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Technical Note

Large-scale interface shear testing of sandbag dykematerials

T. Krahn1, J. Blatz2, M. Alfaro3 and R. J. Bathurst4

1Engineer, Built Alternatives Inc., P.O. Box 22, Anola, Manitoba, Canada, Telephone: +1 204 866 3521,

Telefax: +1 204 866 3287, E-mail: [email protected] Professor, Department of Civil Engineering, University of Manitoba, Winnipeg,

Manitoba R3T 5V6, Canada, Telephone: +1 204 474 9816, Telefax: +1 204 474 7513,

E-mail: [email protected] Professor, Department of Civil Engineering, University of Manitoba, Winnipeg,

Manitoba R3T 5V6, Canada, Telephone: +1 204 474 8155, Telefax: +1 204 474 7513,

E-mail: [email protected] and Research Director, GeoEngineering Centre at Queen’s-RMC, Civil Engineering

Department, Royal Military College of Canada, Kingston, Ontario K7K 7B4, Canada,

Telephone: +1 613 541 6000 ext 6479/6347/6391, Telefax: +1 613 545–8336,

E-mail: [email protected]

Received 23 October 2006, revised 12 January 2007, accepted 16 January 2007

ABSTRACT: This paper presents results of large-scale direct shear testing of six interfaces with

materials used to construct sandbag dykes for flood protection structures. Two additional test series

using intact sandbags were also conducted to examine the influence of sandbag-to-sandbag

interaction in typical stacked sandbag dyke construction and sandbag-to-sod (grass) contact

representing the interface condition at the dyke foundation. The interface test results are

summarised using Mohr–Coulomb shear strength parameters and interface shear stiffness

parameters. The data presented in this paper are necessary for stability analyses of instrumented

and monitored sandbag dyke structures that are currently under way as part of the development of

a new guidance document for the design of sandbag dykes for temporary flood protection works in

Canada.

KEYWORDS: Geosynthetics, Direct shear, Testing, Sandbags, Dykes, Flood protection

REFERENCE: Krahn, T., Blatz, J., Alfaro, M. & Bathurst, R. J. (2007). Large-scale interface shear

testing of sandbag dyke materials. Geosynthetics International, 14, No. 2, 119–126

[doi: 10.1680/gein.2007.14.2.119]

1. INTRODUCTION

Temporary secondary dykes are an integral component of

the flood control system for the City of Winnipeg, located

in the province of Manitoba, Canada. In 1997 a 90-year

flood of the Red River that flows through the city

seriously challenged the integrity of clay dykes con-

structed immediately prior to the floods and temporary

sandbag dykes constructed by volunteers. This event

prompted a review of the design template used by the city

for temporary sandbag dykes and investigation of strate-

gies to improve the stability of these systems, particularly

if anticipated larger floods occur in the future.

A cross-section of the standard sandbag dyke design for

temporary flood control is illustrated in Figure 1. The

stability of these structures can be related to six different

interface types constructed with four different materials.

The materials include sandbags manufactured from woven

slit film polypropylene (WSFPP) geotextile, polyethylene

sheeting (PES) used as an internal water flow cut-off, a

clean pit run sand used to fill the bags, and grass (sod) at

the base of the dyke. The base material is generally

variable, and during typical flood periods can be frozen

and include snow and ice. It can be argued that the

interface identified as WSFPP on sand is not a potential

slip interface. However, during dyke construction sandbags

may be damaged, and this interface condition could

develop.

Geosynthetics International, 2007, 14, No. 2

1191072-6349 # 2007 Thomas Telford Ltd

A review of the literature showed that previous labora-

tory investigations of sandbag performance are sparse.

Kalumba and Scheele (1999) conducted interface shear

tests on three different sands in contact with a single

nonwoven needle-punched geotextile. They found that the

interface shear strength was lower than the shear strength

of the soil tested alone under the same confining pressure.

