- Email: [email protected]
Landslides and residual strength in marl profiles in Israel
Engineering Geology 89 (2007) 36 – 46 www.elsevier.com/locate/enggeo
Landslides and residual strength in marl profiles in Israel Sam Frydman a,⁎, Mark Talesnick a , Samuel Geffen b , Asia Shvarzman c a
Department of Structural Engineering and Construction Management, Faculty of Civil and Environmental Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel b Samuel Geffen - Soils Engineering Ltd., Kfar Saba, Israel c Faculty of Civil Engineering, Negev Academic College of Engineering, Beer-Sheva, Israel Received 13 March 2006; received in revised form 27 July 2006; accepted 6 September 2006 Available online 1 November 2006
Abstract Many areas in northern Israel show evidence of old landslides in marly profiles, and new slides are often activated in these regions. Such a slide occurred in the town of Rechasim, near Haifa, during an exceptionally wet winter in 1992. The present paper analyzes the causes for this slide, and suggests that it was due to a combination of downslope flow in the overlying, severely cracked chalk–limestone layer, and mobilization of residual strength at the marl–chalk interface. The residual strength of Israeli marly soils has been studied, and it is shown that while many of these consist predominantly of clay-sized (b2 μm) particles, and may, therefore, expect to have a very low residual friction angle, the amount of clay sized carbonate in the soil is of major influence on the residual friction angle. When the clay-sized material includes less than about 11% carbonate, the clay particles dominate the shearing mechanism and result in a residual friction angle of the order of 12°; this was the case at Rechasim. However, when more than about 30% of the clay-sized material is carbonate, the carbonates control the shearing mechanism, and residual friction angles can be as high as 30°. Results of a preliminary study of the change in residual strength with time are also presented. © 2006 Elsevier B.V. All rights reserved. Keywords: Landslides; Marl; Residual strength
1. Introduction During the winter of 1991–92 in Israel, record rainfall fell, corresponding to about twice the yearly average since local recording began. During this period, a landslide occurred at the construction site for a new residential area in the town of Rechasim, close to Haifa, in the north of Israel. Work being carried out at the site included two independent projects — one at the top of the slope, where 6 buildings were in the process of construction, and the second, at the base of the slope, ⁎ Corresponding author. Tel.: +972 4 8293320; fax: +972 4 8293323. E-mail address: [email protected] (S. Frydman). 0013-7952/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2006.09.009
where pre-construction excavations had been carried out. Fig. 1 shows the area several days after the slide. The large cracks forming the main landslide scarp can be seen passing through the two left-most buildings, and additional cracks are observed passing through the third building. Fig. 2 shows a closer view of some of the damage in the upper project site, with the slide surface passing through the second left-most building and the partial collapse of the building itself. In order to stabilize the landslide, a large portion of the excavated area was refilled, following which slip movements appeared to effectively cease. The three leftmost buildings were irrecoverably damaged, and had to be destroyed, but it was hoped that the other three
S. Frydman et al. / Engineering Geology 89 (2007) 36–46
37
Fig. 1. Aerial photograph of Rechasim landslide area, and locations of borings.
buildings could be occupied. Five months after the landslide, an investigation was initiated in order to study the causes for the landslide, and the stability of the remaining three buildings. In the progress of this study, attention became focused on the residual strength properties of the marls which formed a dominant part of the geological profile at the site. In view of previous cases of slope instabilities which have occurred in marl profiles in Israel (e.g. Wiseman et al., 1970), the Ministry of Housing and Construction extended the scope of the investigation in order to better understand the residual strength of Israeli marls and clays. The present paper presents results of this investigation, and applies these to help explain the slide at Rechasim.
2. The slide at Rechasim In July of 1992, five months after occurrence of the slide, three borings (1,2,3) were carried out at the site, at locations indicated in Fig. 1. Inclinometer tubes were installed in borings 1 and 2. Fig. 3 shows the geotechnical profile at the site following refilling, as indicated by the borings and geodetic measurements. The top 10–14 m is comprised of severely cracked chalk and limestone, and this is underlain by marl (a mixture of clay minerals and calcium carbonate (calcite), each accounting for not less than 25% by weight). The soil surface, and the layer interface are almost parallel, and slope at about 15° to the
Fig. 2. Structural damage and soil cracking.
38
S. Frydman et al. / Engineering Geology 89 (2007) 36–46
Fig. 3. Assumed cross-section of the slope and subsurface profile, based on borings.
horizontal. Fig. 4 shows moisture contents and consistency limits measured at the time of the site investigation. As the borings were performed in July, the profile had dried out from its wet, winter condition, but a zone of high moisture contents was still observed at the top of the marl. The marl layer is of very high plasticity at its top (liquid limit above 90), decreasing in plasticity with depth. The clay mineral of the marl at this site, as for most Israeli clay (Kassiff et al., 1969), is predominantly montmorillonite. The consistency limits indicate that the marl consists mainly of clay at the top of the layer, while the calcite content increases with depth. Inclinometer measurements began on 22 July. Measurements in boring 1 are shown in Fig. 5, and similar results were obtained in boring 2. Fig. 5 shows incremental displacements and accumulative movement from the bottom of the inclinometer tube to its top. It is seen that even
though measurements began about 5 months after refilling the excavation, when it appeared that movement had ceased, slow, continuous movement actually continued, with the top, chalk layer moving as a more or less solid block on top of the marl layer, at a depth of 14 m. Measurements continued in boring 1 until March 1997, by which time a movement of about 10 mm had developed. Attempts to continue measurements after this date were unsuccessful, as it was found impossible to lower the inclinometer probe below a depth of 14 m. Apparently the tube had bent excessively or broken at this depth, preventing any further penetration of the probe. An estimate of the development of shear strain with time at 14 m depth was made by dividing the incremental displacement by the incremental measurement length at this depth, as shown in Fig. 6. It is seen that shear strain was continuing more or less linearly, at a rate of about 0.5%/year, when measurements were discontinued, indicating a continuous, creep movement of the slope, with no sign of stabilization, during the 5 year measurement period. This creep was evident, also, from cracks which continued to reappear in the buildings following repair works. 3. Evaluation of the instability at Rechasim
Fig. 4. Variations of moisture content and consistency limits with depth.
