The settlement behavior of soft natural clays under a 14-year embankment load is investigated by obtaining the high-quality undisturbed samples of soft natural clays beneath a constructed embankment. It is found that the conditional way of determining the settlement by combining the layerwise summation method of calculating settlement, the Osterberg method of determining the embankment load-induced stress in subsoil, and the compression parameters determined from high-quality undisturbed samples overestimates the settlement, compared with the field observations. The key cause is that the Osterberg method overestimates the vertical superimposed stress in this case. The modified method of determining the vertical stress in subsoil suggested by the authors is adopted for improving the accuracy of vertical superimposed stress calculated. The balance among errors in the settlement calculation model, determination of vertical superimposed stress induced by embankment loads, and measurement of compression parameters are also discussed.
National Natural Science Foundation of China51678157Fok Ying Tong Education Foundation1610701. Introduction
It has been well recognized that the constructions of embankments over the soft natural clay are often beset by settlement problems at various degrees [1–4]. Settlement analysis of soft ground with natural clays under embankment loads inevitably involves three issues: calculation model; determination of vertical superimposed stress in subsoil (σs′); and measurements of calculation parameters.
The layerwise summation method is generally adopted as a simple way of calculating embankment load induced settlements with the one-dimensional assumption [3–6]. The soil profile is subdivided into layers, and the mean values of effective stress and soil compression index at each corresponding layer are used for settlement calculations. The Osterberg method is often adopted as a conventional way of determining the vertical superimposed stress (σs′) in subsoils [1, 7]. The Osterberg method was established based on the assumptions of instantaneous loading from embankment constructions, perfectly flexible loaded area along the base of embankments, and linear elastic theory of soils [8, 9]. On the contrary, the compression parameters are generally obtained from the experimental results of one-dimensional incremental load consolidation tests performed on undisturbed specimens sampled from different soil layers below ground surface [1, 5]. It has been well documented that the laboratory observations of compressibility may be significantly affected by sample disturbance [10–13], consolidation stress paths [14–17], testing methods [18–20], soil microstructure [21, 22], etc.
Note that the probable errors caused by the assumptions in calculating model, the determinations of σs′, and the observations in compression parameters should be in balance for obtaining the settlements with an accepted accuracy. Several researchers reported that the degrees of accuracy in the calculated results vary from good agreement [23–25] to poor agreement with the field observations [26–29]. A case study is a powerful way of understanding the key factors responsible for difference between calculated and observed settlements for natural clays induced by embankment loads. Note that case studies have been little reported in China on embankment load-induced settlements over soft natural clays.
The first-phase engineering project of Huaihe River Waterway had been constructed for three years from 1999 to 2002. The planning on the second-phase project of Huaihe River Waterway provides us an opportunity to investigate the settlement behavior of natural clays under a 14-year embankment load. This study obtained undisturbed samples at different depths below ground surface from two holes at the central point of the embankment and out of the embankment, respectively. The physical properties of samples from two holes are compared to identify that the samples out of the embankment can be used to measure compressibility of natural clays before embankment constructions. Then, the settlement under embankment loads is analyzed using the Osterberg method. The calculated settlement is compared with the observation to check the behavior of σs′ determined by the Osterberg method in this case, and a modified way suggested by Wang et al. [30] is adopted for improving the accuracy of calculations on σs′. The values of σs′ obtained by the Osterberg method and the modified way are compared with the yield stresses by one-dimensional consolidation tests. Finally, the most probable factors of controlling the calculations of settlements are discussed.
2. Project Background and Measured Settlement
Figure 1 shows the location of the embankment investigated which locates at Huiahe River Waterway. The Huaihe River is a flood-prone river in China that originates from Tongbai Mountain, flows through Henan and Anhui Provinces, and pools up into the Hongze Lake in Jiangsu Province. The Huaihe River Waterway was designed to link up Hongze Lake with Yellow Sea for enhancing the flood discharge of Huaihe River and Hongze Lake. The first-phase waterway project was constructed from 1999 to 2002 for increasing the floor control standard of the Hongze Lake from once every 50 years to once every 100 years. The first-phase of the waterway project was about 162.3 km. There are three different geological deposits along the waterway, stiff deposit (K3–K57), soft deposit (K57–K108), and sand deposit (K108–K162) [31].
Location of embankment and sampling.