Specimen size has been cited as an important issue in

geosynthetic interface shear testing. Devices having mini-

mum dimensions of 300 mm 3 300 mm are recommended

for these tests to reduce boundary effects during shear

(Bove 1990). The ASTM D 5321 method of test for shear

testing of soil/geosynthetic interfaces recommends

300 mm as the minimum horizontal shear box dimension

for these tests, and this recommendation has been fol-

lowed by others (e.g. Criley and Saint John 1997).

Lohani et al. (2006) and Aqil et al. (2006) investigated

the load–deformation response of stacked soil bags sub-

jected to vertical compression and lateral shear under a

range of normal loads. Aqil et al. commented on the very

low interface shear friction angle at the horizontal inter-

face between vertically stacked bags due to the smooth

geotextile surfaces. Lohani et al. showed that the vertical

stiffness of stacked soil bags increased with increasing

strength and stiffness of the geotextile used to make the

soil bags. At large vertical deformations, however, the

effect of initial soil density of the infill soil was seen to

diminish. During lateral shear tests the shear strength of

the soil infill was only partly mobilised owing to confine-

ment effects by the encapsulating geotextile. However, the

normal loads applied in these tests were very much higher

than the vertical loads applicable to the sandbag structures

used as temporary flood protection works in Winnipeg.

The writers are currently engaged in a research pro-

gramme that has the objective to update the City of

Winnipeg guidance document for rapid construction of

sandbag dykes (Krahn et al. 2005). As a first step in this

larger programme, a laboratory investigation was carried

out to quantify the interface properties of the component

materials identified above. The test programme was

carried out using a large-scale shear box apparatus (1 m 3

1 m in plan area) (Figure 2). This apparatus allowed tests

to be carried out using full-size sandbag specimens and to

minimise specimen size effects when interface shear test-

ing was carried out with sheet materials, and with sheet

materials and sod in contact. These parameters are being

using to evaluate the local stability of sandbag dykes

during construction and loading. The results of this phase

of the larger research programme will be reported in

future publications.

2. TEST MATERIALS

The combinations of materials used to form the interfaces

in this investigation are summarised in Table 1. A descrip-

tion of the individual materials follows below.

PES on sod WSFPP on sod

PES on PES

WSFPP on sand

PES on WSFPP

WSFPP on WSFPP1.20 to 4.25 m(typical)

Figure 1. City of Winnipeg sandbag dyke construction

template showing six interface surfaces (after City of

Winnipeg 1997) (Notes: PES polyethylene sheeting; WSFPP

woven slit film polypropylene geotextile)

Figure 2. Large-scale (1.0 m 3 1.0 m 3 1.0 m) direct shear

apparatus

Table 1. Test programme showing interface shear surfaces investigated

Test materials PES WSFPP

geotextile

Sand Sod Sandbags

PES 3 3 3

WSFPP

geotextile

3 3 3 3

Sand 3 3a

Sod 3 3 3

Sandbags 3 3

Notes: PES ¼ polyethylene sheeting; WSFPP ¼ woven slit film polypropylene.aBenchmark used for comparison purposes only

120 Krahn et al.


2.1. Polyethylene sheeting (PES)

The PES material is a readily available 0.15 mm thick

(0.006 in) vapour barrier polyethylene sheeting material

that meets the Canada General Standards Board specifica-

tion for this class of material (CAN/CGSB 51.34-M86).

2.2. Woven slit film polypropylene (WSFPP)

geotextile

The WSFPP geotextile bags used by the City of Winnipeg

for sandbag dykes are purchased in bulk, usually in

bundles of 1000. The current suppliers of these products

are located in China, and use several manufacturing

plants. The bags are generally made from ends and scraps

left over from larger, more valuable textile products. This

means that there is limited information available on the

material properties of the bag material, and product

consistency is likely to be poor.

Specimen sheets used for the large-scale tests were

obtained from WSFPP samples used to bundle the bags

for shipping. The local distributor for the City of Winni-

peg Public Works department indicated that it was com-

mon practice for the manufacturer to use the same

materials for packaging as that used to manufacture

cheaper products such as the sandbags. All the specimens

tested in this programme were taken from the same stock.

