CHAPTER 6
Discussion of Results
1.1.Introduction
Weak foundation soils
6.1. Optimal Vertical Distance of Geosynthetic
In many cases the total thicknesses of pavement are very deep. For that, this will need an embankment as improved subgrade or subbase layer. The inclusion of geosynthetics in this zone as a reinforcement……….
6.2. Stress Concentration RatioCAO WEI PING’s WORK
The simplest starting point for analysis of arching is a two dimensional (2D) plane strain model. Some findings of Cao Wei-Ping’s experimental work and then the use of finite element as a comparison are a good description to understand wellthe arching behavior. The important parameter when soil arching occurred is stress concentration ratio, n.
Cao Wei-Ping et al.(2007) have conducted the experimental work for 15 models test.The PLAXIS Finite Element Method using Mohr Coulomb (MC) model based on soil material data at Qiantang River Beach will also presented simultaneously with the experimental work in other to give a comparison.
Parameter data for modeling as follows:
6.2.1. Influence of embankment height on stress concentration ratio
During test the soft soil support was imitated by using the water bag at max. pressure of 17.51 kPa. Test 7 describes the vertical stress both on above soft soil and top of pile during embankment filling. The stresses increase linearly with height of embankment. Moreover, the stresses on top of pile would be higher than those of sub soil.
Fig. x. Influence of embankment height doe to vertical stress during embankment filling test 7
Another important finding is during consolidation process as imitated by discharging of watergradually in the water bags from this test. When the consolidation process was going on,the surface of sub soil will go slow down, and it means the reducing vertical support occurred over the surface of sub soil. It turns out that the stress at the top of pile increase until certain maximum value and then gradually going down and more stable circumstance afterwards. It means that the stress concentration ratio, n, would be changing during the consolidation process as described Fig.b) which higher of height to clear span ratio, h/s, would result in the stress concentration ratio to be higher than those of smaller h/s.
Fig.x. the variation of vertical stress and stress concentration ratio
a). Vertical stress during water discharge test 7b).Influence h/s on stress concentration ratio
Fig xx above shows that it is strongly dependent on pile-subsoil relative displacement, Sc, (Sc=8-13 mm), at which the soil arching developed most effectively and beyond which there may be less effective arching.
6.2.2. Differential Settlement and Critical Height of Embankment
In tests 1-4, when h/s 1.4, the embankment height was relatively ‘low’ and no completed soil arching, it shows that the surface of embankment was non-uniform implying differential settlement occurred on the top of the embankment. Furthermore, in Tests 5-7, when h/s > 1.6, the embankment height was relatively ‘high’, the settlements in the base of embankment were non uniform, but at surface of embankment remained almost flat. Both situations when pile-soil relative displacement S =35 mm as depicted below:
Fig. x. Differential settlement (a) Test 1 and (b) Test 7
Mean while as verification for calculation settlement at surface of embankment using finite element for test 1 (h/s=0.7) and Test 7 (h/s=2) as shown in Table x below.
Table x settlement at surface of embankment
Test number / Settlement at surface of embankmentMeasurement / Finite element method
= 34 o / = 44 o
Above of pile / Between piles / Above of pile / Between piles / Above of pile / Between piles
Test 1 (h/s=0.7) / 11 / 37 / 13.73 / 35 / 13.5 / 34.5
Test 7 (h/s=2.0) / 40 / 39 / 29.63 / 29.63 / 29.75 / 29.77
Test results above suggest that the height of the equal settlement plane, hc, is about 1.4-1.6 times of the cap beam clear spacing (hc= (1.4-1.6)s. Furthermore, to ensure that no differential settlement occurs at the surface of embankment, the embankment height of 1.6s is necessary.
6.2.3. Influence of geosynthetics reinforcement on stress concentration ratio
The influence of ‘catenary’ geosynthetics reinforcements using single layer with differential tensile strength at two cases (low and height embankment) due to stress concentration ratio will be presented from Cao Wei-Ping et al.’s experimental work.
