Rigorous control of the degree of compaction on-site is the decisive step that validates the construction quality of embankments, sub-bases, and pavement foundation layers. Among the standardized direct methods, the in-situ density test by the sand cone method stands out as the metrological reference procedure for determining the dry unit weight of compacted soil. This article details laboratory and field calibration protocols, the step-by-step excavation procedure, compaction degree calculations, and critical sources of error that compromise the technical acceptance of work fronts.
1. What is the test and what problem does it solve?
The in-situ density test by the sand cone method consists of determining the dry unit weight of a soil in its compacted state in the field, by simultaneously measuring the mass of excavated soil and the volume of the corresponding hole, which is filled with a standardized sand of known unit weight.
In the construction of road infrastructure, airports, and industrial platforms, structural stability and the prevention of differential settlements directly depend on the level of densification achieved by compaction equipment (compactors). The test solves the fundamental problem of verifying whether the actual density obtained in the field reaches the reference value stipulated in the project (generally expressed as a percentage of the maximum dry unit weight obtained in the Standard or Modified Proctor test).
2. What is it for and when should it be used?
The systematic application of the sand cone test is mandatory in the following work fronts:
• Daily technological control of earthwork layers (embankments, trenches, building foundations, and platforms).
• Acceptance and release of sub-base and base layers in road paving before the placement of asphalt or concrete surfacing.
• Compliance verification in transition zones of engineering structures (cuts and bridge abutments).
• QA/QC audits for validating the compaction efficiency of different types of compactors (sheepsfoot, smooth vibratory, or pneumatic).
www.cambaboost.com
3. Reference standards
The execution of the test must strictly comply with the requirements of the technical standards applicable to the contract:
• NP 141: Soils - Determination of in-situ unit weight (Sand-bottle method).
• ASTM D1556 / D1556M: Standard Test Method for Density and Unit Weight of Soil in Place by Sand-Cone Method.
• AASHTO T 191: Density of Soil In-Place by the Sand-Cone Method.
• LNEC E 195: Soils - Determination of in-situ unit weight.
• DNIT 141/2010 - ME: Soils - Determination of in-situ unit weight by the sand-bottle method.
4. Prior equipment calibration
The reliability of the sand cone test entirely depends on a rigorous prior laboratory calibration process, divided into two essential phases:
1. Calibration of sand unit weight (g_sand):
A calibrated container of known volume (Vcontainer) is used. The sand cone apparatus (bottle coupled to the conical base) is filled with calibrated sand (usually clean, dry, and uniform quartz sand, passing the 2.0 mm sieve and retained on the 0.125 mm sieve). The assembly is inverted over the container until the flow ceases. The sand that filled the container is weighed, and gsand = Msand / Vcontainer is calculated.
2. Calibration of sand mass retained in the cone (M_cone):
The mass of sand exclusively required to fill the volume of the cone and the base plate is determined by opening the valve over a clean, flat surface. This mass M_cone will be deducted in all field tests to isolate only the sand that penetrated the excavated hole.
5. Equipment and materials
Field equipment must be kept clean, calibrated, and protected against moisture and impacts:
• Sand cone apparatus consisting of a transparent or metallic bottle of adequate capacity (usually 4 liters) and a metallic conical valve with a quick-opening device.
• Square or circular metallic base plate with a central hole coinciding with the opening of the conical valve.
• Manual excavation tools: short chisel, trowel, small scoop, soft brush, and brush for complete material collection.
• Airtight containers or sturdy plastic bags for collecting and transporting the moist sample without moisture loss.
• Field balance with a minimum capacity of 10 kg and an accuracy of 0.1 g to 1.0 g.
• Laboratory oven regulated at 110 °C ± 5 °C for water content determination.
• Control sieves, light hammer, and metallic ruler.
6. Step-by-step procedure
The execution of the test in the field follows the following rigorous operational sequence:
- Surface preparation: Level an area of approximately 30 x 30 cm at the test location, removing loose or dry surface materials, to obtain a flat and firm surface.
- Base plate installation: Firmly seat the metallic plate on the leveled surface, ensuring full contact with the soil.
- Hole excavation: Excavate the soil through the central hole of the base plate with the aid of the chisel and trowel. The depth of the hole should correspond to the thickness of the compacted layer (typically 15 cm to 25 cm). The diameter of the hole should coincide with the diameter of the plate's hole.
