1 INTRODUCTION

Associated with construction in temperate climateregions, foundations can be successfully built in com-pacted tamped soils (Krutov et al. 1985).

The importance of this technique is that due to thefoundation load, a compacted soil of higher density, sta-bility and lower moisture content comparing to the nat-ural one is formed below the base and around the sides.Consequently, the ground at the base becomes stableagainst changes in climate conditions, water effects andtemperature changes, in particular (Guidance… 1966).The foundation load transferred to the ground is firstaccepted by the consolidated soil, then distributed overa larger area and is transferred to the ground of naturalcomposition resulting in bearing capacity of the base.

Tamping technique developed rapidly in the early70s in France. It is considered to be the most effectivefor foundation base preparation. Nevertheless, mechan-ical soil compaction is not common practice in the per-mafrost area. Heavy tamping is known to be used forfill materials in Canada. In Magadan region, Russia,heavy tamping was conducted for prethawed groundcompaction (Evtikhiev 1977). Until very recently notechnology was recognized for tamping in northernconstruction.

The most common technique of unfrozen and thawground strengthening is ground freezing with the use of seasonally cooling devices. But the result may notalways be effective. For instance, the active layer willpermanently freeze and thaw in spite of its freezingtechnique. If the ground is frost susceptible upon freez-ing and thaw collapsible upon thawing, then the foun-dations are placed lower than the calculated depth of

seasonal thaw. The ground improved by this tampingtechnique would allow using shallow and surface foun-dations resting on compacted ground of active layer.

Besides, development of tamping technique may beurgent in the following permafrost conditions:

– when deep technogenic taliks are widespread andtheir freezing is economically inefficient,

– when strongly salted ground is common or cryo-pegs exist and these may not always be artificiallyfrozen,

– in case of global climate warming any artificialfreezing is ineffective.

Ground compaction by tamping is special in perma-frost areas. Underlying permafrost is practically non-deformable under dynamic load, a compactedlayer, therefore, should be considered as a finite thick-ness layer.

To reveal the technological parameters and com-paction regime as well as the factors affecting on groundcompaction results the experimental works have beencarried out in different ground conditions in Yakutsk.

Yakutsk is located in the continuous permafrostarea with permafrost thickness of about 250 m. Adepth of seasonal thaw varies from 2.0 to 3.0 m reach-ing its maximum in September–October. The meanannual air temperature is �10.2°C.

2 TEST PROCEDURE

Prior to tests the factual seasonal thaw depth, grounddensity and moisture content were determined, com-paction points were set up evenly spaced at 5 m intervals.

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Experience of active layer tamping in cryolithozone

N.P. SemenovaMinistry of Construction & Architecture of the Republic of Sakha (Yakutia), Yakutsk, Russia

A.N. TseevaYakut Design Research Institute for Construction, Yakutsk, Russia

N.B. KutvitskayaGersevanov Scientific Research Institute of Underground Structures, Moscow, Russia

ABSTRACT: The ground of the active layer is not normally used as a base in the continuous permafrostregions. A building load is transferred directly to permafrost without being distributed in the active layer. For thispurpose deep foundations are mainly used, i.e. piles and column foundations. Improvement of the active layerconditions allows using effective shallow and surface foundations or those in tamped down foundation pits. Toreduce the thaw settlement ability and avoid active layer frost susceptibility a well-known technique of groundcompaction by heavy tamping may be used. This paper presents test results of ground improvement technique inYakutsk. Experimental works were carried out in different permafrost conditions and demonstrated peculiaritiesof the active layer compaction by tamping.

Permafrost, Phillips, Springman & Arenson (eds)© 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7

To observe vertical ground displacements weinstalled metal marks. Thus, metal mark in the center ofcompaction point determined the tamped down foun-dation pit bottom lowering. For observation ground dis-placements around the foundation pit metal marks wereinstalled at distances of 1.5, 1.75, 2 m from the com-paction point center. All vertical displacements of theground were measured by leveling after each cycle ofstrokes.

Tamping was conducted with the ground of naturalcomposition and moisture content unchanged. Atamper was fixed on the track crane-excavator arm inorder to provide its free drop in a given point.

The tamper had a form of truncated pyramid 1.0 mhigh, measuring 1.2 � 1.2 m at the top and 0.6 �0.6 m at the bottom manufactured from sheet steel10 mm thick and filled with concrete (Fig. 1). Its masswas 2500 kg. The weight and size were taken from theassumption that specific statistic pressure would benot less than 0.03 MPa at the base of tamper, and notless than 0.05 MPa when tamping in coarse material(Krutov et al. 1985). Besides, the effect of strokeenergy was assigned to be no higher than 100 kilojoulesto prevent cracking in the underlying permafrost.

Following the finishing of work, monoliths of com-pacted ground were sampled in the pits to determine

moisture content and density throughout thaw zonedepth. The ground was sampled every 0.25–0.50 mvertically to the lower limit of permafrost thawing.When tamping in crushed stone the ground densitywas determined by volume substitution.

