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Doygun olmayan siltli zeminlerin tekrarlı yükler altındaki davranışı

The behaviour of unsaturated silty soils under cyclic loads

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  1. Tez No: 998127
  2. Yazar: KADİR KOCAMAN
  3. Danışmanlar: PROF. DR. AŞKIN ÖZOCAK
  4. Tez Türü: Doktora
  5. Konular: İnşaat Mühendisliği, Civil Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 2026
  8. Dil: Türkçe
  9. Üniversite: Sakarya Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: İnşaat Mühendisliği Ana Bilim Dalı
  12. Bilim Dalı: Geoteknik Bilim Dalı
  13. Sayfa Sayısı: Belirtilmemiş.

Özet

Türkiye'de ve dünyada gerçekleşen büyük depremlerde kumlu ve siltli alüvyal sahaların deprem kuvvetleri etkisinde sıvılaşması, zemin davranışının daha ayrıntılı incelenmesini ve farklı iyileştirme yaklaşımlarının geliştirilmesine katkı sağlamıştır. Özellikle 2003 Miyagi depreminde, doygun kum ve siltlerin yanı sıra kısmi doygun zeminlerde de büyük deformasyonların gözlenmesi, doygunluk derecesinin çevrimsel davranış üzerindeki etkisini araştıran çalışmaları hızlandırmıştır. Tekrarlı gerilmeler altında aşırı boşluk suyu basınçlarının gelişimini sınırlayarak veya geciktirerek yenilmeyi önlemek amacıyla zemine hava enjeksiyonu yapılması, son on yılda sıklıkla çalışılan yöntemlerden biri olmuştur. Bununla birlikte, son yıllarda doygun olmayan zeminlerin sabit ve tekrarlı yükler etkisindeki davranışını araştıran çalışmalar artış gösterse de geoteknik uygulamalarda maksimum güvenlik düzeyini temsil eden doygun zemin varsayımı üzerine bina edilmeye devam edilmektedir. Bu varsayım çoğu durumda ekonomik olmayan aşırı tasarımlara yol açabilmektedir. Gerçek saha koşullarında ise yüzeye yakın zemin tabakalarının önemli bir bölümünün doygun olmadığı ve bu koşullardaki özellikle çevrimsel davranışın tasarımlara yeterince yansıtılamadığı dikkate alındığında, doygun olmayan zemin davranışına ilişkin sahada kolay kullanılabilir düzeltmelere ihtiyaç bulunmaktadır. Bunun için doygun olmayan zeminlerin çevrimsel davranışının daha ayrıntılı biçimde ele alınması gerekmektedir. Bu tezde, siltli zeminin doygun olmayan koşullardaki çevrimsel davranışının laboratuvarda incelenmesiyle, doygunluk derecesinin (S_r) tekrarlı yükler altındaki gerilme ve deformasyon davranışına olan etkisinin ortaya konulması amaçlanmıştır. Bu amaçla Adapazarı havzasından elde edilen doğal siltli zeminden, %40 rölatif sıkılıkta (D_r) nemli tokmaklama yöntemiyle hazırlanmış silindirik numuneler üzerinde üç eksenli deneyler gerçekleştirilmiştir. Çalışma kapsamında, geleneksel