Construction and Building Materids, Vol. 10, No. 4, 281-286, 1996 pp.

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Investigations on soft clay subgrade

In the last two decades geotextiles are being increasingly used as tensioned/tensile members. When used for reinforcement functions, a geotextile has to yield high resistance at low strains, especially for structures where allowable rut depths are low and the life span of the structure is long. This necessitates the use of high modulus geotextiles with adequate roughness. The present study aims at investigating the efficacy of the geocell alternative and the effect of the cell geometry and relative density of the backfill. Experimental and finite element procedures have been adopted to study the above parameters. Soft saturated marine clay was used as subgrade while sand was used as backfill material. Monotonic loading was applied in plate load tests. The experimental results were simulated in a three-dimensional finite element (FE) procedure using ANSYS, a general purpose package. Considerable improvement in the load and reduction of settlement was

Geotextiles are being increasingly used in road con- struction, tank foundations and several other reinforce- ment (tensile or tensioned members) applications. Numerous pavement design methods have been sug- gested’--’ for unpaved roads which are characterized by high allowable rut depths, low volume of traffic and no vehicle wander. Nagaraju and Mhaiskar” have sug- gested the use of soil filled tubes for paved roads, while Kazerani and Jamnejads have suggested the use of geo- cells for paved roads. De Garidel et al.6 have used geo- cells of width (a) and height (b) ratio (i.e. a/b) of 0.5 and reported that the bearing capacity can be improved up to three times. Several other investigators have reported the use of geocells of different a/b ratio with varying degrees of benefit. Bush et al.7 have reported the use of geogrid geocell embankment on soft soil.

For paved road applications (characterized by low allowable rut depth and high volume of traffic) the geo- textile has to yield high resistance at low strains, i.e. be of high modulus. High modulus geotextiles are not available in India. Besides, the cost of low modulus geo- textiles is prohibitively high. Therefore, other alterna- tives need to be investigated.

Geocells which have axisymmetric cells can prove to be effective for such applications. Limited investigations have been carried on a geocell structure resting on a soft clay subgrade. Therefore, the present study was aimed

281

These factors are geocell geometry, effect of strength of the geocell wall and relative density of the backfill. Experimental and finite element investigations have been used to study the behaviour of the geocell struc- ture and stress distribution resulting from it.

Experimental study

An experimental model was fabricated to study the influence of cell geometry and relative density of the backfill on the geocell structure. The setup was designed to carry full-scale loading.

Two mild steel tanks (0.85 m by 0.75 m by 0.60 m (height) and 1.15 m by 1.05 m by 0.75 cm (height)) were used for the experiments. The tanks were placed under a loading frame and were filled with marine clay up to a depth of 0.48 m. The geocell was directly placed on this clay subgrade. A loading plate 0.25 by 0.35 m was used to represent the contact area due to a dual as- sembly subjected to a 80 kN load and a 620 kPa tyre pressure. Figure I shows the experimental setup. The subgrade (marine clay) was placed in eight layers, each of 0.06 m thickness. Each layer was compacted by giving three blows from a modified Proctor hammer over a plate having an area of 0.0225 m2. The soil required for each layer was weighed to achieve a dry density of 10.4 kN/m3 at a moisture content of 2%

282

48cm ! I Clay subgrade

Outlet k I-

\ 85cm ) Steel

tank

Figure 1 Experimental set-up

before compacting. The dimensions of the frame were 0.62 m by 0.75 m by 0.08 m (height). The frame used for the larger tank had dimensions of 1.10 m by 1 .OO m by 0.08 m (height). The space between the tank and the frame was filled with sand to facilitate saturation. Once compacted, the tank was flooded for seven days to attain maximum saturation. This method of com- paction and saturation yielded a degree of satruation. (S,) and undrained cohesion (c,) of 93% and 10 kPa respectively. Such a method of compaction and satura- tion has been used in the past to simulate soft clay subgrade.