3 TEST RESULTS

At the first stage of studies, we had to establish tech-nological parameters for ground tamping and to re-veal the main peculiarities of compaction of the finitethickness layer.

The site was composed of sandy and clayey ground.Weakly moistened salt-free sand from silty to mediumgrained, with prevailing fine sand did not have anyorganic admixtures. Clayey ground represented by loamand sandy loam occurred beneath the soil-vegetationcover to a depth of 0.8 m. Locally salted ground withorganic material inclusions was plastic upon thawingand could flow depending on the organic content.

During test period (September–October 1998) theactive layer was 2.0–2.4 m thick. Figure 2 shows atrench after tamping in sandy loam. Figure 3 presentsa dependence of the bottom lowering of tamped downfoundation pit on a number of tamper strokes. Thedensity of natural dry sandy loam (rd � 15.0…17.0 kN/m3) was higher than the dry sand density(rd � 12.6…13.9 kN/m3), but as can be seen fromFigure 3, sandy loam can be compacted better thansand with the same number of strokes.

When tamping according to marks installed outsidethe compacted zone, vertical downward displacementswere observed. Vertical upward displacements ofmarks started after 40 strokes in the first turn. Markslocated at a distance of 1.5 m from the center of thecompaction point.

To study the processes occurring in unfrozenground underlain by incompressible permafrost

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Figure 1. Test experiments. Figure 2. Trench in sandy loam.

further tamping was conducted reaching 100 strokes,squeezing of the ground around the foundation pitincreased both in height and according to distributionzone. In this case the foundation pit bottom loweredby 0.45–0.5 m in weakly moistened loose sands andby 0.8–0.9 m in wet sandy loams, respectively.

Following the end of tamping, the ground was sam-pled to determine its density and moisture contentdirectly below and outside the compacted site.

The data on sandy loam compaction are presentedin Table 1.

Table 1 shows that a large number of strokesresulted in ground thinning, i.e. the ground thinningoccurred directly under the tamper, some increase indensity was observed at a lower limit of compactedlayer (at a distance of 1 m from the center) and signif-icant decrease in density in the upper layers.

The investigations carried out on the active layercompaction showed the following:

– a number of strokes is the main criterion for tamp-ing effectiveness,

– tamping should be conducted prior to groundsqueezing.

The results obtained were taken into account duringtests at the next site in November 2000. The upperground freezes to a depth of 0.2–0.5 m at that time in Yakutsk. Thus, an intermediate unfrozen layer wascompacted underlain by non-deformed base and over-lain by strong frozen plate. Two sites with differentground conditions were distinguished at this location.

The first site from the surface to a depth of 1.2 mwas represented by fill material (sand-gravel mixture)with an average moisture content of 21%, sand loamand silt loam occurred below with moisture contentW � 35…110%. After the first 10-stroke cycle afrozen layer was broken through. In total 40 strokeswere made and squeezing of the ground started at adepth of foundation pit being 60 cm.

Table 2 shows the values of ground density and mois-ture content before and after compaction. As shown inthe table, dry density in the upper layers (to a depth of1.5 m) increased substantially up to 17.1 kN/m3. Themeasurements demonstrated ground thinning andincrease in moisture content in the lower inundatedclayey ground.

Dynamic load on water-saturated ground in closedsystem was completely transferred to the water, andthe results demonstrated zero effective pressure, water-saturated ground was recognized incompressible. The

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n

200

300 1

400

500 2

600

700

800

20 40 60 80 100

S,mm

Figure 3. Dependence of the lowering of foundation pitbottom on a number of strokes. 1 – in weakly moistenedsand; 2 – in wet sandy loam.

Table 1. Physical properties of the sandy loam.

Ground properties

Below the center 1 m from the center 2 m from the center Natural composition of foundation pit of foundation pit of foundation pit

Depth (m) W(%) rd (kN/m3) W(%) rd (kN/m3) W(%) rd (kN/m3) W(%) rd (kN/m3)

0.5 25 15.0 – – 17 14.0 18 1.480.75 – – – – 16 14.1 13 1.451 16 17.1 16 14.7 23 15.2 12 1.401.25 – – 24 15.0 20 15.0 17 1.411.5 21 15.5 21 15.2 16 14.5 14 1.431.75 – – 5.0 13.8 23 14.7 35 1.432 27 15.0 5.0 13.2 28 15.4 27 1.47

Table 2. Physical properties of the ground.

Ground properties

Natural composition After compaction

Depth (m) W�o� (%) rd (kN/m3) W�o� (%) rd (kN/m3)

0.5 27 14.2 15 17.11.0 23 15.6 16 17.81.5 44 11.8 23 15.62.0 33 13.0 53 10.12.5 90 7.3 138 5.23.0 110 6.3 – –

compacted zone probably developed to the sidefinally causing squeezing of the ground to the surfaceeven in the presence of the upper stable ground plate.

At the second site to a depth of 3 m salt-free sandyloam with organic admixture and an average densityof 13 kN/m3 occurred underlain by fine sand. A depthof seasonal thaw was measured to be 2.0–2.5 m.

The main purpose of tamping is improvement ofstructural properties of the active layer. Therefore, toincrease density, decrease moisture content and, con-sequently, to decrease frost susceptibility coarse mate-rial (crushed stone) was tamped in loose ground at the second site.

For this, 0.5 m3 of dry crushed stone was filled in the0.52 m deep depression after 20 strokes and tamped inthe base again by 20 strokes. In this case, the founda-tion pit bottom lowered by 0.53 m.

Table 3 illustrates the effectiveness of wet sandy loamtamping, the density increased from 13.0–13.4 kN/m3 to15.1–15.8 kN/m3 and moisture content became approx-imately equal throughout the unfrozen zone.

The results obtained permitted to find character-istics of the active layer compaction in permafrostconditions:

– to compact water-saturated ground water removalshould be provided,

– to compact loose wet ground tamping in of drycrushed stone is effective,

– 30–40 tamper strokes changed unfrozen groundproperties towards the upper limit of the incom-pressible layer, i.e. permafrost.

The third series of tests was carried out in August 2001.To place surface plate foundation essential was com-pacting of the layer of dry loose fill sand 3 m thick. Twosites were distinguished at this locality with differentground conditions in the upper layers: the first – withdense upper layer to a depth of 0.4–0.6 m consisting ofsand-gravel mixture with high content of clayey parti-cles, dry density was 21.8 kN/m3; the second – loosenon compacted layer with dry density 13.1 kN/m3.

The ground was tamped by 5-stroke cycles and sam-pled to determine its properties after compaction. In

order to observe vertical ground displacements metalmarks were installed around test foundation pits (Fig. 4).

At the first site with dense upper sands the first 5-stroke cycle caused lowering of the foundation pit bot-tom by 190 mm and maximum heaving of the groundsurface around the foundation pit by 7–8 mm inducedby displacements of the upper dense gravel-sandy layerto the sides and upward, which was broken through after15 strokes. Vertical displacements of marks stoppedafter 25 strokes. The last cycle of tamper strokes pro-duced lowering of the foundation pit bottom K to775 mm. Table 4 shows vertical ground displacementsafter each cycle of strokes.

The data from Table 5 demonstrated that after 30tamper strokes the ground properties changed to 2.5 m

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Table 3. Physical properties of the ground.

Ground properties

Natural composition After compaction

Depth W�o� �d W�o� �d (m) (%) (kN/m3) (%) (kN/m3)

0.5 31 13.4 25 15.81.0 30 13.0 25 15.11.5 15 – 22 15.72.0 19 16.0 22 16.0

Table 4. Vertical ground displacements when tampingdown the foundation pit K, mm.

Number of strokes

Points 5 10 15 20 25 30

BottomK �190 �320 �461 �590 �710 �775T1 �2 0 �5 �1 0 0T2 �8 �2 �2 �1 0 �1T3 �7 0 �3 0 0 �1

Table 5. Physical properties of the ground.

Compacted ground below the center of

Natural ground foundation pit K1

Depth �d W �d W, (m) (kN/m3) (%) (kN/m3) (%)

0.5 21.2 3.0 21.0 3.01.0 14.1 7.0 15.8 6.02.0 14.2 6.0 14.9 7.02.5 14.1 9.0 17.1 5.03.0 14.7 6.0 14.7 5.0

T1 T3

2000

T2

2000 2000

K

Figure 4. Scheme of location of T1…T3 marks aroundfoundation pit K.

depth. At that, the ground density increased by 1.0–3.0 kN/m3 with natural moisture content preserved.

At the site with upper loose dry sands after each cycleof strokes the foundation pit was filled with sandy-gravel mixture with a moisture content W � 15%. Asthis takes place, the ground density below the founda-tion pit bottom increased by 4.0–6.0 kN/m3, comparingto the natural density, and averaged 20.2 kN/m3. Naturalground moisture content W � 6.0% changed on aver-age to W � 10%.

Tests conducted at the third site showed the effec-tiveness of compaction of weakly moistened loosesands with tamping in wet coarse material.

4 CONCLUSIONS

The investigations showed that an active layer compaction by tamping established the following conclusions:

– active layer compacts under dynamic load thatleads to intensive upward squeezing out of theground,

– tamping may be effective prior to start of groundsqueezing,

– in weakly moistened plastic sands and sandy loamsground squeezing out starts after practically thesame number of strokes, i.e. 40 strokes,

– when water is present in the layer above the per-mafrost table, compaction is possible only afterwater removal, and

– loose weakly moistened sands may be effectivelycompacted by tamping in wet coarse material.

REFERENCES

Evtikhiev, A.L. 1977. Pile installation in constructionunder northern conditions (in Russian). Leningrad:Stroyizdat.

Guidance on soil compaction in industrial and civil con-struction (in Russian) Construction Press. 1966.Moscow: NIIOMTP GOSSTROI USSR.

Guidance on foundation design and construction in tampedout foundation pits (in Russian). 1981. Moscow:Stroyizdat.

Krutov, V.I., Bagdasarov, Y.A. & Rabinovich, I.G. 1985.Foundations in tamped out foundation pits (inRussian). Moscow: Stroyizdat.

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