üç eksenli hücrede doygun (S_r=%100) ve kısmi doygun (%80<S_r<%100) numuneler, doygun olmayan zeminler için tasarlanan çift duvarlı USPv2 üç eksenli deney sisteminde ise kılcal gerilme (1, 2, 5 ve 15 kPa) ve net gerilme kontrollü (50, 100 ve 150 kPa) olarak doygun olmayan (S_r=%45-58) numuneler test edilmiştir. Kısmi doygun deneylerde doygunluk derecesi B değeri üzerinden düzenlenmiştir. Ayrıca hacimsel şekil değiştirme numune üzerine yerleştirilen iki adet yerel eksenel bir adet radyal deformasyon ölçer ile ölçülmüştür. USPv2 ile aynı anda kılcal gerilme ve net gerilme kontrollü olarak uygulanmış ve drenajsız koşullarda monotonik ve çevrimsel yükleme altında gelişen eksenel ve hacimsel şekil değiştirme ile boşluk hava ve boşluk suyu basınçları ölçülmüştür. Deney sonuçları, kılcal gerilme artışıyla doygunluk derecesinde meydana gelen azalmanın monotonik ve çevrimsel dayanımı yükselttiğini, özellikle çevrimsel deneylerde yenilme mekanizmasını değiştirdiğini göstermiştir. Monotonik yükler altında, doygun olmayan numunelerde maksimum deviatör gerilme değerleri artmakla birlikte, doygunluk derecesinden bağımsız olarak tüm numunelerde aynı kayma direnci açısı (ϕ^'=33.8°) ve aynı kritik durum eğimi (M=1.41) edilmiştir. Kılcal gerilme artışı, zeminde ilave bir sürtünme açısı (ϕ^b=26.5°) oluşturarak görünür kohezyon (c_a) değerini yükseltmiştir. Çevrimsel yükler altında, doygunluk derecesi azaldıkça ölçülen hacimsel şekil değiştirme (ε_v) artmıştır. Bununla birlikte her bir doygunluk derecesi için ölçülen ε_v değerleri uygulanan çevrimsel gerilme oranından (CSR) bağımsız olarak karakteristik bir değere yakınsamıştır. Sonuçlar literatürde temiz kumlar için doygunluk derecesine bağlı olarak Okamura ve Soga (2006) tarafından geliştirilen potansiyel hacimsel şekil değiştirme yaklaşımıyla uyumludur. Üç eksenli çevrimsel deneylerde doygunluk derecesi azaldıkça çevrimsel yenilme için gerekli yükleme çevrimi sayısı (N) artmış, CSR-N düzleminde elde edilen çevrimsel direnç eğrileri yukarı ötelenmiştir. Doygunluk derecesindeki S_r=%100 →%53 değişim için çevrimsel direnç yaklaşık üç katına yükselmiştir. S_r<%50 durumunda ise çevrimsel yenilme gerçekleşmemiştir. S_r=%50-60 aralığında, deformasyona dayalı yenilme gerçekleşirken Farklı doygunluktaki çevrimsel direnç oranlarının (CRR(S_r)), doygun duruma göre normalize edilmesiyle normalize çevrimsel direnç (NCR) geliştirilmiştir. NCR-S_r ilişkisi kullanılarak, siltli zeminlerde herhangi bir doygunluk derecesindeki çevrimsel direnci doygunluk derecesine bağlı düzenleyen doygunluk derecesi düzeltme katsayısı K_Sr önerilmiştir.