The inclinometer data indicate that the slide occurred at the top of the marl layer, probably at the interface with the chalk–limestone layer. On first consideration, this appears strange, considering the mild interface slope of about 15°. Saturated Israeli clays, including marls, have been found to have an effective friction angle, ϕ′, of at least 25° and a small effective cohesion (Frydman, 2000). The slide appears to have resulted from a combination of the heavy rainfall, and excavation of the slope, exposing the chalk–marl interface at the excavation base (see Fig. 3), and removing support for the upper chalk layer. The heavy rain led to saturation of the cracked chalk–limestone layer, and the top of the marl. Due to the very low permeability of the marl, there would be expected to be very limited penetration of rain water down into the marl. The top surface of the layer
S. Frydman et al. / Engineering Geology 89 (2007) 36–46
39
Fig. 5. Inclinometer measurements in boring 1.
would be likely to have reached saturation, and the excess water presumably flowed down the slope, within the cracks of the chalk–limestone. Two processes, individually or together, appear to have contributed to the onset of the slide: (a) Development of saturated flow in the upper, cracked layer, and resulting pore pressure at the top of the marl layer. For such flow, more or less parallel to the chalk–marl interface, the factor of safety, F, may be estimated for infinite slope or sliding block conditions, as: F ¼ ðgsub =gt Þðtg/ V=tgbÞ where γsub and γt are the submerged and total unit weights, respectively, of the saturated upper cracked layer,
ϕ′ is the effective friction angle of the saturated marl, and β is the inclination of the interface. In the present case, assuming ϕ′ = 25° and β = 15°, a factor of safety, F, slightly above 1 results. Evidence of flow in the cracked upper layer was observed at the site, with water streaming out of the excavated face. However, the flow did not appear to be occurring over the full thickness of the layer, and flow, alone, would therefore not explain the slide. (b) Geological investigations of the area and study of existing geological maps following the slide (Saltzman, personal communication) indicated that the area is an old landslide area, with evidence of old slide scarps. It would, then, be likely that the operational strength parameters of the saturated marl are the residual, rather than the peak, values (Skempton, 1985). Consequently, a study of the residual strength of the in-situ marl was initiated. 4. Residual strength of Israeli clays and marls
Fig. 6. Development of shear strain with time at the marl–chalk layer interface.
Lupini et al. (1981) and Skempton (1964, 1985) presented a conceptual model for the development of residual strength conditions, in which it is suggested that the major cause for the decrease in strength to a residual value, at large shear deformation, is the increased parallel orientation of platy, clay particles located along the slip surface. They further suggested that at a residual
40
S. Frydman et al. / Engineering Geology 89 (2007) 36–46
indicated that approximately 65% of the material is clay sized, and according to Fig. 7, the residual friction angle of this material would be expected to be of the order of 12°. Consequently, the slide which occurred at Rechasim may be explained by assuming that the operational strength at the top of the marl layer corresponded to the residual, rather than the peak condition. However, as will be shown below, it appears that Fig. 7 may not be universally applicable to all soils. Further discussion of the residual strength of Israeli marls in general, and of the Rechasim marl in particular, is presented below. As a result of the slide at Rechasim, and of other landslides and instability phenomena which have occurred previously in marl slopes in the Galilee (e.g. Wiseman et al., 1970), the Israel Ministry of Housing and Construction initiated a research investigation into the residual strength of Israeli clays and marls (Shvarzman, 1995). A number of natural and artificial soils were tested; properties are listed in Table 1. Soils 1–8 are natural soils; soil 2 was from Rechasim, from the top of the marl layer, at a depth of 14 m. Soil 9, labelled “clay” was obtained from the clay-size fraction (− 2 μm) of Mizra soil (soil 1), by sedimentation. Soils 10–13 (soils “A”–“D”) were prepared by mixing this clay with various amounts of laboratory grade CaCO3, of particle size 2–5 μm. Residual strength is generally experimentally evaluated in either ring shear tests, or reversal direct shear tests. Both procedures were employed in this investigation. As has been demonstrated (e.g. Bishop et al., 1971), residual strength is independent of initial soil structure, and so remolded samples were tested. The procedure employed for the reversal direct shear tests involved inserting soil at about the liquid limit into the direct shear ring, applying
Fig. 7. Residual friction angle, ϕ′r versus percent clay size fraction (b2 μm), (after Skempton, 1985) (Printed with permission of ICE).
condition, the shear strength could be expressed by a linear Mohr Coulomb envelope, in terms of residual parameters cr and ϕr, and that, in general, the residual cohesion, cr is very small and can be neglected. The residual shear strength was therefore expressed by the equation: sr = σ′ tanϕr. The above authors found that for a large range of clays, ϕr could be related to the percent clay sized material (smaller than 2 μm) in the soil. They suggested that for soils with predominantly clay sized (i.e. platy) material, shear deformation along a slip surface takes place mainly in a sliding mode, whereas if the particles are predominantly rotund, the deformation mode is turbulent. At intermediate clay-size percentages, a transitional deformation mode was hypothesized. This model has become widely accepted, and their relationship between ϕr and percent clay-size fraction (Fig. 7) is often used for estimating the residual friction angles of soils. Hydrometer tests on samples taken from the top of the marl layer at Rechasim Table 1 Soil properties Soil Soil source no.
Clay minerals Quartz % Liquid Plasticity CaCO3 CaCO3 in CaCO3 in Sand Silt Clay (montmorillonite) % limit % index % % −0.074 mm − 2 μm size % size % size % fraction % fraction %
1 2 3 4 5 6 7 8 9 10 11 12 13
45–55
20–25 5
25–35 15–20 45–55
15 5 25–30
Mizra Rechasim B-2 Beit Shemesh B-13 Beit Shemesh B-90 Rechasim Ramat David Migdal Haemek Bikah Clay Soil “A” Soil “B” Soil “C” Soil “D”
66 101 75 61 66 73 57 58
40 64 53 39 36 45 25 26
10 43 20 38 55 15 37 44 21 22 44 100
9 26 14 19 55 12 32 32.5 20 21 43 100
3 7 11 16 32 3 29 29 3 20 21 50 100
1.5 5.7 3 2.1 2.4 5.6 10.1 1.2
36.8 28.9 14.5 21.4 21.1 30.4 32.7 34.0
1.3 1.3 1
32.6 32.2 23
61.7 65.4 82.5 76.5 76.5 64 57.2 64.8 100 66.1 66.5 76 100
S. Frydman et al. / Engineering Geology 89 (2007) 36–46
41
consolidation in stages, cutting along the proposed shear surface with a fine cheese wire, and then shearing. Shear was applied to a relative displacement of about 6 mm and then reversed and sheared back and forth to a displacement of about 10 mm. The reversal procedure was repeated until a constant shear resistance was achieved. Displacement rates were varied between 0.213 mm/min and 0.000068 mm/min, and the rate was found to have no influence on the resulting residual shear strength. Ring shear tests were carried out in a Bromhead apparatus. In order to minimize friction resulting from top cap settlement into the specimen ring during shear, the “flush” test procedure (Stark and Vettel, 1992) was employed To reduce consolidation settlement, samples were prepared at, or slightly below, the plastic limit. Shearing was carried out at a rate of 0.024 deg/min under normal stresses in the range 25–200 kPa. Several of the tests were performed as multi-stage tests. Overall, similar results were obtained from the ring shear and the reversal direct shear procedures. Figs. 8 and 9 show typical results from ring shear and reversal direct shear tests respectively, on Rechasim B-2 soil. The residual friction angle, ϕr, (defined as arctan sr/σ) as a function of normal stress, σ, for a number of
Fig. 9. Results of reversal direct shear tests, soil 2. (a) Shear stress– shear displacement curves. (b) Residual strength envelope.
