Soil Parameters 1

[ONENGINEER]

If no further in-situ/lab testing can not be carried out, what could be an experienced engineer's assessment of the till materials investigated by test pitting to a depth of 4.5m.

The walls were stable until 2 hrs before ther pits were backfilled (refer to the attachment), no water was encountered and pocket penetrometer (although may not be accurate)on the excavated wall showed readings of more than 4.5 tsf.
Atterberg test showed the soil to be non plastic with LL = 13%. Moisture content of 9% and the till is expected to be clayeye silty sand sandy silt with gravels, cobbles and boulders. I have assumed a phi value of 50 but need to know how much could C be for shoring design.

 

[mconti]

50° is to much usually 35-38° and c=0KPa (granular soil)

 

[ONENGINEER]

The SPT has been assumed as 50 and a phi value of 40 degrees. Could a C value of 0.15 tsf be assumed for the soil which shows pocket pentrometer reading of 4.5 tsf. Sorry, there was a mistake in my original question about phi value.

 

[Ron]

Agree with mconti....phi about 35, c=0

Be careful with open excavations in such soil. As with other granular soils, walls of excavation can suddenly collapse as moisture reduces from exposed face.

 

[ONENGINEER]

The test pit was dug with difficulties of hard ground. The till was often peeled by the backhoe as if the materials were weak sandstone. I left these excavated materials in water and soaked for few days. The sheet lumps did not disintegrate in water and the grains show sticked together while in water. Based on these observation could a C' = 6 kPa be assumed for these materials? Thank you again.

 

[cvg]

sands and gravels should be assumed to have zero cohesion. granted you may have minor fractions of clay and silt, but you should use a conservative estimate of zero