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 | |
• 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
- Introduction
- Concept of Rig-Based Ground Assessment
- Required Rig Parameters
- Drilling Resistance Index
- DRI to Equivalent SPT-N
- Bearing Capacity by Soil/Rock Type
- Ultimate and Allowable Bearing Capacity
- Complete Calculation Workflow
- Worked Engineering Example
- Application in Multi-Strata Ground
- Field Data Collection and Quality Control
- Importance of Site Calibration
- Limitations of the Method
- Engineering Dos and Don'ts
- Practical Applications
- Frequently Asked Questions
- Conclusion
1. Introduction
Foundation engineering depends fundamentally on understanding the ground beneath a structure. Traditional geotechnical investigations normally rely on boreholes, Standard Penetration Tests (SPT), Cone Penetration Tests (CPT), laboratory testing and engineering interpretation. However, modern drilling and piling rigs can also generate valuable information while drilling.
Parameters such as torque, crowd force, penetration rate and rotational speed vary with changes in the resistance of the material being drilled. When these parameters are continuously recorded, they can provide a useful supplementary indication of changes in subsurface resistance.
The SR-115 Rig-Based Multi-Strata Bearing Capacity Calculator uses this concept as an empirical screening and decision-support method. The calculator first determines a simplified Drilling Resistance Index, then maps that index to an equivalent SPT-N value and finally applies soil/rock-specific empirical relationships.
2. Concept of Rig-Based Ground Assessment
During drilling, the rig must overcome resistance offered by the ground. This resistance is affected by several factors, including:
- soil density and consistency;
- grain size and grading;
- cohesion and friction;
- rock weathering;
- rock strength;
- fracturing and discontinuities;
- drilling tool or bit characteristics;
- flushing conditions;
- drilling speed;
- operator control;
- rig configuration and hydraulic characteristics.
Consequently, an increase in drilling resistance may indicate a change from soft soil to dense soil, gravel, weathered rock or stronger rock. Conversely, a sudden reduction in resistance may indicate a weaker or more compressible stratum.
3. Required Rig Parameters
The calculator uses the following primary drilling parameters:
| Parameter | Unit | Engineering Significance |
|---|---|---|
| Depth | m | Depth corresponding to the drilling observation. |
| Torque | kN·m | Rotational resistance encountered by the drilling system. |
| Crowd Force | kN | Downward thrust applied to advance the drilling tool. |
| RPM | rev/min | Rotational speed of the drilling tool. |
| Penetration Rate | mm/min | Rate at which the drill advances into the ground. |
| Soil/Rock Type | Category | Determines which empirical bearing-capacity relationship is applied. |
| Calibration Factor, k | Dimensionless | Adjusts the equivalent SPT-N relationship based on field calibration. |
| Factor of Safety | Dimensionless | Used to convert the selected ultimate value into an allowable value where applicable. |
The calculator specifically requires torque, crowd force, RPM and penetration rate for the rig input calculation. The depth is primarily used to identify the location of the result.
4. Drilling Resistance Index (DRI)
The first computational step is to calculate the simplified Drilling Resistance Index (DRI).
Where:
- Crowd Force is expressed in kN.
- Torque is expressed in kN·m.
- Penetration Rate is expressed in mm/min.
The basic engineering interpretation is straightforward: if greater thrust and greater torque are required while the penetration rate is low, the calculated DRI becomes higher.
4.1 Why penetration rate is important
Consider two drilling observations with approximately the same torque and crowd force. If one section is penetrated rapidly while another section requires substantially more time, the second section will generate a higher DRI.
This makes penetration rate an important component of the simplified index.
4.2 Engineering caution regarding DRI
DRI should not automatically be interpreted as a direct measurement of soil strength. Actual drilling resistance is affected by machine, tool, drilling technique and ground conditions.
Therefore, DRI becomes significantly more useful when it is compared against actual geotechnical investigation results from the same project.