For the case of low embankment height namely h/s=0.7, Test 1 is without catenary geosynthetic reinforcement, whereas Test 8-10 using geosynthetics of biaxial tensile strength at 8 % axial strain are 0.35 kN/m, 1.4 kN/m, 22.5 kN/m respectively. Another case is high embankment namely h/s=1.8, Test 6 is no reinforcement of geosynthetic, whereas Test 11-13 with reinforcements using biaxial geosynthetics as mentioned above. The results for two cases as depicted in Fig.x. below:
Fig. x. Influence of geosynthetic reinforcement due to stress concentration ratio (a) for h/s=0.7 and (b) for h/s=1.8
In the case of h/s=0.7, the inclusion of a reinforcement using a tensile strength of 22.5 kN/m can increase the maximum stress concentration ration from 2.8 to 3.9, equivalent to 39%. Besides that for the case h/s=1.8, the maximum stress concentration ratio was increased from 7.6 to 18.5, an increase of 143%. The increment of stress concentration ratio on top of cap beam because the load carried out by geosynthetics is transferred to top of beam. The higher tensile strength will contribute the higher stress over cap beam that means stress concentration ratio is increased.
Table x below suggests the influence of geosynthetics reinforcements with different tensile strengths due to settlements when pile-subsoil relative displacement s=35 mm.
Table x. Influence of tensile strength due to settlement at surface of embankment
Description / Model / Low embankment h/s=0.7 / Heigh embankment h/s=1.8Test no. / 1 / 8 / 9 / 10 / 6 / 11 / 12 / 13
Tensile strength, kN/m / - / 0.35 / 1.4 / 22.5 / - / 0.35 / 1.4 / 22.5
Max. settlement, mm / Test / 37 / 36 / 33 / 29 / 38 / 38 / 36 / 35
MC / 35
Max.diff. settlement, mm / Test / 26 / 24 / 21 / 19 / < 2 / < 2 / < 2 / < 2
MC / 22
It is clear that the inclusion of geosynthetics as a reinforcement will reduce the differential settlement at surface of embankment for both cases low and high embankment. The use of higher tensile strengthswould contribute the smaller settlement as well differential settlement at surface of embankment.
6.2.4. Comparison stress concentration ratio from experimental results with some methods
It is useful to understand the parameter of stress concentration ratio for arching phenomenon between experimental results and some methods as well finite element method. Several methods have already existed such as Method of Low et al.(1994), Method of Terzaghi (1943), British Standard BS8006.
Consequently, only the results of Tests 1-7 can be compared with analytical methods. Unfortunately the stress concentration ratio that obtained from these methods arenot be related to the pile-subsoil relative displacement. Parameters of fill material are assumed c=0 kPa, =44o and = 15.5 kN/m3. Resume of equations for analytical methods as shown in Table x below:
Table x. Equations in analytical method
Method / Stress on pile, p / Stress on subsoil, s / RemarksLow et al. / / / =b/(s+b)
=0.8
s = piles clear spacing
Terzaghi / / / K= 0.7
Valid for h/s 2
BS8006 / Case 2D…
Case 3D… / / ac=1.95 h/b- 0.18 for end-bearing piles
ac=1.55 h/b- 0.07 for friction piles
hc=1.4 s
Then, stress concentration ratio can be obtained easily by n=p /s.
Here, the comparison is only intended for embankment without reinforcement namely from Test 1 with h/s=0.7 to Test 7 with h/s=2.0 as depicted in Fig. x below
Fig. x. Comparison between test results, some methods and Finite Element Method for stress concentration ratio: (a)Test 1, (b)Test 2,(c)Test 3,(d)Test 4,(e)Test 5,(f)Test 6 and (g)Test 7
Figure x above shows that Terzaghi method is always over-prediction for the stress concentration ratio whereas the BS8006 method suggests strongly under-estimate. Besides that Low et al method gives slightly larger result for low embankment h/s1.4 but this method gives good agreement with high embankment h/s > 1.4.
