611
Optimization of compaction zoning in loess embankments
Limin Zhang, Xueming Yu, and Ting Hu
Abstract : Extensive centrifuge model tests were carried out to investigate optimum compaction density zones within loess embankments 30 and 63.8 m high. In these tests, water infiltration into the slopes and upstream storm water ponds were modelled to simulate the most unfavourable working conditions. Displacement contours in the cross sections were measured and crest settlements were monitored during the tests. Finally, the optimum compaction zoning was analysed based on the comprehensive evaluation of the total crest settlement, the postconstruction settlement, and the slope stability of the
embankments. It was indicated that a properly designed high loess embankment could perform satisfactorily with the lower part compacted looser than the conventional code requirements. In terms of total crest settlement and embankment cracking, the degree of compaction K of loess fills should not be less than 0.85 for 30 m high embankments and not less than 0.90 for 63.8 m high embankments. In terms of the postconstruction settlement (<0.2–0.3 m) and construction feasibility, it is expedient to compact the lower one third to K = 0.85, and the top two thirds to K = 0.90 for 63.8 m high embankments. For 30 m high embankments, the bottom zone with K = 0.85 can make up two thirds of the height.Key words : embankment, centrifuge test, compaction, settlement, stability, loess.
Résumé : Un ensemble d’essais de modélisation en centrifugeuse a été mené pour étudier les zones de compaction optimum à l’intérieur de remblais en loess hauts de 30 et 63,8 m. Dans ces essais, l’eau d’infiltration le long des pentes et les retenues d’orage amont ont été modélisées de manière à simuler les conditions de service les plus défavorables. Les lignes de contour des déplacements dans les sections transversales ont été mesurées et les tassements en crête ont été enregistrés pendant les essais. Finalement, les zones de compaction optimum ont été analysées à partir d’une évaluation complète du tassement total en crête, du tassement après construction et de la stabilité des pentes des remblais. On a trouvé qu’un remblai en loess de hauteur importante mais bien con?u, avec une partie inférieure compactée de manière plus lache que les spécifications
conventionnelles, pouvait se comporter de fa?on satisfaisante. En termes de tassement totale en crête et de fissuration
d’ouvrage, le degré de compaction K des matériaux loessiques ne devrait pas être inférieur à 0,85 pour des remblais de 30 m de haut et 0,90 pour 63,8 m. En termes de tassement après construction (<0,2 à 0,3 m) et de faisabilité, il est commode de compacter le tiers inférieur à K = 0,85 et les deux tiers supérieurs à K = 0,90 pour les remblais de 63,8 m de haut. Pour les ouvrages de 30 m de haut, la zone basse avec K = 0,85 peut s’étendre jusqu’aux deux tiers de la hauteur.Mots clés : remblai, essai en centrifugeuse, compaction, tassement, stabilité, loess.[Traduit par la Rédaction]
Introduction
The density of compacted fill materials is the key to the stabil-ity and settlement control of embankments. As a result, com-paction densities for highway and railway embankments are clearly specified in design codes. In the United States, the AASHTO standard M57-80 (AASHTO 1986a ) suggests that the degree of compaction K , defined as the ratio of construc-tion dry density ?d to the maximum dry density ?dmax , be greater than 0.95 both for embankment fill and for subgrade using AASHTO test designation T-99 (1986b ) or ASTM test designation D-698 standard compaction method. Further-
more, most states have their own specifications according to their local situations (Behbahani 1974). Some, like Florida and Ohio, require K = 1.0 using AASHTO T-99 method C.Some others have flexible control of the degree of compaction in the range 0.85–1.00 using the same methods. The modified Proctor test method AASHTO T-180 is also adopted in Alaska, Maine, and Rhode Island;the soils should be com-pacted to K = 0.90–0.95. In China, the highway construction specifications (China Department of Transportation 1987)recommend the modified Proctor compaction test methods similar to AASHTO test designation T-180 or ASTM test des-ignation D-1557 and require a degree of compaction K ? 0.93for subgrades and K ? 0.90 for embankment fills.
Most earthworks are only a few metres high, and the stan-dard specifications are applicable. However, the specifica-tions may not be feasible in some special cases. On the “Loess plateau” in northern and northwestern China, loess soils cover a vast land of about 600 000 km 2. As a typical geographic characteristic, numerous erosion gullies disperse on the landscape. Many high embankments were built across these
Received June 27, 1997. Accepted May 12, 1998.
L. Zhang. Department of Civil Engineering, University of Florida, Gainesville, FL 32611-6580, U.S.A.
X. Yu and T. Hu. Department of Hydraulic Engineering, Sichuan Union University, Chengdu, Sichuan 610065, People’s Republic of China.
C a n . G e o t e c h . J .
D o w n l o a d e d f r o m w w w .n r c r e s e a r c h p r e s s .c o m b y S h i j i a z h u a n g R a i l w a y C o l l e g e o n 03/01/13F o r p e r s o n a l u s e o n l y .
612
Can. Geotech. J. Vol. 35, 1998
gullies in highway and railway engineering. The cost of a high embankment is usually within half that of a concrete bridge,since the construction materials for an embankment are imme-diately available on the site and labour costs are compara-tively low. Furthermore, most of this plateau area is arid or semiarid, with annual precipitation of only around 300mm.Therefore, the runoff in the gullies is usually within the drain-age capacity of the culverts built across embankments.
A major problem encountered in constructing embank-ments in this area was the compaction of fill materials. First,the gullies are usually V-shaped with steep side slopes. Some gullies are as deep as 60 m and the side slopes are steep cliffs.The ramps for modern construction equipment were fre-quently unable to reach the bottom of the gullies. Conse-quently, the initial part of spreading and compaction jobs had to be carried out manually. The compaction thus obtained was frequently found insufficient until the elevation reached the machine-accessible level.
To justify the practice of special construction needs, a research project was initiated in association with the recon-struction of the Xian–Lanzhou freeway. It aimed to optimize construction density zones within an embankment to meet the stability and settlement (including wetting compression)requirements with feasible construction techniques. This paper reports the results of a series of centrifuge model tests performed for this purpose.