Fig. 8. Results of ring shear tests, soil 2. (a) Shear stress–angular displacement curves. (b) Residual strength envelope.
the soils, is shown in Fig. 10. ϕr is seen to decrease with σ at low normal stresses, and then to reach a constant value. This non-uniformity of ϕr implies non-linearity
42
S. Frydman et al. / Engineering Geology 89 (2007) 36–46
Fig. 10. Residual friction angle versus normal stress.
of the residual strength envelope, which has been noted previously (e.g. Kenney, 1977; Skempton, 1985). It appears that as CaCO3 content increases, the envelope becomes more linear. The relationship between ϕr (corresponding to the straight portion of the envelope), and percent clay-size fraction is shown in Fig. 11, in which CaCO3 contents in the clay-size fraction are marked next to each point. No clear relationship is seen between ϕr and percent clay size fraction, but it is evident that the low friction angles correspond to low carbonate contents, while the high friction angles correspond to high carbonate contents. The influence of the carbonate content is shown in Fig. 12a–c. Fig. 12a and b show that there is no clear relationship between ϕr and overall carbonate content or carbonate content in the fine (passing #200 sieve — 0.074 mm) fraction of the soil. Fig. 12c shows a very
Fig. 12. Residual friction angle versus total carbonate content. (a) Overall (b) in fine fraction (c) in clay size fraction.
Fig. 11. Residual friction angle versus percent clay size fraction.
clear relationship between ϕr and carbonate content in the clay-size fraction of the soil. For these soils, the amount of carbonate in the clay-size fraction has a
S. Frydman et al. / Engineering Geology 89 (2007) 36–46
greater influence on ϕr than does the percentage of claysize material itself. It should be noted that for all of these soils, there is about 60% or more of clay-size material, and, according to Fig. 7, the actual amount would not be expected to have any significant influence on ϕr. It appears that the residual friction angle (over the straight portion of the strength envelope) of Israeli clays and marls, with less than about 11% carbonate of clay size, is of the order of 12°, consistent with that expected from Fig. 7. For this range of carbonate content, the shearing mode is, apparently, controlled by the platy, clay particles, and involves a sliding mechanism. As the amount of non-platy carbonate in the clay-size fraction increases up to about 30%, the shear mode goes from sliding to transitional to turbulent, and the value of ϕr increases to about 30°. The above observations of the influence of calcium carbonate on the residual friction angle seem to explain why certain old landslide areas in marly deposits are presently stable, despite the fact that Fig. 7 would predict low residual strength values and consequent instability, while others, such as Rechasim, actually slid, consistent with the residual strength values predicted from Fig. 7. 5. The influence of time on residual strength The above discussion has assumed that the operative shear strength in prefailed slopes is the residual strength, as measured in the laboratory. Such an approach to shear strength on old slide surfaces assumes that no strength recovery occurs during the (often very) considerable time period which has elapsed since development of the slip surface. Thixotropic strength increase in remolded and compacted clays has been previously reported by numerous researchers (e.g. Moretto, 1948; Skempton and Northey, 1952; Kruyt, 1952; Mitchell, 1960; Amir, 1961). These investigations studied cases in which compacted or remolded soil specimens were allowed to age for various periods before shearing. Gibo et al. (2002) reported tests in which two soils were first sheared to residual conditions in a ring-shear apparatus, then reconsolidated for two days without allowing further shear displacements, and finally re-sheared. They found that for the soil containing large amounts of silt and sand and only 10% clay-size particles, significant strength recovery was observed for low normal stresses (below 100 kPa). On the other hand, for the soil containing 73% clay-size particles which were predominantly montmorillonite, negligible strength recovery was observed. Although the authors didn't suggest so, it is possible that this lack of strength recovery may have
43
been due to the short waiting period between initial shearing and re-shearing stages. As part of the present study of residual strength of Israeli soils, limited testing was carried out in order to obtain an initial indication of the time effect, using the following testing procedure. A remolded test sample was prepared by mixing the dry soil with distilled water to its liquid limit, and then consolidating it for 24 h under a normal stress of 100 kPa. With the normal, consolidation stress held constant, the submerged sample was cut along its potential shear plane, and sheared in direct shear using the stress-reversal technique, until the residual condition was developed. At this stage, in order not to tie up direct shear devices for long periods of time, the sample was quickly unloaded, removed from the shear device, and transferred (still in the direct shear rings) to a compression device where it was submerged and subjected to the same normal stress as that applied during shearing. Following a waiting period of up to five months, it was transferred back to the direct shear device, the normal stress was reapplied and held for 48 h, and the sample was then re-sheared. These transfers of the samples may have had a detrimental effect on their strengths, but could not be avoided due to equipment limitations. Tests with waiting periods of less than 144 h were left in the direct shear device during the whole procedure, and were not transferred to a different compression apparatus. Tests of this nature were performed on soils 1, 2, and 9. Some additional comments regarding this procedure are warranted: (a) Under field conditions, shear stresses may remain on the slip surface throughout the time elapsed between establishment of residual conditions and re-shear, in contrast to the conditions applied to the laboratory samples. These in-situ shear stresses would possibly result in some creep on the slip surface, and the resulting strength on re-shearing may be less than that measured in the present laboratory procedure. Consequently, the results of the tests described here may be considered to be on the high side. (b) It is known (e.g. Anson and Hawkins, 1998) that pore fluid composition has an effect on soil shear strength in general, and residual strength in particular. In the Israeli scenario, the marl slopes of interest are inevitably well above water table, and normally unsaturated, while wetting, accompanied by instability phenomena, is usually a result of heavy rainfall. Consequently, it was considered that the use of distilled water for soaking the samples would be most representative of field conditions. No tests were performed to study possible chemical
44
S. Frydman et al. / Engineering Geology 89 (2007) 36–46
Fig. 13. Re-shearing stress-displacement curve for soil 9, following a waiting period of 720 h.
changes in the samples (e.g. dissolution, precipitation) during the extended waiting periods.
quickly during re-shearing, with strength reverting back to the residual value. Consequently, definition of the strength recovery in terms of the ratio of re-sheared peak to residual strengths may be misleading; the strength increase may be relevant over such a short displacement range that it is insignificant in practical terms. An alternative measure of the strength recovery, which may better describe its effectiveness, is defined here as the recovered energy per unit area of sheared surface, E. This energy is the area of the triangular zone of increased strength on the stress-displacement curve (e.g. DEF in Fig. 13). Fig. 14a shows E as a function of waiting time for the different soils, together with trendlines for soils 1 and 9 only. According to Fig. 14a, the more plastic soil 9 developed lower recovered energy, but its rate of increase was higher than that of soil 1. The result obtained from the test on soil 9 after a waiting period of 4 h showed very small strength
Fig. 13 shows a typical test result, obtained on a sample of soil 9. After the sample was sheared to a residual condition, corresponding to a shear stress of 20–22 kPa (line AB), it was unloaded (line BC), and left under the normal stress for 720 h. It was then re-sheared. It is seen that during the re-shearing, the shear stress developed a new peak value of 35 kPa (point E), but then, with additional shearing, the strength decreased, reverting to the residual value. The displacement range over which strength was above the residual value has been labeled Δ. In this test, the ratio of re-sheared peak strength to residual strength was 1.62; the value of Δ over which re-sheared strength was greater than residual strength was 1 mm. The test results (Table 2) indicate that there is a recovery of shear strength with time following development of residual conditions, which appears to be dependent on the soil properties. It is observed, however, that the strength recovery is dissipated quite Table 2 Results of re-shearing tests following residual conditions Soil
Waiting period (h)
Residual Strength (kPa)
Strength increase peak/resid
Δ (mm)
1 1 1 1 2 9 9 9
12 48 144 3600 720 4 20 720
22 22 22 22 20 21 21 21
1.36 1.45 1.82 2.05 1.6 1.1 1.19 1.62
0.85 0.85 1.12 5.0 1.8 0.1 0.6 1.0
Fig. 14. Recovered energy per unit area as a function of waiting period.
