Underpasses

Underpasses can refer to shallow tunnel construction; it called cut and cover construction, Cut and cover construction is used when the tunnel profile is shallow and the excavation from the surface is possible, economical, and acceptable.  Cut  and cover construction is used for underpasses,

Two types of construction are employed to build cut and cover tunnels; bottom-up and top-down

The cut-and-cover tunnel is usually designed as a rigid frame, In urban areas, due to the limited available space, the tunnel is usually constructed within a neat excavation line using braced or tied back excavation supporting walls

Bottom-Up Construction

conventional “bottom-up” construction, a trench is excavated from the surface within which the tunnel is constructed and then the trench is backfilled and the surface restored afterward. The trench can be formed using open cut (sides sloped back and unsupported), or with vertical faces using an excavation support system. In bottom-up construction, the tunnel is completed before it is covered up and the surface reinstated.

Conventional bottom-up sequence of construction in  Figure 4 generally consists of the following  steps:

Step 1: Excavation

Step 2:  Construction of the tunnel structure by constructing the floor

Step 3:  Compete construction of the walls

Step 4:  Compete construction of the roof,  apply waterproofing as required

Step 5: Back filling to final grade and restoring the ground surface.

Bottom-up construction offers several advantages

•              It is a conventional construction method well understood by contractors.

•              Waterproofing can be applied to the outside surface of the structure.

•              The inside of the excavation is easily accessible for the construction equipment and the delivery, storage and placement of materials.

•              Drainage systems can be installed outside the structure to channel water or divert it away  from the structure.

Disadvantages of bottom-up construction include:

•               Somewhat larger footprint required for construction than for top-down construction.

•              The ground surface can not be restored to its final condition until construction is complete.

•             Some times requires temporary support or relocation of utilities.

•               May require de watering that could have adverse affects on surrounding infrastructure

Top-Down Construction

With top-down construction the tunnel walls are constructed first, usually using slurry  walls, although secant pile walls are also used.  In this method the support of excavation is often the final  structural tunnel walls. Secondary finishing walls are provided upon completion of the construction. Next the roof is constructed and tied into the support of excavation walls. The surface is then reinstated before the completion of the construction. The remainder of the excavation is completed under the protection of the top slab. Upon the completion of the excavation, the floor is completed and tied into the walls. The tunnel finishes are installed within the completed structure. For wider tunnels, temporary or permanent piles or wall elements are sometimes installed along the center of the proposed tunnel to reduce the span  of the roof and floors of the tunnel

Top-down sequence of construction in figure 6 generally consists of the following steps

Step 1 : Installation of excavation support structural walls, such as slurry walls ,secant, tangent pile walls

Step 2: Excavation to the level of the bottom of the underpass top slab

Step 3:  Construction and waterproofing of the tunnel top slab tying it to the support of excavation walls

Step 4:  Excavation of underpass interior and  completing the interior facing including the secondary walls

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Top-down construction offers several advantages in comparison to bottom-up construction

•              It allows early restoration of the ground surface above the underpass

•              The temporary support of excavation walls are used as the permanent structural walls

•              The structural slabs will act as internal bracing for the support of excavation thus reducing the amount of tie backs required

•              It requires somewhat less width for the construction area

•              Easier construction of roof since it can be cast on prepared grade rather than using bottom forms

•              It may result in lower cost for the tunnel by the elimination of the separate, cast-in-place concrete walls within the excavation and reducing the need for tie backs and internal bracing

•              It may result in shorter construction duration by overlapping construction activities

Disadvantages of top-down construction include

•              Inability to install external waterproofing outside the tunnel walls.

•              More complicated connections for the roof, floor and base slabs.

•              Potential water leakage at the joints between the slabs and the walls

•              Limited spaces for excavation and construction of the bottom slab

Selection It is difficult to generalize the use of a particular construction method since each project is unique and has  any number of constraints and variables that should be evaluated when selecting a construction method