Ảnh hưởng của hạ mực nước ngầm trong hố đào đến các kết cấu hiện hữu

Influence of excavation dewatering on existing structures

Tóm tắt:

Xuất hiện một cấu trúc địa chất như là đất cát bão hòa nước và mực nước ngầm nông luôn đòi hỏi thực hiện các biện pháp khử nước đối với các hố đào sâu. Tuy nhiên, nhiều vấn đề phát sinh bởi bơm hút nước quá mức như là sự chênh lệch mực nước bên trong và ngoài công trình, cát chảy, lún nền, chuyển vị kết cấu tường chắn, giảm độ cứng và biến dạng hệ giằng chống, nứt kết cấu công trình nhà lân cận. Do đó, nghiên cứu nhằm mục đích đánh giá tác động của việc khử nước như thế nào đến kết cấu chắn giữ hố đào và nhà lân cận để giảm các thiệt hại có thể xảy ra. Trong trường hợp nghiên cứu, phân tích phần tử hữu hạn với Plaxis 2D được áp dụng để mô phỏng quá trình thi công đào hầm. Kết quả phân tích và dữ liệu quan trắc chuyển vị tường vây tương đối khớp. Hơn nữa, kết quả nghiên cứu cũng chỉ ra rằng khử nước gây ra thiệt hại đến các kết cấu; do đó, biện pháp khử nước hố đào nên được xem xét và kiểm soát kỹ lưỡng bởi các kỹ sư Địa kỹ thuật kinh nghiệm.

Abstract:

Appearing a geological profile such as saturated sandy soil and groundwater table at shallow depths always requires conducting dewatering methods for deep excavations. However, many problems occurred due to excessive groundwater withdrawal, such as differences in groundwater level between inside and outside the site, sand boil, ground settlement, retaining wall movement, strength reduction and deformation of the bracing system, and cracks in adjacent structures. Therefore, the study aims to assess how dewatering impacts earth-retaining structures and existing houses to minimize possible damages. In the case study, the finite element analysis with Plaxis 2D is applied to simulate the process of basement excavation execution. The analysis result and the observed data regarding the retaining wall’s displacement are well-matched. In addition, the study result also shows that excavation dewatering caused damage to structures; therefore, it should be thoroughly considered and controlled by experienced geotechnical engineers.
Keywords: Damages; dewatering; influence; groundwater; excavation

Introduction

Many know that excavation dewatering can cause subsidence and crack in surrounding houses, but how many know which phenomenon between sand boil and consolidated settlement is more dangerous? Although ground settlement induced by pumping to lower the groundwater level occurs more commonly than sand boil in practice, it does not have an immediate effect like the sand boil, and the influence zone is also narrower. Theoretically, the principle of drainage causes consolidation of the ground. However, if there is no significant ground load, the settlement mechanism is just drainage, and it takes quite a long time to reach the settlement amplitude. Significantly, the foundations of neighbouring houses are also slow to be affected.

On the contrary, once the sand boil appears, within a few days, a large amount of sand is washed out of the ground by forced flow from the system of pump wells placed inside the excavation work. As a result, the ground becomes large voids. The situation gradually increases over time, then induces shallow foundation structures, including the bearing capacity decreases, and the settlement process increases and transmits to the beams, columns, and walls. These are then cracked gradually with increasing crack width. In this study, a real case observed and compared with the Plaxis 2D finite element model is used to warn the problem of the sand boil due to excavation dewatering that needs to be investigated, calculated, designed, and predicted to reduce possible damages and losses.

Dewatering methods

Dewatering is an almost mandatory item in the construction of deep excavations due to the nature of the soil. Geological cross-sections are typical in urban areas near rivers, so the formation properties almost contain sediment. The method of dewatering is to construct a point well system with a depth, diameter, and quantity to ensure the drying of the excavation surface. In terms of construction methods, moving excavators and the process of reinforced concrete construction of the foundation structure also need to be prepared with a dry surface, thereby dewatering and controlling the groundwater level inside the excavatSion is necessary. Install a suitable point well system depending on the specific geological conditions and the characteristics of the excavation work.