Özet (Çeviri)

Liquefaction of sandy and silty alluvial deposits during strong earthquakes in Türkiye and worldwide has highlighted the need for a more detailed understanding of soil behaviour and the development of alternative mitigation methods. In particular, the 2003 Miyagi earthquake provided clear evidence that not only saturated sands and silts, but also partially saturated deposits can undergo significant deformation under cyclic loading. This observation has stimulated research into the cyclic response of unsaturated soils and the role of the degree of saturation. One promising mitigation concept is to deliberately introduce air into pores either by gas injection to limit or delay the build-up of excess pore water pressures under cyclic loading. Although numerous experimental and numerical studies on the monotonic and cyclic behaviour of unsaturated soils have been published over the last two decades, geotechnical design practice still largely relies on the conservative assumption of fully saturated soils, often leading to overly safe and uneconomical designs. Especially near the surface, a substantial part of natural deposits is partially saturated, and the cyclic response of such deposits is not explicitly accounted for in most current liquefaction assessments. Therefore, a clear need for correction schemes that allow cyclic resistance, classically defined for saturated conditions, to be adjusted for realistic in-situ degrees of saturation in a rational and practically usable manner. This thesis aims to quantify the effect of the degree of saturation (S_r ) on the monotonic and cyclic behaviour of a silty soil under laboratory conditions and, ultimately, to derive a saturation-dependent correction factor that can be used to modify cyclic resistance ratios obtained for saturated states. To this end, a comprehensive triaxial testing programme was carried out on a natural silt obtained from the Adapazarı basin, which is well known that was liquefied during the 1999 Kocaeli earthquake. All specimens were prepared at a uniform relative density of approximately 40% by moist tamping in five identical layers to minimise fabric variability. The test program combined three different saturation regimes: (a) saturated specimens with S_r=100%, (b) partially saturated specimens with 80%<S_r<100%, and (c) unsaturated specimens with 45%≤S_r≤58%. Saturated and partially saturated tests were conducted in a conventional triaxial cell. The degree of saturation in partially saturated specimens was controlled and monitored through the Skempton B-value, which was adjusted to target ranges before shearing. For these tests, undrained monotonic and undrained cyclic loading conditions were imposed under different effective confining pressures. To capture the volumetric behaviour in partially saturated specimens, local deformation was measured using two axial and one radial local displacement transducers mounted directly on the specimen surface, allowing for an approximate computation of the instantaneous volumetric strain. Unsaturated tests were performed in a double-walled USPv2 unsaturated triaxial system designed explicitly for the independent control of matric suction and net stress. In this system, matric suction s=u_a-u_w (where u_a and u_w are pore air and pore water pressures, respectively) was imposed at four target levels (1, 2, 5 and 15 kPa), while net confining stress σ_“net”=σ-u_a was applied at 50, 100 and 150 kPa. Under undrained monotonic and cyclic loading, the system allows simultaneous measurement of axial and volumetric strains and of both pore air and pore water pressures, so that the evolution of suction, net stress and Bishop's effective stress can be tracked throughout loading. The unsaturated specimens in this programme covered a degree of saturation range between about 45% and 58%, thus representing practically relevant“partially saturated”to“unsaturated”states. Analysis of the monotonic triaxial tests revealed that increasing suction and decreasing degree of saturation resulted in an apparent increase in peak deviator stress under otherwise similar stress paths. Despite this increase in strength, all specimens were found to converge towards essentially the same critical state line in the q–p^' plane, with a critical state slope M=1.41 corresponding to an effective friction angle ϕ^'=〖33.8〗^∘. In other words, the internal friction mechanism of the silt remained practically unchanged, while suction mainly manifested as an increase in apparent cohesion. This is consistent with the observation that the Mohr–Coulomb failure envelopes obtained from both saturated and unsaturated tests could be represented by a nearly constant friction angle ϕ^'=〖33.8〗^∘and a suction-dependent cohesive intercept. The contribution of matric suction to the shear strength is related to the additional friction angle ϕ^b=〖26.5〗^∘ that arises under suction. As suction increased from 1 to 15 kPa, the apparent cohesion c_a increased progressively. At the same time, the change in ϕ^' remained minor, confirming that the primary role of suction in silty soil is to provide an additional cohesion component rather than to alter the frictional resistance at critical state. The cyclic tests revealed a much more profound influence of the degree of saturation on the failure mechanism. Under undrained cyclic loading, saturated specimens exhibited classical liquefaction behaviour: excess pore water pressure ratios r_u approached 0.9–1.0 within a relatively small number of cycles, and axial double-amplitude strain DA_(5%) was attained shortly before or concurrently with this pore pressure build-up. In