Derin zemin karıştırma kolonlarının yol dolgusu altındaki kohezyonlu ve kohezyonsuz zeminlerin düşey deplasmanı üzerindeki etkisinin incelenmesi
Investigation of the effect of deep soil mixing columns on the vertical displacement of cohesive and cohesionless soils under embankment
Yazar: ESMA ALAN
Danışman: DOÇ. DR. MÜGE BALKAYA
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Zemin Mekaniği ve Geoteknik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2022
Konu: İnşaat Mühendisliği = Civil Engineering
Dizin: Derin zemin karıştırma yöntemi = Deep soil mixing ; Sayısal analiz = Numerical analysis ; Zemin iyileştirme = Soil improvement

Özet:
Derin zemin karıştırma yönteminde zayıf dayanımlı zeminler bağlayıcı özellik gösteren malzemeler ile mekanik olarak karıştırılarak mühendislik özellikleri iyileştirilir. İyileştirilmiş zeminlerin iyileştirilmemiş zemine oranla dayanımları daha yüksek, geçirgenlikleri daha düşük ve yük altında sıkışabilirlikleri daha azdır. Bu durum düşük dayanımlı zeminleri daha ideal bir temel zemini yapmaktadır. 
Derin zemin karıştırma yöntemi bağlayıcı malzeme türüne (çimento, kireç, uçucu kül gibi) veya karıştırma yöntemine (kuru/ıslak, döner/jet tabanlı, burgu tabanlı veya bıçak tabanlı) göre sınıflandırılırlar. Çok sert veya sıkı olmayan ve içinde kaya parçaları gibi engellerin olmadığı her türlü zemine uygulanabilirler. Günümüzde kullanımı yaygınlaşan bu yöntem kara veya deniz ortamında yapılan çalışmalarda kullanılabilmektedir. Karayolu ve demiryolu dolguları, bina ve fabrika temelleri, beton blok tipi deniz kaplamaları, iskeleler, tank, silo, dayanma duvarı temelleri uygulama alanlarından bazılarıdır.
Bu çalışma kapsamında Fulambarkar ve diğ. (2021) tarafından yapılan çalışmada kullanılan zemin parametreleri ve hesap modeli kullanılarak analiz modeli doğrulanmıştır. Doğrulanan model üzerinde, DZK kolonları olmadan yukuşak kil ve siltli gevşek kum zemin üzerine yapılan yol dolgusu altında oluşan düşey deplasman değeri sayısal analizler ile hesaplanmıştır. Daha sonra iyileştirme alan oranı (ar) %35'te sabit tutulurken D=65cm için s=0.97m, D=80cm için s=1.20m ve D=100cm için s=1.50m durumlarına göre analizler yapılmış ve düşey deplasman değeri üzerindeki etkileri incelenmiştir. İkinci durumda D=80cm ve s=1.20m için yani iyileştirme alan oranı %35 iken kolon boyu (L) 7.0m, 9.0m, 10.0m ve 10.5m değerleri için analizler yapılmış olup kolonun zayıf dayanımlı zemin içinde kaldıkça kolon boyunun oturma değeri üzerindeki etkisi ve sıkı kum birime girmesi durumunda oturma değeri üzerindeki etkisi incelenmiştir. Son olarak D=80cm ve kolon boyu 9.0m'de sabit tutularak ar=%15, ar=%35, ar=%45 ve ar=%78.5 değerleri için analizler yapılmış ve oturma değeri üzerindeki etkisi incelenmiştir. Tüm analiz sonuçları göz önüne alındığında DZK kolonlarının düşey deplasman üzerindeki iyileştirme etkisi yumuşak kil zeminde siltli gevşek kum zemine oranla daha yüksektir. Alan oranı sabit tutulduğunda düşey deplasmandaki en büyük azalma siltli gevşek kum zeminde yumuşak kil zemine kıyasla daha büyük çaptaki kolonlar ile sağlanabilmiştir. Her iki zemin türünde de zayıf dayanımlı zemin içinde kalındığı sürece kolon boyu arttıkça düşey deplasman değeri azalmıştır. Zayıf zemin tamamen iyileştikten sonra nispeten daha sağlam zemine kolon boyunun uzatılması düşey deplasman değerini fazla etkilememektedir. Siltli gevşek kum zeminde her ne kadar iyileştirme alan oranı arttıkça düşey deplasman azalsa da, yumuşak kil zeminde iyileştirme alan oranı %35 değerinde düşey deplasman değerindeki azalma maksimum değeri bulmuş ve bu değerden sonra iyileştirme alan oranının artırılması düşey deplasmanı kayda değer şekilde etkilememiştir.