5. Conversion of DRI to Equivalent SPT-N
After determining DRI, the calculator assigns a base equivalent SPT-N value using a stepwise empirical relationship.
| DRI Range | Base Equivalent N |
|---|---|
| DRI < 50 | 3 |
| 50 ≤ DRI < 150 | 10 |
| 150 ≤ DRI < 300 | 22 |
| 300 ≤ DRI < 600 | 40 |
| DRI ≥ 600 | 60 |
The calculator subsequently applies the calibration factor:
where k is the user-defined calibration factor.
5.1 Why calibration is necessary
A drilling rig does not behave like an SPT hammer. Two different rigs can produce different torque and penetration characteristics while drilling the same material. Similarly, changing the drilling tool, bit condition, flushing system or operating procedure can change the recorded parameters.
Therefore, a project-specific relationship is preferable whenever sufficient borehole and SPT data are available.
6. Bearing Capacity Estimation by Soil and Rock Type
Once the corrected equivalent N-value has been obtained, the calculator selects an empirical relationship based on the identified soil or rock type.
| Ground Type | Relationship Used by Calculator | Engineering Interpretation |
|---|---|---|
| Cohesionless Sand | qa ≈ 10N | Preliminary correlation for sand. |
| Cohesive Clay | Cu ≈ 5N qult ≈ 5.7Cu | Uses estimated undrained shear strength followed by an ultimate-strength relationship. |
| Silty Sand / Sand with Fines | qa ≈ 9N | Reduced empirical value relative to clean sand. |
| Gravel / Sandy Gravel | qa ≈ 16N | Higher estimated value associated with dense granular interlock. |
| Silt | Cu ≈ 4N qult ≈ 5Cu | Fine-grained soil correlation. |
| Mixed / Silty Clay | qa ≈ 7N | Approximate intermediate relationship. |
| Weathered / Soft Rock | qa ≈ 600 + 20N | Empirical preliminary weak-rock relationship. |
| Hard Rock | qa ≈ 1600 + 30N | Empirical preliminary high-resistance rock relationship. |
The relationships above are the specific empirical relationships implemented in the calculator. They should therefore be regarded as tool methodology, not universal code equations applicable to every foundation condition.
7. Ultimate and Allowable Bearing Capacity
Bearing capacity terminology must be understood correctly before interpreting calculator output.
7.1 Ultimate bearing capacity
Ultimate bearing capacity represents the theoretical pressure associated with the selected failure criterion under the adopted calculation model.
7.2 Allowable bearing capacity
The allowable bearing pressure is generally obtained by applying an appropriate factor of safety to the relevant ultimate value.
However, the calculator's implementation varies depending upon the selected soil/rock correlation. For several categories, the empirical relationship directly provides a quantity labelled as allowable bearing capacity and the corresponding ultimate value is obtained by multiplying by the selected FOS.
8. Complete Calculation Workflow
1Collect drilling data
Record depth, torque, crowd force, RPM and penetration rate at suitable depth intervals. Preferably record the data continuously through an automated monitoring system.
2Calculate DRI
3Determine base equivalent N
Compare the calculated DRI with the empirical DRI ranges and assign the corresponding base N-value.
4Apply calibration factor
5Identify the ground type
Select sand, clay, silty sand, gravel, silt, mixed soil, weathered/soft rock or hard rock based on actual site investigation and drilling observations.
6Apply empirical bearing-capacity relationship
The calculator applies the relationship associated with the selected ground type.
7Apply factor of safety where applicable
The selected FOS is used in the calculation of the corresponding allowable or ultimate value according to the implemented methodology.
8Review against actual investigation
Compare the calculated estimate with borehole logs, SPT/CPT results, laboratory data, groundwater conditions and foundation load-test results before adopting a design value.
9. Worked Engineering Example
Consider a hypothetical drilling observation with the following parameters:
| Parameter | Value |
|---|---|
| Depth | 18.0 m |
| Torque | 120 kN·m |
| Crowd Force | 180 kN |
| Penetration Rate | 60 mm/min |
| RPM | 25 rpm |
| Ground Type | Cohesionless Sand |
| Calibration Factor | 1.0 |
| FOS | 2.5 |
Step 1 – Calculate DRI
DRI = 360
Therefore:
Step 2 – Determine equivalent N
A DRI of 360 falls within the 300–600 range. The corresponding base equivalent N-value is therefore:
With k = 1.0:
Step 3 – Estimate bearing capacity for sand
For the sand relationship implemented by the calculator:
qa ≈ 10 × 40
qa ≈ 400 kPa
The corresponding calculator-derived ultimate value using FOS = 2.5 would be:
qult = 400 × 2.5
qult = 1000 kPa
10. Application in Multi-Strata Ground Conditions
One of the most useful aspects of continuous drilling data is the ability to observe changes with depth.