S. Frydman et al. / Engineering Geology 89 (2007) 36–46
recovery, over a very small displacement range, Δ. Consequently, the calculated recovered energy may not be accurate and Fig. 14b shows the relationships between E and waiting time without this result. In this case the lines are reasonably parallel, with the results for soil 9 still lying below those of soil 1. Further investigation is required in order to verify whether there is significance to the features noted in Fig. 14b. According to the data, the soil of higher plasticity (soil 9) would need more time in order to develop the same recovered energy (which may be considered as recovered resistance to shear) as that developed in the less plastic soil over a shorter waiting time. This is consistent with the observation of Gibo et al. (2002) that a low plasticity, silty soil (ML) showed significant strength recovery after a two day waiting period, whereas a highly plastic, montmorillonite clay (CH) showed negligible recovery. On the other hand, it is contrary to the observations which have been made regarding thixotropic strength regain in compacted and remolded soils. For example, Skempton and Northey (1952), who studied thixotropic strength regain in kaolin, illite and bentonite soils, reported, that “kaolin shows almost no thixotropy and illite shows only a small effect. In contrast, the bentonite shows a remarkable regain at very short time intervals and it is not possible to suggest an upper limit for this material since the strength continued to increase throughout the experiment.” Amir (1961) found, from tests on Israeli, montmorillonitic clays, that thixotropic strengthening of highly plastic clay is higher than that of silty clay. Clearly, the subject of strength gain with elapsed time from residual conditions is not yet clear, and requires further investigation and testing of a range of soils. 6. Conclusions The study of a landslide in a 15° slope at Rechasim, near Haifa, has indicated that the slide developed at the interface between the overlying, cracked chalk – limestone layer and the lower, highly plastic marl, more or less parallel to the slope surface. The slide followed exceptionally heavy rainfall, accompanied by execution of an excavation at the base of the slope exposing the marl layer. It appears to have been a result of a combination of downslope flow in the upper cracked layer, and mobilization of residual strength at the top of the marl. A study of residual strength of Israeli clays and marls has shown that it is related to the carbonate content in the clay-size fraction of the soil. A significant portion of the carbonates encountered in the marly soils may be of clay size, and this fine, carbonate
45
material appears to have a major effect on ϕr. The influence of the carbonate content in the material passing the #200 sieve (0.074 mm) on ϕr has been noted previously by Hawkins and McDonald (1992, 1993). In the present investigation, it has been found that a more significant effect may be attributed to the clay-sized carbonates. When the clay-size fraction of the soil contains less than about 11% carbonates, the platy clay particles dominate the shearing mechanism, whereas if more than about 30% are non-platy carbonates, they control the shearing mechanism. The clay sized carbonates also influence the shape of the residual strength envelope; a more curved envelope is obtained for low carbonate contents, and the envelope becomes increasingly linear with increasing amounts of carbonate in the clay-size fraction. Some preliminary findings regarding increase in strength from the residual condition with elapsed time have been presented. The increase in shearing resistance has been defined in terms of the “recovered energy per unit area of sheared surface”. Initial tests indicated that this recovered energy increases linearly with time on a log–log basis, and that it develops less quickly for more plasic soil. However these observations are based on few tests, and further study of this subject is warranted. References Amir, J., 1961. Influence of test duration on the strength of clay. MSc thesis, Technion. Anson, R.W.W., Hawkins, A.B., 1998. The effect of calcium ions in pore water on the residual shear strength of kaolinite and sodium montmorillonite. Geotechnique 48 (6), 787–800. Bishop, A.W., Greens, G.E., Garga, V.K., Andersen, A., 1971. A new ring shear apparatus and application to the measurement of residual strength. Geotechnique 21, 273–328. Frydman, S., 2000. Shear strength of Israeli soils. Israel Journal of Earth-Sciences 49, 55–64. Gibo, S., Egashira, K., Ohtsubo, M., Nakamura, S., 2002. Strength recovery from residual state in reactivated landslides. Geotechnique 52 (9), 683–686. Hawkins, A.B., McDonald, C., 1992. Decalcification and residual shear strength reduction in Fuller's earth clay. Geotechnique 42, 453–464. Hawkins, A.B., McDonald, C., 1993. The influence of granular calcareous particles on the residual shear strength of Fuller's earth clay. Quarterly Journal of Engineering Geology 26, 321–325. Kassiff, G., Livneh, M., Wiseman, G., 1969. Pavements on Expansive Soils. Jerusalem Academic Press. Kenney, T.C., 1977. Residual strength of mineral mixtures. Proc., 9th Intl. Conf. Soil Mech. and Foundn. Engg., Tokyo, vol. 1, pp. 155–160. Kruyt, H., 1952. Colloid Science, Irreversible Systems. Elsevier, NY. Lupini, J.F., Skinner, A.E., Vaughan, P.R., 1981. Drained residual strength of cohesive soils. Geotechnique 31, 181–213. Mitchell, J.K., 1960. Fundamental aspects of thixotropy in soils. Journal of the Soil Mechanics and Foundations Division, ASCE 86 (SM3), 19–53.
46
S. Frydman et al. / Engineering Geology 89 (2007) 36–46
Moretto, O., 1948. Effect of natural hardening on the unconfined compression strength of remoulded clays. Proc., 2nd International Conf. on Soil Mechanics and Foundn Engng., vol. 1, pp. 137–145. Shvarzman, A., 1995. Residual strength of cohesive soils in Israel. MSc thesis, Technion. Skempton, A.W., 1964. Long-term stability of clay slopes. Geotechnique 14, 77–102. Skempton, A.W., 1985. Residual strength of clays in landslides, folded strata and the laboratory. Geotechnique 35, 3–18.
Skempton, A.W., Northey, R.D., 1952. The sensitivity of clays. Geotechnique 3 (1), 30–54. Stark, T.D., Vettel, J.J., 1992. Bromhead ring shear test procedure. Geotechnical Testing Journal 15 (1), 24–32. Wiseman, G., Hayati, G., Frydman, S., Aisenstein, B., David, D., Flexer, A., 1970. A study of a landslide in the Galilee, Israel. Proc. Congress of Intl. Assoc. of Engg. Geology, Paris, vol. 1, pp. 50–61.