Visual inspection of WSFPP samples taken from the bags

and the packing material did not reveal any visual

differences aside from overall dimensions. A single test on

a representative specimen of geotextile gave a mass per

unit area of 100 g/m2 (ASTM D 5261) and a wide-width

strip tensile strength of 5 kN/m (ASTM D 4595).

It is also common practice for the manufacturers to treat

the WSFPP with a surfactant to reduce static electricity

produced by rubbing that occurs during shipping and

handling. In all cases the surfactant was still present on

the material used in this test programme.

2.3. Granular material (sand)

A washed pit run sand from a local quarry near Kingston

was selected because it has a gradation that is similar to

the washed pit run sand materials used in the Winnipeg

area. The soil used in this investigation is classified as SP

according to the Unified Soil Classification System and

has the following particle sizes: D10 ¼ 0.09 mm, D30 ¼0.4 mm, D50 ¼ 1 mm and D60 ¼ 1.3 mm. The soil was

placed at low moisture content (approximately 3%). In

practice, the water content at the time of filling of the

sandbags during a flood event will vary, because the sand

is typically stockpiled at city public works yards. The

effect of moisture content and soil density was not

examined in this investigation. Direct shear tests were

carried out in the large-scale direct shear box on 1 m3

specimens of this sand compacted to an initial density of

about 1800 kg/m3 (relative density of 60%). The cohesion

and friction angle at large deformations were 10 kPa and

308, respectively (Table 2). These numbers are useful as a

benchmark for comparison with the interface shear

strength values discussed later. Table2.Summary

ofMohr–Coulombstrength

parameters

Testseries

Interface

Peakat

<30mm

displacement

150mm

displacement

(post-peak)

Residual

displacement

Ranka

(Norm

alstress

� n¼

24–85kPa)

c(kPa)

�(deg)

R2

c(kPa)

�(deg)

R2

c(kPa)

�(deg)

R2

Peak

150mm

displacement

Residual

1PES

onPES

613

0.88

416

1.00

514

1.00

6–8

7–8

7–8

2PES

onWSFPP

415

0.99

413

1.00

313

1.00

7–9

99

3WSFPPonWSFPP

419

1.00

417

1.00

219

1.00

6–8

6–7

6–8

4WSFPPonsand

725

1.00

10

24

1.00

825

0.99

3–4

44

5Sandonsandb

10

34

1.00

10

31

1.00

10

30

1.00

12

2

6PES

onsod

712

0.95

713

0.95

713

0.95

7–9

6–8

6–7

7WSFPPonsod

623

1.00

725

0.99

923

1.00

55

5

8Sandbagsonsod

822

0.99

929

1.00

827

1.00

3–4

33

9Sandbagsonsandbags

14

24

0.97

13

39

1.00

12

43

1.00

21

1

Notes:

a1¼

highestshearstrength;

bbenchmarkusedforcomparisonpurposesonly;R2¼

coefficientofdetermination;PES

¼polyethylenesheeting;WSFPP¼

woven

slit

film

polypropylenegeotextile.

Large-scale interface shear testing of sandbag dyke materials 121


2.4. Grass sod

There are a large variety of plants and grasses present

along the riverbanks where sandbag dykes are built in the

Red River valley. For simplicity, Kentucky blue grass sod

was chosen for the large-scale direct shear tests conducted

for this project. Kentucky blue grass is a common variety

used in the Winnipeg area, and this provided a known

baseline that can be compared against other grass varieties

in future work, should that be necessary.

3. EXPERIMENTAL PROGRAMME

3.1. General

The large size of the direct shear apparatus used in this

investigation allows interface properties to be quantified

for systems that exhibit discrete block behaviour, in this

case filled sandbags and filled sandbags in contact with

sod. It is not possible to capture field-scale interface

performance using conventional bench-scale direct shear

equipment (i.e. typically 50 mm 3 50 mm or 100 mm 3

100 mm in plan area). An additional limitation of a

traditional bench-scale direct shear apparatus is the influ-

ence of boundary effects on measured behaviour. Bound-

ary effects are due to local distortion of the sheet products

based on how the specimen materials are attached to the

test device. In the large-scale device used here, the inter-

face area is large enough that local distortions near the

specimen edges that would otherwise influence macro-

scale response are reduced. Finally, it should be noted that

the shear tests were carried out in general accordance with

the ASTM D 5321 method of test.

