Evaluation of Different Methods for Cone Resistance Assessment from Dynamic Penetration Tests

Abstract

Dynamic penetration tests (DPT) are largely applied for soil in situ characterization. Cone resistance assessment from DPT is usually based on driving formulas derived from Newtonian approach. These are nevertheless often considered inaccurate and unable to fully describe dynamic penetration phenomenon. Wave equation approach provides a more rigorous analysis but remains less common for practical reasons (requiring instrumentation and a more complex analysis). Historically, driving formulas derived from Newtonian approach are often preferred for its simplicity. Thus, these formulas still play a major role in DPT interpretation and their use is recommended by current standards. It is important to evaluate different methods derived from wave equation and also to verify if a simple approach, such as Dutch formula (DF), combined with good practice (e.g., energy measurement, skin friction control) can produce satisfactory results. For that, laboratory and field tests were performed employing instrumented dynamic penetrometers (a lightweight and a super-heavy dynamic penetrometer). The main contribution of this paper is demonstrating the importance of method applied when determining resistance from DPT and providing comparison between common methods when testing different type of soils. Results showed that on average DF results were comparable to those from cone penetration test (CPT) resistance (diverging by no more than 15%). Wave equation methods results were comparable (±30%) to CPT cone resistance in most cases when testing shallower nonplastic layers. CASE method underestimated by more than 50% the resistance for shallower layers. DF including energy measurements produce lower results than all methods examined and up to 50% inferior to CPT. This work shows that, for shallow soils, DF method does not produce less accurate for assessment of soil resistance that wave equation-based methods. Other key aspect brought to light is that wave equation-based methods considerably overestimated resistance in deeper plastic saturated layers which suggests that skin friction correction application should also be considered in such conditions for these methods. When only unsaturated zones are considered, these methods tend to underestimate resistance: Simplified method and tip force integration method produced results 18% and 24% inferior to CPT whereas DF with energy measurement and other wave equation methods significantly underestimated resistance (on more than 40%).