Landslides and residual strength in marl profiles in Israel Sam Frydman a,⁎, Mark Talesnick a , Samuel Geffen b , Asia Shvarzman c a
Department of Structural Engineering and Construction Management, Faculty of Civil and Environmental Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel b Samuel Geffen - Soils Engineering Ltd., Kfar Saba, Israel c Faculty of Civil Engineering, Negev Academic College of Engineering, Beer-Sheva, Israel Received 13 March 2006; received in revised form 27 July 2006; accepted 6 September 2006 Available online 1 November 2006
Abstract Many areas in northern Israel show evidence of old landslides in marly profiles, and new slides are often activated in these regions. Such a slide occurred in the town of Rechasim, near Haifa, during an exceptionally wet winter in 1992. The present paper analyzes the causes for this slide, and suggests that it was due to a combination of downslope flow in the overlying, severely cracked chalk–limestone layer, and mobilization of residual strength at the marl–chalk interface. The residual strength of Israeli marly soils has been studied, and it is shown that while many of these consist predominantly of clay-sized (b2 μm) particles, and may, therefore, expect to have a very low residual friction angle, the amount of clay sized carbonate in the soil is of major influence on the residual friction angle. When the clay-sized material includes less than about 11% carbonate, the clay particles dominate the shearing mechanism and result in a residual friction angle of the order of 12°; this was the case at Rechasim. However, when more than about 30% of the clay-sized material is carbonate, the carbonates control the shearing mechanism, and residual friction angles can be as high as 30°. Results of a preliminary study of the change in residual strength with time are also presented. © 2006 Elsevier B.V. All rights reserved. Keywords: Landslides; Marl; Residual strength
1. Introduction During the winter of 1991–92 in Israel, record rainfall fell, corresponding to about twice the yearly average since local recording began. During this period, a landslide occurred at the construction site for a new residential area in the town of Rechasim, close to Haifa, in the north of Israel. Work being carried out at the site included two independent projects — one at the top of the slope, where 6 buildings were in the process of construction, and the second, at the base of the slope, ⁎ Corresponding author. Tel.: +972 4 8293320; fax: +972 4 8293323. E-mail address: [email protected] (S. Frydman). 0013-7952/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2006.09.009
where pre-construction excavations had been carried out. Fig. 1 shows the area several days after the slide. The large cracks forming the main landslide scarp can be seen passing through the two left-most buildings, and additional cracks are observed passing through the third building. Fig. 2 shows a closer view of some of the damage in the upper project site, with the slide surface passing through the second left-most building and the partial collapse of the building itself. In order to stabilize the landslide, a large portion of the excavated area was refilled, following which slip movements appeared to effectively cease. The three leftmost buildings were irrecoverably damaged, and had to be destroyed, but it was hoped that the other three
S. Frydman et al. / Engineering Geology 89 (2007) 36–46
37
Fig. 1. Aerial photograph of Rechasim landslide area, and locations of borings.
buildings could be occupied. Five months after the landslide, an investigation was initiated in order to study the causes for the landslide, and the stability of the remaining three buildings. In the progress of this study, attention became focused on the residual strength properties of the marls which formed a dominant part of the geological profile at the site. In view of previous cases of slope instabilities which have occurred in marl profiles in Israel (e.g. Wiseman et al., 1970), the Ministry of Housing and Construction extended the scope of the investigation in order to better understand the residual strength of Israeli marls and clays. The present paper presents results of this investigation, and applies these to help explain the slide at Rechasim.
2. The slide at Rechasim In July of 1992, five months after occurrence of the slide, three borings (1,2,3) were carried out at the site, at locations indicated in Fig. 1. Inclinometer tubes were installed in borings 1 and 2. Fig. 3 shows the geotechnical profile at the site following refilling, as indicated by the borings and geodetic measurements. The top 10–14 m is comprised of severely cracked chalk and limestone, and this is underlain by marl (a mixture of clay minerals and calcium carbonate (calcite), each accounting for not less than 25% by weight). The soil surface, and the layer interface are almost parallel, and slope at about 15° to the
Fig. 2. Structural damage and soil cracking.
38
S. Frydman et al. / Engineering Geology 89 (2007) 36–46
Fig. 3. Assumed cross-section of the slope and subsurface profile, based on borings.
horizontal. Fig. 4 shows moisture contents and consistency limits measured at the time of the site investigation. As the borings were performed in July, the profile had dried out from its wet, winter condition, but a zone of high moisture contents was still observed at the top of the marl. The marl layer is of very high plasticity at its top (liquid limit above 90), decreasing in plasticity with depth. The clay mineral of the marl at this site, as for most Israeli clay (Kassiff et al., 1969), is predominantly montmorillonite. The consistency limits indicate that the marl consists mainly of clay at the top of the layer, while the calcite content increases with depth. Inclinometer measurements began on 22 July. Measurements in boring 1 are shown in Fig. 5, and similar results were obtained in boring 2. Fig. 5 shows incremental displacements and accumulative movement from the bottom of the inclinometer tube to its top. It is seen that even
though measurements began about 5 months after refilling the excavation, when it appeared that movement had ceased, slow, continuous movement actually continued, with the top, chalk layer moving as a more or less solid block on top of the marl layer, at a depth of 14 m. Measurements continued in boring 1 until March 1997, by which time a movement of about 10 mm had developed. Attempts to continue measurements after this date were unsuccessful, as it was found impossible to lower the inclinometer probe below a depth of 14 m. Apparently the tube had bent excessively or broken at this depth, preventing any further penetration of the probe. An estimate of the development of shear strain with time at 14 m depth was made by dividing the incremental displacement by the incremental measurement length at this depth, as shown in Fig. 6. It is seen that shear strain was continuing more or less linearly, at a rate of about 0.5%/year, when measurements were discontinued, indicating a continuous, creep movement of the slope, with no sign of stabilization, during the 5 year measurement period. This creep was evident, also, from cracks which continued to reappear in the buildings following repair works. 3. Evaluation of the instability at Rechasim
Fig. 4. Variations of moisture content and consistency limits with depth.