The soft deposit in the first-phase of the waterway project has a length of about 52 km. Based on the experience and stability analysis, the embankments over soft deposit were stage-constructed without any reinforcement of soft foundation. To monitor the magnitude and the rate of the soft foundation settlement, the surface settlement gauges were installed during the construction. Figure 2 shows the development of surface settlement at the centerline of embankment with elapsed time during and after construction, based on the observations reported by JPEIRI [32]. It can be seen that the surface settlement at the end of construction is 1.35 m. Note some settlement gauges were destroyed or damaged caused by the flooding emergency in July 2003. The reconstruction of the monitor system was conducted in August 2005. The observations were continued to December of 2013. As shown in Figure 2, the maximum postconstruction settlement under the embankment load is recorded as 1.7 m in December 2013 [32]. Figure 3 shows the settlement rate based on the settlement-time curve in Figure 2. It can be seen that the settlement rate decreases with the increase in elapsed time after construction. The settlement rate varies from the 2.11 mm/d in August 2002 to 0.78 mm/d in August 2005, 0.50 mm/d in July 2006, 0.41 mm/d in October 2010, and 0.26 mm/d in December 2013.
Development of surface settlement with elapsed time during and after construction.
Development of settlement rate with elapsed time during and after construction.
Nowadays, the State Council of China [33] has approved of the project for further reducing the frequency of major downstream flooding of Hongze Lake to once every 300 years. The second-phase of Huaihe River Waterway project will broaden and deepen the entire channel and strengthen the existing embankments on the basis of first-phase project [31].
3. Undisturbed Samples of Soft Deposit and Physical Properties
Based on the information of site investigation reported by JPEIRI [34, 35], a typical subsoil profile can be illustrated in Figure 4(a). The subsoils beneath the embankment can be divided to eight soil layers. The top layer is a thin crust (TC) of approximately 1.0 m thick; below the TC, there are four soft clay layers (labeled as SC-1, SC-2, SC-3, and SC-4) identified by detailed physical tests described later; their average thicknesses are 3.5 m, 6.4 m, 6.0 m, and 7.4 m, respectively. These soft deposits were laid down during the quaternary period [36]. Next are three layers of stiff sand clay (SSC-1, SSC-2, SSC-3) extending down to 37.9 m depth, and their N values of standard penetration test (SPT) are 8, 45, and 50, respectively.
Test site N88K: (a) cross section of embankment and soil profile before construction; (b) borehole location.
The planning on the second-phase project of Huaihe River Waterway calls for further detail site investigations by performing laboratory tests on high-quality undisturbed samples. Taking this opportunity, we obtained the undisturbed samples of soft natural clays beneath the embankment constructed at K85.5 section of the first-phase engineering project. For comparing the physical and mechanical behaviors of undisturbed samples with and without embankment loads, the undisturbed samples of soft natural clays under the ground surface out of the embankment (50 m distance from the centerline) were also sampled, as shown in Figure 4(b).
A thin-wall free-piston sampler was used to obtain two holes of undisturbed samples at the depths of 3 m, 6 m, 9 m, 12 m, and 15 m below ground surface. The hole under the center of embankment was termed as Em borehole, and the hole out of the embankment is termed as Gr borehole, as shown in Figure 4(b). The number after the borehole represents the depth of the samples. For example, the Em_U9m represents the undisturbed sample obtained from a depth of 9 m at the embankment borehole.
For evaluating the sample quality, the method suggested by Lacasse et al. [10] was adopted with measuring volumetric strain (ε_{v0}) at the effective overburden stress (σv0′). The ε_{v0} was calculated as follows: ε_{v0} = (e_{0} − e_{v0})/(1 + e_{0}) × 100%. The term e_{0} and e_{v0} represent the initial void ratio and the void ratio under σv0′, respectively. The values of σv0′ were calculated by γ′H, where γ′ is the effective unit weight of soil and H is thickness of the soil layer. As suggested by Lacasse et al. [10], the sample quality can be classified as good or fair corresponding to the values of ε_{v0} within 0–2% or 2–4%, respectively. Figure 5 shows the sample qualities of the undisturbed samples from Gr borehole and Em borehole, and the most values of ε_{v0} varied from 1.4% to 4%. It can be concluded that the qualities of investigated samples are good or fair. Note that embankment loads were not considered in calculating σv0′ for undisturbed samples from the Em borehole. Embankment loads will induce vertical superimposed stress in subsoils and decrease the values of e_{0}. On the contrary, the embankment loads will cause destructuration of natural soft deposits [13], consequently resulting in the decrease in the values of e_{v0}. More studies are required to investigate the balance among the variation in e_{0} and e_{v0}, the destructuration, and the vertical superimposed stress for evaluating sample quality of natural clays subjected to embankment loads.