Two types of geotextiles were used for forming the geocells, namely needle punched nonwoven and woven slit film types. Initial experiments (loose state, relative density = 15%~) were carried out with needle punched nonwoven only. This data was then used to optimize the geocell geometry. Only the optimized sizes were used for the woven slit film type and nonwoven geocells in the dense experiments. The cell heights used were 100, 150, 200, 2.50 mm with u/b ratios of 0.5, 1 .O, 1.5, 2.0 in the loose states and 0.33, 0.5 and 1.0 in the dense states. Geocells were especially prepared for these experiments.

The nonwoven geocells were made by bonding (one inch length) with a heat bonded adhesive (AIKAMELT 20), while the woven geocells were stitched by nylon threads. The geocells were expanded and anchored before filling it with sand backfill. The sand was filled at two different relative densities, viz. 15% and 80”/0, to study the effect of relative density. The loose density (15%) was achieved by pouring sand through a free fall of 0.15 m, while the dense state (80%) was achieved by rodding the sand with a tamping rod 5 kN in weight 25 times for a geocell of u/b ratio of 0.5 or 100 times for a geocell of u/b ratio of 1.0 for a geocell height of 100 mm. Similar compaction efforts were imparted for geo- cells of different sizes. These efforts were determined from initial experiments. These are commonly used methods to compact the sand in the laboratory. Load was applied by a hydraulic jack and measured by a load

cell of 20 kN capacity. The settlement was recorded by three dial gauges (0.01 mm least count) symmetrically placed on the plate. The load was applied in steps until the footing (plate) exhibited failure. Settlement corre- sponding to each increment was recorded until the rate of settlement was less than 1.0 mm/min (to simulate quick, undrained loading conditions). Pressure trans- ducers were placed at the geocellisoft clay subgrade interface to observe the interface stresses. Horizontal and vertical strain gauges were mounted on the geocell wall below the loaded area to study the nature of the geocell wall deformation. Mounting the strain gauges, maintaining them while placing the geocell and filling it with sand caused many strain gauges to be inactive during the test. However useful observations were obtained in a few tests.

The sand used as backfill had a minimum density of 16.05 kN/m3, a maximum density of 18.1 kN/m3, and a uniformity coefficient of 4.6, while the marine clay used as subgrade had a liquid limit of 80%, a plastic limit of 40%, an optimum moisture content of 32% and a maximum dry density of 13.85 kN/m3.

The nonwoven geotextile used for the geocell was made up of polypropylene fibres and had a weight per square metre of 300 g, a breaking load of 8.35 kN/m, a failure strain of 40%, a thickness of 2.35 mm, a burst strength of 100 kPa and a seam strength 0.35 kN for a 10 cm length of seam when the geocell was formed. The woven slit film type geotextile had a weight per square metre of 136.5 g, breaking load of 6.5 kN/m, failure strain of lo%, thickness of 0.4 mm and a seam strength of 0.16 kN.

Finite element investigations

ANSYS, a general purpose finite element (FE) package was used for the present study. It is based on classical engineering concepts and documented FE and numerical techniques.

The typical three-dimensional grid used for the FE

analysis is shown in Figure 2. To the right of the figure are values of the dimensions of the model, viewing posi- tion etc. for the grid. As the problem was symmetric only the symmetric quarter was simulated in the analy- sis. An eight-noded iso-parametric solid element was

ANSYS 4.3A

1 June 1992

13 58 50

Plot No. 1

Prep 7 elements

xv=1 YV=l

ZV=l

Distance 49.623 XF = 27.5

YF = -29.0

ZF = -25.0

Precise hidden

Figure 2 Typical finite element mesh

Table 1 Summary of properties used in FE analysis

Modulus of elasticity of unreinforced sand 700 kPa Poisson’s ratio of unreinforced sand 0.35 Modulus of elasticity of reinforced sand0 15 cm layer 2100 kPa Poisson’s ratio of reinforced sand 0.17 Modulus of elasticity of clay subgrade 210 kPa Poisson’s ratio of clay subgrade 0.48

aModuli mentioned are those for which a fairly good fit was obtained with experimental results

used to simulate the geocell and the clay subgrade layers in a three-dimensional analysis. The element had anisotropic and plasticity capabilities.

