Estimation of In-situ Shear Strength Parameters of Weathered Granitic (Masado) Slopes Using Lightweight Dynamic Cone Penetrometer

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i) Professor, Graduate School of Engineering, Hiroshima University (ttuchida＠hiroshima-u.ac.jp).ii) Lecturer, Department of Civil and Construction Engineering, School of Engineering and Science, Curtin University.iii) Associate Professor, Kure National Colledge of Technology.iv) Shimizu Corporation, Tokyo, Japan.

The manuscript for this paper was received for review on June 2, 2010; approved on April 12, 2011.Written discussions on this paper should be submitted before January 1, 2012 to the Japanese Geotechnical Society, 4-38-2, Sengoku,Bunkyo-ku, Tokyo 112-0011, Japan. Upon request the closing date may be extended one month.

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SOILS AND FOUNDATIONS Vol. 51, No. 3, 497–512, June 2011Japanese Geotechnical Society

ESTIMATION OF IN-SITU SHEAR STRENGTH PARAMETERS OFWEATHERED GRANITIC (MASADO) SLOPES USINGLIGHTWEIGHT DYNAMIC CONE PENETROMETER

TAKASHI TSUCHIDAi), A. M. R. G. ATHAPATHTHUii), SEIJI KANOiii) and KAZUAKI SUGAiv)

ABSTRACT

A major problem in the geotechnical approach to the stability of natural slopes is that there is usually little informa-tion on the in-situ geotechnical conditions, because of the extreme di‹culty associated with ground investigations ofsteep slopes covered with vegetation. In this study, a lightweight dynamic cone penetration test (LWDCPT) has beenintroduced for a geotechnical survey of natural weathered granitic (Masado) slopes. Based on a series of direct sheartests, the internal friction angle and the apparent cohesion of reconstituted Masado soil were found to be fairly closelyrelated to the void ratio and the degree of saturation in the soil. From the laboratory calibration tests, an equation wascreated to relate the dynamic cone resistance (qd) and the void ratio of reconstituted Masado under diŠerent degrees ofsaturation. Equations were developed to calculate the internal friction angle and the apparent cohesion from the valueof qd for Masado at a known degree of saturation. LWDCPT and direct shear tests were carried out on undisturbedsamples taken from a natural Masado slope. The estimated internal friction angle and the apparent cohesion calculatedwith the value of qd in the LWDCPTs agreed fairly well with those of tests on the undisturbed samples obtained inlaboratory shear tests.

Key words: cohesion, internal friction angle, natural slope, penetration test, slope stability, weathered granite (IGC:C3/D6)

INTRODUCTION

Masado, a residual sandy soil of heavily weatheredgranite, is widely spread in western Japan. Due to thediŠerential weathering processes over the years, Masadoproˆles exhibit erratic weathering fronts with spatiallyvarying material properties (Nishida, 1981). For naturalslopes of Masado soil, landslide disasters induced by tor-rential rainfall are common. As many as 32,000 naturalslopes are considered to be susceptible to landslides inHiroshima Prefecture alone (an area of 8,477 km2),which is the highest number for any prefecture in Japan(Hiroshima Prefecture, 2010). Most of these slopes arecovered with layers of Masado from 0.5 to 5 m in thick-ness. The occurrence of failures is mainly due to the risein the groundwater table in the slopes or the loss of the in-bound shear strength of the Masado soil due to intenseand continuous rainfall during heavy rainy seasons(Aboshi and Sokobiki, 1972; Murata and Moriwaki,1990).

The risk assessment systems presently employed bymost prefectural governments, including Hiroshima

Prefecture, use rainfall indexes. In Hiroshima Prefec-ture, the prefectural land is divided into 350 unit areas of5 square kilometers, and criteria using rainfall indexes,such as the soil water index and rainfall intensity, are de-termined for each unit area on the basis of past records oflandslide disasters due to heavy rain. In this system,warnings of expected landslides and orders to evacuateare made for each unit area; however, the risk of individ-ual slopes in each area cannot be given. Due to this res-triction, although warnings or orders to evacuate havebeen made, no landslides occur in most of these cases.This results in a situation where people in an area subject-ed to such warnings or evacuation orders will sometimesnot heed them and are reluctant to leave their homes.