Sample quality classification.
Table 1 shows some physical properties of the samples investigated. The liquid limits (wL) were measured using the Casagrande method according to Head [18]. The plastic limit tests were also conducted in accordance with rolling thread suggested by Head [18]. It can be seen that the investigated clays have a wide spectrum of wL, ranging from 50% to 81%. Figure 6 shows the plasticity chart, indicating that all samples lie above the A-line defined by the Unified Soil Classification System [37]. The Funing soft clays can be classified as high plasticity clay (CH).
The oedometer test for undisturbed samples.
Sample
G_{s} (g/cm^{3})
wn (%)
wL (%)
wp (%)
wn/wL
Consolidation stress (kPa)
Gr_U3
2.72
49.8
50
20
0.99
6.25-12.5-25-50-100-200-400-800-1600
Gr _U6
2.72
59.8
60
23
0.99
Gr _U9
2.70
59.7
60
23
0.99
Gr _U12
2.70
56.7
68
23
0.83
Gr _U15
2.70
57.5
71
24
0.81
Em_U6
2.70
41.0
50
20
0.82
Em_U9
2.69
46.9
60
22
0.78
Em_U12
2.70
47.9
68
23
0.70
Em_U15
2.69
48.9
66
24
0.74
Plasticity chart of investigated soils.
From Figure 6, it can be also seen that the investigated clays can be divided into four groups with the values of wL: (1) wL = 50%–51%; (2) wL = 58%–60%; (3) wL = 68%–70%; and (4) wL = 80%–81%. It is interesting to note that the wL value of Gr_U3m is approximately equal to that of Em_U6m. That is, the sample of Em_U6m with embankment loads is identical to that of Gr_U3m without embankment loads, with considering the total settlement of 3.05 m during and after the construction of embankment. Meanwhile, the value of wL for Gr_U6m is almost exactly the same as that of the Em_U9m. Furthermore, the wL values of Gr_U12/15m are in good agreement with the values for Em_U12/15m. Detail information on embankment load-induced changes in subsoil layers can be seen in the later section.
On the contrary, Figure 7 shows the particle size distributions curves for all tested soil samples, which were measured by hydrometer analysis as described by Head [18]. It is interesting to note that there are four type of patterns in the plot, corresponding to wL = 50%–51%; wL = 58%–60%; wL = 68%–70%; and wL = 80%–81%, respectively. It can be seen that the grade curve of Gr_U3m is almost identical to the Em_U6m. For the Gr_U6m, its grade distribution is consistent with the Em_U9m. The grade distribution curves of Gr_U12/15m are almost the same as those of the sample of Em_U12/15m. Hence, the Em_U6m, Em_U9m, and Em_U12/15m can be considered belonging to the SC-1 layer (1.0 m–4.5 m), SC-2 layer (4.5 m–10.4 m), and SC-3 layer (10.4 m–16.9 m) in preconstruction stage, respectively. Note that the samples of Em_U3m are the embankment fill materials [31].
Grading curve of investigated soils.
The above physical properties of the Atterberg limits and particle size distributions indicate that the foundations can be considered as horizontally homogeneous before embankment was constructed. That is, the samples out of the embankment can be used to measure compressibility of natural clays for calculating embankment load induced settlement.
4. Assessment on Embankment Settlements
It is recognized that the natural sedimentary clays are generally subjected to soil structure effects developed during the depositional and the postdepositional processes [1, 38–43], resulting in the vertical yield stress (σp′) often being larger than the effective overburden pressure. The soil structure results relatively the small deformation of natural clays under effective vertical stress up to the consolidation yield stress. The compressibility of clays increases dramatically when the applied load exceeds the yield stress [39, 44–47]. The settlement of natural sedimentary clays under σv′ can be calculated using the following equations:(1)S=H1+e0Crlog10σv0′+σs′σv0′,whenσv0′≤σv′≤σp′,S=H1+e0Crlog10σp′σv0′+Cclog10σv0′+σs′σp′,whenσv′>σp′,where the effective vertical stress (σv′) in subsoil can be calculated as σv0′+σs′. σs′ represents vertical superimposed stress and Cr represents the compression index at the preyield zone when σv0′≤σv′≤σp′ and is expressed as Cr=Δe/logσv′/σv0′. Cc is the compression index at the postyield state when σv′>σp′ and is obtained by Cc=Δe/logσv′/σp′.