The geocell layer filled with sand and subgrade (clay) layer were modelled as isotropic, elastic-perfectly plas- tic, non-dilatant materials (associated flow rule was used). The Drucker Prager yield criterion which uses the outer cone approximation to Mohr’s Coulomb theory was adopted for the analysis. Due to uncertainties in the boundary constraints and difficulties in modelling the stress-strain relationship for the soil-geocell composite system, the geocell layer was considered as a layer with an equivalent stiffness. Kazerani and Jamnejads have used a similar approach. In order to compare the increase in modulus over the unreinforced case the sand layer was also considered as a layer with an equivalent stiffness with modulus constant with depth.

Kazerani and Jamnejad introduced a Clegg hammers to measure the in situ modulus of the geocell layer. Due to the confinement of the soil by the geocell wall in the reinforced case the Poisson’s ratio is also reduced. In the present study the modulus of elasticity of the reinforced and unreinforced sand was increased in steps in the FE

analysis, until a fairly good fit was obtained in the load settlement diagrams drawn from experimental results The properties used in the FE analysis are listed in Table 1.

Results and discussions

Load settlement curves were drawn for all static load tests‘ to study the effect of reinforcing the sand with a geocell. In order to clearly identify the failure, the load settlement curves were drawn in dimensionless form of plAyB versus w/B, as suggested by De Beer (where p is the pressure on the plate, A is a dimensionless factor which is a function of the settlement ratio w/B and dimensions of the plate, B is the width of the plate, yis the density of the underlying soil and w is the settlement of the plate.

Figure 3 shows load settlement curves for nonwoven geocells with varying u/b ratio backfilled with loose sand (relative density = 15%) resting on a soft clay sub- grade (c = 10 kPa). Figure 4 shows load settlement curves plotted from tests with dense sand (relative den- sity = 80%), other factors remaining same. Ultimate bearing capacity (UBC) values obtained from Figurers 3 and 4 are plotted in Figure 5. It is seen from Figure 5 that for loose sand, at an u/b ratio of 1.5 marginal ben-

u/b ratio of 2.0 the reinforced sand behaves as if it is

alb d- 0.5 e 1.0 + 1.5 -x- 2.0 _._ Unreinforced

(clay only) _ _ _ _ _ _ Unreinforced

(clay + loose sand)

100 I I I111111 I I I,,,,

1 2 3 4567910 20 30 40506070 102

Figure 3 Load settlement curves for non-woven geocells with varying u/b ratio backfilled with loose sand (relative density = 15%) resting on a soft clay subgrade (c = 10 kPa)

Note: curve for geocell (a/b = 2.0) is not shown since it nearly merges with unreinforced case

+ Geocell (a/b = 0.51 + Geocell (a/b = 1.0) d Geocell (a/b = 1.51 _x_ Unreinforced

(clay + sand)

10

Load (plAyB)

Figure 4 Load settlement curves for nonwoven geocells of varying a/b ratio resting on a soft clay subgrade (c = 10 kPa) backfilled with dense sand (relative density = 80%)

I-‘” -.-.-.-

-x- Geocell backfilled with dense sand

110 - p Geocell backfilled

with loose sand

- - - Unreinforced (clay + dense sand)

- Unreinforced (clay c = 1OkPa)

10 - a 5 I I I

0 0.5 1.0 1.5 2.0 2.5 a/b ratio

cl

Figure 5 Variation of ultimate bearing capacity with u/b ratio for non- woven geocells backfilled with sand resting on a soft clay subgrade (c = 10 kPa)

unreinforced. It is also seen that the difference in UBC

between loose and dense sands (reinforced) at higher a/b

284

ratios is small. However, at lower u/b ratios the differ- ence in the UBC is substantial, clearly indicating the beneficial effects of compacting the sand to higher rela- tive density and lower a/b ratios with a maximum ratio of 1.0.