3.2. Large-scale direct shear test apparatus

The large-scale direct shear test apparatus used in this

testing programme is a custom-built device with a shear

box 1 m 3 1 m 3 1 m in size. The box comprises equal

top and bottom halves constructed from 250 mm high

steel C-sections. The 1 m 3 1 m shear plane is located at

the mid-height of the shear box. Steel plates at the sliding

interface surface between the top and bottom boxes are

treated with a specially formulated industrial coating to

minimise friction and abrasion. Horizontal load and stroke

are provided by a 222 kN capacity hydraulic actuator. A

stiffened rigid steel loading plate is used to apply a

vertical load to the specimen using a 222 kN capacity

hydraulic actuator. The bottom box is seated on a set of

rollers. The apparatus is instrumented to record displace-

ment (horizontal shear) of the lower box while the top box

is restrained horizontally. Vertical and horizontal loads are

recorded using load cells, one mounted at the end of the

vertical actuator piston, and the other opposite the top box

in line with the horizontal connection with the test frame.

Displacement transducers located at the corners of the

vertical loading plate are used to record vertical movement

of the test specimens. The apparatus configuration and

loading are identical to those found in a conventional

shear box apparatus but at a larger scale. Actuator control

and data acquisition are carried out using a personal

computer and ancillary control cards and custom-designed

software.

3.3. Specimen preparation

Sheets of 16 mm and 24 mm thick plywood were used to

secure the sheet specimens during testing. The bottom

plywood sheets were seated on a timber crib that was used

to fill the lower half of the shear box. Plywood sheets

were cut to 1 m square size to fit snugly into the shear

box. A minimum of three plywood sheets was used above

and below the interface surfaces to support the test speci-

mens and to ensure that the shear interfaces remained flat

and continuous during testing. Sheet metal flashing (1 mm

nominal thickness) was used as a shim material between

the plywood sheets to adjust the interface location so that

it was level and parallel with the shear plane of the test

apparatus.

The sheet materials tested (PES and WSFPP types)

were cut into 1.2 m square pieces that were then trimmed

at the corners and folded over the plywood sheet support.

They were then attached to the plywood sheet by stapling

the sheet on the reverse side of the plywood. This ensured

that the specimen sheets were taut over the shear surface.

For sandbag/sod interface shear testing, the sod was

affixed to a sheet of plywood using deck screws placed on

a 100 mm square grid pattern. The top 6 mm of the screw

head was left protruding from the plywood to engage and

hold the soil at the base of the sod layer. This arrangement

minimised slippage between the sod and the plywood

interface without interfering with the interface surface at

the top of the sod layer. Figure 3 shows the sod in the

bottom half of the direct shear apparatus before a test.

Figure 4 shows filled sandbags placed over the sod layer

with the top half of the shear box removed following

shearing.

3.4. Normal load and shear displacement rate

The normal loads used for this research were selected to

match the range of sandbag dyke heights used in typical

flood protection efforts in the Red River Valley (Figure 1).

Dykes smaller than 1.20 m are generally stable, and hence

Figure 3. Sod layer ready for interface testing in large-scale

direct shear apparatus

122 Krahn et al.


1.20 m was taken as the minimum dyke height in this

investigation. The maximum dyke height selected was

4.25 m, although there were cases during the 1997 flood

where higher dykes were constructed to protect some

residential homes. This value has been suggested as the

maximum temporary sandbag height for future flood

protection works in Winnipeg, with the expectation that,

where higher structures are required, engineered clay

dykes will be built. The pressure at the base of the dyke

was calculated at the centre of the structure using an

estimated saturated unit weight for sandbags of 20 kN/m3,

which gives a range of foundation pressure from 24 to

85 kPa. Based on these calculations, each shear test

configuration in the laboratory test programme was car-

ried out at normal pressures of 25, 75 and 125 kPa.

