Excavation-caused extra deformation of existing masonry residence in soft soil region

Growing need for construction of infrastructures and buildings in fast urbanization process creates challenges of interaction between buildings under construction and adjacent existing buildings. This paper presents the mitigation of contradiction between two parties who are involved the interaction using civil engineering techniques. Through the in-depth analysis of the results of monitoring surveys and enhanced accuracy and reliability of surveys, a better understanding of the behavior of deformable buildings is achieved. Combination with the original construction documents, the two parties agree that both of them are responsible for building damages and a better understanding for the rehabilitation of the existing buildings is focused on. Two cases studies are used to demonstrate and describe the importance of better understanding of the behavior of existing buildings and their rehabilitations. The objective of this study is to insight into mechanisms of soil-structure interaction for buildings adjacent to deep excavations, which can result in a damage in existing masonry residence, and to take the optimized measures to make deep excavations safety and economic and adjacent buildings keep good serviceability in urban areas with soft soil conditions.


Abstract.
Growing need for construction of infrastructures and buildings in fast urbanization process creates challenges of interaction between buildings under construction and adjacent existing buildings. This paper presents the mitigation of contradiction between two parties who are involved the interaction using civil engineering techniques. Through the in-depth analysis of the results of monitoring surveys and enhanced accuracy and reliability of surveys, a better understanding of the behavior of deformable buildings is achieved. Combination with the original construction documents, the two parties agree that both of them are responsible for building damages and a better understanding for the rehabilitation of the existing buildings is focused on. Two cases studies are used to demonstrate and describe the importance of better understanding of the behavior of existing buildings and their rehabilitations. The objective of this study is to insight into mechanisms of soil-structure interaction for buildings adjacent to deep excavations, which can result in a damage in existing masonry residence, and to take the optimized measures to make deep excavations safety and economic and adjacent buildings keep good serviceability in urban areas with soft soil conditions.

Instruction
It is common that some of the existing buildings affected by new construction with excavation nearby are not satisfied with serviceability required. With the high speed development of urbanization in China in the past three decades, the distances between new building constructions and infrastructures and their adjacent existing buildings become smaller and smaller inevitably. In addition, the evaluation of adverse effect caused by adjacent new construction, construction time, budget, and housing privatization do not match the speedy development of urbanization. These results in conflicts between new construction and adjacent existing buildings, especially in soft soil region.
This paper focuses on the elaboration of the interaction between a new construction and adjacent existing buildings from geotechnical engineering and structural engineering, geodesy monitoring, and evaluation methods. Then the effect of new construction on the existing buildings could be described reasonably and quantitatively. The conflicts between the new construction's party and existing buildings' party are mitigated by the reliable evaluations. Then, the two parties would concentrate on rehabilitations or strengthening for the existing buildings if it is necessary rather than arguing with each other. Furthermore, the party of new constructions will take effective measures to compensate the party of existing buildings based on the field monitoring and analysis, as well as strategies for maintenance. Finally, the existing building serviceability is controlled within the allowable condition.
In soft soil region, for instance Shanghai, the conflict is more obvious than other regions due to geological reason. The physical and mechanical properties of typical stratum in Shanghai are listed in table 1.
Two case studies presented in this paper are from Shanghai residences in China. Both are in concrete raft foundation with 2.0 m embedded below ground surface. The residences are masonry structure with concrete ring beams on the floor elevations and concrete columns in the cross positions of brick walls. The floors are of precast hollow slabs, but concrete cast-in-place slabs for kitchens and washrooms. Both had severe cracks on the residence walls. The cracks are also on the ground surface around them. The crack widths on bearing brick walls and self-weighted walls are over limitation (GB/T 50344-2004, DG/TJ 108-79-2008, and DG/TJ 08-804-2005) in diagonal direction, irregular cracks exhibit on the ceramic tiles attached on the decorated layer in kitchen and wash-room, as well as excessive deformations appear on the frame of windows and doors.     It is noted that the Tongji University checked the previous system before monitoring to ensure that the results are reliable. Assume the residence is rigid body despite of moment deformation, combined with settlement data in different periods, we can deduce the initial residence inclination before new construction begins, during of excavation of adjacent new buildings, and after excavation. The analysis results are listed in table 3.
Consider that the inclination of residence 1 is approaching the criteria of 10 ‰ (JGJ 125-99,2004), we calibrated the Total station (RTS112SR5L) made by Germany and monitored 10 lines. Each line had been monitored three times. Each time we read three coordinates. This technical measure can migrate the errors from measurement system and limited data. The residence inclination on April 15, 2016 is shown in figure  3.

3.1
Result Analysis Residence 1 and 1# of residence 2 have 5‰ initial inclinations (table 5). Compared with other monitored buildings, they have more increments of inclination. Maximum inclination rates are 2.82‰ and 0.19‰, respectively.
Maximum settlement rates during two months filed monitoring by Tongji University are 0.090 and 0.043 mm/d, respectively. From the slope of settlement rate, they are not stable. Tang and Zhao (2012) found that the soft layer under rafts of residences is not convergent due to constant slope of deformation based on laboratory Triaxial tests. Assume the residence has 10‰ inclination, the maximum allowable inclination (JGJ 125-99), residence 1 has 100 mm differential settlement northward and residence 2 has 210 mm westward. And according to elastic theory, we can deduce the differential stress on the rafts according to equation (1).
Where S is settlement, P is average stress on the raft based on the loads of superstructure and selfweight of foundation, usually take 100 kPa, E is soil elastic modulus, and H is thickness of compressive layers.
Assume the residences have 150 mm average settlements (based on local experiences) after two decades service, then the settlements for residences 1 and 2 after adjacent new constructions are 250 mm and 360 mm, respectively. S0 = 150 mm, S1 = 250 mm for residence 1, S1 = 360 mm for residence 2, P0 = 100 kPa. . Thus, P1 = 240 kPa. Similarly, we can get P1 = 240 kPa for residence 2. Therefore, estimated stress increments due to inclinations are 67 kPa and 140 kPa for residences 1 and 2, respectively. These stress increments will lead to more extra differential settlements and inclinations.
On the basis of the settlement rate, inclination rate vs. initial condition, as well as the deformation caused by different period conducted by scientific analysis, we can better understand the results of monitoring surveys and two parties realize their responsibilities for the present condition of the residences. Then both par-ties will concentrate on how to improve the serviceability of the residences.
We also employed the finite element software to evaluate the bearing capacity and seismic performance of the residences (the process is omitted here). The result is almost satisfied with the requirements (GB 50009-2012, GB 50011-2010, and GB 50007-2011).

3.2
Strategies for Rehabilitation There are various measures to avoid existing buildings adjacent new constructions from excessive deformation. For instance, underpin under existing shallow foundations, install diaphragms around existing foundation to isolate existing buildings and new constructions, mitigate excavation deformation itself, and mitigate existing building settlements by effective methods.

Conclusion
New construction should make sure that adjacent existing building to be in normal service situation by various permanent or temporary measures although it is not difficult to achieve. In soft soil region, multistory brick masonry buildings with shallow foundations are very sensitive to adjacent new constructions which are with deep foundations accompany with deep excavations. Two case studies show that extra 5~7‰ inclination happens easily if they have initial defects, such as initial inclination. Evaluation of extra deformation and structure safety should be done before the new construction begins. It is recommended to protect existing buildings or mitigate deformation of excavation of new construction if the evaluation exhibits that the existing building is vulnerable.
It is important to understand the existing building behavior to assure field monitoring accuracy, reasonable data process.