Ultimate bearing capacity values obtained from investigations on nonwoven and woven geocells filled with dense sand (relative sand density = 15%) having various ratios u/b and cell heights (b) resting on a soft clay subgrade (c = 10 kPa) are summarized in Tuble 2.

It is seen from this table that although the woven geocell has a lower seam strength compared to the non- woven geocell (refer properties) it has yielded higher ultimate bearing capacity values by virtue of its higher modulus (lower strain at failure). The difference in the ultimate bearing capacity values is larger for lower geocell heights while it reduces for higher geocell heights, indicating clearly that the modulus plays an important role compared to the seam strength, for the subgrade strength and geocell heights considered.

It is also seen from Tuble 2 that for both the cases of woven and nonwoven geocells the ultimate bearing capacity increased with decreased u/b ratio. No opti- mum u/b ratio was observed. Therefore, a graph of ulti- mate bearing capacity versus b/B ratio for various u/b

ratios was plotted (Figure 6). It should be noted that for a given b/B ratio, the u/b ratio was varied by varying cr. The u values for various u/b ratios for a constant b/B ratio are marked on this graph. Values of ultimate bear- ing capacity for other widths were interpolated. Also shown in Figure 6 are Ultimate Bearing Capacity for unreinforced sand resting on a soft clay subgrade (c, = 10 kPa) of varying thickness (H). Ultimate bearing capacity values for H/B ratios greater than 0.8 were cal- culated using the approach suggested by Meyerhof and Hannas. It is seen from Figure 6 that the critical H/B

ratio (beyond which there is no increase in bearing

260 -

240 -

3 220 -

9 200- i

; 180- .Y :: 160- B u

i 140- 120-

0 Interpolated values + Geocell (a/b = 0.33) -x- Geocell (a/b = 0.50) --b Geocell (a/b = 1 .OO) -+- Geocell (a/b = 1.50) -o- Geocell (a/b = 2.00) -*- Theory

U.R. -.- Experimental - Unreinforced

ir Horizontal inclusion

0 IOO- 0 j 80-

D 60- *’ I in = 2.54cm

Note: values marked arc “a” values

01 I I I I I I I 0 0.5 I .o I..5 2.0 2.5 3.0 3.5

b/B orH/B ratio

Figure 6 Variation of ultimate bearing capacity y” with H/B ratio for unreinforced, horizontal (non-woven) inclusion, geocell (non-woven) reinforced soil resting on a soft clay subgrade (c = 10 kPa) backfilled with dense sand (0 = 38”)

capacity) is 2.8 for the unreinforced case. Ultimate bear- ing capacity values for the same a values were joined by smooth curves. A critical b/B ratio of 0.625 was observed from this graph. The reduction in the critical b/B ratio achieved by reinforcing the sand with a geocell clearly indicates the beneficial effects and the savings realized therein. Even at this reduced b/B ratio there is an improvement in ultimate bearing capacity of 1.31 times compared to the ultimate bearing capacity corre- sponding to the critical H/B ratio (unreinforced case). A similar trend is also seen for nonwoven geocells.

Figure 7 shows the variation of ultimate bearing capacity with B/u ratio (where B is the width of the plate and u is the width of the geocell) for an u/b ratio of 0.5. From Figure 7 it is clear that the B/u ratio shows an optimum value of around 3.5. This together with the b/B critical ratio of B/u ratio shows an optimum value of around 3.5. This 0.625 obtained early can be consid- ered as optimization of the geocell geometry. A similar trend is also seen for nonwoven geocells.

