Flat Dilatometer Test (DMT)

Marchetti’s Flat Dilatometer Test (DMT): A Comprehensive Guide

The Flat Dilatometer Test (DMT), developed by Marchetti, utilizes a steel membrane that is pneumatically expanded against the soil at 0.2-meter intervals. During testing, precise pressure readings are recorded, providing critical insights into soil stiffness and stress history. These readings are then correlated with various geotechnical parameters, including:

• Overconsolidation ratio (OCR)

• Constrained modulus

• Undrained shear strength

• Soil behavior type

• Friction angle

Why Choose DMT?

The DMT is a valuable complement to other in-situ geotechnical tools like the Cone Penetration Test (CPT) and Seismic CPTu (SCPTu). What sets the DMT apart is its exceptional sensitivity to horizontal stress, made possible by its blade-shaped probe design. This unique feature enables the DMT to capture high-resolution data, even in challenging geological conditions.

Thanks to its robust blade construction, the DMT can be advanced into the ground using either static push or hammering techniques, making it suitable for a wide range of soil types, from ‘semiliquid’ soils to dense or hard soils.

Additionally, samples can be rapidly collected in nearby locations using direct push technology upon request.

Key Benefits of the DMT

1. Standardized and repeatable operations: Ensures reliable and reproducible results.

2. High sensitivity to horizontal stress: Outperforms many in-situ geotechnical tests in this aspect.

3. Faster data collection: Captures data at a higher resolution compared to conventional methods like the Standard Penetration Test (SPT) and soil sampling.

4. Real-time data availability: Facilitates quicker decision-making during field investigations.

5. Quick dissipation tests: Particularly effective for identifying niche soil layers such as silts.

6. Dynamic push advancement: Enables the tool to penetrate hard soils or dense strata using hammering techniques.

Challenges of Using the DMT

While the DMT offers numerous advantages, it is important to consider some of its challenges:

1. Limited penetration in very dense soils or gravels: The blade design may encounter difficulty in penetrating highly compact or coarse-grained soils.

2. Specialized equipment and expertise required: The DMT relies on precise calibration and skilled operators to ensure accurate results.

3. Relatively high initial investment: The setup costs, including equipment and training, can be higher than traditional methods like SPT.

4. Difficulty in interpreting results for very soft clays or loose sands: In these cases, the membrane may not expand uniformly, affecting data reliability.

5. Not ideal for very shallow depths: The DMT performs best at depths where static or dynamic pushing can be effectively employed.

When Does the DMT Have the Most Value?

The DMT provides unmatched value in specific geotechnical scenarios where detailed soil stress and stiffness profiles are critical. Key applications include:

• Soft and cohesive soils: The DMT is ideal for assessing soil behavior in clays, silts, and organic soils due to its sensitivity to horizontal stress.

• Sites requiring settlement prediction: High-resolution stiffness data allows for more accurate settlement estimates in foundation and embankment design.

• Liquefaction studies: The DMT is highly effective in determining liquefaction potential, particularly in loose sandy soils.

• Infrastructure projects: It is commonly used for evaluating subgrade modulus in pavement and diaphragm wall design.

• Slope stability analysis: The ability to detect slip surfaces makes it invaluable for clay slope assessments.

• Compaction control: The DMT provides precise measurements for quality control during compaction operations.

• Challenging geological conditions: Its versatility and robustness make it suitable for a wide range of soil conditions, from soft silts to hard clays.

By understanding the benefits and limitations of the DMT, engineers can leverage its strengths to achieve accurate, site-specific geotechnical insights, ensuring successful project outcomes.