Why Is the sequential excavation method (SEM) preferred where existing structures such as sewers or subways can't be relocated?

Tunneling near existing structures, such as sewers and subways, usually presents a number of challenges. As tunnel boring machines (TBM) advance through the intended bore path, the surrounding soil is disturbed and removed in the process. This disturbance affects the strength and stability of the soil matrix, resulting in soil movement which can cause settlement of existing nearby structures.

Settlement can eventually lead to cracks and, in extreme cases, failure or collapse of the existing structure.

Additionally, the soil near these existing structures helps to provide a means of load transfer and support; thus placing the surrounding soil under constant pressure. Removal of soil near these structures without proper wall supports can create a hazardous working environment for construction personnel, and may even lead to collapse of the excavated area. (Read Soil Types and How They Affect Trenchless Construction.)

The sequential excavation method (SEM) addresses these shortcomings by allowing excavations to be performed in a segmented and controlled manner. During SEM, the tunnel is divided and excavated in several relatively small sections. Each segment is carefully monitored via measurement instrumentation installed in the lining and boreholes in the vicinity of the excavation; thus, allowing contractors to observe soil movements and provide appropriate supports in accordance with the existing ground conditions.

Due to the nature of its operation, SEM is regularly referred to as a design-as-you-go approach.

SEM is, therefore, ideal when tunneling near existing structures and infrastructure as it gives engineers and contractors greater control over the behavior of the ground conditions. As each segment is excavated, the installed instrumentation and monitoring systems provide valuable insight into the deformation characteristics of the surrounding soil.

With this knowledge, adequate support systems, such as girders, welded-wire fabric reinforcement, and shotcrete, can be provided for each section to minimize adverse soil movement. (Learn more with The Difference Between Guniteand Shotcrete Explained.)

In other words, the strength of the surrounding ground is mobilized to the maximum extent possible for each segment.

When properly executed, SEM can be used to construct several large and complex tunnel structures using conventional construction equipment without disturbing large sections of soil which can result in damage to adjacent structures.