Description of soil properties
Loess is an aeolian soil. The central Gansu Province of China is covered by a thick layer of loesses that are identified mostly as Tertiary deposits Q 3 or Quaternary deposits Q 4. The natural dry density is usually between 1140 and 1690 kg/m 3.The natural moisture content varies from 7.0 to 23.0%, and the liquid and plastic limits are 21.7–32.5% and 14–21%,respectively. The friction angle and apparent cohesion from consolidated–undrained tests are 23–39? and 10–65 kPa,respectively. To investigate the mechanical properties of the loesses and perform centrifuge tests, a sample soil was taken at Jinin, about 100 km east of Lanzhou City. Geologically,this soil was accumulated in the later Tertiary period and was identified as Q 3. The silt fraction (particle diameter d =0.074–0.002 mm) constitutes up to 79% of the soil and the clay fraction (d < 0.002 mm) 16%. The coefficient of uniformity (C u = D 60/D 10) and coefficient of curvature (C c =(D 30)2/(D 60D 10)) are 20.34 and 1.93, respectively. The liquid and plastic limits are ?L = 30% and ?P = 17%, respectively.The specific gravity is G = 2.75. In the Unified Soil Classifi-cation System, the soil is termed well-graded CL.
The modified Proctor compaction tests were carried out according to AASHTO test designation T-180 or ASTM test designation D-1557. The maximum dry density was ?d =1910 kg/m 3, the void ratio was e min = 0.44, and the corre-sponding optimum moisture content was ?op = 12.5%.
Most loesses are of a collapsive nature. The shear strength and deformation modulus of collapsive soils are sensitive to moisture content. Large collapsive deformation may develop upon wetting and last for an extended time, causing damage to road embankments (Knight and Dehlen 1963), deep fills (Kropp et al. 1994; Noorany and Stanley 1994; Vicente et al.1994), and other structures. To account for this, wetting was simulated in the model tests. Furthermore, extensive consoli-dated–undrained triaxial tests on four different densities of samples were conducted both in the natural moisture state and in the saturated state. In the triaxial tests, all the samples were prepared to the optimum moisture content. The vacuum satu-ration technique was then employed to prepare the saturated samples. The peak friction angle and apparent cohesion of the soil at four different densities are listed in Table 1 (total stress parameters). The apparent cohesion of the loess in Table 1 is relatively high. However, it is reasonable for the loess soil at the corresponding densities. A statistical study (Zhang et al.1993) of several hundred soil samples in the region revealed that the average dry density, apparent cohesion, and friction angle were 1680 kg/m 3, 87.2 kPa, and 30.1?, respectively, for the Q 2 loess deposits and 1300 kg/m 3, 29.4 kPa, and 25.0?,respectively, for the Q 3 and Q 4 loess soils. The in situ dry densities are mostly less than those of the compacted soils in this study. Yet, vertical cliffs more than 10 m high are com-mon and may last for dozens of years.
Centrifuge tests and results
Test procedures
A typical loess embankment is shown in Fig. 1. Usually,berms are designed with 10–15 m height intervals. Within the embankment, the degree of compaction should not be less than 93% for the road bed 2–3 m beneath the pavement struc-tures, and not less than 90% for the other parts of the embank-ment. In the model tests, the pavement and the subgrade layer were neglected, since their thicknesses were very small com-pared with the embankment fills. The actual combination slopes were also simplified to single slopes for simplicity of sample preparation.
The geotechnical centrifuge at Sichuan Union University has a 25 g -ton effective capacity and a 1.55 m radius from the centre of the model to the axis. The model container is 490mm long, 290 mm wide, and 330 mm high in inside dimen-Table 1.Friction angle and apparent cohesion of the loess samples (total stress parameter).Dry density (kg/m 3):1800172016301530Degree of compaction:0.94
0.90
0.85
0.80
——————————————————————————————————————–—Moisture state:
Unsaturated Saturated Unsaturated Saturated Unsaturated Saturated Unsaturated Saturated Unit weight (kN/m 3)19.8521.0319.0120.5317.9319.9716.8619.34Friction angle (°)
34.6030.0033.0028.0031.3027.0029.5021.00Apparent cohesion (kPa)
144.00
66.00
65.00
60.00
58.00
54.00
50.00
40.00
C a n . G e o t e c h . J .
D o w n l o a d e d f r o m w w w .n r c r e s e a r c h p r e s s .c o m b y S h i j i a z h u a n g R a i l w a y C o l l e g e o n 03/01/13F o r p e r s o n a l u s e o n l y .
Zhang et al.
613
sions, with a detachable perspex glass wall for model obser-vation. To simulate highway embankments of 63.8 m height,models of 300 mm height were centrifuged to 212.7g .
In preparing samples, the aggregations in the model soils were first broken up and the soils were prepared to the required moisture content. Then they were placed in water-tight containers for at least 24 h for moisture uniformity. Each model embankment was constructed layer by layer in six lifts.In each lift, the required amount of soil was compacted to the specified height according to the design density. At the end of sample construction, the transparent perspex glass was detached. Pin markers 25 mm long were inserted on the side of the model in 40 mm spacing (see Fig. 2) and their initial horizontal and vertical positions were measured manually.Finally, the perspex glass and two displacement transducers were mounted and the model was ready for testing. Shown in Fig. 2 are the setups of the test package.