The compression parameters (C_{r}, C_{c}, σp′) were determined by one-dimensional incremental load consolidation tests on undisturbed natural clay obtained from ground borehole. All the specimens had a diameter of 61.8 mm and an initial height of 40 mm. The loading steps ranging from 6.25 kPa to 1600 kPa by doubling the load for each increment and the duration of every loading increment was about 3 days, following Zeng et al. [48]. Detailed test program is listed in Table 1.
It can be seen from Figure 8 that the compression curves of undisturbed samples show a typical inverse ‘S’ shape as a result of the effects of soil structure. The relationship between σp′ and σv0′ is shown in Figure 9. The consolidation yield stress (σp′) for all the investigated specimens was determined by the Casagrande method. σp′ values were all larger than σv0′ values due to the soil structure during depositional and postdepositional processes [40]. The values of C_{r}, C_{c}, and σs′ are shown in Table 2. Note the values of σs′ in Table 2 were calculated by the Osterberg method.
Compression curves of natural clay.
Comparison between the yield stress and initial effective stress.
The calculated results.
Soil
Depth (m)
H (m)
e_{0}
C_{c}
C_{r}
σ_{pi} (kPa)
σ_{v0i} (kPa)
σ_{si} (kPa)
S (m)
SC-1
1.0–4.5
3.5
1.39
0.47
0.07
63
20
121
0.29
SC-2
4.5–6.9
2.4
1.60
0.72
0.04
99
39
115
0.14
6.9–10.9
4.0
1.61
0.60
0.09
73
49
110
0.35
SC-3
10.9–13.9
3.0
1.54
0.64
0.08
90
82
102
0.24
13.9–16.9
3.0
1.58
0.74
0.11
113
102
95
0.21
SC-4
16.9–24.3
7.4
1.54∗
0.44∗
0.2∗
156
156
85
0.28
SSC-1
24.3–25.9
1.6
0.85∗
0.12∗
0.07∗
167
167
78
0.02
SSC-2
25.9–31.1
5.2
0.53∗
0.04∗
0.03∗
189
189
77
0.02
SSC-3
31.1–37.9
6.8
0.65∗
0.1∗
0.09∗
225
225
71
0.05
Total
1.59
∗Obtained from the site survey (Jiangsu Province Engineering Investigation and Research Institute, 2010, 2013).
Figure 10 shows the comparisons between the calculated settlements and field measurements. It can be seen that the calculated settlement is approximately 14% larger than the field after construction measurement of 1.7 m. Note that the Osterberg method of determining σs′ is based on the assumption of instantaneous loading, perfectly flexible loaded area along the base, and soil elasticity. These assumptions are far from real condition of embankment [9, 49]. The assumptions in the Osterberg method results in oversimplifying actual embankment condition, leading to the significant miscalculations of embankment load induced settlements, as reported by Wang et al. [30].
Comparisons of settlements between the calculated and the measured values.
5. Vertical Superimposed Stress in Subsoil
In the calculation of embankment load induced settlements, quantitatively evaluating the contact stress along the base (σb′) and vertical superimposed stress in subsoil (σs′) is great importance [8]. Wang et al. [30] proposed a modified way of determining σs′ for overcoming the assumptions of instantaneous loading, perfectly flexible loaded area along the base, and soil elasticity in traditional approach using the geometric parameters of embankment and the friction angle of fill material (ϕ). Moreover, a reduction coefficient of 0.85 is suggested for considering the effect of elastoplastic nature of subsoil. Note the geometric parameters in Figure 4 and ϕ = 30° [31] were adopted for analysis in this study.
The typical comparisons of predicted contact stresses along base between the modified method and traditional method are shown in Figure 11. It can be seen that the contact stresses predicted by modified method are close to a bell-shaped distribution that yields lower stresses at the central portion of embankment and higher stresses near outer edge by comparison with the traditional method. Figure 12 depicts the typical comparison of vertical superimposed stresses at embankment boreholes. It can be observed that the modified method yields lower stress than the traditional Osterberg method, up to approximately 24 kPa at the centerline of the embankment. These discrepancies are attributed to the effects of the contact stress along the base and elastoplastic behavior of subsoil on load transfer [30].
Contact stress along the base.
