Native soil Parameters

The table below has been copied from USBR and indicates native trench wall material properties. The link is give in the previous blog.

Material Type 1: Based on SPT, Cone Penetrometer Test (CPT), or in-place field density tests, the trench wall material has an E′n ≥ 2500. Minimum clearance between the pipe and the trench is all that is required.

Material Type 2: Based on SPT, CPT, or in-place field density tests, the trench wall material has an E′n ≥ 500 < 2500. The trench width will vary depending on the deflection requirements and the ratio of the stiffness between the embedment material and the trench wall material.

Material Type 3: When the trench walls are extremely soft (E′n < 500) and provide minimal pipe support.

When Material Type 3 soil conditions are identified prior to construction, design methods can be used to avoid a five pipe diameter wide trench. These methods are:
1. Specify only rigid pipe on an improved foundation.
 2. Increase the pipe stiffness factor (EI/r3 ) by increasing the wall thickness or increasing the thickness of mortar coatings, or both. Note: The haunch area still requires compaction.
3. Encase the pipe in reinforced concrete.
4. Completely remove Material Type 3 and replace with a suitable compacted material to a calculated trench width that will prevent excessive deflection.

E'n values can also be taken from the following: