Applicability of near-surface soil improvement with hydraulic binders in Sweden considering the influence of freeze/thaw-cycles on the strength of stabilised clay

Fine-grained soils are often not suitable as subsoil for roads or railways or other large-scale construction because of their frost susceptibility. The engineering properties as well as the frost durability of such soils can be improved by mixing with hydraulic binder, which is used in countries with moderate climate. This paper presents a laboratory study of a Swedish clay soil stabilised with a by-product originated hydraulic binder. The procedure and interpretation of the study considers the country-specific boundary conditions of Sweden. The study contains two different binder contents (4 and 7%) and unstabilised clay, three different curing times (14, 28 and 90 days) before twelve freeze/thaw-cycles as well as a subsequent curing time (28 days). The curing conditions were adopted to cold climate, i. e. +4°C. The unconfined compressive strength (UCS) was used as a measure of strength. The results show that this soil gains strength by stabilisation with this binder even at this cold curing temperature and that an increase is still remarkable after freeze/thaw-cycles. The time after freeze/thaw-cycles may allow a continued curing which is indicated by a somewhat higher strength.

Aufgrund ihrer Frostempfindlichkeit eignen sich feinkörnige Böden oftmals nicht als Unterbau für Straßen oder Bahntrassen. Die Eigenschaften dieser Böden können durch Stabilisierung mit hydraulischen Bindemitteln deutlich verbessert werden, was in Ländern mit warmgemäßigtem Klima wie Deutschland üblich ist. In dem vorliegenden Beitrag wird eine Laborstudie an einem schwedischen Ton präsentiert, der mit einem Recyclingbindemittel stabilisiert wurde. Die Durchführung und Interpretation der Studie berücksichtigt landestypische Randbedingungen von Schweden. Die Studie umfasst zwei verschiedene Bindemittelgehalte (4 und 7%) sowie unbehandelten Ton, drei verschiedene Erhärtungszeiten (14, 28 und 90 Tage) vor zwölf Frost- Tau-Wechseln sowie eine anschließende Erhärtungszeit. Die Bedingungen während des Erhärtens entsprachen kalten Klimaverhältnissen, d. h. +4°C. Als Maß für die Festigkeitsentwicklung wurde die einaxiale Druckfestigkeit verwendet. Die Ergebnisse zeigen, dass die Festigkeit des Bodens durch Stabilisierung deutlich zunimmt und dass eine Zunahme auch nach Frost-Tau-Wechseln sichtbar bleibt. Die Nacherhärtungszeit könnte eine weitere Erhärtung ermöglichen, was die etwas höheren Festigkeitswerte vermuten lassen.

Fine-grained soils are often not suitable as subsoil for roads or railways or other large-scale constructions due to their sensitivity to settlements as well as their frost susceptibility. The engineering properties as well as the frost durability of such soils can be improved by stabilising it with hydraulic binders. Stabilisation is quite often used in countries with moderate climate, but seldom in cold climate. This publication presents a laboratory study of a Swedish silty sand stabilised with Multicem, a cement type containing 50% cement kiln dust. The unconfined compressive strength (UCS) was used as a measure of strength. The study investigates different binder contents and different curing times. The UCS was measured before and after 12 freeze–thaw cycles as well as after a subsequent curing time (28 d). The curing conditions were adapted to conditions as given in northern countries – that is, +4°C. The results show that the strength gained by stabilisation is sufficient even at this cold curing temperature. The strength after the freeze–thaw cycles is still significant higher than without stabilisation. The recovering time after the freeze–thaw cycles may allow a continued curing, which is indicated by a higher strength. This remaining strength should become subject for further investigation.

Terrasstabilisering används fortfarande lite i Sverige, trots att idén är gammal och användningen är stor runt om i världen. Osäkerheten om frostpåverkan på stabiliserat jordmaterial är en huvudanledning för detta. Det här forsknings-projektet riktar sig mot att studera frost-stabiliteten av stabiliserad jord i de fall där stabiliseringen har skett med hydrauliska bindemedel, alltså kalk, cement, eller industriella restprodukter med liknande egenskaper, som t.ex. flygaska eller slagg. Resultat från labb-försök visar att en hållfasthetstillväxt sker även vid låga temperaturer och att hållfastheten direkt efter frys-tö-påverkan fortfarande är högre än hos det ursprungliga ostabiliserade materialet. Även en viss efter-härdning verkar ske. Generella slutsatsen är att terrasstabilisering kan löna sig även i nordiskt klimat.

