Modified Broms' Method for Formation of a Working Platform on Very Soft Soil Using Geotubes
- Ben Lewis
- Feb 20
- 4 min read
Introduction
Construction on very soft soils presents significant challenges due to inadequate bearing capacity and excessive settlement. These conditions make it difficult for equipment and personnel to conduct soil improvement works. Traditional approaches to constructing working platforms include cement mixing, deep soil mixing, and pile-supported platforms, but these methods can be costly and time-intensive.
A widely known technique, Broms' Method, involves placing a geotextile layer over the soft soil, followed by the controlled deposition of sand berms to distribute loads. However, this approach has limitations, including construction uncertainties and instability of the sand berms due to uncontrolled spreading.
To overcome these challenges, the Modified Broms Method introduces geotextile tubes to confine sand berms. This ensures a more stable working platform with improved constructability and predictable performance.
Methodology: Modified Broms' Approach
The Modified Broms' Method enhances the original approach by incorporating geotextile tubes to stabilise the sand berms. The process consists of the following key steps:
Placement of Geotextile: A high-strength geotextile layer is deployed over the soft soil, secured at the boundaries to provide a load-bearing surface.
Geotextile Tube Installation: Geotextile tubes are positioned at specified intervals over the geotextile layer.
Sand Infilling: Sand is pumped into the geotextile tubes, forming sand berms confined within the tubes.
Gradual Load Distribution: The tubes act as rigid berms that distribute loads evenly, allowing for a controlled settlement.
Additional Sand Placement: Thin layers of sand are added progressively over the geotextile tubes to complete the working platform.
This modification addresses three key problems associated with the original Broms' Method:
Prevents instability caused by sand spreading unpredictably.
Improves construction control by confining sand within geotextile tubes.
Enhances platform load-bearing capacity, making it suitable for heavy construction equipment.
Theoretical Model & Analytical Approach
The stability of the working platform is governed by geotextile tensile strength, soil reaction forces, and vertical load distribution. The study proposes a new analytical solution to evaluate:
Tensile strain in the geotextile during deformation.
Settlement profiles based on soil compressibility and applied loads.
Bearing pressure distribution across the platform.
Key Analytical Assumptions
The system is modelled in plane strain conditions.
The geotextile deforms into a parabolic shape under sand berms.
The working platform is assumed to be in undrained conditions initially.
The unit weights of soil and sand are considered uniform.
The geotextile's weight is negligible compared to imposed loads.
The ultimate stability is evaluated at the point where full sand loading is applied.
The analytical solution was validated against Finite Element Analysis (FEA) and field trials, demonstrating good agreement between predicted and measured values.
Field Trial & Experimental Validation
A full-scale field trial was conducted in Singapore, where a working platform was constructed over 12.5 m of marine clay slurry. The site covered 3,600 m², and high-strength woven geotextile (Tencate HPa380) was used.
Key Observations
The installed geotextile tubes maintained a consistent cross-section, preventing sand berm collapse.
The deformed geotextile profile closely matched theoretical predictions.
Settlement was controlled, ensuring uniform consolidation across the platform.
Field Measured vs. Predicted Values
These results confirm that the proposed analytical method accurately predicts platform behaviour.
Design Guidelines & Considerations
For designers considering the Modified Broms' Method, key design parameters include:
Geotextile Selection
Minimum tensile stiffness: > 675 kN/m for heavy construction loads.
Ultimate tensile strain: At least 6% elongation.
Geotextile Tube Spacing & Sand Berm Dimensions
Contact width (L₁): 2 m
Spacing between tubes (L₂): 5–8 m
Sand tube height: 0.8 m (field trial)
Loading Conditions
Maximum bearing pressure: 20–40 kN/m².
Expected settlement: 0.3–0.5 m, depending on soil conditions.
Construction Sequence
Ensure pre-stretched geotextile installation.
Pump sand into tubes incrementally to avoid differential settlement.
Conduct post-construction settlement monitoring.
Comparative Advantages Over Conventional Broms' Method
Enhanced control over sand berm dimensions.
Eliminates construction uncertainties due to uncontrolled sand spreading.
Higher bearing capacity, allowing the use of heavy construction machinery.
Comparison with Other Methods
Limitations & Future Developments
Soil variability: Requires site-specific testing.
Initial consolidation time: Some settlement monitoring is necessary.
Tensile strain limits: Geotextile must be carefully selected to prevent rupture.
Conclusion
The Modified Broms' Method offers a practical, cost-effective solution for constructing working platforms over very soft soils. By incorporating geotextile tubes, it enhances construction efficiency, stability, and predictability, making it an attractive alternative to traditional methods.
Key Takeaways for Designers:
Ideal for mine sites, land reclamation, and infrastructure projects.
Geotextile selection and spacing optimisation are crucial.
Validated through analytical modelling, FEA, and field trials.
Reduces construction uncertainty, making it practical for large-scale applications.
By adopting this approach, designers and engineers can improve site access, reduce settlement risks, and optimise construction costs on challenging soft soils.
To see the referenced paper in full, sign up to the best geosynthetics community at GeoKonect and view it free among an array of other geosynthetics technical content. Direct link to the paper below:
Comments