Models with flat slopes (1:1.75) were tested first. During
each test series, the centrifuge was accelerated to the 212.7g test acceleration for simulating the end-of-construction per-formance of the embankment, and maintained at that acceler-ation for 47 min, approximately equivalent to 4.05 years in the prototype. Afterward, the centrifuge was stopped and the displacement readings of the markers were taken. Either dis-placement contours or locus could be plotted based on the end-of-spin and initial readings taken from the markers across the embankment profile. After the first operation, the model surfaces were infiltrated slowly with water in amounts equiv-alent to the annual precipitation. For instance, 1.41 mm 3 of water in a 1.0 mm 2 plan area was equivalent to 300 mm of annual precipitation in the prototypes. For a 300 mm high model with a 1:1.5 side slope, the model plan area was 0.48 ×0.29 = 0.139 m 3 and the water required to model the annual rainfall was 196.3 mL. This amount of water was sprinkled uniformly over the model surface without surface runoff (to the safe side). If the model was centrifuged to 212.7g acceler-ation again and maintained for 18 min (approximately 1.55years in prototype scale), further displacements would develop in the model due to wetting of subsurface soils and the markers were measured again. Following that, the wetted part of the model was cut and the model was trimmed to a 1:1.5 slope. The same procedures were repeated at 1:1.2,1:0.75, 1:0.5, and 1:0.3 slopes until the model embankment finally failed. In most of the tests, storm-water ponds equiva-lent to 23.4 m depth in prototypes were maintained during the tests to simulate the worst working conditions.
Manual measurements of the displacement markers gave an accuracy of only 0.1 mm in model scale. For 200g models,the accuracy would be 20 mm in prototype scale. This was acceptable considering the nature of earthworks and that the maximum measured displacement was up to 4.2 m. After the centrifuge stopped, the model would rebound. The marker measurements then would underestimate the end-of-construc-tion displacements. However, the postconstruction settle-ments would be less affected, since all the displacement increments were measured at the same situation (stationary).The simulation of unsaturated soil is a special problem in centrifuge model tests. Among other factors affecting flow in unsaturated soil, it was demonstrated by Arulanandan et al.(1988) and Cook and Mitchell (1991) that similitude is achieved for five of eight nondimensional numbers governing the flow. Among the five numbers, the capillary effects are similar in prototypes and centrifuge models, which ensures that the moisture-content profile of the model unsaturated soil column will be geometrically similar to that of the prototype.
Fig. 1. Typical standard embankment profile.
Fig. 2. Setups of the centrifuge models.
C a n . G e o t e c h . J .
D o w n l o a d e d f r o m w w w .n r c r e s e a r c h p r e s s .c o m b y S h i j i a z h u a n g R a i l w a y C o l l e g e o n 03/01/13F o r p e r s o n a l u s e o n l y .
614
Can. Geotech. J. Vol. 35, 1998
This was essential to simulate the surface soil collapse upon wetting for this study. In addition, the fluid flow in the test situation was very slow and the flow was approximately lam-inar. Therefore, Darcy’s law remained valid, even though the similitude of Reynold’s number was not achieved. This implied that the time scaling law for saturated soil was approximately applicable to the unsaturated soils in this study. Similar applications can also be found in the centrifuge tests by Cook and Mitchell (1991), Mitchell and Stratton (1994), Hellawell and Savvidou (1994), and Atkinson and Taylor (1994).
The scheduled test series is listed in Table 2. Nine different density combinations were considered for 63.8 m high embankments and two combinations were considered for 30m high embankments. In each test series, three to five embankment slopes and different test conditions were involved. These combinations were expected to supply enough data to reveal optimum compaction zoning at mini-mum cost. A total of 61 centrifuge operations were performed.Total displacements
The accumulated embankment displacements were calculated by subtracting the initial readings of the markers prior to the first spinning from the latest marker readings after the current spinning. Figure 3 presents the typical measured settlement contours and horizontal-displacement contours from the model series M-7. Significant increases in settlement and hor-izontal displacement were observed due to upstream ponding.The displacements increased steadily as the slopes became steeper. The maximum settlements and horizontal displace-ments of all the tests are summarized in Tables 3–6.
Shown in Table 3 are the results of the single-density embankments with the degree of compaction ranging from 0.80 to 0.94. Fill density was found to be very critical to the settlement and horizontal displacement of these embank-ments. For instance, at a 1:1.75 slope, the maximum settle-ment 4.05 years after construction almost doubled with every 0.05 decrease in the degree of compaction. At K = 0.94 (?d =1800 kg/m 3), the maximum settlement to height ratio was only 0.41%. This ratio increased to 0.90, 1.90, and 3.83% as the degree of compaction decreased to K = 0.90 (?d =1720kg/m 3), K = 0.85 (?d = 1630 kg/m 3), and K = 0.80(?d = 1530 kg/m 3), respectively. The horizontal displacement increased accordingly, but at a slower pace.
The upstream storm ponds and slope-water infiltration had a significant influence on the embankment displace-ments. Water infiltration into a slope alone increased the crest settlement by means of wetting and increased unit weight, yet it did not cause considerable increases in horizon-tal deformation, as shown in models M-1 and M-4. How-ever, in the presence of a storm pond, the fills in the capillary zone and the saturated zone below the phreatic table would be weakened, which would in turn cause large vertical and horizontal displacements and possibly loss of stability. The impacts of storm ponds and slope-water infil-tration were compared in models M-3 and M-4. Small hori-zontal displacements were observed in model M-4 which involved only slope infiltration, and the embankment was still stable at a slope of 1:0.5. However, large lateral move-ments were observed in model M-3 after upstream impound-ing. Therefore, emphasis was given to the simulation of the effects of storm ponds.
Since a looser soil is more susceptible to disintegration of structures upon wetting than a denser soil, it is very important to protect the slopes in the lower part of embankments where the density might be smaller. In some embankments along the Xian–Lanzhou freeway, asphalt-coated membranes were used as shown in Fig. 4. A culvert was sometimes not fur-Table 2.Schedule of the centrifuge model tests.Model Centrifuge Prototype Zone dry Zone Crest Slope Test height acceleration
height density height width Pond water series (mm)(g )(m)(kg/m 3)(m)(m)formation infiltration M-00.30212.763.8180063.8012.8Yes Yes M-1
0.30212.763.8163042.5412.8No Yes 172021.27M-20.30212.763.8172063.8012.8Yes Yes M-30.30212.763.8163063.8012.8Yes No M-40.30212.763.8163063.8012.8No Yes M-50.30212.763.8172021.2712.8Yes Yes 163042.54M-60.30212.763.8172042.5412.8Yes Yes 163021.27M-70.30212.763.8172042.5412.8Yes No 153021.27M-80.30212.763.8172021.2712.8Yes No 153042.54M-90.30212.763.8153063.8012.8Yes No M-100.15200.030.0172020.0012.8Yes No 163010.00M-11
0.15
200.0
30.0
172010.0012.8
Yes
No
1630
20.00
C a n . G e o t e c h . J .