The inclinometer data indicate that the slide occurred at the top of the marl layer, probably at the interface with the chalk–limestone layer. On first consideration, this appears strange, considering the mild interface slope of about 15°. Saturated Israeli clays, including marls, have been found to have an effective friction angle, ϕ′, of at least 25° and a small effective cohesion (Frydman, 2000). The slide appears to have resulted from a combination of the heavy rainfall, and excavation of the slope, exposing the chalk–marl interface at the excavation base (see Fig. 3), and removing support for the upper chalk layer. The heavy rain led to saturation of the cracked chalk–limestone layer, and the top of the marl. Due to the very low permeability of the marl, there would be expected to be very limited penetration of rain water down into the marl. The top surface of the layer
S. Frydman et al. / Engineering Geology 89 (2007) 36–46
39
Fig. 5. Inclinometer measurements in boring 1.
would be likely to have reached saturation, and the excess water presumably flowed down the slope, within the cracks of the chalk–limestone. Two processes, individually or together, appear to have contributed to the onset of the slide: (a) Development of saturated flow in the upper, cracked layer, and resulting pore pressure at the top of the marl layer. For such flow, more or less parallel to the chalk–marl interface, the factor of safety, F, may be estimated for infinite slope or sliding block conditions, as: F ¼ ðgsub =gt Þðtg/ V=tgbÞ where γsub and γt are the submerged and total unit weights, respectively, of the saturated upper cracked layer,
ϕ′ is the effective friction angle of the saturated marl, and β is the inclination of the interface. In the present case, assuming ϕ′ = 25° and β = 15°, a factor of safety, F, slightly above 1 results. Evidence of flow in the cracked upper layer was observed at the site, with water streaming out of the excavated face. However, the flow did not appear to be occurring over the full thickness of the layer, and flow, alone, would therefore not explain the slide. (b) Geological investigations of the area and study of existing geological maps following the slide (Saltzman, personal communication) indicated that the area is an old landslide area, with evidence of old slide scarps. It would, then, be likely that the operational strength parameters of the saturated marl are the residual, rather than the peak, values (Skempton, 1985). Consequently, a study of the residual strength of the in-situ marl was initiated. 4. Residual strength of Israeli clays and marls
Fig. 6. Development of shear strain with time at the marl–chalk layer interface.
Lupini et al. (1981) and Skempton (1964, 1985) presented a conceptual model for the development of residual strength conditions, in which it is suggested that the major cause for the decrease in strength to a residual value, at large shear deformation, is the increased parallel orientation of platy, clay particles located along the slip surface. They further suggested that at a residual
40
S. Frydman et al. / Engineering Geology 89 (2007) 36–46
indicated that approximately 65% of the material is clay sized, and according to Fig. 7, the residual friction angle of this material would be expected to be of the order of 12°. Consequently, the slide which occurred at Rechasim may be explained by assuming that the operational strength at the top of the marl layer corresponded to the residual, rather than the peak condition. However, as will be shown below, it appears that Fig. 7 may not be universally applicable to all soils. Further discussion of the residual strength of Israeli marls in general, and of the Rechasim marl in particular, is presented below. As a result of the slide at Rechasim, and of other landslides and instability phenomena which have occurred previously in marl slopes in the Galilee (e.g. Wiseman et al., 1970), the Israel Ministry of Housing and Construction initiated a research investigation into the residual strength of Israeli clays and marls (Shvarzman, 1995). A number of natural and artificial soils were tested; properties are listed in Table 1. Soils 1–8 are natural soils; soil 2 was from Rechasim, from the top of the marl layer, at a depth of 14 m. Soil 9, labelled “clay” was obtained from the clay-size fraction (− 2 μm) of Mizra soil (soil 1), by sedimentation. Soils 10–13 (soils “A”–“D”) were prepared by mixing this clay with various amounts of laboratory grade CaCO3, of particle size 2–5 μm. Residual strength is generally experimentally evaluated in either ring shear tests, or reversal direct shear tests. Both procedures were employed in this investigation. As has been demonstrated (e.g. Bishop et al., 1971), residual strength is independent of initial soil structure, and so remolded samples were tested. The procedure employed for the reversal direct shear tests involved inserting soil at about the liquid limit into the direct shear ring, applying
Fig. 7. Residual friction angle, ϕ′r versus percent clay size fraction (b2 μm), (after Skempton, 1985) (Printed with permission of ICE).
condition, the shear strength could be expressed by a linear Mohr Coulomb envelope, in terms of residual parameters cr and ϕr, and that, in general, the residual cohesion, cr is very small and can be neglected. The residual shear strength was therefore expressed by the equation: sr = σ′ tanϕr. The above authors found that for a large range of clays, ϕr could be related to the percent clay sized material (smaller than 2 μm) in the soil. They suggested that for soils with predominantly clay sized (i.e. platy) material, shear deformation along a slip surface takes place mainly in a sliding mode, whereas if the particles are predominantly rotund, the deformation mode is turbulent. At intermediate clay-size percentages, a transitional deformation mode was hypothesized. This model has become widely accepted, and their relationship between ϕr and percent clay-size fraction (Fig. 7) is often used for estimating the residual friction angles of soils. Hydrometer tests on samples taken from the top of the marl layer at Rechasim Table 1 Soil properties Soil Soil source no.
Clay minerals Quartz % Liquid Plasticity CaCO3 CaCO3 in CaCO3 in Sand Silt Clay (montmorillonite) % limit % index % % −0.074 mm − 2 μm size % size % size % fraction % fraction %
1 2 3 4 5 6 7 8 9 10 11 12 13
45–55
20–25 5
25–35 15–20 45–55
15 5 25–30
Mizra Rechasim B-2 Beit Shemesh B-13 Beit Shemesh B-90 Rechasim Ramat David Migdal Haemek Bikah Clay Soil “A” Soil “B” Soil “C” Soil “D”
66 101 75 61 66 73 57 58
40 64 53 39 36 45 25 26
10 43 20 38 55 15 37 44 21 22 44 100
9 26 14 19 55 12 32 32.5 20 21 43 100
3 7 11 16 32 3 29 29 3 20 21 50 100
1.5 5.7 3 2.1 2.4 5.6 10.1 1.2
36.8 28.9 14.5 21.4 21.1 30.4 32.7 34.0
1.3 1.3 1
32.6 32.2 23
61.7 65.4 82.5 76.5 76.5 64 57.2 64.8 100 66.1 66.5 76 100
S. Frydman et al. / Engineering Geology 89 (2007) 36–46
41
consolidation in stages, cutting along the proposed shear surface with a fine cheese wire, and then shearing. Shear was applied to a relative displacement of about 6 mm and then reversed and sheared back and forth to a displacement of about 10 mm. The reversal procedure was repeated until a constant shear resistance was achieved. Displacement rates were varied between 0.213 mm/min and 0.000068 mm/min, and the rate was found to have no influence on the resulting residual shear strength. Ring shear tests were carried out in a Bromhead apparatus. In order to minimize friction resulting from top cap settlement into the specimen ring during shear, the “flush” test procedure (Stark and Vettel, 1992) was employed To reduce consolidation settlement, samples were prepared at, or slightly below, the plastic limit. Shearing was carried out at a rate of 0.024 deg/min under normal stresses in the range 25–200 kPa. Several of the tests were performed as multi-stage tests. Overall, similar results were obtained from the ring shear and the reversal direct shear procedures. Figs. 8 and 9 show typical results from ring shear and reversal direct shear tests respectively, on Rechasim B-2 soil. The residual friction angle, ϕr, (defined as arctan sr/σ) as a function of normal stress, σ, for a number of
Fig. 9. Results of reversal direct shear tests, soil 2. (a) Shear stress– shear displacement curves. (b) Residual strength envelope.