Figure 8 shows load settlement curves for a woven geocell (height of geocell, b = 150 mm) backfilled with dense sand underlain by a soft clay (c = 10 kPa) as obtained from FE (ANSYS) and experimental investiga- tions. The curves show a fairly good fit at an equivalent modulus of 2100 kPa in the FE investigations. Similar

Table 2 Ultimate bearing capacity values obtained from experimental investigations

Sr. Geocell ulh no. ratio

h Ultimate Bearing Remarks (mm) bearing capacity

capacity ratio

(kPa)

1 UR 100 31.4

2 Nonwoven (NW) 0.5

1 NW 1.0 4 NW 1.5 5 NW 2.0 6 Woven (W) 0.5 7 8 9

10 II 12 13 14 15 16

17 18 19 20 31 22 23

24 25 76 ‘I 28

W W W

UR NW NW NW W W W

UR NW NW NW

W W W

UR NW NW NW W

1.0 1.5 2.0

0.33 0.50 1.00 0.33 0.50 1 .oo

_

0.33 0.50 1 .oo 0.33 050 1 .oo

0.33 0.50 1 .oo 0.33

-

2.33

Unreinforced (UR) clay and

dense sand

100 87.3 100 65.9 100 52.9 100 43.3 100 99.0 100 77.0 100 53.0 100 39.4

1.76 1.41 1.16 2.65 2.06 1.42 1.05

150 41.8 UR 150 122.2 2.92 150 98.0 2.34 150 62.3 1.50 150 156.6 3.78 150 118.8 2.86 150 70.2 1.69

200 44.8 UR 200 118.8 2.65 200 98.7 2.20 _

200 54.4 1.21 200 132.2 2.95 200 102.0 2.28 200 67.8 1.51

250 48.0 UR 250 109.8 2.28 _

250 90.0 I .88 250 51.0 1.06 250 108.0 2.25

285

"0 1 2 3 4 S 6 B/a

Figure 7 Variation of ultimate bearing capacity, yU for various B/a ratios at different a/b ratios for non-woven geocells

I IO 100

Loi (p/A yB) Figure 8 Load settlement curves for woven geocell (b = 150 mm, u/b = 0.5) backfilled with dense sand underlain by soft clay (c - 10 kPa) -as obtained from FE and experimental investigations

curves (not shown here) drawn for the unreinforced case have shown a good fit at an equivalent modulus of 700 kPa. The increase in the modulus upto three times is clearly indicative of the benefit obtained by reinforc- ing the sand with geocell.

Figures 9 and 10 show the vertical stress distribution contours for the two cases described above, viz. 150 mm thick unreinforced dense sand on a soft clay and 150 mm (height, b) woven geocell reinforced dense sand on a soft clay. It is seen from Figures 9 and 10 that when the sand is reinforced with a geocell, the pressure bulb is nearly contained in the geocell layer, thereby clearly indicating that the stress transferred to the clay sub- grade is negligible. In contrast to this the pressure bulb in the unreinforced case extends well beyond the dense sand layer.

Conclusion

The experimental and finite element (ANSYS) studies carried out on sand reinforced with geocell of varying heights and a/b ratios backfilled with loose/dense sand resting on a soft clay subgrade indicate that:

1 It is advantageous to compact the backfill to a dense state. As the a/b ratio decreases the ultimate bearing

ANSYS 4.3 A 8 June 1992 14 07 55 Plot No. I Post 1 stress steps-44 Interactions = 5

G/z) DMX= 1.669 ShiN = -0.247422 sMx=o.o47117

g:; ZV=l Distance 49.623 XP = 27.5 YP = -29.0 ZP = -25.0 E$e2Fdden

B:-O:18 c=-o.13 D=-O.OlI E = -0.03

Stress values are in kg/cm’ F=0.02

Figure 9 Vertical stress contours for unreinforced dense sand (H = 100 mm). Underlain by soft clay subgrade (c = 10 kPa)