A shear rate of 5 mm/min was used in the large-scale

direct shear testing, which resulted in a test duration of

1 h to achieve 300 mm of horizontal displacement. The

ASTM D 3080 method of test states that this rate is

sufficiently slow to ensure that excess porewater pressures

for the worst case of fully saturated sand specimens are

dissipated during shear. None of the materials was fully

saturated in this investigation; however, as a precaution

and to employ a consistent test methodology the value of

5 mm/min was used for all tests.

4. TEST RESULTS

4.1. General

For brevity, only plots from selected interface shear test

results are presented here (Figures 5 to 8). The peak shear

stress, shear stress at 15 cm displacement and residual

stress values are indicated by the solid symbols in each

plot. Some of the stress–displacement plots show stick–

slip behaviour (e.g. at about 3 cm displacement for the test

at 75 kPa normal stress in Figure 6 and at 5–10 cm

displacement in Figure 7). This behaviour did not influ-

ence the interpretation of the results, because target

strength values were taken from the trace of the backbone

curve in each test. The frequency of stick–slip events

during shear testing was observed to be very much higher

for smaller specimen tests using a 60 mm 3 60 mm shear

box apparatus (Krahn 2005). This is further evidence that

larger shear box equipment is required for the type of

interface shear testing reported here.

The peak shear strength values were taken over the 0 to

3 cm displacement range in each curve. In some cases the

maximum value occurred at larger deformations. The

3 cm displacement criterion was selected because it was

Figure 4. Filled sandbags on sod after shearing in large-scale

direct shear apparatusHorizontal shear deformation (cm)

She

ar s

tres

s (k

Pa)

0

10

20

30

40

50

25 kPa

75 kPa

σn 125 kPa�

0 5 10 15 20 25 303

Figure 5. WSFPP geotextile/geotextile interface shear stress–

displacement curves

Horizontal shear deformation (cm)

She

ar s

tres

s (k

Pa)

0

10

20

30

40

50

60

70

80

25 kPa

75 kPa

3

σn 125 kPa�

0 5 10 15 20 25 30

Figure 6. WSFPP geotextile/sod interface shear stress–

displacement curves


She

ar s

tres

s (k

Pa)

0

10

20

30

40

50

60

70

80

25 kPa

75 kPa

3

σn 125 kPa�

0 5 10 15 20 25 30

Figure 7. Sandbag/sod interface shear stress–displacement

curves



observed that under operational conditions the accumula-

tion of 3 cm of lateral displacement at each layer in the

dyke could result in excessive leakage and structure

distortion, resulting in a breach of the sandbag dyke

(Krahn et al. 2005).

Post-peak strength values are of interest to designers

because it is possible that critical mechanisms with a

stepped geometry can develop in a sandbag dyke. Both

peak and large-deformation shear strengths can be mobi-

lised along the shear paths defining these critical mechan-

isms. Furthermore, the critical failure mechanism may

change as the dyke deforms.

In order to characterise the post-peak response, strength

values at 15 cm displacement and at the end of the tests

were extracted from the stress–displacement curves. The

value of 15 cm displacement is arbitrary. The shear

strengths at the end of each test (typically at 25 to 30 cm

displacement) are identified as residual shear strength

values in this paper.

The strength values at each normal load using the

criteria described above were used to generate Mohr–

Coulomb (M-C) strength parameters for each interface

type. These data are summarised in Table 2. The regres-

sion coefficients varied from 0.88 to 1.00, with most

datasets corresponding to values of 1.00. Hence linear

M-C interface shear envelopes corresponding to the range

of normal stresses applied are accurate models for the

interfaces investigated.

4.2. Mohr–Coulomb interface shear strength

The following observations can be noted from the data in

Table 2.

The peak apparent cohesion value (4 kPa) and peak

friction angle (198) for the sandbag geotextile-to-geotextile

interface shear tests were less than values measured for

the sandbag-to-sandbag interface (14 kPa and 248). Hence

there was greater interface shear strength recorded for the

sandbag-to-sandbag test configuration than for the geotex-

tile material alone. The difference in shear strength is

much larger when Coulomb strength values at 15 cm

displacement and at residual values are compared. The

difference is due to the test configuration for the sandbag-

to-sandbag tests, which results in system dilation as the

top row of sandbags moves up and over the lower row of

sandbags. This discrete block mechanism does not develop

when the geotextile sheet material is tested in isolation.