Fig. 8. Results of ring shear tests, soil 2. (a) Shear stress–angular displacement curves. (b) Residual strength envelope.
the soils, is shown in Fig. 10. ϕr is seen to decrease with σ at low normal stresses, and then to reach a constant value. This non-uniformity of ϕr implies non-linearity
42
S. Frydman et al. / Engineering Geology 89 (2007) 36–46
Fig. 10. Residual friction angle versus normal stress.
of the residual strength envelope, which has been noted previously (e.g. Kenney, 1977; Skempton, 1985). It appears that as CaCO3 content increases, the envelope becomes more linear. The relationship between ϕr (corresponding to the straight portion of the envelope), and percent clay-size fraction is shown in Fig. 11, in which CaCO3 contents in the clay-size fraction are marked next to each point. No clear relationship is seen between ϕr and percent clay size fraction, but it is evident that the low friction angles correspond to low carbonate contents, while the high friction angles correspond to high carbonate contents. The influence of the carbonate content is shown in Fig. 12a–c. Fig. 12a and b show that there is no clear relationship between ϕr and overall carbonate content or carbonate content in the fine (passing #200 sieve — 0.074 mm) fraction of the soil. Fig. 12c shows a very
Fig. 12. Residual friction angle versus total carbonate content. (a) Overall (b) in fine fraction (c) in clay size fraction.
Fig. 11. Residual friction angle versus percent clay size fraction.
clear relationship between ϕr and carbonate content in the clay-size fraction of the soil. For these soils, the amount of carbonate in the clay-size fraction has a
S. Frydman et al. / Engineering Geology 89 (2007) 36–46
greater influence on ϕr than does the percentage of claysize material itself. It should be noted that for all of these soils, there is about 60% or more of clay-size material, and, according to Fig. 7, the actual amount would not be expected to have any significant influence on ϕr. It appears that the residual friction angle (over the straight portion of the strength envelope) of Israeli clays and marls, with less than about 11% carbonate of clay size, is of the order of 12°, consistent with that expected from Fig. 7. For this range of carbonate content, the shearing mode is, apparently, controlled by the platy, clay particles, and involves a sliding mechanism. As the amount of non-platy carbonate in the clay-size fraction increases up to about 30%, the shear mode goes from sliding to transitional to turbulent, and the value of ϕr increases to about 30°. The above observations of the influence of calcium carbonate on the residual friction angle seem to explain why certain old landslide areas in marly deposits are presently stable, despite the fact that Fig. 7 would predict low residual strength values and consequent instability, while others, such as Rechasim, actually slid, consistent with the residual strength values predicted from Fig. 7. 5. The influence of time on residual strength The above discussion has assumed that the operative shear strength in prefailed slopes is the residual strength, as measured in the laboratory. Such an approach to shear strength on old slide surfaces assumes that no strength recovery occurs during the (often very) considerable time period which has elapsed since development of the slip surface. Thixotropic strength increase in remolded and compacted clays has been previously reported by numerous researchers (e.g. Moretto, 1948; Skempton and Northey, 1952; Kruyt, 1952; Mitchell, 1960; Amir, 1961). These investigations studied cases in which compacted or remolded soil specimens were allowed to age for various periods before shearing. Gibo et al. (2002) reported tests in which two soils were first sheared to residual conditions in a ring-shear apparatus, then reconsolidated for two days without allowing further shear displacements, and finally re-sheared. They found that for the soil containing large amounts of silt and sand and only 10% clay-size particles, significant strength recovery was observed for low normal stresses (below 100 kPa). On the other hand, for the soil containing 73% clay-size particles which were predominantly montmorillonite, negligible strength recovery was observed. Although the authors didn't suggest so, it is possible that this lack of strength recovery may have
43
been due to the short waiting period between initial shearing and re-shearing stages. As part of the present study of residual strength of Israeli soils, limited testing was carried out in order to obtain an initial indication of the time effect, using the following testing procedure. A remolded test sample was prepared by mixing the dry soil with distilled water to its liquid limit, and then consolidating it for 24 h under a normal stress of 100 kPa. With the normal, consolidation stress held constant, the submerged sample was cut along its potential shear plane, and sheared in direct shear using the stress-reversal technique, until the residual condition was developed. At this stage, in order not to tie up direct shear devices for long periods of time, the sample was quickly unloaded, removed from the shear device, and transferred (still in the direct shear rings) to a compression device where it was submerged and subjected to the same normal stress as that applied during shearing. Following a waiting period of up to five months, it was transferred back to the direct shear device, the normal stress was reapplied and held for 48 h, and the sample was then re-sheared. These transfers of the samples may have had a detrimental effect on their strengths, but could not be avoided due to equipment limitations. Tests with waiting periods of less than 144 h were left in the direct shear device during the whole procedure, and were not transferred to a different compression apparatus. Tests of this nature were performed on soils 1, 2, and 9. Some additional comments regarding this procedure are warranted: (a) Under field conditions, shear stresses may remain on the slip surface throughout the time elapsed between establishment of residual conditions and re-shear, in contrast to the conditions applied to the laboratory samples. These in-situ shear stresses would possibly result in some creep on the slip surface, and the resulting strength on re-shearing may be less than that measured in the present laboratory procedure. Consequently, the results of the tests described here may be considered to be on the high side. (b) It is known (e.g. Anson and Hawkins, 1998) that pore fluid composition has an effect on soil shear strength in general, and residual strength in particular. In the Israeli scenario, the marl slopes of interest are inevitably well above water table, and normally unsaturated, while wetting, accompanied by instability phenomena, is usually a result of heavy rainfall. Consequently, it was considered that the use of distilled water for soaking the samples would be most representative of field conditions. No tests were performed to study possible chemical
44
S. Frydman et al. / Engineering Geology 89 (2007) 36–46
Fig. 13. Re-shearing stress-displacement curve for soil 9, following a waiting period of 720 h.
changes in the samples (e.g. dissolution, precipitation) during the extended waiting periods.
quickly during re-shearing, with strength reverting back to the residual value. Consequently, definition of the strength recovery in terms of the ratio of re-sheared peak to residual strengths may be misleading; the strength increase may be relevant over such a short displacement range that it is insignificant in practical terms. An alternative measure of the strength recovery, which may better describe its effectiveness, is defined here as the recovered energy per unit area of sheared surface, E. This energy is the area of the triangular zone of increased strength on the stress-displacement curve (e.g. DEF in Fig. 13). Fig. 14a shows E as a function of waiting time for the different soils, together with trendlines for soils 1 and 9 only. According to Fig. 14a, the more plastic soil 9 developed lower recovered energy, but its rate of increase was higher than that of soil 1. The result obtained from the test on soil 9 after a waiting period of 4 h showed very small strength
Fig. 13 shows a typical test result, obtained on a sample of soil 9. After the sample was sheared to a residual condition, corresponding to a shear stress of 20–22 kPa (line AB), it was unloaded (line BC), and left under the normal stress for 720 h. It was then re-sheared. It is seen that during the re-shearing, the shear stress developed a new peak value of 35 kPa (point E), but then, with additional shearing, the strength decreased, reverting to the residual value. The displacement range over which strength was above the residual value has been labeled Δ. In this test, the ratio of re-sheared peak strength to residual strength was 1.62; the value of Δ over which re-sheared strength was greater than residual strength was 1 mm. The test results (Table 2) indicate that there is a recovery of shear strength with time following development of residual conditions, which appears to be dependent on the soil properties. It is observed, however, that the strength recovery is dissipated quite Table 2 Results of re-shearing tests following residual conditions Soil
Waiting period (h)
Residual Strength (kPa)
Strength increase peak/resid
Δ (mm)
1 1 1 1 2 9 9 9
12 48 144 3600 720 4 20 720
22 22 22 22 20 21 21 21
1.36 1.45 1.82 2.05 1.6 1.1 1.19 1.62
0.85 0.85 1.12 5.0 1.8 0.1 0.6 1.0
Fig. 14. Recovered energy per unit area as a function of waiting period.