The typical effective stress distribution with depth for two boreholes: (a) ground; (b) embankment.
The calculated settlements with the modified method of determining σs′ by Wang et al. [30] are presented in Figure 10. It can be seen that the predicted settlements using the modified method of determining σs′ are consistent with the field measurements. Figure 13 presents the varied depths of the soil stratification beneath embankment determined by the calculated settlements with the modified method in Table 2 and recorded construction settlements. It is encouraged to find that the varied depths of soil stratification are consistent with the results of liquid limit and particle size distributions tests.
Soil stratification by the proposed method.
Note the values of σs′ obtained by the Osterberg method and the modified way are compared with the yield stress determined by one-dimensional incremental load consolidation tests on undisturbed samples of natural clay obtained from embankment borehole, as shown in Table 2. Figure 14 depicts that the predicted effective stress using the modified method yields slightly higher than the yield stresses. This result is attributed to the uncompleted primary consolidation in the soft foundation. The calculated results with the Osterberg method are also shown in the same figure, again indicating that the Osterberg method may significantly overestimate the values of σs′.
Comparisons between the calculated effective stress and the yield stress.
6. Key Factors on Settlement Calculations
For the classical layerwise summation method, the embankment behaves like a sample tested in an oedometer condition, which neglects lateral displacements. The effect of accuracy of the vertical superimposed stress (σs′) on settlements is evaluated as shown in Figure 14, but the predicted settlements with the modified method still slightly smaller than the field measurements, as shown in Figure 10. This discrepancy would be induced by the neglect of lateral displacements, the effect of strain rate, the sample disturbance, and the stress paths on compression behavior [1].
In practice, the field consolidation strains are generally higher than the values obtained from conventional consolidation tests, and the rate effects are associated with the C_{c}/(1 + e_{0}) [50]. Leroueil et al. [1] reported that the effect of strain rate is small for the clay with low C_{c}/(1 + e_{0}) values between the 0.2–0.25. The values of C_{c}/(1 + e_{0}) for investigated natural Funing soft clay in Table 2 (vary from 0.19 to 0.28) reveals a minor effect of strain rate in this study.
On the contrary, the natural Funing clay has a yield stress ratio greater than 1.0 (Figure 9) that attributed to the development of the resistance of soil structure during the depositional and the postdepositional processes. The soil structure of natural clay can be easily disturbed during sampling and handing. The sample disturbance reduces the consolidation yield stress and the compressibility of soil in the postyield state that results in the underestimation of the settlements [1, 13].
The following two factors may be responsible for the discrepancy on settlements using traditional method: the neglect of lateral displacements and the effect of consolidation stress path on mechanical behaviors of natural clays. In practice, the neglect of lateral displacements resulted in an underestimation of the settlements induced by the embankment load and the compression parameters calibrated from oedometer tests result in overestimation of settlements [1, 16, 20]. As shown in Figure 10, the former factor has a much significant influence on the calculation of settlement than the latter factor in this work.
7. Conclusions
This study performs a case study on the settlement behavior of soft natural clays induced by a 14-year embankment load. The predicted settlements obtained by different methods are compared with field observations. The main conclusions are summarized as follows:
The settlement predicted by combining layerwise summation method of settlement calculation model, the modified method of determining vertical superimposed stress suggested by Wang et al. [30], and compression parameters measured from undisturbed samples obtained by a thin-wall free-piston sampler is in agreement with the field observation
The difference in predicted settlement between traditional method and the modified approach can be attributed to the overestimated vertical superimposed stress in subsoil determined by the Osterberg method
The agreement in settlement between predicted value and field observation is a result from the balance among the errors in calculation model, vertical superimposed stress, and compression parameters
NotationsC_{c}:
Compression index at postyield state of natural clay
C_{r}:
Compression index at preyield state of natural clay
e_{0}:
Initial void ratio
e_{1}:
Void ratio after consolidation
G_{s}:
Density of soil particles
H:
Thickness of the soil layer
S:
Settlement
wn:
Natural water content
wL:
Liquid limit
wp:
Plastic limit
σ_{b}:
Contact stress along the base
σp′:
Yield stress
σs′:
Vertical superimposed stress caused by embankment load
σv0′:
Effective overburden stress
Δε_{v}:
Volumetric strain at the effective overburden stress
γ′:
Unit weight of soil.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This study was supported by the National Natural Science Foundation of China (Grant no. 51678157) and Fok Ying Tung Education Foundation (Grant no. 161070).
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