D o w n l o a d e d f r o m w w w .n r c r e s e a r c h p r e s s .c o m b y S h i j i a z h u a n g R a i l w a y C o l l e g e o n 03/01/13F o r p e r s o n a l u s e o n l y .
Zhang et al.615
nished, or was installed at a high elevation to decrease its length and the earth pressure acting on it. To protect the slope against the storm pond, an asphalt-coated membrane was applied to the slope surface below the culvert elevation. An additional layer of soil was then compacted at the surface to protect the membrane.
In the centrifuge model tests, most of the consolidation set-tlement occurred within 4.05 years after the end of construc-tion. From the local practice, it was found that an embankment is not likely to crack due to differential settle-ment if the maximum settlement is not larger than 1.0% of the height. On the other hand, an embankment is most likely to crack if its maximum settlement exceeds 3.0% of its height. Therefore, it would be favourable if the maximum settlement is controlled within 1.0% of the height. According to the measurements in Table 6, the settlement of a 30 m high embankment was within 1.0% of its height, even when the zone with degree of compaction K = 0.85 made up two-thirds of the total height. For the 63.8 m high uniform embankments (Table 3), the degree of compaction should be no less than K = 0.90 to satisfy the criterion. In the density combinations with looser fills as shown in Tables 4 and 5,even the one with the top two thirds compacted to K = 0.90could not make the settlement to height ratio less than 1.0%.
Fig. 3. Typical settlement and horizontal-displacement contours (in centimetres) measured by the markers in model series M-7. (a , b ) No storm water pond. (c –h ) Upstream pond water depth of 23.4 m.
C a n . G e o t e c h . J .
D o w n l o a d e d f r o m w w w .n r c r e s e a r c h p r e s s .c o m b y S h i j i a z h u a n g R a i l w a y C o l l e g e o n 03/01/13F o r p e r s o n a l u s e o n l y .
616Can. Geotech. J. Vol. 35, 1998
Table https://www.doczj.com/doc/0718252363.html,parison of embankments compacted uniformly at degrees of compaction of 0.94,0.90,0.85,and 0.80.
Upstream Maximum Degree of Total Time pond Slope Maximum horizontal Test compaction height elapsed depth water settlement displacement
series ρd /ρdmax
(m)Slope (years)(m)infiltration (m)(m)Remarks
M-0
0.94
63.80
1:1.50 4.63No No 0.2600.0331:1.507.1523.4Yes 0.3600.0481:1.208.7023.4Yes 0.4000.0591:1.0010.2523.4Yes 0.6500.0781:0.6313.3523.4Yes 4.200 3.053Failed
M-20.9063.80
1:1.75 4.05No No 0.5740.1491:1.75 5.6023.4No 0.5740.3191:1.507.1523.4No 0.6810.4251:1.208.7023.4No 0.6810.4681:0.7510.2523.4No 0.7110.9151:0.7510.7523.4Yes 0.830—Failed
M-30.8563.80
1:1.75 4.05No No 1.2120.1701:1.75 5.6023.4No 1.4250.8081:1.507.1523.4No 1.6380.915Cracking 1:1.208.7023.4No 1.851 1.042Cracking 1:1.0010.2523.4No 2.425 1.360Cracking
M-40.8563.80
1:1.75 4.05No No 1,3820.1701:1.50 5.60No Yes 1.6300.2131:1.207.15No Yes 1.9140.2341:0.758.70No Yes 2.4040.2341:0.5010.25No Yes 2.8080.298M-90.8063.80
1:1.75 4.05No No 2.4460.2131:1.75 5.6023.4No 3.2330.8081:1.507.1523.4No 3.744 1.0801:1.20
8.70
23.4
No
3.999
1.170
Table https://www.doczj.com/doc/0718252363.html,parisons of embankments compacted with 0.90and 0.85degree of compaction combinations and H =63.8m.
Upstream Maximum Degree of Total Time pond Slope Maximum horizontal Test compaction height elapsed depth water settlement displacement
series ρd /ρdmax
(m)Slope (years)(m)infiltration (m)(m)Remarks
M-1
0.85at top
63.80
1:1.75
4.05No No 0.8510.191two-thirds H ;0.90at 1:1.75
5.60No Yes 1.2760.191bottom one-third H
1:1.507.15No Yes 1.6170.2341:1.208.70No Yes 1.7650.2551:0.7510.25No Yes 1.9780.3831:0.5011.80No Yes 2.1270.3191:0.3013.35No Yes 2.276—Failed
M-5
0.90at top
63.80
1:1.75 4.05No No 1.2550.213one-third H ;0.85at 1:1.75 5.6023.4Yes 1.5530.489bottom two-thirds H
1:1.507.1523.4Yes 1.7650.6591:1.208.7023.4Yes 2.2120.8721:0.7510.2523.4Yes 2.978 1.255Cracking
M-6
0.90at top
63.80
1:1.75 4.05No No 0.7230.149two-thirds H ;0.85at 1:1.75 5.6023.4Yes 0.8510.213bottom one-third H
1:1.507.1523.4Yes 0.9570.3191:1.208.7023.4Yes 1.0640.5961:0.80
10.25
23.4
Yes
1.106
1.234
Failed
C a n . G e o t e c h . J .
D o w n l o a d e d f r o m w w w .n r c r e s e a r c h p r e s s .c o m b y S h i j i a z h u a n g R a i l w a y C o l l e g e o n 03/01/13F o r p e r s o n a l u s e o n l y .
Zhang et al.