ANSYS 4.3A 11 June 1992 11 0458 Plot No. 1 Post 1 stress Steps = 4 Interactions = 5 SV (a@ S global DMX = 1.592 SMN = -0.244212 SMX = 0.045279

xv=1 YV=l ZV=l Distance 5 1.868 XF = 27.5 YF=-31.5 ZF = -25.0 Precise hidden A = -0.23 B = -0.18 c = -0.13 D = -0.08 E = -0.03

Stress values are in kg/cm* F = 0.02

Figure 10 Vertical stress contours for woven geocell (b = 150 mm; u/b = 0.5) backfilled with dense sand resting on a soft clay subgrade (c = 10 kPa)

capacity (UBC) and the bearing capacity ratio (BCR)

increase. For the range of geocell heights and a/b ratios con- sidered the modulus of the geotextile used to form the geocell plays an important role compared to its seam strength. The b/B ratio shows a critical value of 0.625 for woven geocells in contrast to the H/B critical value of 2.8 for the unreinforced case. Even at this reduced b/B critical value suggested earlier there is an improvement of 1.31 times compared to the unrein- forced case. An optimum B/a ratio of 3.5 is seen for the geocells having an u/b ratio of 0.5. This together with the b/B critical ratio suggested earlier can be considered an optimization of the geocell geometry. On the basis of the finite element investigations it has been shown that in the reinforced case the modulus

286

increases up to three times over the unreinforced case. 6 At the critical geocell height the pressure bulb is con-

tained within the geocell layer, thus transferring lower stresses to the underlying clay layer.

However, these investigations have to be supple- mented by field studies to establish the design modulus of reinforced sand. Such studies for determining the modulus/structural coefficient (for pavement applica- tions) are in progress. The present study clearly indi- cates the benefit gained by reinforcing the sand with a geocell and suggests the optimum geometry of the geocell.

Acknowledgement

The authors are greatly indebted to Prof. B. Nag (ex- Director), IIT Bombay and Dr J. V. N. Rao, Principal, Sardar Pate1 College of Engineering for their encour- agement. The authors also wish to thank the CAD centre, IIT, Bombay and the Soil Mechanics Laboratory staff for their assistance during the various stages of the investigations.

References

Giroud. J. P. and Noiray. L. Geotextile reinforced unpaved road design. J. Geotech. Eng. Div. ASCE. 1981. 107, GTY, 1233-1254 Sellmeijer J. B., Kenter C. J. and Van den Berg. J. Calculation method for a fabric reinforced road. In Proc. Second Internutiotlul Conftirencc 011 Geote.~tiles, Las Vegas, A. A. Balkema. Rotterdam. 1982. pp. 393-398 Milligan. G. W E.. Jewell. R. A.. Houlbsy G. T. and Burd H. J. A new approach to design of unpaved roads - Part I. Ground Eng.. April 25529 1989 Nagaraju S.S. and Mhaiskar S. Y Experiments on the use of geotextiles and geocomposltes in paved roads. In Pruc. Indian Geotechnrcul Conftrence. Vol I. Indian Geotechnical Society. pp. 89-92 Kazerani, B and Jamnejad. G. H. Polymer grid cell reinforce- ment in construction of low cost highways: an experiment. In Proc Geo.\ynthetic.s ‘97 con/Lwnce. Vol. 1. IFAI. USA. 1987, pp. 58M8 De Garldel. R. and Morel. G. New strengthening techniques by textile elements for low-volume roads. In Proc. Third Int. Conf on Geotex-tikr. Vienna, Austria. 1986. pp. 1027-1032 Bush, D. 1.. Jenner, C. G. and Basett, R. H. The design and con- struction of a geocell foundation mattress supporting embank- ments over soti ground Gc,ore.*r Geon?c,nlh~. 1990. 9. X3-98 De Beer. E. E. Experimental determination of the shape factors and bearing factors for sand. Geotzchn 1970, 4, 387411 Meyerhof G. G. and Hanna A. M., Ultimate bearing capacity of foundation on layered soils under inclined loads. Gun. Geotrch. J. 1978, 15, 565-572.