Abstract:
When the foundation soil can safely support the loads of the structure, a shallow foundation is built. However, if the bearing capacity of the soil is insufficient, the structural loads must be transferred to the deeper solid layer. In this case, deep foundations are designed. However, considering the construction time and cost, this method is sometimes not applicable. This situation has led geotechnical engineers to look for various alternative methods. Soil improvement is one of these alternative methods.
With soil improvement, the engineering properties of the soil are improved. For example, with improved soil, shear strength increases while permeability and compressibility decrease. Soil improvement can be superficial or deep. Compaction, sand drains, improvement with additives, jet grout, and deep soil mixing are some of these methods.
The deep soil mixing method is based on mechanical mixing of weak soil with binders such as cement in the field. A soil improved by deep soil mixing has higher strength, lower permeability, and lower compressibility than a weak soil. Deep soil mixing method is classified according to binder material type and mixing method. It can be applied to any type of soil that is not very stiff, dense and does not contain rock fragments.
According to some sources, the method of deep soil mixing was first used in Japan, according to other sources the method was first used in America. The beginnings of today's modern application began in America and it is still accepted in America as a developing technology. Of course, this development depends on the availability of equipment and experienced workers.
Depending on the application method of the deep mix, it can be a mechanical mix, a mix with high injection pressure, or a combination of both methods. In the mechanical method, the binder material is mixed with the soil under low pressure. The binder can be added to the soil dry or mixed with water as a slurry. In this case, the method is referred to as dry or wet. In the wet method, there are four different types: wet rotary shaft, wet rotary end, wet jet end and wet vertical panel.
Improving weak and highly compressible soils, reducing the liquefaction potential of soils, supporting excavations, creating impermeable walls against groundwater, and improving the environment are some of the applications of deep soil mixing. They can be used for temporary tasks such as supporting excavations and slopes, and for permanent tasks such as supporting embankments and breakwaters. The diameter and center-to-center distances of the columns can be designed as desired, and they can be formed in block, wall, grid and group column arrangements.
Soil can be improved by deep soil mixing up to 40m. Increases the strength of low strength soils from 0.5 to 4 MPa. It can be used on many types of soils and its application is simple. However, it cannot be applied to solid, hard and rock-like soils. The improved soil strength may differ from the values provided in the design, depending on the existing soil properties.
The models used in this study have been validated using the soil properties and the analysis method used in Fulambarkar et al. (2021). The system consists of a road embankment constructed on deep soil mixing columns located in soft clay soil. The soil at the construction site consists of a surface crust of 1.0m underlain by  a 9.0m thick soft compressible clay followed by a compact sand unit. The groundwater level is 1.50m below the ground surface. The road embankment constructed on the soft clay layer has  a height of 5.0m, and the side slopes of the embankment have an inclination of  2H:1V, with  H being the horizontal distance and V being the vertical distance.
While the displacements were kept constant in three directions at the base of the model (zmin), displacements were allowed in all three directions at the upper boundary (zmax). Only vertical displacements are allowed in the xmin, xmax, ymin, and ymax planes. Groundwater drainage is allowed along the z-plane, while it is kept constant in the x- and y-axes due to model symmetry. The fill layer and deep soil mixing columns are modeled using the Mohr-Coulomb material model, and the soft clay layer is modeled using the Modified Cam Clay material model. The Rint value between the smooth deep soil mixing columns and the soil was assumed to be 0.67. Due to the symmetry of the system, half of the fill layer is modeled to reduce analysis time. To facilitate meshing, the 0.6m diameter deep soil mixing columns were converted to equivalent square columns and assumed to have an edge length of 0.886D (D=column diameter). The center-to-center spacing of the columns is 0.97m and the column length is 9.0m, with an area ratio of 30%. To maintain stability and reduce displacement on weak soils, the fill layer was placed in stages. According to the RDSO (2020) guidelines for the phased construction of embankment in soft soils, a distribution of excess pore water pressure (PWP) of at least 30% is allowed in each phase. In this case, the waiting time is adjusted to reduce the excess pore water pressure formed in the previous phase by at least 30%. Once the construction of the fill layer was complete, the traffic load was applied as 12 kPa. The FE was then analyzed until a consolidation of 90% was reached. The results of the analyses showed that the largest difference between the values obtained by this study and Fulambarkar et al. (2021) was 9.69%. These differences are assumed to be within the acceptable limits (<10%). For this reason, the model procedures followed in this study have been validated by Fulambarkar et al. (2021). The further studies have been performed using the validated model.