617
Table https://www.doczj.com/doc/0718252363.html,parisons of embankments compacted with 0.90and 0.80degree of compaction combinations and H =63.80m.aa
Upstream
Maximum Degree of Total Time pond Slope Maximum horizontal Test compaction height elapsed depth water settlement displacement
series ρd /ρdmax
(m)Slope (years)(m)infiltration (m)(m)Remarks
M-7
0.90at top
63.80
1:1.75 4.05No No 1.1270.191two-thirds H ;
1:1.75 5.6023.4No 1.2980.4250.80at bottom one-third H
1:1.507.1523.4No 1.5320.4681:1.208.7023.4No 1.7020.5531:0.7510.2523.4No 2.3400.936Failed
M-8
0.90at top 63.80
1:1.75 4.05No No 1.4040.255one-third H ;
1:1.75 5.6023.4No 1.6590.4680.80at bottom two-thirds H
1:1.507.1523.4No 1.8080.4681:1.20
8.70
23.4
No
2.106
0.745
Table https://www.doczj.com/doc/0718252363.html,parisons of embankments compacted with 0.90and 0.85degree of compaction combinations and H =30m.
Upstream
Maximum Degree of Total Time pond Slope
Maximum horizontal Test compaction height elapsed depth water settlement displacement
series ρd /ρdmax
(m)Slope (years)(m)infiltration (m)(m)Remarks
M-10
0.90at top two-thirds H ;30.00
1:1.75 4.05No No 0.080.080.85at bottom one third H
1:1.75 5.6012.0No 0.140.241:1.507.1512.0No 0.180.241:1.208.7012.0No 0.180.281:1.0010.1512.0No 0.260.30Cracking
M-10
0.90at top one-third H ;
30.00
1:1.75 4.05No No 0.060.060.85at bottom two-thirds H
1:1.75 5.6012.0No 0.230.141:1.507.1512.0No 0.240.181:1.208.7012.0No 0.260.341:0.80
10.15
12.0
No
0.30
0.40
Fig. 4. Sketch of the protection of a slope by an asphalt-coated mem-brane.
Nevertheless, it is the postconstruction settlement rather than the maximum total settlement that determines the serviceabil-ity of embankments.
Postconstruction settlement
If slope stability of embankments is ensured, the settlement that occurs during construction and the possible minor cracks due to differential settlement are not critical to the service-ability of embankments. The postconstruction settlement that occurs after the completion of embankment or pavement con-struction is then the major concern of designers. If excessive,postconstruction settlement may damage the pavement and subgrades and increase maintenance costs. To compensate for the settlement, extra height and, if required, extra width should be considered.
The total postconstruction settlement consists of two parts:the conventional primary and secondary consolidations, and the collapsive settlement. In the consolidation process, the excess pore-air and pore-water pressures dissipate with time and eventually return to their values prior to loading (Fred-lund and Rahardjo 1993). Upon wetting by rainfall or surface
C a n . G e o t e c h . J .
D o w n l o a d e d f r o m w w w .n r c r e s e a r c h p r e s s .c o m b y S h i j i a z h u a n g R a i l w a y C o l l e g e o n 03/01/13F o r p e r s o n a l u s e o n l y .
618Can. Geotech. J. Vol. 35, 1998
runoff, the silty soil will lose suction and generate collapsive settlement under the overburden pressure. Lawton et al.(1992) reported that sand–silt–clay mixtures tended to exhibit strong collapse potential if their clay fraction was between 10and 40%. In this study, the clay fraction (16%) was in this range and collapsive deformation would be expected.
Obviously, both parts of the postconstruction settlement are difficult to estimate because of the complicated nature of the unsaturated compacted fills. As a result, the compensation fill thickness is usually empirically estimated in practical designs. Centrifuge tests offer a good tool to circumvent the unknown properties in handling unsaturated soils. The settle-ment process can be measured from such models directly. In the following, the postconstruction settlements starting from the end of construction and from one-half year after the end of construction were estimated based on the displacement transducer records and the marker measurements. The latter were considered because pavement works usually would not start until after one-half year of consolidation.
A hyperbolic relation was used to simulate the crest settle-ment process of the centrifuge models:t
[1]
S t =
S ? + t
where S is the total settlement, t is the time from the begin-ning of construction, S t is the settlement at time t , and ? is a coefficient that can be calculated using the settlements at two points on a monitored curve by the displacement transducers:t 1t 2(S 2 – S 1)[2]
? =
S 1t 2 – S 2t 1
where S 1 and S 2 are the settlements at times t 1 and t 2, respec-tively. If the construction period is t c years, then the postcon-struction settlement starting from the end of construction (S c )is
?
[3]
S c =
S ? + t c
and the postconstruction settlement starting from one-half year after the end of construction (S c ′) is
?
[4]S c ′ =
S ? + t c + 0.5In the centrifuge tests, the time required to reach 212.7g test acceleration was 13 min, and the equivalent prototype construction period was approximately 0.307 years. Pre-sented in Table 7 are the results of postconstruction settle-ment of 63.8 m high embankments starting from the end of construction, and one-half year after the end of construction.The embankments compacted to K = 0.90 or 0.94 yielded very small postconstruction settlements and met the design requirements well. If an embankment was compacted to K =0.80 as in M-9, however, the postconstruction settlement would be excessive. Even the replacement of the top two-thirds zone to K = 0.90 degree of compaction (M-7) could not decrease the postconstruction settlement to S c < 0.3 m. Yet,the postconstruction settlement from one-half year after the end of construction could be controlled below 0.2 m in this way. Similar situations occurred when the whole embank-ment was compacted to K = 0.85 (M-4), or the top one-third was compacted to K = 0.90 but the bottom two-thirds was compacted only to K = 0.85 (M-5). These two combinations were widely practiced in the 1960s and 1970s for many unclassified roads. In both combinations M-1 and M-6, the postconstruction settlements after the end of construction were less than 0.3 m, and the postconstruction settlements from one-half year after the end of construction were all less than 0.2 m. In the light of construction feasibility, the combi-nation M-6 offers a viable choice. In this combination, the top two-thirds of the embankment was compacted to K = 0.90while the bottom one-third was compacted only to K = 0.85,
Table 7.Expected postconstruction settlements starting from the end of construction and 0.5years after the end of construction.
