In the cut and lower method the soil is excavated section-wise in-between the panels, while the lining elements are forced into the in situ soil simultaneously on both sides.

Insertion of linear shoring support elements with roller frame in practice: The uninterrupted force-locked connection between the trench shoring and the subsoil is guaranteed.

For the installation of pipelines and sewers in areas with groundwater, the trench has to be kept free of water. There are three water retention methods available for that purpose:

• Open dewatering

• Groundwater lowering using shallow tube wells (well-points)

• Groundwater cut-off (cement grouting, freezing process, chemical process, …)

Subsequently, the first two options will be considered in detail.

(Image: Groundwater lowering using wells – Detail: …

Open dewatering involves simultaneously collecting accruing groundwater water during the excavation process. This process is performed at about 20 in to 40 in (50 cm to 100 cm) ahead of the excavation, and requires stable types of soil (such as rock), coarse gravel or cohesive types of soil and generally low permeable soils to be effective. The groundwater can be directly pumped or allowed to seep to sumps at the boundaries of the trench, where it …

(Image: Groundwater lowering using wells)

In the groundwater lowering method, vertical wells as well as required piping and pumps are installed around the planned trench or construction pit (in the case of large construction pits also within the pit) prior to the excavation process.

Compared to the open water retention method, groundwater lowering by means of wells allows for significantly deeper lowerings. The design type and dimensioning of the well …

(Image: Installation of wellpoints for groundwater lowering using gravity drainage) (Image: Groundwater lowering using gravity drainage- mode of operation) (Image: Installation of wellpoints for groundwater lowering using gravity drainage)

Basically, we distinguish between gravity drainage and vacuum drainage systems.

In the case of gravity drainage, the lowest groundwater level can be found in the well pipe. Consequently, due to gravity, the water …

Instead of vertical wells, horizontally arranged filters or drainage pipes can also be installed next to the excavation (or even below larger excavations).

The so-called horizontal water collection significantly reduces the water quantity to be pumped. However, the additional costs for the installation of this system are only economical for long-term construction measures.

The construction of sewers is still often carried out in the form of open cut construction. Depending on the design, this results in different pipe-soil systems with different external loads on the sewer.

After completing this module, you will have a sound knowledge of:

• rules and regulations;
• requirements for planning, tendering and construction of the pipeline zone, the shoring and the components and building materials;
• procedures for dewatering and
• process sequences of the open construction method.

(Image: Open cut trench working space)

The trench width has always been the subject of discussions due to its impact on the economic efficiency and the quality of pipe installation.

The trench width should be dimensioned in such a way as to provide sufficient working space around the pipe.

The working space is considered to be the area at the base of the trench measuring from the outside edges of the pipe to the inside edge of the shoring or the trench …

(Image: Inadequate working space at the side of the pipe (no arrow)) (Image: Inadequate working space at the side of the pipe – Proper compaction is impossible)

The minimum trench width defines the minimum working space in the trench needed to enable proper compaction of the haunch zone and of the embedment.

A definition of the minimum trench width can be found for example in the European standard [EN1610:2015].

The minimum trench width is a function of both the nominal pipe size and the trench depth.

(Image: Minimum trench width subject to the nominal pipe size) (Image: Minimum trench width subject to the …

The abbreviation for the nominal size of the pipe is DN (Diameter Nominal) or ID (Internal Diameter). It defines the parameter for circular cross sections in millimeters (without specifying the unit mm). The nominal size approximately corresponds to the clear diameter.

Non-circular cross sections are classified into DN groups with the help of …

Standards, specifications and installation guidelines used in the United States are specific and unique for each pipe material, therefore, unlike the European standards, there is no general approach documented in one single standard related to minimum trench width calculation. As a result, the most common practice in the US is for the engineers and contractors to follow the specifications imposed by the pipe manufacturer, or the specifications prescribed …

The following formula can be used as a general guide to compute the minimum trench width for circular concrete pipe installation using the open cut method.

Sample calculation:

O.D. = 24” ID + 3 in wall thickness x 2 = 30 in

Total trench width = O.D. + (O.D./6 x2) = 30 in+ (30/6 x 2) = 30 in + 10 in = 40 in min

The table below presents the method to select the minimum trench width for HDPE pipe installation using the open cut method for stable ground conditions [[Plastic Pipe Institute]]

AASHTO Section 30 requires a minimum trench width of not less than: Wmin = 1.5 x OD + 12 in (300 mm)

ASTM D 2321 requires a minimum trench width of the greater of either the pipe OD + 16 in (400 …

The table below presents the method to compute the minimum trench width for polyvinyl chloride (PVC) pipe installation using the open cut method.

The table below presents the minimum trench width as a function of pipe outside diameter for glassfiber reinforced pipe (GRP) installation using the open cut method.

The table below presents the minimum trench width as a function of pipe outside diameter for centrifugally cast glass-fiber reinforced polymer mortar (CCFRPM) pipe installation using the open cut method.