Field Estimationof Fine Soil Materials Strength

[ONENGINEER]

A test pit was dug to 4.2 m. From top to bottom, it was 1.6 m silty clayey sand, 0.9 m peat/organic silt, and 1.7 m mineral silt. The bottom of pit was examined immediately after excavation and it was stiff silt but after half an hour the silt materials spalled and caved in. GWL was at top of silt layer. The silt on the wall appeared to be firm. A photo is attached.
What does the spalling of wall suggest to an engineer. Could the silt be soft or.. Thanks.

 

[ONENGINEER]

The intention is to design a cofferdam and I am trying to estimate soil parameters.

 

[GeoPaveTraffic]

Looks like the silt is behaving as a mostly non-cohesive material. Caving with time should be expected as the negative pore pressure induced by the excavation dissipates.

Additionally, the inflow of water may be causing the silt to cave.

You are going to need other tests to estimate design parameters.

Mike Lambert

 

[ONENGINEER]

Thanks for the feedback. CPT profile is available at a distance of 25 m. However, the question is if the silt was stiff, would it cave in after half an hour or could that imply the silt was originally soft.

 

[GeoPaveTraffic]

To me it implies that the silt is neither soft nor stiff. Instead it is loose, medium dense, dense, etc.

Mike Lambert

 

[PEinc]

The ground water is slowly draining from the silt and causing it to collapse into the excavation. I would expect this silt to be very hard to dewater because of its low permeability. Soil like this, but usually not as dense, is often referred to as "bull's liver." The sheeting design will need to consider hydrostatic pressure. Steel sheet piling would probably be more suitable than soldier beams and lagging.

http://www.PeirceEngineering.com

 

[Riggly]

First of all, are you sure this material was silt or very fine sand (I said that because there are some very fine sand that could be seen in the field as silt)? You also said that the bottom of the pit was examined immediately. Did someone went in (would be very unsafe), or was just by visual inspection appeared stiff? Or was it that undisturbed material retrieved from the bottom of the pit was stiff?
Normally, as mentioned previously, granular soil will behave like this below the groundwater level; the walls will collapse, even decent bearing soils. Fine grain sands and non-plastic silts typically tend to be more susceptible to collapse. Because there is no confinement,once the surrounding material is removed, these soils may appear loose, but they could be of a much denser nature in the undisturbed state.

It is very difficult and unreliable to look at soil of this nature and predict its strength. Use SPT or maybe use dynamic cone penetrometer to help estimate the strength of the material. If those are not available, you have to use some very conservative estimates.