Postconstruction settlement
Postconstruction settlement
at 1:1.50slope (m)
at 1:1.20slope (m)
———————————————
———————————–————
From 0.5years
From 0.5years
Test Height From the end after the end From the end after the end series (m)Degree of compaction of construction of construction
of construction of construction
M-063.800.940.031
0.0120.038
0.016M-263.800.900.0590.0240.0650.027M-463.800.850.4270.1910.5310.241M-963.800.80
1.4850.729 1.7140.858M-163.800.85at top two-thirds H ;0.2250.091
0.2630.1130.90at bottom one-third H M-563.800.90at top one-third H ;0.3600.1560.4940.2170.85bottom two-thirds H M-663.800.90at top two-thirds H ;0.1420.0590.1740.0730.85at bottom one-third H M-763.800.90at top two-thirds H ;0.3210.1400.3730.1720.80at bottom one-third H M-8
63.80
0.90at top one-third H ;
0.562
0.261
0.711
0.335
0.80at bottom two-thirds H
C a n . G e o t e c h . J .
D o w n l o a d e d f r o m w w w .n r c r e s e a r c h p r e s s .c o m b y S h i j i a z h u a n g R a i l w a y C o l l e g e o n 03/01/13F o r p e r s o n a l u s e o n l y .
Zhang et al.
619
which could be achieved by hand-operated equipment if com-paction machinery was not accessible.
The postconstruction settlements of the 30 m high embank-ments were not analysed here, since even the total crest settle-ment did not exceed 0.3 m and the postconstruction settlement would be much smaller. In this case, the density combination with the bottom two thirds compacted to K =0.85 and the top one third compacted to K = 0.90 offered an economical choice.
In all the tests described above, large collapsive displace-ments were observed at all densities of slopes in the upstream impounded areas. It suggested that wetting-induced deforma-tions still develop even if the loess fills were compacted at the optimum moisture content ?op . This was different from the clayey sand in the study carried out by Lawton et al. (1989),where collapse and swelling could be eliminated for all over-burden pressure by setting the compaction moisture to ? ??op . Therefore, in addition to compaction, the surface drain-age measures for loess slopes are also very important to pre-vent the surface soil from excessive water infiltration.
In the empirical determination of the compensation fill thickness in the loess area concerned, the compensation fill is usually taken as 1.0–1.5% of the embankment height H . More specifically, Zuo (1988) suggested that the thickness of the compensation fill be 1–2% of H if the degrees of compaction of the surface course, base course, and embankment fill are no less than 0.95, 0.90, and 0.85, respectively; 0.5–1.0% of H if the degrees of compaction are larger than 0.95, 0.90, and 0.90,respectively; and 0.1–0.5% of H if the degrees of compaction are larger than 0.97, 0.95, and 0.90, respectively. Practices in recent years prove that this empirical method is not appropri-ate. One or two years after completion, elevated areas were found in the middle of many high embankments along the road axis due to the design compensation fill thickness being much larger than the actual settlement in these areas (Zhang et al. 1993). Recent highway and railway practices utilize more stringent settlement control. For instance, the postconstruction settlement after pavement or rail works have been completed should not be more than 0.2–0.3 m, whatever the embankment height or foundation soils. Accordingly, the compensation fill thickness should not be more than 0.2–0.3 m or a specified permissible postconstruction settlement. As demonstrated in Table 7, this requirement can be achieved for 63.8 m high embankments by either a sound compaction (K ? 0.90 for the
whole cross section) or a zoned compaction and a delay in pavement work (i.e., one-half year consolidation period).Slope stability
The rotational equilibrium analysis of multilayer embank-ment (REAME) program developed by Huang (1982) was employed to calculate the safety factor of the test embank-ments. This program is based on the simplified Bishop method and assumes cylindrical failures that fit well the field observations of loess slopes and centrifuge test results.
Shown in Fig. 5 are the possible working situations created by the centrifuge models. If slopes are flushed, the surface layer would be considered saturated and the saturated soil parameters would apply. During the tests, the average thick-ness of saturated surface soil layers was measured as within 10 mm in the models or 2.13 m in the prototypes. If storm water was retained upstream of the embankments, the phreatic table in the embankments was simplified as a straight line from the upstream water table to the downstream toe. The total stress method was used in the stability analysis because of the complicated nature of the unsaturated soil zones involved. The soil parameters at natural moisture contents were used for the unsaturated fill zones II and III (see Fig. 5),and saturated soil parameters were used for the zone beneath the phreatic table (zone IV) and the surface saturated zone (zone I) by rainfall infiltration. This analysis avoided the measurements of pore-air and pore-water pressures required for an effective analysis for the unsaturated zones. The peak strength parameters for different densities of saturated and unsaturated soils are listed in Table 1.
For stability evaluation of the loess embankments, it is required that the safety factor F s of a slope under a basic load combination be no less than 1.25 by the Chinese high-way construction specifications (China Department of Trans-portation 1987). Presented in Table 8 are the results of slope stability analysis of some 63.8 m high embankments.According to this table, a 1:1.50 slope would be sufficient to satisfy F s ? 1.25, even if soils with ?d = 1530 kg/m 3 (K =0.80) were used solely (model M-9) or as most of the fill material (model M-8), and a 1:1.2 slope would suffice if soils with ?d = 1630 kg/m 3 (K = 0.85) were used primarily (models M-3 and M-5). Furthermore, the 30 m high embank-ments in models M-10 and M-11 would still be stable (F s ?1.25) at a slope of 1:0.5 if their slopes were not flushed.
Fig. 5. Sketch of the slope stability analysis.
C a n . G e o t e c h . J .
D o w n l o a d e d f r o m w w w .n r c r e s e a r c h p r e s s .c o m b y S h i j i a z h u a n g R a i l w a y C o l l e g e o n 03/01/13F o r p e r s o n a l u s e o n l y .
