Samsun-Çarşamba mavi kilinin sıkışma davranışına örselenmenin etkisi

Özet

Zeminlerin ek bir yüke maruz kaldıklarında sergileyecekleri zamana bağlı sıkışma davranışının önceden belirlenmesi geoteknik mühendisliği açısından büyük önem taşımaktadır. Yumuşak kil zeminler üzerinde inşa edilecek yapının ömrüne bağlı olarak mühendisin, toplam oturma hesabına birincil konsolidasyonun yanı sıra ikincil ve üçüncül sıkışma miktarlarını da katması gerekebilmektedir. Üçüncül sıkışmalar, her kil zemin türünde görülmemekle birlikte, özellikle organik zeminlerde büyük mertebelere ulaşabilmektedir. Bu çalışmada, üçüncül sıkışma davranışı sergileyen SamsunÇarşamba Mavi Kili ile çalışılmıştır. Dane çatısının örselenmesinin üçüncül sıkışmalar üzerindeki etkisi araştırılmıştır. Bu kapsamda örselenmemiş ve yoğrulmuş kil numuneleri üzerinde klasik konsolidasyon deneyleri ve 7 adet uzun süreli konsolidasyon deneyi yapılmıştır. Deneyler sonucunda; örselenmenin birincil konsolidasyonun tamamlanma $t_p$ süresini uzattığı, $c_v$ katsayısını düşürdüğü, $sigma^{prime}_p$ önkonsolidasyon basıncını azaltarak belirsizleştirdiği, tekrar sıkışma indisini $C_r$ artırdığı, sıkışma indisini $C_c$ azalttığı belirlenmiştir. Ayrıca, örselenmeyle örselenmemiş numunelerde belirlenen ikincil sıkışmanın tamamlanma süresinin $t_s$ kısaldığı, ikincil sıkışma katsayısı $C_{alpha}$ değerinin %30 oranında arttığı, $C_{alpha}/C_c$ oranının etkilenmediği görülmüştür. Örselenmenin üçüncül sıkışma katsayısını $C_{alphaepsilon t}$ düşük gerilmelerde 4.5 kat artırdığı, gerilmenin artmasıyla bu artışın kaybolduğu bulunmuştur. Uzun süreli konsolidasyon deneylerinde, ikincil ve üçüncül sıkışmalar sırasında örselenmemiş ve yoğrulmuş numunelerin sıkışma eğrilerinde Samsun Çarşamba Mavi Kilinde yaşlanma, yıkanma veya çimentolanma yönünde herhangi bir bulguya rastlanmamıştır.

Abstract

In Turkey, housing problem due to rapid increase of human population and immigration to big cities enforces the municipalities to allow the sites having unfavourable geotechnical soils for settlement. Moreover, the requirement for large areas makes the lowlands with thick alluvial soil deposits be used for the structures like airports. Generally, high ground water causes low bearing capacity and high compressibility in soft clays in these types of soil deposits. One of the examples of these types of problems in Turkey is the settlement problem at Samsun- Carsamba International Airport that has been constructed on Samsun-Carsamba plain of Black Sea region. During the construction stage of the airport the site explorations illustrated that the settlement problems would occur in the soil profile at the runway sections containing a soft blue clay layer named as Samsun-Carsamba blue clay. It was decided to improve the clay layer by preloading that existed below the 20 000 m2 area at the beginning of the runway. The settlement observations during the one year preloading period showed that tertiary compression would also occur not only in organic soils and peat, but also in soft clays. Wet sieve analysis illustrated that the grain size distribution of Samsun-Carsamba blue clay is 41% silt and 59% clay. The mineralogical investigations on the samples illustrated that clay grain size minerals consist of 40 - 50% Ca-montmorillonite, 30% calcite + quartz and 10% kaolin. The index properties of the clay are that consistency limits; liquid limit $w_L$ = 73 %, plastic limit $w_p$ = 29 % specific unit weight γs = 27.6 kN/m3, and natural water content $w_n$ = 65%. According to the Unified Soil Classification System the clay has been classified as high plasticity clay CH. Generally, the secondary compression of a soil at a constant effective stress is commonly attributed to collapsing of the soil fabric. On the other hand, what causes the tertiary compression is still not well defined. In the scope of this study the effect of disturbance on time dependent compression behaviour of Samsun-Carsamba blue clay has been investigated. For this purpose 2 conventional consolidation tests and 7 long-term consolidation tests that lasted 3 to 8 months were performed on undisturbed and remoulded samples. In the conventional consolidation tests where Load Increment Ratio LIR=1 and Load Increment Duration LID=24 hours, it has been found that remoulding decreased the initial void ratio. Furthermore, by remoulding, end of primary (EOP) e-log $sigma^{prime}_v$ compression curves shift to downward, preconsolidation pressure becomes uncertain and at every stress level primary consolidation last longer. Moreover remoulding of Samsun-Carsamba blue clay sample increases recompression index $(C_r)$ and decreases compression index $(C-c)$. At every stress level remoulding causes a decrease on consolidation coefficient $(c_v)$ values for undisturbed and remoulded samples. On undisturbed samples around preconsolidation pressures the 2 ~ 3 times reduction of $c_v$ has been observed, whereas on remolded samples this has not been the case. On undisturbed and remoulded samples, 7 longterm consolidation tests have been performed. Each test included a creep test that lasted 70 - 234 days at various stress levels ranging from 25 kPa to 400 kPa. As a result, the dispersed structure at remoulded sample illustrated more resistance towards flowing than the flocculated structure at undisturbed sample, so remoulding increases the primary consolidation time $(t_p)$. Furthermore, remoulding decreases the end of secondary compression $(t_s)$ or the initial time of tertiary compression. By remoulding the variation of secondary compression coefficient $(C_{alpha})$ and $C_{alpha}/C_c$ ratio is negligible. For undisturbed samples as the effective stress increases, tertiary compression ratio ($C_{alphaepsilon t}$) gradually increases whilst for remoulded samples the greater $C_{alphaepsilon t}$ values at lower stress levels decreases. During a long-term consolidation test up to the creep test stress level loading steps were carried out according to LIR = 1. To investigate the post compression behaviour of Samsun-Carsamba blue clay after creep tests, the samples have been loaded by small load increments ( LIR < 1 ) gradually but no aging, leaching or cementing effects have been observed on $epsilon_v-logsigma^{prime}_v$ graphs of undisturbed and remoulded samples.