Iglesias et al (1999) introduce a terminology namely Ground Reaction Curve. This curve describes a curve of vertical stress over subsoil. When stress on top of pile is at a maximum value, the vertical stress over subsoil is at a minimum value. By normalized of subsoil movement downwards to pile clear spacing, they conclude that critical values lay between 2 and 6 %. Meanwhile, from curves from Figure x above, we can look that the critical values between 8-14 mm. With using pile clear spacing 60 cm, it means that the critical values are 1.3% to 2.3 %.
6.3. Load Transfer PlatformCek Hassandi Abdullah et al 200x
In Malaysia particularly nearby Gebeng Highway, Cek Hassandi et al.(200x) have already conducted a field test using the various load transfer platform (LTP) on top of aggregate piers(called ‘geopiers’) on soft soil. It would have showed the performance of several load transfer platform used in the field test. The rammed aggregate piers as columns used in the field test are ‘floating piles’ over soft soil. Test embankment was approximately 90 m long, 14.5 wide, and 3.5 high. The side slopes of the embankment were 1V:1.5 H.
There are three types of load transfer platform (LTP) performed in the field test, namely:
- Geosynthetic-reinforced LTP with two layers of geogrid (catenary LTP)
- Geosynthetic-reinforced LTP with three or more layers of geogrid (beam LTP)
- Reinforced concrete LTP
Two control sections were also provided in other to give a comparison in which these sections are embankment without reinforcement. Layout and dimension of different LTP as shown in Fig. x. below
(a)
(b)
Fig. x. Layout of LTP sections and control sections, (a)Plan view (b)Schematic sections of LTP
The geosynthetic-reinforced LTPs consist of an aggregate layer with geogrid reinforcement within the aggregate. Section 1 has a 1.5 m thick beam LTP with four layers of a biaxial extruded polypropylene geogrid (Tensar SS20) reinforcement spaced at 0.3 m apart vertically within the LTP, and supported on geopiers spaced in a 3.25 m center-to-center square pattern. Section 2 has a 1.0 m thick beam LTP spaced at 0.3 m apart vertically, and supported on geopiers in spaced a 2.5 m center-to-center square pattern. Section 3 has a catenary LTP with two layers of uniaxial woven polyester high-strength geogrid coated with polyvinyl chloride (Miragrid 24XT) spaced at 75 mm apart vertically placed in different direction (longitudinal and transverse) of embankment. This section was also supported on geopiers spaced at 2.5 m. Last section (section 4) was supported on 0.3 m thick continuous steel-reinforced concrete LTP over geopiers spaced of 2.5 m.
Table x. Properties of geogrids used in the field test
Property / Biaxial extruded geogrid / High-strength uniaxial geogridMachine direction / Cross-machine direction / Machine direction / Cross-machine direction
Tensile strength at ultimate(kN/m)
Tensile strength at 5% strain (kN/m)
Tensile modulus (kN/m)
Grid aperture (mm) / 20
14
280
39 / 20
14
280
39 / 370.3
93.3
1870
101 / 43.6
17.5
350
17.8
Mass/unit area (kg/m2) / 0.220 / 1.289
The selected aggregate used in the LTPs consisted of well-graded crushed granitic rock with fine material that was less than 3 %. This aggregate is normally used as a sub-base layer for road pavements. Then, over this aggregate blanket, the embankment was constructed using gravelly sandy clay.
The rammed aggregate pier, called geopier, is a relatively new intermediate-depth columnar foundation introduced in the construction industry (Fox and Cowell,1998). Typically, the drilled holes extend between 2 and 8 m below subsoil surface. In this field test, the initial drilled diameter of the geopiers was 0.75 m and the initial depth of the drilled hole for the geopiers was 5.5 m.
