Shear strength of soils

Programming & Analysis
1

As mentioned in the PA video lecture “Geotechnical Reports - Part II”, clay soils have high shear strength, while granular soils have low shear strength. This helps explain why clay is the least susceptible soil type to liquefaction, and when excavating a site with clay soil, the sides of the excavation can stand on their own without intervention.

However, if we refer to Table 7.2 “Unified Soil Classification System” in the book “Site Planning and Design Handbook,” second edition, page 229, sandy soil classifications such as SW, SP, SM, and SC have better shear strength than clay soil classifications such as CL and CH. On the other hand, MH and CH are equally poor, and ML and CL are equally fair for shear strength.

Doesn’t this seem a contradiction between these 2 study materials? I got confused by the information in that table.

@kiaragalicinao I would appreciate it if you could please provide any insight on this. Thank you very much in advance.

2

Great question @ylourdes84! The key is that these two sources are discussing shear strength in different contexts and aspects of soil behavior. In general, well-compacted granular soils (such as dense sands and gravels) can have high shear strength due to particle interlock, which is what the Unified Soil Classification System table reflects. However, the PA video is emphasizing soil behavior under saturated conditions and during excavation. Cohesive clay soils have cohesion that allows excavation walls to temporarily stand on their own and makes them much less susceptible to liquefaction, whereas sandy soils rely more on friction and can lose strength when saturated or disturbed.

Kiara Galicinao, AIA, NCARB
Product Coordinator
Black Spectacles