Subsoil stabilisation is only seldom applied in Sweden in spite of the fact that the idea is already old and used all over the world. One main reason for this is the uncertainty about the frost-stability of the stabilised soil material. This research project focusses on frost-resistance of soil stabilised with hydraulic binder, meaning lime, cement or industrial residual products, e.g. fly-ash or slag. Lab experiments showed increasing strength even at low temperature. Moreover, the strength directly after freeze-thaw still exceeds the non-stabilised soil. In addition, the experiments indicate a hardening process even after freeze-thaw cycles. The general conclusion is that subsoil stabilisation can be worthwhile even in Nordic climate.

Fine-grained soils are normally not suitable as subbase in road, railway, and other largescale constructions due to their compressibility, low shear strength, and/or frost susceptibility. Common procedures to improve the subbase are soil replacement, lightweight fill, or insulation. The engineering properties of fine-grained soils can also be improved in situ by mixing them with hydraulic binders. A SWOT analysis based on literature study indicates that near-surface soil stabilisation has opportunities as a method. However, one major weakness of the method of near-surface soil stabilisation is the question of strength reduction as a consequence of freeze-thaw cycles. It is unclear how near zero curing temperature followed by freeze-thaw cycles, representing conditions in northern countries, might influence the curing as well as the ultimate strength of the stabilised soil. The impact of freeze-thaw cycles on the engineering properties of stabilised soils was investigated with focus on the reduction of strength. In these laboratory investigations, curing took place at temperatures of +4°C with freeze-thaw cycles interrupting the curing period. Variations in the laboratory set up contained different soil types, binders, binder contents, and curing times before the freeze-thaw cycles. The results show, that the strength increases with curing time in spite of the near zero temperature; but for one binder, a strength-over-time development with decrease after 90 days has been observed in the cold curing conditions in the experiments. In general, the strength gained during curing was reduced by freeze-thaw cycles. However, strength was regained during curing afterwards, and reached a higher level of strength than the original soil before stabilisation.

Silt is a highly frost susceptible soil. In moderate climate conditions in opposite to cold climates, it is common to improve silt and other fine-grained soils by mixing with hydraulic binder. Here, a laboratory study of a Swedish clayey silt stabilised with Petrit T, a by-product from sponge iron production is presented. The samples were cured at +4°C for 14, 28 and 90 days, representing conditions in northern Sweden. Further samples were exposed to 12 freeze-thaw-cycles as well as subsequent curing time. A third of the samples were cured under a sur-charge, while for another third the surcharge has been applied during the freeze-thaw-cycles and subsequent curing. The last third had no surcharge. Focus is given here on the stress-strain-relations obtained during unconfined compression tests to establish the strength (UCS). The results show differences in both stiffness and strength, with lower values of both for the samples that had endured freeze-thaw-cycles. The results of the samples with surcharge show higher stiff ness and strength than those without surcharge at the same testing time.

Feinkörnige weiche Böden werden in Ländern mit gemäßigtem Klima häufig mit hydraulischen Bindemitteln behandelt, um z.B. die Tragfähigkeit zu erhöhen oder die Frostemp-fi ndlichkeit zu verringern. Frost während der Erhärtungszeit beeinfl usst die Erhärtungsreaktion hydraulischer Bindemittel und kann den Effekt der Verbesserung beeinfl ussen. Um den Einfluss einer niedrigen Lagerungstemperatur und Frost-Tau-Wechseln während der Erhärtung zu unter-suchen, wurde eine Laborstudie durchgeführt. Probekörper eines schwedischen Schluffs wurde mit einem Recyclingbindemittel, Petrit T, stabilisiert. Die Erhärtung fand bei +4°C fur 14, 28 bzw. 90 Tage statt. Anschließend wurden einaxiale Druckversuche zur Aufzeichnung der Span-nungs-Dehnungs-Kurve und zur Ermittlung der Druckfestigkeit durchgeführt. Weitere Proben bei gleicher Erhärtungszeit wurden zwölf Frost-Tau-Wechsel ausgesetzt. Zusätzlich wurden Proben untersucht, die nach zwölf Frost-Tau-Wechseln und einer weiteren Erhärtungszeit von 28 Tagen ausgesetzt wurden Ein Drittel der Proben hatte eine Auflast während der gesamten Zeit, ein Drittel für die Dauer der Frost-Tau-Wechsel und der Nacherhärtungszeit, das weitere Drittel hat-te keine Auflast.Die Ergebnisse zeigen Unterschiede in der Steifig- wie auch in der Festigkeit. Proben ohne Frost-Tau-Wechsel erreichen die höchsten Festigkeiten im Vergleich zu Proben, die Frost-Tau-Wechseln ausgesetzt waren, Die Auflast hat einen deutlichen Einfluss auf die Fes-tigkeit und Steifi gkeit der Proben nach Frosteinfl uss.