To apply a geotechnical approach to this problem, weneed to know the geographical conditions of the slope,the in-situ shear strength, and the changes in strength dueto an increase in the degree of saturation or the rise in thegroundwater table in the slope induced by possible heavyrains. Using these data and an adequate analysis method,the risk of individual slopes can be estimated one by one,on the basis of their failure mechanisms, thereby provid-

498498 TSUCHIDA ET AL.

ing more accurate risk assessments and more reliablewarnings and evacuation orders.

One major problem in the geotechnical approach tonatural slopes is that there has been little information onthe geotechnical conditions of natural slopes, due to theextreme di‹culty associated with ground investigationsof steep slopes covered with vegetation. In this study, alightweight dynamic cone penetration test (LWDCPT) isintroduced for a geotechnical survey of natural slopes ofMasado soil. To use LWDCPTs for natural Masadoslopes, the relationship between the shear strengthparameter and the penetration resistance in theLWDCPTs must be known. A series of laboratory testson reconstituted Masado samples in calibration chambersfor LWDCPTs, a êld investigation, and laboratoryshear tests on undisturbed samples obtained from thetested site were conducted. The results of these tests showthat the LWDCPT device can be a practical tool for geo-technical investigations of natural slopes, owing to itslight weight, the simplicity of its handling by one person,and its high accuracy.

Masado is a residual sandy soil of heavily weatheredgranite and is common in the western part of Japan.Masado soil is known to retain large matrix suction andshear strength in unsaturated conditions, but loses thisstrength drastically with an increase in the degree of satu-ration (Nishida, 1981). Although some studies have beenconducted on Masado soil in the past, the shear strengthcharacteristics have not been well documented in litera-ture. Some attempts have been made to study the eŠect ofmatrix suction on the shear strength of Masado soil forvarious stages of degrees of saturation and void ratios.The geotechnical ˆndings of similar sandy soils cannot bereadily applicable, due to the speciˆc characteristics ofMasado soil.

Most of the investigations of the shear strength ofMasado soil have dealt with the material in its reconstit-uted (disturbed) state, owing to the di‹culties associatedwith sampling undisturbed specimens. However, thereare some studies in which undisturbed specimens havebeen used to measure the shear strength parameters ofMasado soil. Unfortunately, details on the degree of dis-turbance or the quality of the extruding process of theseundisturbed specimens have not been given in the pub-lished literature. The following subsections discuss someof the ˆndings from past studies.

Onitsuka et al. (1985) conducted direct shear tests oncompacted and undisturbed specimens of decomposedgranite. Larger peak stress values were reported for theundisturbed specimens than for the compacted soil speci-mens. However, the residual stress levels were similar inboth cases. This was explained by the way the soil struc-ture of the undisturbed specimens is disturbed during theshear process and gradually changes into a structure simi-lar to that of compacted soil. Onitsuka et al. also showedthat the ˆner the soil particles, the more the cohesion isin‰uenced by disturbance and compaction. Murata andYasufuku (1987) investigated the shear characteristics ofundisturbed and reconstituted decomposed granite with

special reference to the degree of weathering. They foundthat the diŠerence between the shear characteristics of un-disturbed and disturbed specimens appears remarkably inlow stress regions. They also showed that the internalfriction angle depends heavily on the degree of weather-ing. Nakayama et al. (1988), Yagi et al. (1988), and Katoet al. (2002) also discussed the shearing characteristics ofMasado soil in its undisturbed state. Dissanyake (2002)and Thi (2005) also conducted series of direct shear testson Masado soil in its reconstituted state.

It has been reported that many obstacles were encoun-tered while trying to determine the shear strength ofMasado soil in the laboratory, because of the di‹cultiesassociated with extruding undisturbed samples from thesite of the natural slopes (Murata and Moriwaki, 1990).One option for overcoming such di‹culties is to carry outêld tests to determine the shear parameters. Recently,the use of in situ tests together with laboratory tests hasbecome an expedient and cost-eŠective way to determinethe strength parameters of Masado soil.

Presently, conventional êld tests, such as standardpenetration tests (SPT), cone penetration tests (CPT),etc., are not available for determining the shear strengthof natural slopes with gradients of more than 259. Theportable dynamic cone penetration test (PDCPT) hasbeen widely used in Japan as a practical substitution forthe SPT test (Japan Geotechnical Society, 1995). In thistest, a cone, 25 mm in diameter, is penetrated by a freefall blow from a 5-kg weight at a blow count of Nd for apenetration of 10 cm. However, a method for determin-ing the strength parameters of Masado soil using Nd

values has not yet been established. Accordingly, inHiroshima Prefecture, no reliable slope stability analyseshave been carried out for most of the 32,000 naturalslopes susceptible to heavy rains. Therefore, there is aneed for an e‹cient êld device that can be handledeasily, is portable and cost eŠective, and can determinethe strength parameters needed for stability analyses.

