in the entire area of 92.5 m2in the corners of the grid with a side of 150 cm×150 cm.

Punching tests with A=5 000 cm2were conducted to determine the total deformation modulus E0of a sand bed.Punching test has shown that in compacting using the above method deformation modulus corresponds to 28.5 MPa.

Later，filled and compacted bed were served for testing the prototype shell-type foundation.

During the construction of the first three residential buildings and laboratory building in Igarka city on the shell-type foundations，an intermediate layer (bed)was made from sandy gravel.

Construction of experimental buildings in Igarka was performed at watered taliks in the permafrost zone with deformation modulus of E0=3，4 MPa.This area built was within the old riverbed with silt.Keeping in mind the geological conditions of the building site and the territory relief，works on large-block hard rock fill for the purpose of“reinforcement”of base liquid mass under the bed shell-type foundations and creation of a single mark of territory of development planning were made originally.

Works to fill a bed of gravel-sand mixture with a moisture between 10%and 12%were carried out for the planned，construction site for development.

Moistening of soil mixture was made by a watering machine.

The fill was realized layer by layer by 15～20 cm with compacting by a loaded dump-body truck.The rolling density was determined visually and was considered to be achieved after complete lack of track tread of a wheel dump-truck.After its completion to the designed level，works on the arrangement of sand-cement inserts under internal spaces of shell-type foundation were made for laying-out axis of buildings.Schemes are shown in Fig.2.

A layer of 10 cm sand was prepared and poured on the prepared base，after that formwork was set on two constraint axes of the building of its transverse direction in the first half of a shift，then the sand-cement solution(M25)was filled and compacted by an internal vibrator.(Fig.2(a)).

Formwork allowed creating an insert on the internal form of shell-type foundation folds.In 3 hours，the formwork was removed and rearranged to the following foundation folds，and the process was further repeated.

Leveling course of the same sand-cement mixture was deposited on the inserts surface and after that precast reinforced concrete foundation elementswere mounted，then armatures welding，and joints concreting between them were made.

Fig.2 Scheme for filling internal spaces shell-type foundation:a-filling using the panel formwork;b-the same after installation of prefabricated elements，c-filling through holes in the horizontal beams mounted prefabricated shell-type foundation1-formwork;2，5-solution;3-board in the places of prefabricated elements are jointed;4-box;5-bucket;6-horizontal beam with holes for grouting.

The sequence of production for filling the internal foundation spaces changed at laying of the second building，as well as the laboratory building.At the beginning，elements of foundation were installed along the transverse laying-out axis of building.Thereafter，wooden boards and gutters were fastened at the ends of foundation folds(Fig.2(b)).Then through the gutters and gaps between the fold elements，solution was poured and its compacting was produced(Fig.3).

Fig.3 Filling the internal foundation space

The degree of filling of the folds internal spaces by the mixture was monitored to displace their surplus from boxes and gaps between elements［1］.

The third scheme was also developed with the aim of filling the internal shell-type foundation spaces by solution.

The prefabricated elements of foundation along transverse laying-out axes of building are necessary to be erected at the prepared base where in horizontal folds beams，it is necessary to provide two δ=100 mm holes between internal reinforcing stiffening ribs (Fig.2(c)).The injection of complex solution (sand，cement，lime)by M25 hose through the holes above is made after prefabricated elements have been installed in the design position.

Filling the internal spaces with solution can be controlled by its displacement in a neighbouring hole while compaction by vibrators，which is passed inside after the space complete filling.

The metal formwork was fabricated for the manufacture of prefabricated elements of the shell-type foundation，in which a reinforcing cage was installed (Fig.4).

Its sides were closed after installation of the reinforcing cage into the formwork，and the concrete pouring occurred with W/C=0.7 m B20.

Concrete compaction was made by vibrators，which were fastened to the inclined sides of the formwork，two on each side.

A steaming cap was used for concrete durability (Fig.5)，in which the steam from the concrete section was supplied by a hose.Product Steaming was produced up to 80%of concrete strength，then the cap was raised，the formwork sides were opened and products produced were removed(Fig.6 and Fig.7).The prefabricated elements were delivered and packed at a construction site(Fig.8).During the construction of a 4-storey frame-panel administrative and amenity building(AAB)in the Norilsk industrial district(Oganer) on the surface shell-type foundation，the latter was made in a monolithic form(Fig.9).

Fig.5 Lifting steaming cap:1-cap;2-formwork with reinforced concrete element

Fig.6 Opening formwork sides after steaming

Fig.7 Lifting and entrucking the element

Fig.8 Prefabricated elements at a construction site

Fig.9 The monolithic structure of shell-type foundation1-foundation fold;2-horizontal slab;3-vertical stiffening ribs;4-broadening under column base;5-horizontal fold beam;6-insert in the inner foundation spaces;MJ-movement joint.

