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Dieses Kapitel befasst sich mit den mechanischen Eigenschaften von Löß, einer Bodenart, die bei unterschiedlichem Feuchtigkeitsgehalt und begrenzenden Druckverhältnissen für den Bau von U-Bahn-Stationen nach der PBA-Methode (Pile-Beam-Arch) von entscheidender Bedeutung ist. Die Studie präsentiert uniaxiale und triaxiale Kompressionstestergebnisse, die zeigen, wie unterschiedliche Feuchtigkeitsniveaus die uneingeschränkte Druckfestigkeit, den Elastizitätsmodul, den Zusammenhalt und den inneren Reibungswinkel von Löß beeinflussen. Zu den wichtigsten Erkenntnissen zählen die deutliche Verringerung der Druckfestigkeit und des Elastikmoduls bei steigendem Feuchtigkeitsgehalt, was die Bedeutung der Überwachung und Kontrolle des Feuchtigkeitsniveaus während des Bauens zur Gewährleistung von Sicherheit und Stabilität unterstreicht. Die Forschung untersucht auch die Auswirkungen eines begrenzten Drucks auf die mechanischen Eigenschaften von Löß und liefert wertvolle Erkenntnisse für Ingenieure und Bauleiter, die in Lössregionen arbeiten. Die aus dieser Studie gezogenen Schlussfolgerungen enthalten praktische Empfehlungen zur Verbesserung der Planung und des Baus von U-Bahn-Stationen in Gebieten mit Lössboden, die für mehr Stabilität und Sicherheit sorgen.
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Abstract
In order to solve the mechanical characteristics in the construction of pile-beam-arch method in loess stratum, this paper carried out laboratory tests, focusing on the mechanical behavior of loess under different water content. Through uniaxial and triaxial compression tests, the variation patterns of key parameters such as compressive strength, elastic modulus, cohesive force, and internal friction angle of loess samples were analyzed in detail. The results show that the compressive strength and elastic modulus of loess decrease significantly with the increase of water content, and the cohesion and internal friction angle also show a downward trend. These experimental results not only reveal the internal mechanism of the change of mechanical properties of loess with water content, but also provide an important theoretical and experimental basis for the construction of subway stations in the loess area.
1 Introduction
In the construction of subway stations, the PBA method, as a commonly used construction method, has the advantages of fast construction speed and small impact on the surrounding environment, and has been widely used in loess areas. During the construction of this method, the guide tunnel is excavated first, and then the strip (pile) foundation, bottom longitudinal beam, side pile, central column, crown beam, and top longitudinal beam are constructed inside the guide tunnel. Then, excavation and arch construction are carried out, and finally, the soil is excavated layer by layer and the internal structure is constructed in the support system formed by the Pile-Beam-Arch, also known as the PBA method.
However, as a special soil type, the mechanical properties of loess are affected by many factors such as water content and density, which brings many challenges to the construction of pile-beam-arch method. At present, research on the tunnel pile method has achieved certain results in the fine sand layer [1‐3], water rich sand layer [4, 5], and water rich pebble layer. However, research on the tunnel pile method for subway construction in loess layers is rare. Therefore, this article aims to systematically study the changes in mechanical properties of loess under different moisture content conditions through indoor experiments, providing theoretical basis and experimental parameters for the design and construction of subway stations using the pile-beam-arch method in loess areas.
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2 Physical Parameters of the Loess
Sample the undisturbed loess at the station location, the basic physical parameters of the sample were measured, as given in Table 1.
Table 1
Physical parameters of samples
Physical parameters
ρ/g.cm−3
w/%
ρsat/g.cm−3
wsat/%
Verage value
1.72
23.3
1.88
35.07
In order to study the effects of different moisture content on the mechanical properties of loess, soil samples were humidified and placed in a vacuum saturator for moisture. After many times weighing and adjusting, the moisture content of soil samples reached 8%, 16%, 23%, 30% and saturation state 35%, respectively.
3 Experimental Results
Conduct uniaxial and triaxial compression tests on samples with different moisture contents. Figure 1 shows the uniaxial stress–strain diagram of the samples under different rate of water content. Figure 2 shows the triaxial stress–strain curve of the sample under different rate of water content and confining pressure conditions.
Fig. 1
Uniaxial stress–strain diagram of samples under different rate of water content
3.1 Unconfined Compressive Strength Under Different Moisture Content
The following conclusion can be drawn from Fig. 3. The lower the water content of loess is, the greater its unconfined compressive strength will be. When the water content is lower, the faster its unconfined compressive strength will increase, as shown in.
Fig. 3
Relation between compressive strength and moisture content
3.2 Variation of Elastic Modulus of Loess Under Different Water Content
According to the lower stress–strain diagram of loess under different rate of water content, the elastic modulus of loess under various water content conditions is obtained, the curve relationship is shown in Fig. 4. It can be seen that when the moisture content of the sample increases, the elastic modulus of loess will decrease. When the moisture content rises from 0 to 8%, the elastic modulus decreases by 90.82%. When the moisture content rises from 30 to 35%, it only decreases by 20.94%, indicating that the greater the moisture content, the slower the change rate of elastic modulus and the smaller the slope of the curve.
Fig. 4
Relation between moisture content and elastic modulus
3.3 Changes of Cohesive Force and Internal Friction Angle of Loess Under Different Water Content
The cohesive force and internal friction angle of loess under triaxial pressure were summarized, and the variation law of mechanical properties of loess under triaxial pressure was obtained.
As shown in Figs. 5 and 6, with the continuous increase of water content, the cohesion and internal friction angle of the specimen decreased, and the decreasing trend of cohesion gradually slowed down with the increase of water content. However, the decreasing trend of internal friction angle remains unchanged as water content increases, and the slope can be approximated as a constant value of −0.65.
3.4 Variation of Triaxial Strength of Loess Under Different Water Content
As can be seen from Fig. 7, under the same confining pressure, the triaxial compressive strength of loess gradually decreases with the increase of water content, and the lower the water content, the increasing range of triaxial compressive strength increases. With the water content increasing from 8 to 35%, the triaxial compressive strength decreases by 80.4% under 50 kPa confining pressure. Under 150 kPa confining pressure, the triaxial compressive strength decreases by 76.3%. Under 250 kPa confining pressure, the triaxial compressive strength decreased by 75.6%.
Fig. 7
Triaxial compressive strength of samples with different moisture content
As can be seen from Fig. 7, the effect of water content on the triaxial compressive strength of soil is very significant. When the loess with low water content is humidified, the triaxial compressive strength of the loess will drop sharply, thus bringing great adverse effects on the safety of the project.
4 Conclusion
In this paper, after undergoing uniaxial and triaxial compression tests, the changes of mechanical properties of loess under different water content conditions were deeply explored, and it was found that:
As the confining pressure increases, the compressive strength of loess increases and the strain during failure increases. However, the increase of rate of water content leads to the weakening of compressive strength, the decrease of failure strain and the decrease of elastic modulus, and the decreasing trend of elastic modulus becomes slower with the increase of water content.
The increase of water content reduces the cohesion and internal friction Angle of loess, especially the decrease of strength of low water content loess after humidification, which easily leads to soil instability and structural damage. During the construction, the moisture content of loess should be strictly monitored and controlled to ensure the construction safety.
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