Direct Shear Test - Issues/Questions

[sdeaton75]

When I review the direct shear testing schema, there are some things that don't make sense to me. When you run a direct shear test, there is a consolidation stage and a shearing stage. In the shearing stage you can calculate peak cohesion/peak friction and assuming it was sheared to a residual strength level you can calculate residual cohesion and residual friction. However, when you are shearing it you only have the 1 shearing stage - not one for peak and one for residual. ASTM does not mention anything about this, nor does AGS. Nor does anything I have found on the internet. Can someone explain why we have a residual stage?

[AC107]

From our lab manager... If I understand the question correctly, Scott is asking why there can be two shear stress results for a given direct shear specimen. The initial displacement run (uninterrupted displacement in a consistent direction of travel) would result in a peak shear strength for that specimen under the tested conditions. If the material has sufficient clay content or was overconsolidated / relatively dense compared to the applied normal stress, we should expect a post-peak shear strength loss with additional displacement. If the specimen has sufficient clay content and sufficient displacement occurs (e.g., “large” displacement), we would then expect the shear strength to reduce to a relatively minimum value for that tested condition. That minimum shear strength is typically referred to as “large displacement shear strength” and approximates the residual shear strength obtained from a torsional ring shear test. The reason some do not consider it a true residual shear strength value is that the direct shear inherently requires a change in direction of travel for each successive run and therefore may not fully align clay particles like the torsional ring shear that has a consistent direction of travel. Therefore, for a single direct shear specimen you could have multiple shear strength values. I would argue peak, post-peak (subjective to interpret), and large-displacement (pseudo residual). I have seen a proposed repeated direct shear test ASTM standard but I don’t believe it was ever published / finalized. The most widely accepted reference that I know is still the USACE, EM 1110-2-1906, Appendix IXA (attached for reference). If you want a deeper dive, I’m sure Bernardo at VT would oblige. — Allen Cadden / Principal From: Scott Deaton ***@***.***> Sent: Thursday, November 7, 2024 4:14 PM To: DIGGSml/schema-dev ***@***.***> Cc: Subscribed ***@***.***> Subject: [DIGGSml/schema-dev] Direct Shear Test - Issues/Questions (Issue #56) When I review the direct shear testing schema, there are some things that don't make sense to me. When you run a direct shear test, there is a consolidation stage and a shearing stage. In the shearing stage you can calculate peak cohesion/peak friction and assuming it was sheared to a residual strength level you can calculate residual cohesion and residual friction. However, when you are shearing it you only have the 1 shearing stage - not one for peak and one for residual. ASTM does not mention anything about this, nor does AGS. Nor does anything I have found on the internet. Can someone explain why we have a residual stage? image.png (view on web)<https://github.com/user-attachments/assets/20ed1a64-a0b9-43e9-9414-c997ac0dceec> — Reply to this email directly, view it on GitHub<#56>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/AGJD4CMKPCK2PFRLNG5YEQLZ7PJZRAVCNFSM6AAAAABRMDN5NOVHI2DSMVQWIX3LMV43ASLTON2WKOZSGY2DEMRQGU3TQNY>. You are receiving this because you are subscribed to this thread.Message ID: ***@***.******@***.***>>

[sdeaton75]

I understand the peak shear strength and residual shear strength and peak strain and residual strain. I don't understand the having different load deflection series of measurements for a peak loading vs a residual loading.

[dponti]

Several of the strength tests were modified to address AASHTO specs. Could this be the source of the residual strength load/deflection measurements?