Before starting work on filling the intermediate layer under the building，"reinforcement"of upper peat layer has been done.Previously，boraxes heaving was cut，trees were uprooted，a small lake was filled up with the larger particles of rock，which has been under the spot building.

The arrangement of the intermediate layer was made from rock in two stages.

At the first stage，keeping in mind the complex relief of the territory in the micro-district Oganer，preliminary works were performed for soil mixture filling (rock fraction size from 10～15 to 50～60 cm with an admixture of loam).The presence in rock the loam allowed creating an impervious layer in the form of" sand-gravel-clay building mixture"as a result of soil mixture compaction.

The capacity of"sand-gravel-clay building mixture"depending on the construction site relief was 50～80 cm，and in lower areas，including the lake-to 2m.

The wave motion of backfilled surface was observed while filling and compaction of the first two layers in the first stage，especially in the place where site included the lake area.Therefore，the backfilled soil in this area of the lake was removed to its natural base and rocky ground was poured at the bottom of the formed depression(the size of individual particles was to 30～50 cm)，and then with the help of a vibrating roller，the rock was pressed into the pit bottom.Further，several layers of rock were laid(average fraction size to 20～30 cm)，and compacted layer by layer.Fill and compaction were made to a surface mark of the surrounding territory site.This work was carried out at a temperature of－10℃.After replacement of the soil and its compaction(for 3 shifts)，undulating motion of the surface stopped and the whole site of the first fill phase was additionally compacted by a 12-ton vibrating roller.

The density analysis of the fill pad was performed by determining the density in pouring and compacting state.Grain composition，humidity，the content of clay particles were also performed.For example，a quarry density in a natural condition was－1.72 g/cm3and in a compacting one－2.34 g/cm3，humidity－5.7% and the content of clay particles－8.7%.

Later layerwise fill of second stage from pure crushed rocky with fraction sizes of 20～30 cm was carried out at temperature of－20℃，with its compaction by vibrating rollers to the design elevation.Before filling each next layer，the site was cleared from snow in case it had snowed.

Final vertical planning of site was made by fill of the sandy gravel compacted by BelAz with 400 kN load.

Before concreting shell-type foundation，the works for arrangement of sand-cement inserts M25 originally were performed under the internal spaces of shell-type foundation，as shown in Fig.2(a)，in its turn，the inserts served as the base for reinforcement and concreting of the foundation body.

Reinforcement of shell-type foundation was performed by separate and bent reinforcing bars which had been delivered to the construction site from the reinforcement section of reinforcement concrete products factory(Fig.10).

Fig.10 Common view of shell-type foundation reinforcement.

To provide a continuous concreting of a spatial 25 cm thin-walled shell-type foundation，the latter was divided into three areas each of which corresponding to the temperature size unit(Fig.11).

Fig.11 Technological schemes of concreting shell-type foundation:

The laying concrete scheme in the foundation design with two technological breaks was developed for reasons of necessity to constantly alternate works on mounting armature(horizontal beams，wall beam and anvils)with the formwork and concrete works.The necessity of phased concreting was dictated by the condition for achievement the required quality of laying concrete mixture in thin-walled structure of foundation fold of inclined elements which was saturated by armature，with the possibility of further compaction of concrete mixture by vibrators.The first technological place was (Fig.9(a))at the temperature foundation block，including the horizontal slobs，1/3 of the inclined slobs，and the second site 2/3 of the inclined slobs consisted of，the horizontal foundation beams and vertical reinforcing stiffening ribs.After shell-type foundation concreting at the first temperature block，works on the formwork device，installation armature，anvils，wall beam(provided in the initial draft of AAB)and their subsequent concreting，were produced.At the second and third temperature block，all works for reinforcement and concreting of the foundation had the same flow sheet.

Concreting foundation at the first concreting technological place is shown on Fig.12.

Fig.12 Concreting foundation at the first concreting technological place

All works on the site were occurred on the basis of the work production plan developed by the"Promstroi" and"Norilskstroi".

The compaction of concrete mixture was immediately performed after its laying by hand internal vibrators before appearing the signs of laitance in the surface，lack of bubbling air and ending of settling of concrete mixture.The beginning of setting concrete was in the range of 3～6 hours from the moment of its installation in structure.

Favorable temperature and humidity weather conditions during concreting(16～17℃)have contributed to a rapid increase in concrete strength and its cube strength，for example，foundation slab was from 30 to 41.4 MPa in 28 daily age.

The mobility of concrete mixture ranged from 6 to 12 of cone slump.

The installation of columns and slabs was accomplished after complete ending of works and getting the desired concrete strength.