- Excavated soil collection: All material removed from the hole must be immediately placed in an airtight container and weighed to determine its total moist mass (Mmoistsoil).
- Moisture content determination: Take a representative sample of the excavated soil to determine the immediate moisture content (w) in an oven.
- Apparatus positioning: Fill the cone bottle with the previously weighed calibrated sand (Mtotalinitial). Invert the apparatus and fit the cone exactly over the hole in the base plate.
- Valve opening: Open the conical valve without subjecting the apparatus to vibrations. Wait until the sand flow completely ceases (observing through the transparent bottle or controlling the stabilization time).
- Closing and final weighing: Close the valve firmly, remove the cone apparatus, and weigh the remaining sand in the bottle (Mtotalfinal).
7. Calculations and results processing
The mathematical processing of the collected data follows these steps:
1. Calculation of the mass of sand that filled the hole (Msandhole):
Msandhole = Mtotalinitial - Mtotalfinal - M_cone
2. Calculation of the hole volume (V_hole):
Vhole = Msandhole / gsand
Where g_sand is the unit weight of the sand calibrated in the laboratory.
3. Calculation of the in-situ moist unit weight (g_moist):
gmoist = Mmoistsoil / Vhole
4. Calculation of the in-situ dry unit weight (g_dry):
gdry = gmoist / (1 + (w / 100))
Where w is the moisture content expressed as a percentage.
5. Calculation of the Degree of Compaction (DC):
DC = (gdry / gdmax_lab) * 100 (%)
Where gdmaxlab is the maximum dry density reference obtained in the Proctor test (Standard or Modified) corresponding to the layer material.
8. Interpretation of results
The comparison between the DC value obtained in the field and the requirements of the Specifications dictates technical acceptance:
• DC >= Specification (e.g., >= 95% for embankments or >= 100% for bases): Layer approved and suitable for the next construction phase.
• DC < Specification: Layer rejected due to under-compaction. Requires scarification, moisture content correction (water addition or aeration), and re-passing of compactors followed by re-testing.
• Moisture discrepancies: In-situ moisture should be within the optimum range stipulated in the Proctor test (w_opt ± 2%), otherwise falsely acceptable density values may be recorded in structured soils that are vulnerable to future saturation.
9. Common errors and how to avoid them
Frequent operational errors that invalidate the in-situ density test:
• Vibration during sand flow: Passage of trucks or heavy machinery near the test during valve opening causes abnormal sand settlement, falsifying the hole volume. Solution: Stop traffic in the vicinity during execution.
• Loss of soil or moisture during excavation: Removing material slowly without protection causes evaporation or loss of fines. Solution: Immediately place soil in airtight bags.
• Hole with inclined walls or irregular shape: Funnel-shaped holes or those with lateral collapse increase the actual volume compared to the calculated one. Solution: Vertically trim the walls and clean carefully.
• Moist sand or sand contaminated with dust: The presence of moisture in calibrated sand alters its unit weight. Solution: Always use clean sand, oven-dried and cooled before each test.
• Negligence in determining M_cone: Not deducting the sand retained in the cone invalidates the entire hole volume calculation.
10. QA/QC and traceability
In the context of quality control (ISO/IEC 17025) and construction assurance:
• Metrological traceability: Mandatory recording of the balance used, oven thermometer, and lot number of the calibrated sand.
• Weekly apparatus calibration: Re-evaluate the sand unit weight and cone mass whenever there is a change in sand lot or mechanical wear in the valve.
• Coordinates and depth record: Each test must be rigorously georeferenced (station, axis, kilometer, and elevation), accompanied by the original field notebook signed by the laboratory technician and the inspector.
11. Practical application on site
In the field, the sand cone test acts as a validation element for the advancement of work fronts:
• In a road construction project, the release of a granulometrically stabilized sub-base layer typically requires one test for every 200 m² to 500 m² of compacted layer.
• The speed in obtaining moisture content (often complemented by expedited methods for immediate on-site results, with subsequent oven validation) allows the earthwork team to immediately know if the sector has been approved or if it needs corrective intervention before the end of the shift.
12. Limitations and points requiring confirmation