620
Can. Geotech. J. Vol. 35, 1998
The program REAME also correctly predicted the failures that occurred in the centrifuge tests. In model M-2, built uni-formly with ?d = 1720 kg/m 3 (K = 0.90) soil, the 63.8 m high embankment with a 1:0.75 slope was stable when it retained a 23.4 m deep upstream storm water pond, but it failed when the slopes were infiltrated with 300 mm of precipitation water, for which the calculated safety factor was F s = 0.938 or <1.0. Model M-7, in which the top two-thirds of the 63.8 m high embankment was compacted to K = 0.90 and the bottom one-third was compacted to K = 0.80, also failed at a 1:0.75slope. It retained the same upstream storm water pond as in M-2, but the slope was not flushed. The calculated safety fac-tor was F s = 0.998. Note that there were always local failures at the upstream slopes immersed in storm water ponds. These failures were due to the collapse of soils upon wetting, not the general loss of stability of slopes.
As discussed above, stability of slopes is usually not a major concern in loess embankment designs, since compacted loesses typically have high apparent cohesion. In the area mentioned, many high embankments were built to very steep slopes and were longstanding. For instance, special “loess bridges,” embankments with slopes steeper than 1:0.5 slopes,were widely used before 1960 as structures to cross gullies.With the introduction of modern compaction equipment, soils were compacted more densely and embankments were proven stable at even steeper slopes. Consequently, the designs of slopes in this region were determined primarily on the feasibility of construction machinery operation and long-term maintenance needs. The new designs typically adopt slopes no steeper than 1:1.5.
Limitations of the test results
It is worthwhile to note that the centrifuge test methods employed in this paper do not simulate the real stress paths that occur in site construction. In the field, embankments are built layer by layer, and a stress path at any point moves incrementally as the embankment gets higher. In the centri-fuge tests, all the layers were stressed simultaneously when the model was accelerated to the test acceleration, although
the model embankments were also built in multilayers. To model the field conditions realistically, a device for in-flight,step by step construction is necessary. Currently, in-flight construction modelling techniques using sand hoppers (Ran-dolph et al. 1991; Allersma 1994; Allard et al. 1994) and hydraulic filling (Almeida et al. 1985) have been developed.However, in-flight construction modelling devices for cohe-sive soil compaction are not yet available. Better simulation can be obtained after this technique is developed. Also, sprin-kling of water for simulating precipitation should also be car-ried out in flight in future tests to avoid the difference in moisture redistribution at 1g and over 200g .
Presently, the theories for centrifuge modelling of unsatur-ated soils are not yet fully developed. The time-scaling law employed in this paper is of an approximate nature and needs further sophistication in the future.
On the other hand, the present results obtained are on the safe side: they overestimate the settlements at embankment crests but realistically represent the postconstruction consoli-dation and wetting compression processes.
Conclusions
Loess embankments of nine density combinations were simu-lated in a centrifuge. The embankments concerned were 63.8and 30 m high, with slopes varied from 1:1.75 to 1:0.3. The most unfavourable working conditions were modelled. A number of conclusions can be drawn based on the test results:(1) In the case of inaccessible ramps to work sites, a prop-erly designed high loess embankment is able to perform satis-factorily with the lower part compacted more loosely than the conventional code requirements.
(2) In terms of total crest settlement and embankment cracking, the degree of compaction K of loess fills should not be less than 0.85 for 30 m high embankments and 0.90 for 63.8 m high embankments.
(3) In terms of the postconstruction settlement (<0.2–0.3m) and construction feasibility of 63.8 m high embank-ments, it is expedient to compact the lower one third of the embankments to K = 0.85, and the top two thirds to K = 0.90.
Table 8.Results of slope stability analysis using REAME (Huang 1982).aa Safety factor
Upstream ——————–————————————Test Height pond depth
series (m)Degree of compaction
(m)1:1.751:1.501:1.201:0.75M-263.80.90
23.4 1.69 1.63 1.44 1.16———0.94a M-363.80.85
23.4 1.61 1.54 1.36—M-5
63.80.90at top one-third H ;
23.4 1.61 1.53 1.36 1.080.85at bottom two-thirds H M-763.80.90at top two-thirds H ;23.4 1.36 1.54 1.21 1.00b 0.80at bottom one-third H M-863.80.90at top one-third H ;
23.4 1.33 1.31 1.18—0.80at bottom two-thirds H M-9
63.8
0.80
23.4
1.34
1.31
1.19
—
a The slopes were also infiltrated by 300mm precipitation and the embankment failed.b
The test embankment failed.
C a n . G e o t e c h . J .
D o w n l o a d e d f r o m w w w .n r c r e s e a r c h p r e s s .c o m b y S h i j i a z h u a n g R a i l w a y C o l l e g e o n 03/01/13F o r p e r s o n a l u s e o n l y .
Zhang et al.621
For embankments with 30 m height, the bottom zone with K = 0.85 can comprise two-thirds of the height, and the top K = 0.90 zone in turn covers only one-third of the height.(4) Accordingly, the design compensation fill thickness should be equal to the estimated postconstruction settlement,rather than a fixed percentage of embankment height used in the empirical method.
(5) Loess embankments of various densities were observed to be susceptible to wetting compression even if they were compacted at the optimum water content. Surface-drainage measures, in addition to compaction measures, are very impor-tant to prevent excessive water infiltration.
(6) The slopes of loess embankments are determined pri-marily by the construction equipment requirements and main-tenance needs, rather than the stability of slopes.
Acknowledgments
This project was sponsored by the Gansu Provincial Depart-ment of Transportation (GDOT), China. Technical assistance from Professor Xu Xiaoqian, the GDOT chief engineer, is gratefully acknowledged. The first author would also like to thank Dr. M.C. McVay and Dr. F.C. Townsend of the Uni-versity of Florida for encouragement, and Ms. Sharon Oliver who patiently checked the manuscript for English usage. The important input from the reviewers is also acknowledged.
References
AASHTO. 1986a . Standard specification for materials for embank-ments and subgrades, designation M57-80. In Standard specifica-tions for transportation materials and methods of sampling and testing. Part I: Specifications. 14th ed. American Association of State Highway and Transportation Officials (AASHTO), Wash-ington, D.C., pp. 72–73.
