Refusal at 7.5 m and Two Groundwater Readings at a Warehouse Site
A 60 m by 45 m single-story warehouse is planned on a site underlain by about 2 m of gravelly fill of unknown compaction over glacial outwash. Three borings were advanced with hollow-stem augers. In two of them the sampler refused at 7.5 m and 8.2 m, recording 50 blows for 100 mm; the third passed the same elevation without difficulty and reached 20 m, where it logged a soft to firm silty clay from 11 m to the bottom of the hole. The driller recorded groundwater at 3.2 m at the end of drilling and 2.0 m when he returned 24 hours later. The structural engineer wants an allowable bearing value for spread footings at 1.5 m depth within two weeks.
1. Refusal occurred at the same elevation in two of three borings. What does that mean, and what would you do before reporting a bearing stratum at 7.5 m?
Read the worked answer
The refusal is not a bearing stratum yet.
- 50 blows for 100 mm at the same elevation in two borings, while a third boring passes freely, is the signature of a boulder, a boulder field, or a cemented/till layer. A real bedrock surface would not disappear in a boring 20 m away.
- The standard refusal criterion is normally 50 blows for 150 mm (or 100 mm). The driller's count is not a classification of the material.
Actions before anything is reported
- Re-drill 2-3 m off each refusal hole with a tricone bit, Becker hammer, or by driving casing through the obstruction to see whether the hard material continues.
- Core one hole with an HQ-size barrel to identify the material.
- Run a surface geophysical line (MASW or refraction) across the footprint to check whether a continuous refractor exists at 7.5 m; this also shows whether the obstruction is local.
- Check the regional geology for the outwash/till contact, which should be laterally predictable.
- Do not extrapolate the gravelly stratum across 60 m or issue a bearing value assuming it continues. State the uncertainty explicitly in the log and the report.
2. How should the two groundwater readings be interpreted and presented, and what is still missing for design?
Read the worked answer
Neither reading is a groundwater level.
- The 3.2 m reading at the end of drilling is water/fluid standing in the hole.
- The 2.0 m reading 24 hours later is a partially equilibrated hole. In a silty clay, equilibration takes days to weeks; the open hole behaves like a slug test. The hole also mixes perched water in the fill and gravel with deeper water.
Presentation and next step
- Report both readings with time stamps and state that neither is stabilized.
- Install standpipe or vibrating-wire piezometers with a sand pack and a bentonite seal: one screen in the gravel, one in the clay at depth. Read for at least a week, ideally through a wet season.
- Design bearing capacity, settlement, and any below-grade structure or uplift against the highest credible level (for example, 1.5 m), not the 3.2 m end-of-hole reading.
- Ask about off-site wells, nearby basements, and how the fill drains.
3. What additional exploration and testing would you authorize, and how would you lay it out?
Read the worked answer
Layout
- At least one boring within 5 m of each column-line corner, plus one in the center of the largest span; 3 additional holes minimum given the inconsistent refusal.
- Advance by mud rotary or cased methods through the gravel; terminate at 20-25 m or in the clay where the stress increment from the footing falls below roughly 10% of the effective overburden.
In-situ and lab
- Continuous SPT with an automatic hammer and measured energy (report N60); note that hollow-stem auger counts through gravel are unreliable.
- Thin-walled tube samples at 1.5 m intervals in the clay for UU triaxial, consolidation, and index testing.
- Two test pits in the fill to confirm thickness, composition, and whether it can be compacted or must be removed.
- Sieve/hydrometer gradation on the gravel to judge whether it is a suitable bearing stratum and how it will behave under the groundwater.
- If differential settlement is a concern, add shear-wave velocity (MASW) to characterize the modulus profile.