SR-115 Rig Based Multi-Strata Bearing Capacity Calculator

SR‑115 Rig Based Multi‑Strata Bearing Capacity Calculator

Rig Input Parameters
Depth (m)
Torque (kNm)
Crowd Force (kN)
RPM
Penetration Rate (mm/min)
Strata Selection
Soil / Rock Type
Design Parameters
Calibration Factor (k)
Factor of Safety

Engineering Basis:
• DRI derived from torque, crowd & penetration.
• DRI correlated to SPT‑N (field calibration required).
• Empirical bearing capacity correlations from geotechnical practice.
• Final design must be verified by borehole & load test.

SR-115 Rig-Based Multi-Strata Bearing Capacity Calculator

Theoretical Background and Engineering Methodology

This tool provides a quick empirical estimation of allowable and ultimate bearing capacity using real-time drilling parameters from rotary rigs (such as SR-115). It converts rig performance data into an equivalent Standard Penetration Test (SPT) N-value and then applies soil/rock-type-specific correlations to estimate bearing capacity.

1. Introduction

In modern geotechnical practice, Measurement While Drilling (MWD) or rig parameter monitoring offers a continuous, cost-effective way to assess subsurface conditions. By recording torque, crowd (thrust) force, penetration rate, and RPM during drilling, engineers can derive indices that reflect the resistance of the ground — helping to identify strata changes and estimate strength parameters without relying solely on traditional SPT or CPT tests.

            Key Advantage: This method provides real-time data across multiple strata, making it especially useful for variable or layered soil/rock profiles where discrete testing may miss transitions.
        

2. Drilling Resistance Index (DRI)

The calculator starts by computing a simple **Drilling Resistance Index (DRI)**:

DRI = (Crowd Force × Torque) / Penetration Rate

where:

Crowd Force (kN) — downward thrust applied by the rig
Torque (kNm) — rotational resistance
Penetration Rate (mm/min) — rate of drill bit advance

Higher DRI values indicate greater ground resistance (harder material).

3. Correlation to Equivalent SPT N-Value

The DRI is then mapped to an equivalent SPT N-value using a stepwise empirical relationship (calibrated with a user-defined factor k):

DRI Range	Base N-Value
< 50	3
50 – 150	10
150 – 300	22
300 – 600	40
> 600	60

Final N = Base N × Calibration Factor (k)

Note: This is a simplified empirical correlation. In practice, site-specific calibration against actual SPT values is strongly recommended for reliable results.

4. Bearing Capacity Estimation by Soil/Rock Type

Once the equivalent N-value is obtained, the tool applies type-specific empirical correlations commonly used in preliminary geotechnical assessments:

Soil/Rock Type	Correlation Used	Remarks
Cohesionless Sand	qa ≈ 10N (kPa)	Classic Terzaghi-type correlation for sand
Cohesive Clay	Cu ≈ 5N (kPa) qult ≈ 5.7 × Cu	Undrained shear strength and Skempton’s bearing capacity factor
Silty Sand / Sand with fines	qa ≈ 9N	Slightly reduced due to fines content
Gravel / Sandy Gravel	qa ≈ 16N	Higher due to better interlocking
Silt	Cu ≈ 4N qult ≈ 5 × Cu	Lower strength fine-grained soil
Mixed / Silty Clay	qa ≈ 7N	Average for mixed behavior
Weathered / Soft Rock	qa ≈ 600 + 20N	Empirical for weak rock
Hard Rock	qa ≈ 1600 + 30N	High strength rock foundations

5. Allowable vs Ultimate Bearing Capacity

The calculator computes:

Ultimate Bearing Capacity (qult) — theoretical maximum pressure before shear failure
Allowable Bearing Capacity (qa) — qult divided by the selected Factor of Safety (typically 2.5–3.0)

6. Important Assumptions and Limitations

The method is **empirical** and provides preliminary estimates only.
Drilling parameters are influenced by rig type, bit condition, flushing, operator technique, and borehole stability.
SPT N-equivalent is approximate; local calibration is essential.
No direct consideration of groundwater table, foundation shape, depth, or eccentricity.
Does not replace detailed borehole logging, laboratory testing, or plate load tests.
For final design, always verify with conventional geotechnical investigation and load testing.

Engineering Disclaimer:
This calculator is intended as a supplementary tool for experienced geotechnical professionals. It should never be used as the sole basis for foundation design. Final recommendations must be based on comprehensive site investigation, including boreholes, SPT/CPT, laboratory tests, and engineering judgment.

7. Practical Applications

This rig-based approach is particularly valuable for:

Real-time strata identification during piling or drilling works
Preliminary foundation sizing on variable sites
Quality control during deep foundation installation
Optimizing borehole locations and depths

Developed as an educational and decision-support tool • Always apply sound engineering judgment and site-specific validation.

YogiPWD

Rig Based Bearing Capacity Estimator