Jet grout kolonlarının zeminlerdeki oturmalara etkisinin numerik yöntemlerle incelenmesi
The evaluation of effect of jet grout columns to the settlements in soils with numerical methods
Yazar: İREM DÜZEN
Danışman: PROF. DR. AŞKIN ÖZOCAK
Sakarya Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Geoteknik Bilim Dalı
Derece: Yüksek Lisans, 2023
Konu: İnşaat Mühendisliği = Civil Engineering

Özet:
Artan nüfus, kentleşme, yüksek yapı inşaatlarının hızlanması ve deprem riski gibi faktörler zayıf zeminlerde yapılaşmayı beraberinde getirmekte ve zemin iyileştirme uygulamalarının önem kazanmasına neden olmaktadır. Zemin iyileştirme çalışmaları, zayıf zeminin güçlendirilmesinin hedeflendiği yöntemleri kapsamaktadır. Maliyet ve teknoloji kısıtlamaları ile birlikte değerlendirilen farklı yöntemlerde; zeminin yapı yükleri altında oturmasını kontrol etmek, taşıma kapasitesini artırmak, tünel zeminini iyileştirmek, şev duraylılığını artırmak ortak amaçlardan birkaçıdır. Jet grout yöntemi ise yaygın olarak kullanılan bir zemin iyileştirme yöntemidir ve zeminin taşıma gücünü artırma, oturmaları azaltma ve sıvılaşma riskini azaltma gibi avantajları vardır. 
Bu çalışma, jet grout kolonlarının zayıf zemin tabakalarındaki oturmaları azaltmada kullanılmasını ve farklı parametrelerin oturmalar üzerindeki etkisini numerik analizlerle incelemeyi amaçlamaktadır. Bu analizler, Plaxis 2D ve Plaxis 3D yazılımlarıyla gerçekleştirilmiştir. Analizlerde jet groutlar hem tekil kolonlar hem de kompozit ortam ile modellenmiştir. Üç boyutlu Plaxis analizlerinde tekil kolonlar ve kompozit ortam, iki boyutlu Plaxis analizlerinde ise sadece kompozit ortam modellenmiştir. Jet grout boyu, çapı ve aralığı için ise değişken parametreler seçilerek analizler tekrar edilmiştir. Son olarak yumuşak kil etkisinin belirlenebilmesi adına üç boyutlu Plaxis analizlerinde normal kil ve zayıf kile soketlenen kolonlar için iki farklı analiz yürütülmüştür. 
Analizler sonucunda jet grout ile iyileştirmenin zeminlerde oturmaları azalttığı, ancak çap, boy ve aralıkların sonuçlara farklı oranlarda etki ettiği belirlenmiştir. Optimum çözümlerin elde edildiği üç boyutlu analizler sonucunda en uzun jet grout boyunun en düşük aralıklarla yerleştirildiği durumda oturmalarda %22'lere varan düşüşlerin elde edilebildiği tespit edilmiştir. Sonuçlar üzerinde ise en aktif rol oynayan parametrenin çap değil, jet grout kolonlarının aralığı olduğu belirlenmiştir. Optimum çapın 0,8 m olduğu analizlerde daha yüksek çap kullanımının grup etkileri ile performans düşüklüğüne yol açtığı gözlemlenmiştir. Zemin – jet grout kolonlarının kompozit bir bölge ile temsil edildiği durumda ise üç boyutlu ile iki boyutlu analizlerin çok yakınsandığı, dolayısıyla üç boyutlu analizlerin yürütülmesinin pratik açıdan dezavantajlı olacağı yargısına varılmıştır. Ancak kompozit bölge ve tekil kolonlar ile yürütülen analizler kıyaslandığında tekil kolon modellerinde daha yüksek oturma değerleri elde edildiği gözlemlenmiştir. Ek olarak, zayıf tabakalarda yüklemenin daha yüksek oturmalara neden olabileceği, ancak jet grout imalatı ile birlikte normale oranla iyileştirmenin daha iyi performans gösterebileceği yargısına varılmıştır.

Abstract:
With the increase in population in recent years, the increase in the need for existing urbanization activities, the acceleration of high – rise construction or the high risk of earthquakes in our country, which is located within the three major fault lines brings with it the construction on unsuitable (problematic) soils. For this reason, the risk of landslide, earthquake and liquefaction, especially insufficiency of settlement and bearing capacity of the existing construction areas, makes it necessary to carry out geotechnical studies. Within the scope of geotechnical studies, in cases where the engineering properties of the soil are not sufficient for any structure, it is not always possible to change the construction area or to remove the weak soil within the project area and replace it with a more robust material, both in terms of cost and technology. For these reasons, soil improvement studies are of great importance in order to minimize the soil problems that may occur in the structure over time and to make the soil structure suitable.
Soil improvement works include many application areas such as controlling and reducing the settlement under structural loads, increasing the bearing capacity of the soil, sub-foundation improvements, improving the tunnel soil, creating an impermeable curtain, providing slope stability and reinforcement. A soil improvement method that is fast to be applied, reliable, economical and suitable for soil parameters is selected in soil improvements decided within the scope of geotechnical engineering.