contrast, partially saturated and unsaturated specimens showed a markedly different response. As S_r decreased, the development of excess pore water pressure was strongly suppressed; in unsaturated tests with matric suctions of 1–5 kPa, measured r_u values rarely exceeded about 0.6. Although axial and volumetric strains accumulated progressively with the number of cycles, the effective mean stress did not vanish, and classical“zero effective stress”liquefaction did not occur. Consequently, for partially saturated and unsaturated specimens, cyclic failure had to be defined solely in terms of deformation, i.e., reaching DA_(5%), rather than by a pore pressure ratio criterion. A key finding of the study is that volumetric strain at failure exhibits a characteristic behaviour for each degree of saturation. For a given S_r, the volumetric strain at the onset of cyclic failure ε_v converged to a nearly unique value, largely independent of the applied cyclic stress ratio (CSR). As S_r decreased, this characteristic ε_v value increased, indicating that soils with more initial air in the pore space require larger volumetric compaction before reaching a deformation-based failure state. For the unsaturated tests with s=1 kPa and S_r=58%, the experimentally observed volumetric strains at failure were found to be in good agreement with the potential volumetric strain estimated by the empirical approach proposed by Okamura and Soga (2006) for clean sands, adapted here to silty soil and partial saturation. This agreement suggests that the concept of a“potential volumetric strain”governed by initial density and degree of saturation is also applicable to silty soils and can be used to interpret deformation-controlled cyclic failure under unsaturated conditions. In the CSR-N plane, where CSR denotes the cyclic stress ratio and N the number of loading cycles to failure, the influence of the degree of saturation was systematic and pronounced. When the CSR-N curves of all series were plotted together, a clear upward shift was observed as S_r decreased from 100% to approximately 53%. For a reference number of cycles (e.g. N=15 or N=20), the cyclic resistance ratio“CRR”(S_r) at a given degree of saturation increased to roughly three times the saturated value as S_r decreased from 100% to 53%. For degrees of saturation below about 50%, no cyclic failure was observed within the maximum number of applied cycles in this study, suggesting that the tested silt is practically non-liquefiable in this low-saturation range under the adopted loading and stress conditions. In the intermediate range 50%≤S_r≤60%, cyclic failure occurred, but exclusively in a deformation-controlled mode; pore water pressure build-up remained moderate even at the point of failure. To quantify the enhancement of cyclic resistance with decreasing S_r, the cyclic resistance ratios obtained at N=15 and N=20 cycles for different saturation levels were normalised with respect to the corresponding saturated value, yielding a normalised cyclic resistance NCR(S_r). The resulting NCR–S_r relationships for N=15 and N=20 cycles were found to be very similar, indicating that the normalised cyclic resistance is only weakly sensitive to the specific choice of reference cycle number within this range. This observation allowed the definition of a degree of saturation correction factor K_Sr, which depends primarily on S_r and only weakly on N. In practical terms, for the tested silt, the cyclic resistance at a given degree of saturation can be expressed as CRR(S_r)=K_Sr (S_r)“ ”CRR_“sat”. CRR_“sat”is the cyclic resistance under saturated conditions (e.g. as obtained from conventional liquefaction charts or from saturated cyclic triaxial tests), and K_Sr is a saturation-dependent multiplier derived from the laboratory data. For S_r=100%, K_Sr=1.0, while for S_r=53%, values of K_Sr on the order of three were obtained. The proposed correction factor thus provides a simple and practically attractive means of adjusting saturated-based CRR values to account for realistic in-situ degrees of saturation in silty soils. The experimental results presented in this thesis demonstrate that partial saturation can significantly increase the cyclic resistance of silty soils, primarily by limiting pore water pressure build-up and shifting the failure mechanism from pore pressure-driven liquefaction towards deformation-controlled instability. At the same time, the data indicate that this effect is strongly dependent on the degree of saturation and matric suction, and that there is a finite range of S_r for which cyclic failure still occurs, albeit at higher CSR levels and larger volumetric strains than in the saturated case. The proposed saturation correction factor, calibrated for a natural silt from Adapazarı, represents a first step towards incorporating unsaturated cyclic behaviour into practical liquefaction assessments. Its application, however, should be confined to soils and stress conditions similar to those investigated here, and further research is needed to extend and validate the approach for different soil types, stress histories and loading frequencies. Overall, the thesis presents a coherent experimental framework and dataset for understanding and modelling the cyclic behaviour of unsaturated silty soils. It shows that, with appropriate correction factors, it is possible to move beyond the overly conservative assumption of full saturation and towards more realistic, yet still safe, design procedures that explicitly account for the beneficial influence of the saturation degree on cyclic resistance.

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