For example, a borehole may encounter:
- Loose fill or soft soil near ground level;
- Medium dense sand;
- Dense sand/gravel;
- Weathered rock;
- Competent rock.
The drilling response can change significantly as the drilling tool passes through each material.
| Depth Interval | Observed Behaviour | Possible Interpretation |
|---|---|---|
| 0–4 m | High penetration rate, low resistance | Loose/soft material or fill |
| 4–10 m | Moderate resistance | Medium dense soil |
| 10–15 m | High torque and lower penetration | Dense granular stratum |
| 15–18 m | Significant drilling resistance | Weathered rock or hard stratum |
This type of information can be useful when deciding where additional conventional investigation should be concentrated and when identifying potential founding strata.
11. Field Data Collection and Quality Control
11.1 Maintain consistent drilling conditions
For meaningful comparison between depth intervals, maintain reasonably consistent operating conditions wherever practical.
- Use consistent drilling tools where possible.
- Record bit/tool changes.
- Record changes in flushing conditions.
- Record changes in drilling mode.
- Record significant operator interventions.
- Maintain accurate depth referencing.
11.2 Avoid isolated readings
A single spike in torque should not automatically be interpreted as a major geological change. Temporary obstructions, cobbles, reinforcement remnants, tool vibration or drilling instability can generate abnormal readings.
11.3 Use trends rather than individual values
A sustained change in DRI over a meaningful depth interval is generally more informative than one isolated high or low reading.
11.4 Correlate with borehole logs
The most valuable application is to compare rig data with actual recovered strata and geotechnical test results.
12. Importance of Site-Specific Calibration
The calibration factor k is one of the most important features of the calculator because the relationship between rig parameters and ground resistance is not universal.
12.1 Recommended calibration procedure
- Select representative boreholes across the project.
- Record rig parameters at the same depths as conventional investigation points.
- Calculate DRI at those depths.
- Obtain actual SPT-N values where applicable.
- Compare DRI with measured N-values.
- Develop a project-specific correlation.
- Determine a suitable calibration factor or revised relationship.
- Validate the relationship at additional locations.
13. Important Assumptions and Limitations
The calculator is intentionally simplified. Several important foundation-design parameters are not directly incorporated into the basic calculation.
- Groundwater table is not directly included.
- Foundation shape is not directly considered.
- Foundation width and length are not included in the basic empirical calculation.
- Foundation depth effects are not fully incorporated into bearing-capacity theory.
- Eccentric loading is not explicitly considered.
- Inclined loading is not explicitly considered.
- Layer interaction is not explicitly analysed.
- Settlement is not calculated.
- Rig-specific effects may influence DRI.
- Bit wear can influence drilling resistance.
- Flushing conditions can influence penetration behaviour.
- Operator technique can influence the recorded parameters.
- SPT-N equivalence is approximate.
Final Foundation Design Must Not Depend on This Calculator Alone
The calculator should be considered a preliminary engineering and decision-support tool. It does not replace a complete geotechnical investigation.
Final foundation recommendations should be supported by appropriate site investigation, laboratory testing, groundwater assessment, geotechnical analysis and, where required, foundation load testing.
14. Engineering Dos and Don'ts
| Do | Don't |
|---|---|
| Calibrate rig data against actual site investigation results. | Assume equivalent N is identical to measured SPT-N. |
| Record drilling depth accurately. | Use readings without checking depth correspondence. |
| Record tool, bit and flushing changes. | Ignore equipment changes during interpretation. |
| Review DRI trends across depth. | Base geological interpretation on one isolated spike. |
| Compare results with borehole logs. | Classify soil solely from machine parameters. |
| Consider groundwater and settlement separately. | Assume bearing capacity automatically guarantees acceptable settlement. |
| Use engineering judgement. | Use the calculator as the sole basis for foundation approval. |
15. Practical Engineering Applications
A rig-based resistance approach can be useful in several stages of construction and geotechnical assessment.
