Some Basic Questions About Soil Testing

[Vista_Clement]

Hi everyone,

I have some quick questions I hope someone would be kind enough to answer:

1. Are sand replacement tests (SRT) only suitable for shallow trial pits?
2. Is there an equivalent to SRTs for boreholes?
3. Is the SPT (standard penetration test) the preferred method for determining soil unit weight in boreholes?
4. What would "Flush Returns" mean in the context of a borehole record?

Thanks!
Clem

 

[EireChch]

1 - Sand replacement test is used to check the density of fill. I cant rememeber the exact dimensions but it is essentially excavation and 300mm dia circle down to 300mm depth and replacing it with a known quantity of SAND. You can do SRT at surface of fill or excavate 0.5m do a test, excavate another 0.5m and do a test. you can repeat this for ever but you end up and a deep pit. You need to support the pit wall to ensure its stable and you have to properly back fill the pit.

2- No SRT and boreholes are not comparable. Boreholes are for determining the layering of different material and taking sample / doing insitue tests. Boreholes alone are not typically used to verify fill quality. Some boreholes may be done as part of fill verification but they are a secondary verification method.

3- No the SPT is gives an idea of the strength / density of the soil. If you want unit weight you need to take undisturbed samples and work out volume, weight etc.

4 - When drilling a borehole, often times a fluid is pumped in to the borehole to lubricate the drill but and also to stabilize the borehole walls. When stabilizing the fluid is often bentonite water mixture or guargum. There are many others. Typically you pump in fluid and it circulates down the borehole and back up to your tank. If youre drilling in rock and you know youve have pumped in 200L and you have only got back 50L then you know your 'flush return is only 25%". This can indicate that the rock is fractured and the fluid is flowing into the fractures.

 

[GeoEnvGuy]

Agree with EirChch only contributing to point 3, where you have saturated materials you can get a Lab test from the SPT sample to obtain moisture content which can determine the unit weight.

 

[TigerGuy]

SPT samples are somewhat disturbed with respect to volumetric changes resulting from the driving process. I've seen labs use them for density, and even unconfined shear strength testing. While the results are a general indicator of the properties, you need to consider they are not the highest quality values.