S. Frydman et al. / Engineering Geology 89 (2007) 36–46
recovery, over a very small displacement range, Δ. Consequently, the calculated recovered energy may not be accurate and Fig. 14b shows the relationships between E and waiting time without this result. In this case the lines are reasonably parallel, with the results for soil 9 still lying below those of soil 1. Further investigation is required in order to verify whether there is significance to the features noted in Fig. 14b. According to the data, the soil of higher plasticity (soil 9) would need more time in order to develop the same recovered energy (which may be considered as recovered resistance to shear) as that developed in the less plastic soil over a shorter waiting time. This is consistent with the observation of Gibo et al. (2002) that a low plasticity, silty soil (ML) showed significant strength recovery after a two day waiting period, whereas a highly plastic, montmorillonite clay (CH) showed negligible recovery. On the other hand, it is contrary to the observations which have been made regarding thixotropic strength regain in compacted and remolded soils. For example, Skempton and Northey (1952), who studied thixotropic strength regain in kaolin, illite and bentonite soils, reported, that “kaolin shows almost no thixotropy and illite shows only a small effect. In contrast, the bentonite shows a remarkable regain at very short time intervals and it is not possible to suggest an upper limit for this material since the strength continued to increase throughout the experiment.” Amir (1961) found, from tests on Israeli, montmorillonitic clays, that thixotropic strengthening of highly plastic clay is higher than that of silty clay. Clearly, the subject of strength gain with elapsed time from residual conditions is not yet clear, and requires further investigation and testing of a range of soils. 6. Conclusions The study of a landslide in a 15° slope at Rechasim, near Haifa, has indicated that the slide developed at the interface between the overlying, cracked chalk – limestone layer and the lower, highly plastic marl, more or less parallel to the slope surface. The slide followed exceptionally heavy rainfall, accompanied by execution of an excavation at the base of the slope exposing the marl layer. It appears to have been a result of a combination of downslope flow in the upper cracked layer, and mobilization of residual strength at the top of the marl. A study of residual strength of Israeli clays and marls has shown that it is related to the carbonate content in the clay-size fraction of the soil. A significant portion of the carbonates encountered in the marly soils may be of clay size, and this fine, carbonate
45
material appears to have a major effect on ϕr. The influence of the carbonate content in the material passing the #200 sieve (0.074 mm) on ϕr has been noted previously by Hawkins and McDonald (1992, 1993). In the present investigation, it has been found that a more significant effect may be attributed to the clay-sized carbonates. When the clay-size fraction of the soil contains less than about 11% carbonates, the platy clay particles dominate the shearing mechanism, whereas if more than about 30% are non-platy carbonates, they control the shearing mechanism. The clay sized carbonates also influence the shape of the residual strength envelope; a more curved envelope is obtained for low carbonate contents, and the envelope becomes increasingly linear with increasing amounts of carbonate in the clay-size fraction. Some preliminary findings regarding increase in strength from the residual condition with elapsed time have been presented. The increase in shearing resistance has been defined in terms of the “recovered energy per unit area of sheared surface”. Initial tests indicated that this recovered energy increases linearly with time on a log–log basis, and that it develops less quickly for more plasic soil. However these observations are based on few tests, and further study of this subject is warranted. References Amir, J., 1961. Influence of test duration on the strength of clay. MSc thesis, Technion. Anson, R.W.W., Hawkins, A.B., 1998. The effect of calcium ions in pore water on the residual shear strength of kaolinite and sodium montmorillonite. Geotechnique 48 (6), 787–800. Bishop, A.W., Greens, G.E., Garga, V.K., Andersen, A., 1971. A new ring shear apparatus and application to the measurement of residual strength. Geotechnique 21, 273–328. Frydman, S., 2000. Shear strength of Israeli soils. Israel Journal of Earth-Sciences 49, 55–64. Gibo, S., Egashira, K., Ohtsubo, M., Nakamura, S., 2002. Strength recovery from residual state in reactivated landslides. Geotechnique 52 (9), 683–686. Hawkins, A.B., McDonald, C., 1992. Decalcification and residual shear strength reduction in Fuller's earth clay. Geotechnique 42, 453–464. Hawkins, A.B., McDonald, C., 1993. The influence of granular calcareous particles on the residual shear strength of Fuller's earth clay. Quarterly Journal of Engineering Geology 26, 321–325. Kassiff, G., Livneh, M., Wiseman, G., 1969. Pavements on Expansive Soils. Jerusalem Academic Press. Kenney, T.C., 1977. Residual strength of mineral mixtures. Proc., 9th Intl. Conf. Soil Mech. and Foundn. Engg., Tokyo, vol. 1, pp. 155–160. Kruyt, H., 1952. Colloid Science, Irreversible Systems. Elsevier, NY. Lupini, J.F., Skinner, A.E., Vaughan, P.R., 1981. Drained residual strength of cohesive soils. Geotechnique 31, 181–213. Mitchell, J.K., 1960. Fundamental aspects of thixotropy in soils. Journal of the Soil Mechanics and Foundations Division, ASCE 86 (SM3), 19–53.
46
S. Frydman et al. / Engineering Geology 89 (2007) 36–46
Moretto, O., 1948. Effect of natural hardening on the unconfined compression strength of remoulded clays. Proc., 2nd International Conf. on Soil Mechanics and Foundn Engng., vol. 1, pp. 137–145. Shvarzman, A., 1995. Residual strength of cohesive soils in Israel. MSc thesis, Technion. Skempton, A.W., 1964. Long-term stability of clay slopes. Geotechnique 14, 77–102. Skempton, A.W., 1985. Residual strength of clays in landslides, folded strata and the laboratory. Geotechnique 35, 3–18.
Skempton, A.W., Northey, R.D., 1952. The sensitivity of clays. Geotechnique 3 (1), 30–54. Stark, T.D., Vettel, J.J., 1992. Bromhead ring shear test procedure. Geotechnical Testing Journal 15 (1), 24–32. Wiseman, G., Hayati, G., Frydman, S., Aisenstein, B., David, D., Flexer, A., 1970. A study of a landslide in the Galilee, Israel. Proc. Congress of Intl. Assoc. of Engg. Geology, Paris, vol. 1, pp. 50–61.