Also included in the table are shear strength rankings

for the range of normal stresses at the bottom of a sandbag

dyke based on the City of Winnipeg design template (i.e.

24 to 85 kPa). The ranking of interface shear strength

values is very consistent once the 15 cm displacement

criterion is achieved.

The results of all interface shear tests showed that the

benchmark peak shear strength for the sand/sand interface

was the largest (series 5) of all (initial) peak strength

values. The influence of sand and geotextile materials on

interface shear strength can be seen by comparing results

of test series 3, 4 and 5. These tests reveal increasing

shear strength in the same order, indicating that, when

continuous planar interfaces are tested, the presence of the

geotextile reduces sliding resistance. However, after large

deformations between 3 and 15 cm displacement, the

shear strength for the sandbag/sandbag tests (series 9) was

greater than for the sand alone (series 5) in the present

investigation. The explanation for this relative behaviour

is that the sand within the sandbags cannot be placed in a

compacted (dense) state compared with the initial com-

pacted density of the sand in the benchmark sand/sand

tests. However, after large sandbag-to-sandbag shear de-

formations the strength and stiffness of the encapsulating

geotextile more than compensates for the initial lower

density of the sand, and increasing shear capacity of the

sandbags is mobilised at least up to about 30 cm horizon-

tal displacement (see Figure 8).

The interface shear strength for the sandbag/sod inter-

face is always less than for the sandbag/sandbag interface

(compare series 8 and 9). However, the differences be-

tween the sandbag/sod and WSFPP geotextile/sod strength

values are not as great. This relative trend is ascribed to

the amount of specimen dilation that occurs during a test.

The greatest dilation occurs when the interface is com-

posed of sandbags alone (i.e. the bags roll over each other

after large horizontal deformation). In contrast, the inter-

face shear surface with a planar geotextile in contact with

a layer of sod is non-dilatant.

The lowest shear strength values are recorded for those

interfaces that include PES. This may not be unexpected

but, to the best of the writers’ knowledge, interface shear

data for these materials have not been reported in the

literature. For sandbag dykes that are routinely constructed

with PES as a water barrier, these materials may control

dyke stability. PES materials are hydrophobic, so that even

in the presence of water in a field case, the interface

properties determined from the dry tests reported here are

applicable.

The interface shear strength data points from all tests

are summarised in Figures 9–11. To prevent visual clutter,

the M-C envelopes for each dataset are not plotted.

Lower-bound envelopes have been identified in the figures

that can be used as a preliminary (conservative) estimate

of interface shear strength for limit equilibrium-based

stability analyses of sandbag dykes. It can be noted that


She

ar s

tres

s (k

Pa)

0102030405060708090

100110120130140

25 kPa

75 kPa

σn 125 kPa�

30 5 10 15 20 25 30

Figure 8. Sandbag/sandbag interface shear stress–displace-

ment curves

124 Krahn et al.


the design envelope for WSFPP geotextile in contact with

sod is the same as the envelope for the sandbag/sod

interface (i.e. � ¼ 7 + �n tan 228). However, the sandbag/

sandbag design envelope falls above the corresponding

geotextile sheet to geotextile sheet M-C line. Hence using

interface shear strength values for the geotextile sheet

materials alone is likely to be conservative for sandbag

dyke design.

4.3. Interface stiffness

Mohr–Coulomb strength parameters are useful for limit

equilibrium-based stability analysis of sandbag dykes.

However, for more sophisticated analyses using finite

element model or finite difference numerical codes, stiff-

ness values for the interfaces are required. Interface shear

stiffness values were calculated by taking the average

secant slope through the three stress–displacement curves

for each interface type at a value of 0.5 cm. The value of

0.5 cm was selected to represent the initial stiffness of the

interface surfaces but was otherwise arbitrary. The com-

puted values are summarised in Table 3. The data show

that the range of values is narrow compared with stiffness

values for geomaterials that can differ by orders of

magnitude. It is interesting to note that the relative

stiffness values do not rank in the same order as shear

strength values. For example, the sand/sand stiffness value

ranking is relatively lower than the strength value ranking.