The geometry of the road embankment to be placed on site has been determined as specified in the Highway Design Handbook (HDH) (2005). In this case, if the roadway width is 3.5m and the banquette width is 2.5m for the two-lane road, the total width of the road is 12.0m. When the fill height is between 3.0-5.0m, the embankment slope is specified as 2H:1V. In this case, the fill height was determined as 4.0m. Each construction level of the fill layer is specified as 20.0cm, as indicated in KGM (2013). Deep soil mixing columns to be installed in the soft soil were made according to Fulambarkar et al. (2021), with the exception of modeling them as circular sections instead of square sections as indicated in the study.
In this study, firstly, the vertical displacement value under the road embankment constructed on the soft clay layer without deep soil mixing columns was obtained by numerical analysis. Then deep soil mixing columns with different diameters (D) and center to center distances (s) were designed, while the ratio of improvement area (ar) was 35%. In this case, the improvement area ratio was kept constant at 35% and s=0.97m for D=65cm, s=1.20m for D=80cm, and s=1.50m for D=100cm were obtained. Analyzes were performed for these different diameters and center-to-center distances and their effects on the value of the vertical displacement were studied. In the second case, analyzes were performed for D=80cm and s=1.20m, that is, the ratio of improvement areas was 35% and column lengths (L) were 7.0m, 9.0m, 10.0m and 10.5m. Firstly, the effect of increasing DSM column length in soft clay layer on the settlement values was investigated. Then, the DSM cloumn length was extended to the dense sand and its effect on the settlement value was examined. In the third case, where D=80cm and the column length was kept constant at 9.0m, analyzes were performed for the improvement area ratio values of ar=15%, ar=35%, ar=45% and ar=78.5%, and the effects on the settlement value were studied. In this case, the center-to-center distances were 1.83m, 1.20m, 1.06m and 0.80m respectively.
The 3 different analysis methods described were also repeated for the case where a silty loose sand layer of 10m thickness existed instead of the surface crust and the soft clay layer. For the loose sand layer, immediate settlement values were investigated instead of consolidation settlement, and  the Mohr-Coulomb material model was used in the analyses.
While the ratio of improvement areas (ar) was kept constant at 35%, analyzes were performed according to s=0.97m for D=65cm, s=1.20m for D=80cm, and s=1.50m for D=100cm, and their effects on the vertical displacement value were studied. While the maximum improvement was obtained for D=80cm and s=1.20m in soft clay soils, it was obtained for D=100cm and s=1.5m in silty loose sand soils. In the second case, the analyzes were performed for D=80cm and s=1.20m, that is, for an improvement area ratio of 35% and column lengths (L) of 7.0m, 9.0m, 10.0m and 10.5m. For both soil conditions, the change in displacement as a function of column length gave consistent results. Finally, at D=80cm and a constant column length of 9.0m, analyzes were performed for the values of ar=15%, ar=35%, ar=45% and ar=78.5% and the effects on the settlement value were investigated. When the analysis results were examined, it was found that after an improvement area ratio of 35% on soft clay soil, although the distances from the center to the center of the columns decreased, there was no significant effect on the settlement value. On the other hand, on the silty, loose sandy soil, the settlement value continued to decrease as the improvement area ratio increased.
Considering all the analysis results, the improvement effect of deep soil mixing columns on vertical displacement is higher in soft clay soil than in silty loose sand soil. When the area ratio is kept constant, the greatest reduction in vertical displacement can be achieved with larger diameter columns in silty loose sand soils compared to soft clay soil. In both soil types, the value of vertical displacement decreased with increasing column length as long as the weak soil remained. After the weak soil was completely improved, increasing the column length to the relatively stronger soil did not have a significant effect on the vertical displacement value. Although the vertical displacement decreases with increasing improvement area ratio in silty loose sand soil, the decrease of vertical displacement value by 35% in soft clay soil reached the maximum value, and increasing the improvement area ratio after this value did not significantly affect the vertical displacement.