Fig.13 shows a photograph of a building erection and Fig.14 shows the general view of AAB.

Fig.13 Building erection

Fig.14 The general view of administrative and amenity building

The experimental building construction and observation of formation temperature soil condition at their bases were carried out according to the above-mentioned technology of arrangement of spatial ventilated shell-type foundations in Norilsk and Igarka.

A small lake turned out to be under AAB spot while building connection(Fig.15)，water being removed from it，and depression filling with rock compaction.

The presence of positive formation of soil temperature(from 2 to 4℃)below the lake bottom in the future influenced the temperature soil condition of base intermediate layer in the former lake area.Fig.15 shows graphs of temperature soil dynamics under the foundation to a 10 m depth.

Fig.15 Ground temperature profiles beneath the office/service center，Norilskx—x IX.1989;Δ—Δ—IX.1990，o—o—IX.1991;◦—◦—IX.1992;----average depth of pad

Analysis of temperature data showed，first，complete freezing of the fill below the base of the foundation;second，lowering of the ground temperatures in comparison to 1989(before fill placement).At a distance of 7.5 m from the building edge，for example，the ground temperature at 10 m depth decreased from－0.7℃ in September 1989 to－1.0℃in September 1991 and－1.4℃ in September 1992(borehole 7).In borehole10，cooling was respectively from －0.1 to－0.3℃，from 0.1 to－0.7℃ and to－1.15℃(Fig.15).Other boreholes showed similar changes.The talik around BH 10 affected ground temperatures outside the thaw zone:temperatures lowered with increasing distance from the talik.Ground temperatures at 10 m depth in BH 4 located 12.0 m away were－ 0.6 and－0.7℃，respectively.Ground temperature profiles also illustrate that in 1989 the depths of seasonal thaw beneath the building were 4 and 3 m in BH 10 and 4，respectively.In 1991 thaw depths were significantly reduced，to 2.3 and 1.8 m.In other boreholes，thaw depths were within 2.0.In 1992，all boreholes measured the same depths of thaw not exceeding 1.8 m.，and then was 1.3 m.

Along with temperature，settlement observations were conducted which showed that settlements were relatively uniform.The maximum settlement did not exceed 2 cm in 1992.

In Igarka，a heated parking garage was constructed on a ventilated shell-type foundation.The foundation was built of prefabricated and cast-in-situ sections.It was based on a gravel/sand pad of thickness ranging from 0.5 to 1.5 m in lengthwise direction.The garage was a brick structure 6 m in height and 10.4 by 18.0 m in plan area.

The subsoil profile at the construction site was as follows:-ice-rich silty clay with ice lenses 3 to 30 cm in thickness;ice content of up to 40%.Permafrost temperature is－0.8 to－0.9℃.

Details of the foundation are shown in Fig.16.Rectangular ventilating ducts were provided between the folded foundation elements leaning against 160-mm-thick horizontal slabs.

Fig.16 Cross-section of the heated parking garage on a shell-type foundation，Igarka.1-folded element;2-ventilating duct;3-granular fill pad;4-cast-in-situ slab;5-granular infill;6-insulation;7-concrete floor

Fig.17 shows changes in the ground temperatures during four years of operation.The figure illustrates that temperature decreased with depth，and the maximum depth of thaw was 1.3 m.The ground temperature at 7 m depth lowered from －0.9 in 1989 to－1.4℃ in 1992.At a distance of 1.5 m from the side wall axis(south)，the depth of seasonal thaw was 1 m below the base of the fill.

Cooling or warming of the subgrade occurs through the inner spaces of folded elements and the ventilating ducts where air temperature is colder than inside and outside the building.This is confirmed by the data presented in Fig.17 for the summer-winter months.

The thermal regime of the concrete floor has little effect on the subgrade thermal regime［2］.Heat exchange with the ground mainly occurs due to air movement through the ventilating spaces in the foundation during the winter(Fig.17(b)).As is seen in the graph，seasonal thaw progression in the subgrade is mainly influenced by a relatively short season whose length and climatic parameters vary from year to year. In 1992，for example，the period of positive temperatures was about 3 months in length.Maximum positive temperatures(13.8 and 14.7℃)were both observed in August.Mean monthly temperatures for July and September were 4.3 and 2.3℃，respectively.The maximum average monthly temperature in the ducts was close to the outside air temperature only during August，while during the other months it was much lower (Fig.17(a)).

The lower temperatures along the vertical section of the foundation(ducts，beneath the fill pad，and at the top and base of the horizontal slab)compared to the outside air temperature are due to the absence of solar radiation beneath the foundation.An additional effect is from the cold accumulated in the ground during the previous winter，as indicated by±0℃ temperatures in June in the contact zone between the foundation and the fill(Fig.17(a)，2，3 and 4).As a result，the depth of seasonal thaw below the foundation was less than 1.3 m.This explains the observed dynamics of subgrade temperatures and rising of the per-mafrost table under the foundation.