LIGHTWEIGHT DYNAMIC CONEPENETROMETER

The LWDCPT device consists of a dynamic conepenetrometer with variable energy; it was designed anddeveloped in France during the 1990s (Langton, 1999). Aschematic view of the test device is shown in Fig. 1. Itweighs 20 kg and can be operated by one person at almostany location to a depth of 6 m. It mainly consists of ananvil with a strain gauge bridge, a central acquisitionunit, and a dialogue terminal. The hammer is a rebound-type hammer and weighs 1.73 kg. The stainless steel rodsare 14 mm in diameter and 0.5 m in length. Cones of 2, 4,and 10 cm2 in area are available.

The blow from the hammer to the anvil providesenergy input, and a unique microprocessor records thespeed of the hammer and the depth of the penetration.The dynamic cone resistance, (qd), is automatically calcu-lated with a modiêd form of the Dutch Formula, asshown in Eq. (1) (Cassan, 1988; Chaigneau et al., 2000).

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Fig. 1. Schematic view of LWDCPT device Fig. 2(a). Location of ˆled observation site in Hiroshima University

Fig. 2(b). Typical patterns of qd-depth relationship in natural Masado slope(Mt. Gagara observation site in Hiroshima University)

499NATURAL MASADO SLOPE

qd＝1A

･1/2M･V 2

1＋PM

･1x

(1)

wherex＝penetration due to one blow of the hammer (m)A＝area of the cone (m2)M＝mass of the striking part (kg)P＝mass of the struck part (kg)V＝speed of the hammer impact (m/s)On the screen, the dialogue terminal displays not only

the real-time data, both graphically and in tabular form,

but also the dynamic cone resistance and the penetrationdepth. The smallest count that can be read on the screenin the LWDCPT is 0.001 MPa. However, the terminalhas the ability to record cone resistance up to 0.01 MPa.The data recorded in the dialogue terminal can be directlydownloaded to a computer, and the LWDCPT softwarefacilitates the display of smoothed data through aKalman ˆlter, a sliding average of 9 measures, and aweighted average of 50-mm intervals.

Athapaththu et al. (2007b) carried out extensiveLWDCTs at the Mt. Gagara observation site at Hiroshi-ma University, where a rainfall-induced debris ‰ow oc-curred in 1999, and where, since 2000, continuous mea-

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Fig. 3. Grading curves of Masado soil at Mt. Gagara

Fig. 4. Direct shear equipment

500 TSUCHIDA ET AL.

surements of the volumetric groundwater content,ground suction, and rainfall have been conducted (Thi etal., 2003, 2004)., Figure 2(a) shows the location of theobservation site and the area where the LWDCPTs werecarried out. Athapaththu et al. (2007b) found that, asshown in Fig. 2(b), the following six major patterns ofcone resistance versus depth were identiêd in the proˆlesof Masado at Mt. Gagara:

Pattern A: a gradual increase in the penetrationresistance with depth, showing a compara-tively thick weathering front

Pattern B: a gradual increase in the penetrationresistance with depth. The increase ratio ofthe cone resistance with depth is larger thanthat of Pattern A.

Pattern C: a considerably shallower proˆle than thoseof Patterns A and B.

Pattern D: very low values of penetration resistance ofapproximately 1 MPa almost up to 2.0 m,showing that the deposits were transportedby a past landslide.

Pattern E: similar to that of Pattern D, but with athickness approximately 1.2 m less.

Pattern F: the total depth of the Masado is less than0.5 m with a higher penetration resistanceof approximately 10 MPa at the hard stra-tum, showing a small weathering front ofapproximately 0.1 m.

Athapaththu et al. (2007b) also studied the relationshipbetween the blow count, Nd, for a 10-cm penetration inthe PDCPT and the penetration resistance, qd (MPa), inthe LWDCPT, and proposed the following equation:

qd＝12

N 3/4d . (2)

Using the PDCPT data carried out at several sites inHiroshima Prefecture, Athapaththu et al. (2007b) ana-lyzed the proˆles identiêd by LWDCPTs and conˆrmedthat they agreed well with the local geological conditionsat the site as follows:

The authors used the LWDCPT data for geotechnicalsurveys of natural Masado slopes in Hiroshima Prefec-ture.