Normal stress (kPa)

She

ar s

tres

s (k

Pa)

0

5

10

15

20

25

30

35

40

45

PES on WSFPPPES on PESPES on sod

Range for all tests with PES

0 20 40 60 80 100 120 140

τ σ5 tan (11°)� � n

Figure 9. Range of shear strength values for PES interfaces

and lower-bound envelope for design

Normal stress (kPa)

She

ar s

tres

s (k

Pa)

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70WSFPP on WSFPPWSFPP on sandWSFPP on sod

Range for WSFPP on sod

Range for WSFPP onWSFPP

0 20 40 60 80 100 120 140

τ σ7 tan (22°)� � n

τ σ3 tan (18°)� � n

Figure 10. Range of shear strength values for WSFPP

geotextile interfaces and lower-bound envelopes for design

Normal stress (kPa)

She

ar s

tres

s (k

Pa)

0102030405060708090

100110120130140

WSFPP on WSFPP Sandbags on sandbagsSandbags on sod

Range for sandbags on sod

Range for WSFPP on WSFPP

Range for sandbags on sandbags

0 20 40 60 80 100 120 140

τ σ12 tan (24°)� � n

τ σ7 tan (22°)� � nτ σ3 tan (18°)� � n

Figure 11. Range of shear strength values for WSFPP on

WSFPP and sandbag interfaces and lower-bound envelopes

for design

Table 3. Initial secant shear stiffness (Ki) values interpreted from interface shear

stress–displacement plots.

Test series Interface Ki (kPa/mm) Ranka

9 Sandbags on sandbags 3.66 1

4 WSFPP on sand 3.48 2

8 Sandbags on sod 3.43 3

2 PES on WSFPP 3.04 4

5 Sand on sandb 2.79 5

3 WSFPP on WSFPP 2.75 6

7 WSFPP on sod 2.70 7

6 PES on sod 2.58 8

1 PES on PES 2.30 9

Notes: a1 ¼ highest shear stiffness; bbenchmark used for comparison purposes only; PES ¼polyethylene sheeting; WSFPP ¼ woven slit film polypropylene geotextile



Not unexpectedly, the stiffness values corresponding to

two of the interfaces with PES materials are the lowest.

5. CONCLUSIONS

A large-scale direct shear apparatus was used to quantify

the shear behaviour of sandbag dyke interface surfaces.

The test results showed that the interface shear strength of

sandbags in contact with sandbags is very much larger

than that deduced from interface shear testing on sandbag

geotextile sheets. The difference is ascribed to the dilatant

behaviour that occurs as the sandbags move over each

other during large shear deformations.

In terms of shear response, the weakest and least stiff

interfaces for sandbag dykes built according to the City of

Winnipeg 1997 template are those including a PES layer.

Clearly, seating a sandbag dyke directly on a layer of PES

may lead to base sliding of the structure. In current

practice, this arrangement is avoided. Provided that a

horizontal planar surface with PES is not present within

the sandbag structure, then internal failure from the loaded

wet side to the dry side of the sandbag dyke is likely to be

preventable because of the multi-step internal shear me-

chanism that must develop because of the staggered

construction of these systems. Nevertheless, the margin of

safety of temporary sandbag dykes using the current City

of Winnipeg design template has not been established.

This is the topic of ongoing work by the writers.

ACKNOWLEDGEMENTS

The authors are grateful for the financial support from the

Natural Sciences and Engineering Research Council of

Canada and the Secondary Diking Enhancements Program

administered by the Government of Canada, the Province

of Manitoba and the City of Winnipeg. The authors would

also like to thank the staff at Bathurst, Clarabut Geotech-

nical Testing Ltd (BCGT) for providing both technical

assistance and the large-scale direct shear apparatus that

was used for the testing reported here.

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The Editors welcome discussion on all papers published in Geosynthetics International. Please email your contribution to

[email protected] by 15 October 2007.

126 Krahn et al.


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