Fig.17 Graphs of mean monthly temperature(a)and air velocity(b)a-temperatures along the foundation's vertical section:1-outside air，2-duct air，3-below the infill，4-beneath the foundation base，5-on the garage floor;b-air velocity:1-outside the building，2-in the duct;c-vertical section of the foundation:2～5-temperature measurement points

The mean monthly air temperature begins to decrease in September and falls below 0℃ in October (Fig.17(a)，1).This results in lowering of temperatures across the foundation(points 2，3 and 4)and in the subgrade.Starting from October，the temperatures across the foundation are warmer than ambient air due to heat exchange between the building and the subgrade(a heating season starts in September).Ground cooling adjacent to the garage is stronger，as illustrated by the temperature profile for October 1995.In the depth interval from 1 to 3 m(the pad and subgrade)，temperatures beneath the building are colder than in the surrounding ground due to the absence of solar radiation.Below these depths，the foundation soils are warmer than the surrounding ground.

The results of field observations presented above suggest that the combination of open spaces in the spatial foundation(at a 6 m spacing of the folds)，ventilating ducts and granular fill not only maintain the foundation in a frozen state，but also cool its temperatures provided that no thawing is allowed during building’s service life.

2 Conclusion

Building construction can be carried out in areas of permafrost soil distribution on the surface of ventilated shell-type foundations on structurally weak stronglyicy and high-temperature ground.

The internal spaces of shell-type foundations are necessary to be filled with low strength material with strength quality not above M25.

The local broadening with outcomes of armature for the design of monolithic glass-holders for the column is need to provide in horizontal beams of shelltype foundation elements for frame buildings with concentrated loads.

Reinforcement of shell-type foundation elements is reasonable to carry out by enlarged spatial reinforced prefabricated frames.

The outcomes of work armature for subsequent embedment of joints in prefabricated monolithic variant of production it is necessary to provide in manufacture of prefabricated shell-type foundation elements along the perimeter.

The gap setting between adjoining foundation elements is assigned according to the axes pitch of the foundation.The minimum size of the gap，welding of armature outcomes is not provided，should be not less than 30 diameters.In a smaller gap，it is necessary to weld the outcomes of armature.

Shell-type foundations in a prefabricated or cast in situ design are made of heavy-weight concrete classes with compressive strength B20～B30.For the class of buildings on the extent of responsibility:I-B30，and II-IV-B20，and monolithing the joint-from concrete B40 and B30，as appropriate.The reinforcement bar of A-2 and A-3 classes，wire reinforcement is BP used-1 class，for working and structural reinforcement.The grade of concrete in freeze-thaw resistance depends on the class of buildings on the extent of responsibility: for buildings of I class-F300 and F200 for others.The grade of concrete on water permeability must be not less than W6.

Reinforcement of monolithic foundations by reinforcement bars，as well as using the factory reinforcement frames can be performed in winter conditions in slight precipitation areas，and in areas where there are often large snow drifts during a snowstorm-in the warm season.

To provide a continuous concreting cast in situ of shell-type foundation the latter must be divided into separated areas within temperature length and movement joint.

Keeping in mind the necessity of constant alternation of mounting armature，from horizontal beams，wall beams，if they occur，according to the project，and anvils for the columns with erection of formwork is necessary to develop a scheme of foundation concreting with two technological breaks within area.

Phased concreting is necessary to achieve quality of laying concrete mixture in to thin-walled structure of inclined foundation elements，saturated with reinforcement，and the possibility of subsequent compaction by vibrators.All works should be implemented at a site according to work production plan.

Laying concrete mixture in the construction of the shell-type foundation needs to be carried out at the entire elements thickness without gaps，consequently with the laying direction from outer edges of area to the center.Laying concrete mixture in the vertical direction needs to be implemented on 1/3 foundation folds height at the beginning，and further to 2/3 of their height.

Compaction concrete mixture should be carried out immediately after its laying by the deep vibrators before compaction signs(the appearance of laitance at the surface，the lack of air bubbles and finish of settling of concrete mixture).The mobility of the concrete mixture must be between 6-12 cm slump concrete.

Quality control of concreting monolithic foundation should be in accordance with the instructions for bearing reinforcement concrete structures.

［1］ Goncharov Y M，Construction experience of the buildings in the watered taliks［J］.Science and Technology in Yakutia，2006，1 (10):12-16.

［2］ Goncharov Y M，Popovich A P，Surface spatial ventilated foundations at the permafrost［M］.Yakutsk:Permafrost institute after P.I.Melinokov Siberian branch of the Russian Academy of Science，2013.