Mechanical properties of film-bag sand cofferdam: theoretical analysis and numerical study

Figures & data

Figure 1. The equivalent calculation diagram of film-bag sand.

Figure 2. Mohr-Coulomb strength line of reinforced sand filled with membrane bag.

Figure 3. Approximate picture of film bag sand.

Figure 4. Calculation diagram of membrane bag balance under the dead weight load.

Figure 5. Calculation diagram of membrane bag balance under dead weight and external load.

Figure 6. Approximate of the actual shape of film bag sand.

Figure 7. Calculation diagram of actual shape balance of film bag sand.

Figure 8. Calculation diagram of actual shape balance of dead weight and internal and external load.

Figure 9. Calculation diagram of film bag sand shape.

Figure 10. Calculation diagram of tube bag micro-segment.

Figure 11. Shape diagram of tube bag under different filling pressure.

Figure 12. Standard section of sand blowing cofferdam.

Table 1. Parameters of anti-aging polypropylene geotechnical mold bag.

Project		Unit	Index value
Mass per unit area		g/m^2	202.3
Thickness		mm	0.2
Tensile strength	Meridian direction	N/50 mm	2010
	Latitudinal direction	N/50 mm	1808
Trapezoidal tear strength	Meridian direction	N	526
	Latitudinal direction	N	418
Longitude and weft density	Meridian direction	N	16
	Latitudinal direction	N	15
Elongation	Meridian direction	%	22
	Latitudinal direction	%	23
CBR bursting strength		N	4003
Peel strength		– –	20
Permeability coefficient		cm/s	0.079
Equivalent aperture O95		mm	0.15

Table 2. Parameters of model sand.

Materials	Natural weight $\gamma$ ($kN/m^3$)	Internal friction angle $\phi$ (Degree)	Cohesion $C$ ($kPa$)	Compression modulus $E$ ($MPa$)
Silty clay	19.5	10.1	40.6	25.8
Tube bag sand	20.0	18.8	0.5	8.4
Embankment core sand	20.1	20.8	0.9	11.4

Figure 13. Finite element model of pipe bag sand.

Table 3. Parameter Correspondence under Different DP Criteria.

Criterion	Relationship with Mahr Coulomb criterion	$\beta$	${\sigma }_y$
$D{P}_{\hspace{0.17em}}$	Hexagon outer corner circumscribed circle	$\frac{2sin\phi }{\sqrt{3}(3-sin\phi )}$	$\frac{6ccos\phi }{\sqrt{3}(3-sin\phi )}$
$D{P}_1$	Hexagon inner corner circumscribed circle	$\frac{2sin\phi }{\sqrt{3}(3+sin\phi )}$	$\frac{6ccos\phi }{\sqrt{3}(3+sin\phi )}$
$D{P}_2$	Hexagonal inscribed circle	$\frac{sin\phi }{\sqrt{3(3+{sin}^2\phi )}}$	$\frac{3ccos\phi }{\sqrt{3(3+{sin}^2\phi )}}$
$D{P}_3$	A circle equal in area to a hexagon	$\frac{2\sqrt{3}sin\phi }{\sqrt{2\sqrt{3}\pi (9-{sin}^2\phi )}}$	$\frac{6\sqrt{3}ccos\phi }{\sqrt{2\sqrt{3}\pi (9-{sin}^2\phi )}}$
$D{P}_4$	Circle matching Mahr Coulomb criterion	$\frac{sin\phi }{3}$	$ccos\phi$

Figure 14. Calculation results under the gravity with a cross-section height of 1 m.

Figure 15. Calculation results under the gravity with a cross-section height of 0.5 m.

Figure 16. Calculation results of 0.5 m high mold bag sand under dead weight and external load.

Table 4. The theoretical and simulated values of the displacement and stress of the film bag sand under typical working conditions.

Case name	Maximum vertical deformation			Maximum Lateral deformation
	Theoretical value (mm)	Simulated value (mm)	Relative error (%)	Theoretical value (mm)	Simulated value (mm)	Relative error (%)
Cross-section height of 1.0 m under gravity	1.45	1.60	10.34%	0.39	0.34	12.82%
Cross-section height of 0.5 m under gravity	0.88	0.92	4.55%	0.15	0.17	13.33%
Cross-section height of 0.5 m under gravity and external load	2.78	2.60	6.47%	0.59	0.56	5.08%
	Maximum vertical stress			Maximum Lateral stress
Case name	Theoretical value (kPa)	Simulated value (kPa)	Relative error (%)	Theoretical value (kPa)	Simulated value (kPa)	Relative error (%)
Cross-section height of 1.0 m under gravity	30.45	27.80	8.70%	12.21	11.50	5.81%
Cross-section height of 0.5 m under gravity	32.50	30.50	6.15%	8.89	9.79	10.12%
Cross-section height of 0.5 m under gravity and external load	90.42	81.20	10.20%	37.53	33.50	10.74%

Figure 17. Overall calculation model of cofferdam.

Table 5. Conversion cohesion value of mold bag sand.

Inside	1	2	3	4	5	6	7	8
Converted value ($kPa$)	6.85	9.23	11.61	13.99	16.37	18.7	23.51	25.89
Outside	1	2	3	4	5
Converted value ($kPa$)	6.89	14.87	18.06	18.06	18.06

Figure 18. Calculation result with 0.1 m thick concrete slab settlement s (unit: m).

Figure 19. Calculation results with 0.15 m thick concrete slab settlement (unit: m).

Figure 20. Calculation result of weir settlement.