In the case of deep excavations, reinforced concrete diaphragm walls are constructed and deeply embedded in the impermeable soil layer. In this case, the amount of water needed to pump is insignificant. However, it is also necessary to prepare for when the quality of the diaphragm wall is not guaranteed or the diaphragm wall defect so the outside water can still flow into the excavation, thus setting up the pump well system as well. In addition, the number of backup wells also needs to consider.
In the other case, the diaphragm walls end in the aquifer. When pumping from inside the excavation, the outside water continues to flow through the bottom of the diaphragm wall, and here the sand boil is a possible phenomenon. The subsidence rate of the foundation and surrounding structures is alarming because the sand boil occurs very quickly. As shown in Figure 1, the radius of influence increases gradually before, during, and after pumping for a period of days.

Case Study

This section presents an actual site where the sand boil phenomenon caused cracks in house structures due to the dewatering of the excavation. The site dimension is 12m and 65m long, respectively. The design of the building includes three basements, the maximum depth is 16m, and the typical one is 14.5m, located on Tran Hung Dao Street, District 1. The 0.6m thick diaphragm wall was installed 27m deep and bottom-up method with three layers of the strut.
The geological features determined from the four surveyed boreholes are a layer of mud on the surface about 3m thick, below is a layer of sandy clay up to 9m, and the lower part is saturated fine-medium sand. The groundwater table is stable at around 3 meters below the street level. A point well system consists of twenty 200mm diameter wells and a 300mm diameter well with a depth of 24m as shown in Figure 2.

Figure 2. Layout plan of well-point system

During excavation construction, several wells operated to keep the excavation surface dry. The active wells during construction were 50 percent as the quantity designed and the flow fluctuated by 7,200m3/day for about three months. As a result, a large amount of sand was withdrawn from the ground through the wells, shown in Figure 3.
Furthermore, a series of structural cracks appeared in the adjacent houses within an impact radius of nearly 500m, shown in Figure 4. There were not only influences on adjacent structures but also impacts on the earth – retaining wall and bracing system. While the diaphragm wall was affected by large displacement, waling beams of the bracing system were deformed at critical stress, shown in Figure 5.
The inclinometer system was carried out to monitor the diaphragm wall for controlling the horizontal movement during excavation work. This observed data is used to compare with the 2D Plaxis model. Soil parameters for the analysis in the model are taken from the soil investigation report, shown in table 1. Some parameters are adjusted according to empirical formulas, including the soil stiffness.
In the analysis, both the Hardening soil model (HS Model) and Mohr-Coulomb model (MC Model) are well compared with the Inclinometer recorded. This comparison shows that displacement of the diaphragm wall is significant in all three excavation steps at 6.8m, 11.5m, and 14.5m, as shown in Figure 6 (a,b,c).

Figure 3. Sand boil occurred due to dewatering under excavation
Figure 4. Cracks in house structures
Figure 5. Waling beams deformed

Table 1. Soil properties

Figure 6. D-wall’s displacement

Conclusion

It is clear that dewatering induced cracks in house structures under basement construction. Especially, the sand boil caused impacts more dangerously than from consolidated settlement.
The study is to enhance the vitality of dewatering. For such excavation projects are necessary for experienced geotechnical engineers to be involved in both design and construction.

Trần Hoàng Tín
Khoa Xây Dựng, Học viện Hàng Không Việt Nam.

E-mail: tinth@vaa.edu.vn

Nguyễn Thành Đạt
Khoa Xây Dựng, Học viện Hàng Không Việt Nam.

E-mail: datnt@vaa.edu.vn

 

References

[1] Magdi M.E. Zumrawi, Elthaytham Hassan (2016). Effects of excavation dewatering on adjacent structures. University of Khartoum Engineering Journal, 2016.