The stabilization of soft soils with hydraulic binder is a common technique all over the world. By means of stabilization the engineering properties of fine-grained soils can be improved, where settlements or stability problems have to be handled. Lime and cement are usual binders. The use of industrial by-products as binders has been investigated and some are already established on the market in combination with cement, as e.g. fly ash or slag. Nowadays the reduction of CO₂ is in focus, leading to a search for more by-products from the industry that are suitable to use as hydraulic binder. One of these is cement kiln dust (CKD) that is started to be used commercially as a binder in combination with cement.

Fine-grained soils are often frost susceptible i. e. they show frost heave and/or thaw weakening. These typical problems are handled by improvement with hydraulic binder in countries with moderate climate. In countries with cold climate, near surface soilstabilization with hydraulic binder is less used. One reason for this reduced usage are uncertainties about the influence of low temperature on the expected stabilizing effects of the hydraulic binder. Previous research has shown that the strength of the stabilized material is decreased by frost impact, when compared to without frost. New research results have shown a recovering of the strength after a reduction due to frost.

In the present contribution, a multiple linear regression analysis is presented for identifying significant influencing factors for soil stabilization of fine-grained subgrade with hydraulic binders in cold environment. The regression analysis is based on the results from three earlier published laboratory studies, where the unconfined compressive strength (UCS) was used as a measure of strength. The laboratory studies involved different soil types, different binders (mainly by-products), and varying binder contents.

The results from the laboratory studies show that the strength increases during curing in cold environment. After twelve freeze-thaw-cycles the strength is reduced for several samples, compared to without freeze-thaw-cycles, but it is higher than without stabilization. A recovering of strength after a subsequent curing time after the freeze-thaw-cycles is visible in some cases for the stabilized samples. The multiple linear regression analysis presented here shows the influence of the binder content, as well as the freeze-thaw-cycles and the curing time, on the strength.

Fine-grained soils often show a low bearing capacity as well as a high frost susceptibility. These aspects are a challenge for the needs of infrastructure. The addition of hydraulic binder to fine-grained soils is common worldwide to improve the soil properties for engineering purposes. The classical hydraulic binders are lime and cement, but nowadays more and more by-producs are used as well like e. g. fly ash, slag or filter dust. The binders are called “hydraulic” because they react with water. By this reaction new minerals are formed, connecting the soil particles together. This improves the properties of the soil: A strength increase is even visible when the curing takes place in cold environment or after freezing and thawing cycles. Another challenge that occurs in fine-grained sulfidic soils is their possible acidification, when aerated due to e. g. excavation or drainage. Sulfide minerals in contact with oxygen produce sulfuric acid. The low pH caused by this oxidation can mobilize metals from the soil minerals, with harmful consequences for the environment. The addition of lime or calcite is one possible action to improve acid sulphate soils for agricultural and aqua-cultural purposes. The high pH of the lime and calcite increases the buffer capacity of the soil. However, the addition of hydraulic binder to a fine-grained sulfide soil in order to improve both the engineering properties and to buffer the potential acidification is sparsely investigated. In the present publication a field investigation is described where a cement mixture with cement kiln dust (CKD) is used alone and in combination with a calcite-rich by-product from the paper industry to improve a fine-grained sulfide soil for possible usage in earthworks. Samples taken from the surface after one year show a buffering of the potential acidification. Additionally, a strength increase can be seen in the stabilized soil when compressed and stored in a tube in field conditions.