Site investigation indicated a soft silty clay/clayey silt layer as deep as 15 m at some location. This layer is composed of highly plastic clay with natural content between 35% and 61%. Field vane tests indicated that the shear strength ranges from 14 to 60 kPa, with most of tha values less than 25 kPa. Moreover,the soil value of sensitivity varies from 3 to 11.
6.3.1. Settlement of the embankment
This embankment is high embankment (h/s1.4). Therefore, differential settlement at the surface of embankment may be omitted because of two small. Then, total settlement and differential settlement at the bottom of embankment are an important thing to be discussed.
The total settlementresulted from field test and calculation using finite element method (FEM) would be compared as depicted in the following Figure x. The total settlements resulted from FEM only at bottom layer would be compared with those of field measurement.In FEM the compaction and no compaction treatment using load 550 kPa coming from wheel rollerduring the compaction process would be presented. In addition, Mohr Coulomb (MC) model for whole soil material (embankment and subsoil) and Mohr Coulomb (MC) model for embankment combined with Soft Soil Creep (SSC) model for soft soil were also presented.
Table x. Properties material used in Finite Element analysis
Properties / Element and modelGeopier / Fill material / Aggregate blanket / subsoil / subsoil / Geogrid / Concrete Slab
MC / MC / MC / MC / SSC / ElastoPlastic / Elastic
/ kN/m3 / 19 / 18.5 / 21 / 14 / 14 / - / 24
/ [o] / 48 / 35 / 38 / 8 / 8 / - / -
/ [o] / 0 / 0 / 4 / 0 / 0 / - / -
c / kPa / 1 / 1 / 1 / 15 / 15 / - / -
E / MPa / 100 / 200 / 300 / 0.75 / 0.75 / 280, 1870 / 19650
/ [-] / - / - / - / - / 0.1183 / - / -
/ [-] / - / - / - / - / 0.0229 / - / -
/ [-] / - / - / - / - / 0.0058 / - / -
vur /v / [-] / 0.3 / 0.3 / 0.3 / 0.4 / 0.4 / - / 0.15
(a) (b)
(c) (d)
Figure x. total settlement of LTP sections (at centre of square pattern of geopiers) and control sections: (a)Section 1 (b)Section 2 (c)Section 3,4(d)Control sections
From Fig. x. above at Section 1 it shows that the total settlement of geogrid from field measurement at bottom layer would be deformed deeper than at upper layer of geogrid. Mohr Coulomb model using plastic calculation with involving compaction load of 550 kPa can be used to predict the total settlement only for final step of compaction at the end of project execution, although it can’t follow the creep phenomenon on soft soil after completion of project execution. The SSC-model for the soft soil is better and can follow the creep phenomenon when the consolidation calculation type and no compaction load being applied but not for plastic calculation type.
At section 2 it is similar results as at section 1
In case the use of end-bearing pile and load transfer platform of catenary (1 or 2 layers of geosynthetics), Almeida et al.(200x) performed field test at Bara da Tijuda district in Brazil.
6.4.Influence of Column Stiffness
There are some kinds of column used to transfer load from embankment and surcharge to subsoil. These columns can be installed as a type of end-bearing or friction pile.
6.5. Influence of Geosynthetic Stiffness
Geosynthetic will withstand the tensile stress from both vertical stress and horizontal thrust.
6.6. Influence of Loading on ArchingSUZANNE van Eekelen et al.
A full-scale test has been carried out in Gissenburg (so called ‘Kyoto Road’) in the Netherlands. Suzanne van Eekelen at al. (200x) have observed the influence of loading coming from traffic during 3 ½ years after completion of construction by installing cell pressures at surface of piles located above and below geosynthetics to measure vertical stress on the top of piles. They also measured the soil support between piles.