15.1 Preliminary strata identification
Continuous changes in drilling resistance can help identify possible transitions between different materials.
15.2 Piling operations
During piling, drilling records can provide supplementary information about the material encountered along the pile depth.
15.3 Quality control
Where a calibrated relationship exists, unusual drilling behavior can be flagged for further investigation.
15.4 Investigation optimization
Areas showing unusual drilling resistance may be selected for additional boreholes, sampling or testing.
15.5 Preliminary foundation assessment
The method can provide an initial indication of the relative resistance of different foundation strata before detailed design calculations are completed.
16. Understanding the Effect of Each Rig Parameter
Torque
Torque represents rotational resistance. Increasing torque, while other parameters remain constant, increases DRI in the implemented formula.
Crowd Force
Crowd force represents the downward thrust applied to advance the tool. Greater required crowd force increases the calculated DRI.
Penetration Rate
Penetration rate has an inverse relationship in the DRI equation. Lower penetration rates produce higher DRI values for the same torque and crowd force.
RPM
RPM is recorded as an important drilling parameter, but it is not directly included in the current DRI equation. This is an important distinction when interpreting the calculator.
17. Recommended Professional Interpretation Format
For project records, a rig-based assessment should ideally be documented in a structured format.
| Item | Recommended Record |
|---|---|
| Location | Borehole / pile identification and coordinates |
| Depth | Exact drilling depth |
| Torque | Recorded value and unit |
| Crowd | Recorded value and unit |
| Penetration | Recorded value and unit |
| RPM | Recorded value |
| DRI | Calculated value |
| Equivalent N | Base and calibrated value |
| Ground Classification | Based on investigation and drilling evidence |
| Estimated Capacity | Empirical preliminary value |
| Verification | SPT/CPT/laboratory/load-test comparison |
18. Frequently Asked Questions
No. It produces an empirical equivalent N-value. Actual SPT testing remains a separate geotechnical investigation method.
It should not be used as the sole basis for final design. Final design requires appropriate geotechnical investigation and consideration of bearing failure, settlement, groundwater, foundation geometry and other applicable conditions.
Rig characteristics, tools, drilling conditions and local geology affect the relationship between drilling resistance and actual soil/rock properties. Calibration against site data improves project-specific reliability.
RPM is collected as a rig parameter, but the current DRI equation uses crowd force, torque and penetration rate only. Therefore RPM does not directly change the calculated DRI in the present implementation.
It can provide an indication of increased drilling resistance that may be consistent with weathered or harder material, but rock classification should be confirmed using geological and geotechnical evidence such as core recovery, RQD, weathering grade and rock description.
No. Bearing failure and settlement are different design considerations. A foundation may have adequate shear resistance but still experience unacceptable settlement.
The calculator uses the selected factor of safety in converting between the empirical allowable and ultimate values according to the selected ground model.
Check equipment condition, bit condition, drilling mode, flushing, operator intervention, depth referencing and possible obstructions. Abnormal readings should be investigated before being interpreted as a geological change.
19. Conclusion
The SR-115 Rig-Based Multi-Strata Bearing Capacity Calculator represents a practical way of converting continuously available drilling information into a preliminary geotechnical indicator.
The methodology follows a clear sequence:
↓
Drilling Resistance Index (DRI)
↓
Equivalent SPT-N
↓
Calibration Factor
↓
Soil/Rock-Specific Empirical Correlation
↓
Preliminary Bearing Capacity
↓
Geotechnical Verification
Its greatest practical value is not simply producing a single bearing-capacity number. The more important advantage is the possibility of observing how drilling resistance changes with depth and correlating those changes with the actual subsurface profile.
For professional geotechnical work, the strongest approach is therefore to combine rig monitoring + borehole investigation + SPT/CPT + laboratory testing + engineering judgement + load testing where required.
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