AASHTO. 1986b . Standard methods of test for the moisture-density relations of soils using a 2.5 kg rammer and a 305 mm drop,designation T-99. In Standard specifications for transportation materials and methods of sampling and testing. Part II: Methods of sampling and testing. 14th ed. American Association of State Highway and Transportation Officials, Washington, D.C., pp.326–333.
Allard, M.A., Hjortnas-Pedersen, A.G.I., and Out, J.G. 1994. Perfor-mance of a sand hopper. In Proceedings, the International Confer-ence, Centrifuge ’94, Singapore, 31 August – 2 September 1994.Edited by C.F. Leung, F.H. Lee, and T.S. Tan. A.A. Balkema,Rotterdam, The Netherlands, pp. 83–88.
Allersma, H.G.B. 1994. The University of Delft geotechnical centri-fuge. In Proceedings, the International Conference, Centrifuge ’94, Singapore, 31 August – 2 September 1994. Edited by C.F.Leung, F.H. Lee, and T.S. Tan. A.A. Balkema, Rotterdam, The Netherlands, pp. 47–52.
Almeida, M.S.S., Davies, M.C.R., and Parry, R.H.G. 1985. Centri-fuge tests of embankments on strengthened and unstrengthened clay foundations. Géotechnique, 35(4): 425–441.
Arulanandan, K., Thompson, Y .P., Kutter, B.L., Neegoda, N.J.,Muraleetharan, K.K., and Yogachandran, C. 1988. Centrifuge modelling of transport processes for pollutants in soils. Journal of Geotechnical Engineering, ASCE, 114(2): 185–205.
Atkinson, J.H., and Taylor, R.N. 1994. Moisture migration and sta-
bility of iron ore concentrate cargoes. In Proceedings, the Interna-tional Conference, Centrifuge ’94, Singapore, 31 August – 2September 1994. Edited by C.F. Leung, F.H. Lee, and T.S. Tan.A.A. Balkema, Rotterdam, The Netherlands, pp. 417–422.
Behbahani, H. 1974. Current methods and specifications for com-paction of embankment and subgrade in the United States. M.S.thesis, University of Florida, Gainesville, Fla.
China Department of Transportation. 1987. Chinese specifications for highway construction JTJ33-86. People’s Transportation Press, Beijing, China.
Cook, B., and Mitchell, R.J. 1991. Physical modelling of a dissolved contaminant in a unsaturated sand. Canadian Geotechnical Jour-nal, 28: 829–833.
Fredlund, D.G., and Rahardjo, H. 1993. Soil mechanics for unsatur-ated soils. John Wiley & Sons, Inc., New York, pp. 419–439.Hellawell, E.E., and Savvidou, C. 1994. A study of contaminant transport involving density driven flow and hydrodynamic clean up. In Proceedings, the International Conference, Centrifuge ’94,Singapore, 31 August – 2 September 1994. Edited by C.F. Leung,F.H. Lee, and T.S. Tan. A.A Balkema, Rotterdam, The Nether-lands, pp. 357–362.
Huang, Y .H. 1982. Stability analysis of earth slopes. V an Nostrand Reinhold Company, New York, pp. 145–193.
Knight, K., and Dehlen, G. 1963. Failure of a road constructed on a collapsing soil. Proceedings, 3rd Regional Conference for Africa on Soil Mechanics and Foundation Engineering, V ol. 1, pp. 31–33.
Kropp, A.L., McMahon, D.J., and Houston, S.L. 1994. Case history of a collapsible soil fill. In Vertical and horizontal deformations of foundations and embankments. Edited by A.T. Yeung and G.Y .Felio. ASCE Special Publication 40, V ol. 2, pp. 1531–1542.
Lawton, E.C., Fragaszy, R.J., and Hardcastle, J.H. 1989. Collapse of compacted clayey sand. Journal of Geotechnical Engineering,ASCE, 115(9): 1252–1267.
Lawton, E.C., Fragaszy, R.J., and Hetherington, M.D. 1992. Review of wetting-induced collapse in compacted soil. Journal of Geo-technical Engineering, ASCE, 118(9): 1376–1394.
Mitchell, R.J., and Stratton, B.C. 1994. LNAPL penetration into porus media. In Proceedings, the International Conference, Cen-trifuge ’94, Singapore, 31 August – 2 September 1994. Edited by C.F. Leung, F.H. Lee, and T.S. Tan. A.A. Balkema, Rotterdam,The Netherlands, pp. 345–349.
Noorany, I., and Stanley, J.V . 1994. Settlement of compacted fills caused by wetting. In Vertical and horizontal deformations of foundations and embankments. Edited by A.T. Yeung and Y .G.Felio. ASCE Special Publication 40, V ol. 2, pp. 1516–1530.
Randolph, M.F., Jewell, R.J., Stone, K.J.L., and Brown, T.A. 1991.Establishing a new centrifuge facility. In Proceedings, the Interna-tional Conference, Centrifuge ’91, Boulder, Colo., 13–14 June 1991. Edited by H.Y . Ko and F.G. McLean. A.A. Balkema, Rot-terdam, The Netherlands, pp. 3–10.
Vicente, E.E., Diaz, G.M., and Yourman, A.M. 1994. Settlement of deep compacted fills in California. In Vertical and horizontal deformations of foundations and embankments. Edited by A.T.Yeung and Y .G. Felio. ASCE Special Publication 40, V ol. 2, pp.1124–1134.
Zhang, L., Huang, Y ., Xiao, X., and Hu, T. 1993. Highway embank-ments and structures in Gansu loess region. Final Report No. 85-05-01-14 prepared for the Gansu Department of Transportation (GDOT) by Sichuan Union University, Chengdu, China.
Zuo, Z. 1988. The settlement of fills. (In Chinese.) Railway Bed Engineering, 21(6): 15–20.
C a n . G e o t e c h . J .
D o w n l o a d e d f r o m w w w .n r c r e s e a r c h p r e s s .c o m b y S h i j i a z h u a n g R a i l w a y C o l l e g e o n 03/01/13F o r p e r s o n a l u s e o n l y .