The Kyoto road was constructed on 13 m long wooden piles and configured using pile spacing of 1.27 m a grid pattern, concrete pile caps with a height of 0.4 m and 0.3 m diameter. The geogrid reinforcement consisted of two layers uni-axial grid, perpendicular on road axis Fortrac 400/30-30 M and along the axis Fortrac 350/50-30 M. On top of that a 1.15 m high compacted embankment fill of a ‘Hegemann’ (sandy) sludge mixture was constructed. The Hegemann sludge mixture is a mixture of dredged material and additives containing mainly clay and cement with following properties: average unit weight =18.6 kN/m3, friction angle =33.8o and a cohesion of 11.5 kPa. The Kyoto road was build over a 9 m deep of soft soil with reaction modulus k= 477 kN/m3.
Fig. x Layout of the Kyoto Road
Table x shows the properties of fill
wet / dry / avg / w / kv / / ckN/m3 / kN/m3 / kN/m3 / % / m/s / o / kPa
22.2 / 17.0 / 18.6 / 18.1 / 2.1E-9 / 33.8 / 11.5
Table x. Young’s moduli of subsoil
Position / Material / Thickness (m) / E (kN/m2)Top layer / peat / d1= 1.45 / 1077
Lower layer / clay / d2= 1.50 / 2000
The modulus of subgrade reaction,k, can be calculated as follows:k= (E1.E2)/ (E1.d2+E2.d1)= 477kN/m3.
6.6.1. Load Distribution
To compare the calculated and measured load distribution, load parts A, B, C are defined as (Fig. x below):
- Load part A is transferred directly to the pile caps through arching,
- Load part B is transferred through the reinforcement to the pile caps,
- Load part C is resting on the subsoil
Unity for load A, B and C are given in kN.
Fig. x. Load distribution in a piled reinforced embankment (after Suzanne van Eekelen et al.(2008)
Devices TPC t1, t2 and t3 were installed on top of reinforcement. These TPCs measured the pressure imposed directly on the piles. The total pressure cells above reinforcement layer (TPCt1, TPCt2, TPCt3) measure load A, whereas total pressure cell below reinforcement layer (TPCb1) for measuring load (A+B). The vertical load B, which is the load carried by the geosynthetic reinforcement (GR). By means of tensile forces in the geosynthetic, this load is transferred to the piles. The curve presenting load B was determined by subtracting the average measured pressure of TPC t1, TPC t2 and TPC t3 from TPC b1.
The transferred vertical load on each pile was calculated using the equation of *H+p=18.6*1.15+p= 21.39+p kPa,where p is a surcharge. As soon as arching occurred, the load was transferred laterally to the piles. Therefore, the pressure measured at TPCs would be more than 21.39+p. The vertical distance between TPCs and the horizontal line at 21.39 kPa was an indication of arching.
Distribution of load A, B and Calong2 and 3 ½ years for both measured and predicted as shown in Fig. x. below
(a) (b)
Fig. x. Load distribution for A, B and C (a)Vertical stress observed along 2 years (b)Load observed along 3 ½ years
The figure above shows that it took a long time to develop the arching fully. Perhaps, this phenomenon was caused of cementation and settlement process of the fill material. From July 2006 onward, the arching measurements were relatively constant.The fluctuations were mostly due to variation in the weather, moisture content and the alternating periods of traffic.
The prediction of the load acting directly on the piles using BS8006 is better than that of EBGEO, which is much higher than the measured values. However, this value is not so important because for the design purpose for piles the total load is conservatively assumed as the applied load on piles.
When taking into account the support of the subsoil, the EBGEO gives a better approach for predicting the vertical load on geosynthetics than BS8006. It is utmost important as the imposed load on geosynthetics directly determines the tensile forces on the geosynthetics reinforcement.
Finite element…………..
6.6.2. Influence of dynamic load
Traffic started immediately after the completion of construction. Arching needed several months to develop completely (increasing of load A, decreasing of load B, C and pore pressures).
A heavy dynamic load coming from vehicular traffic, however, can cause a sudden short-term decrease of arching (load A decreases and load B increase suddenly). This construction only experiences traffic during working hours, and then usually heavy vehicles. Fig. x below shows a daily arching reduction during the first passages of the day. However, during the periods without traffic the geosynthetics reinforcement has the opportunity to restore again.