With the developing technology, many soil improvement methods have emerged in geotechnical engineering. One of these methods is the jet grout method, which has been widely used in recent years. The jet grout method is preferred because it is used to increase the bearing capacity of the soil, reduce settlements and reduce the risk of liquefaction in case of earthquakes, as well as being more economical compared to other soil improvement methods, shorter application time and a wide range of applications on almost all soils.
This study aims to use jet grout columns to reduce settlements in weak soil layers and to examine the effect of different parameters on settlements by numerical analysis. The load that will cause settlement on the soil is represented by the silo load. The analyzes were repeated separately for both the empty and loaded cases of the silos and the settlements on the soil were calculated. In order to reduce the settlements to permissible levels, the construction of jet grouts has been included. However, since it is thought that the number, diameter, spacing and length of the jet grout will have different effects on settlements, the analyzes were carried out with these variable parameters.
The analyzes carried out within the scope of this thesis were carried out with Plaxis 2D and Plaxis 3D software. In the analyses, jet grouts are modeled with both single columns and composite region. Jet grouts placed under the circular silo are modeled as single columns and composite region in Plaxis 3D. On the other hand, in Plaxis 2D since it is not possible to model jet grouts individually, only the analyzes of the composite region were carried out.
While the three – dimensional analyzes were carried out to compare the effects of the variables in geometry, the two – dimensional analyzes were carried out to observe in which geometric conditions it would be helpful to converge with the three – dimensional and two – dimensional analysis. This inference shows that in some analyzes, by enabling the use of two – dimensional analysis instead of three – dimensional analysis, time and cost savings can be achieved.
The most realistic three – dimensional analysis results helped to decide how the number, spacing, diameter and length of the jet grouts affect the settlements and to choose the optimum parameters. At the same time, jet grouts are modeled as both single columns and composite region in three – dimensional analysis. Thus, in the three – dimensional analysis, a comparison was made between the analysis results of the composite region and the individual columns, and the differences in settlement values between the models were examined. In addition, the extent to which the performance of the jet grout group, which is socketed into different soil layers, will be affected by the geotechnical properties of the soil has been a separate study. For this reason, alternative three – dimensional Plaxis analyzes were performed to observe the effect of soft clay as additional analyzes. These alternative analyzes were applied to observe how settlements would be affected by the presence of a weak layer under the layers in the soil section or the presence of a softer clay layer in the existing soil profile.
As a result of the analyzes, it was determined that the improvement with jet grout reduces the settlements on the soils, but the diameter, length and spacing affect the results at different rates. The implications of all the analyzes made within the scope of the thesis are as follows:
- As a result of the analyzes in which the jet grouts are modeled as single columns as a result of the three – dimensional analyzes in which optimum solutions are obtained, it is seen that up to 22% reductions in settlements can be obtained when the longest jet grout length is placed at the lowest intervals. However, it was concluded that the same situation is not valid for diameter. The optimum diameter size was determined as 0.8 m, and it was determined that the frequent placement of columns with diameters above this would cause group effects. It has also been found that the longest jet grouts cause less settlement, and an increase of 1 unit in length can be compensated by a half unit increase in spacing.
- In the case of modeling the jet grouts as a composite region rather than a single column, it was determined that the change in the column spacing could not be reflected in the results, so it did not change the settlement values and remained ineffective. These results are inconsistent compared to single column analyzes, where the jet grout spacing seems to affect settlement more than the diameter. When the change in the amount of settlement before and after the soil improvement is compared, it is seen that the settlements are reduced from approximately 25 cm to 5 cm, thus a 20% settlement decrease after the improvement with jet grout. In addition, when comparing the size of the Plaxis 3D models, it was determined that lower settlement values were obtained in the composite region than the single column models.
- The model parameters in the 3D analyzes in which the soil - jet grout region was modeled as composite were also applied in the 2D analyzes and it was seen that higher settlements were obtained as a result. In addition, in these 2 – dimensional analyzes, the settlement amount was 28 cm on average before soil improvement, but decreased to 5 cm after improvement. When these results are considered as a percentage, settlement amounts decreased by 19% on average after soil improvement.
- In additional analyzes, in order to observe how the jet grout columns socketed into the soft clay work, the strength parameters of the normal clay with single jet grout column ends were reduced in three – dimensional Plaxis, and it was observed that 26% more settlement occurred in this case. If the clay was softened, the change in the spacing and diameter of the jet grouts began to dominate the settlement amount more. It has been determined that the healing effect is higher in weak clay than in normal clay.
- All the analyzes show that the improvement with jet grout can be represented both with single columns and with the soil - jet grout composite region in two – dimensional and three – dimensional in numerical analysis. However, considering the parametric variables, it has been determined that the most reliable results can be obtained from models created with single columns in three – dimensional analysis. If the composite region is defined, it has been determined that two – dimensional analyzes with almost the same results will be easier instead of three – dimensional analyses. Parametric relationships, on the other hand, showed that the difference in jet grout spacing was more influential on the results than the change in diameter and length. In addition, it was judged that loading in weak layers may cause higher settlements, but with jet grout fabrication, remediation may perform better than normal.