When excavating a 15-meter-deep foundation pit in a dredger fill reclamation area, facing a 6-meter-thick sand layer, selecting an appropriate waterproof curtain requires comprehensive consideration of geological conditions, water-stop effectiveness, construction feasibility, and cost. The following are recommended solutions and analysis for this scenario:

I. Recommended Solution: High-Pressure Jet Grouting Pile Waterproof Curtain

Applicability Analysis

1. Adaptability to Sand Layer Characteristics
   Sand layers (especially medium-coarse sand) have strong permeability, which can easily lead to seepage or piping.High-pressure jet grouting pilescut through the stratum by high-pressure injection of cement grout and mix with sand particles to form a consolidated body (jet grouting pile), which can effectively block water flow and adapt to sand layer geological conditions. Compared with other methods, it has better pile continuity and water-stop effectiveness in dense sand layers.

2. Depth Requirement Match
   The construction depth of high-pressure jet grouting piles can reach more than 20 meters, fully covering the 15-meter foundation pit requirement. If the sand layer is located below the bottom of the foundation pit, it is necessary to ensure that the curtain penetrates 1-2 meters into the lower impermeable layer (such as a clay layer) to prevent seepage around it.

3. Dredger FillAdaptability
   The soil in the dredger fill area is loose and may have uneven settlement. Jet grouting piles can flexibly respond to complex strata by adjusting pressure and grout mix ratio, ensuring the integrity of the curtain.

4. Construction Efficiency and Cost
   Although the cost is higher than that of mixing piles, it is lower than that of underground diaphragm walls, and construction is flexible, requiring no large equipment, making it suitable for sites with limited space. A continuous wall can be formed by multiple rows of piles or swing jet grouting to enhance water-stop reliability.

II. Comparison of Alternative Solutions

1. Triaxial Cement-Soil Mixing Pile

• Advantages: Lower cost, suitable for silty fine sand layers.

• Disadvantages: Poor pile quality in medium-coarse sand layers, prone to pile breakage, and unstable water-stop effectiveness.

• Applicable Scenarios: Recommended only when the sand layer is silty fine sand and the burial depth is relatively shallow, with testing required to verify continuity.

2.Underground Diaphragm Wall

• Advantages: Integrated water-stop and support, extremely high safety.

• Disadvantages: High cost and long construction period, suitable for deep foundation pits in environmentally sensitive surroundings (such as adjacent buildings).

• Applicable Scenarios: Selected when deformation control requirements are extremely high and the budget is sufficient.

3.Grouting Curtain

• Advantages: Flexible adaptation to complex strata.

• Disadvantages: Uniformity is difficult to control, multiple grouting is required, and reliability is relatively low.

• Applicable Scenarios: Assisting other curtains or local reinforcement.

III. Key Implementation Recommendations

1. Curtain Depth Design: Ensure that the curtain penetrates through the sand layer and enters the lower impermeable layer by at least 1-2 meters to form a closed system.

2. Parameter Optimization: Adjust the jet grouting pressure (recommended 20-30 MPa), lifting speed (10-20 cm/min), and grout water-cement ratio (1:1 to 1.5:1) according to the density of the sand layer.

3. Quality Inspection: Use borehole coring andpermeability teststo verify the continuity and impermeability of the curtain (permeability coefficient <= 1 x 10^-6 cm/s).

4. Dewatering Assistance: If there is a locally weakly permeable layer,light wellpoint dewateringcan be used in combination to lower the water level inside the pit.

IV. Conclusion

      High-pressure jet grouting pileis the preferred solution for water-stop treatment of the 6-meter sand layer in a 15-meter foundation pit in a dredger fill area, balancing reliability, adaptability, and economy. If the sand layer is silty fine sand and the budget is limited, the feasibility of triaxial mixing piles can be evaluated, but strict quality control is required. In complex environments, underground diaphragm walls can be combined, but the cost increases significantly. The final solution needs to be determined based on on-site tests and detailed geological survey data.