LABORATORY SHEAR TESTS ON MASADO SOIL

Sample Preparation Testing MethodDue to the diŠerential weathering processes over the

years, Masado soil exhibits a variable composition of sec-ondary minerals and unchanged primary minerals such asquartz, resulting in variable material properties even wi-thin the same weathering grade. The gradation curves forMasado soil are shown in Fig. 3. Masado mainly consistsof sand with a considerable amount of gravel particles in-terconnected with a small fraction of ˆne particles.

A series of direct shear tests was carried out on unsatu-rated Masado soil using a conventional direct shear ap-paratus modiêd for measuring the pore pressure(Athapaththu et al., 2007a). The modiêd direct shear

test apparatus is shown in Fig. 4. A ceramic disk with anair entry value of 200 kPa was sealed onto the bottomhalf of the shear box to facilitate the measuring of thepore pressure. The lower part of the shear box was con-nected to a motor through a gear box for the applicationof horizontal force to the specimen, whereas the upperpart of the shear box was connected to the loading ram.Dial gauges connected to the data logger through trans-ducers were used for measuring the horizontal and thevertical displacements.

The soil samples collected at the Mt. Gagara observa-tion site at Hiroshima University were fairly air dried andpassed through a 2-mm sieve. Remolded specimens wereprepared under diŠerent void ratios and degrees of satu-ration in the laboratory. According to Thi (2005), the insitu void ratios ranged from 0.71 to 1.14, and the thick-ness of the Masado layer ranged from 0.5 m to 5.0 m.The testing program consisted of consolidated directshear tests conducted at initial void ratios of 0.70 to 1.00from 40z to 80z. Drainage was permitted at the upperloading plate and the pore pressure was measured at thebottom. As will be discussed later, it is considered thatthe samples were sheared under an almost constant watercontent condition, when the degree of saturation wasover 70z. When the degree of saturation was over 70z,the water content condition became close to a drainedcondition (Fredlund and Rahardjo, 1993). Direct sheartests with pore water pressure measurements were carried

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Fig. 5. Water retention curve during sample preparation

Fig. 6. Observed water retention curve measured at Mt. Gagara obser-vation site (July 20th–July 27th, 2009)


out in all of the above-mentioned cases , except for thatwith a void ratio of 1.00.

Prior to the tests, the measured amount of soil wasˆlled in three layers and tamped manually by a light-weight pestle (0.24 kg) with light compaction energy toavoid crushing the individual particles and to achieve anormally consolidated state in the specimen. The light-weight pestle was dropped from nearly 1 cm above thesoil sample in the direct shear box. Maximum and mini-mum energy levels were used for the preparation of thedirect shear specimens, namely, for the sample with avoid ratio of e＝0.70, a degree of saturation of Sr＝40zand for the sample with a void ratio of e＝1.00, a degreeof saturation of Sr＝80z, respectively. For e＝0.70 andSr＝40z, the number of blows was 30 per each layer. Thetamping energy varied from 37.7 kJ/m3 (for e＝0.70 andSr＝40z) to 0.0 kJ/m3 (for e＝1.00 and Sr＝80z) de-pending on the test conditions. The applied energy fortamping the samples was less than 6.4z of the energy re-quired for standard proctor compaction tests.

All tests were conducted at normal stress levels of 9.8,19.6, 39.2, and 78.4 kPa. Shearing was achieved byhorizontally displacing the bottom half of the direct shearbox relative to the top half at a constant shear displace-ment rate of 0.2 mm/min, as described in JGS 0561–200(Japanese Geotechnical Society, 2000).

The pore pressure was measured through a ceramicdisk with an air entry value of 200 kPa, mounted belowthe sample by a transducer connected to the data loggerduring the sample preparation, consolidation, and shear-ing stages. The shearing process was carried out until thehorizontal displacement reached approximately 7 mm,and the real-time data were recorded automatically in thedata logger. Under the fully saturated condition, consoli-dated drained (CD) triaxial compression tests were car-ried out, because direct shear tests were not available forthe saturated sample due to mechanical di‹culties.

Pore pressure measurements were recorded during thepreparation of the specimens in each case in order tomake sure the specimens were in a similar condition priorto shearing. Figure 5 shows the water retention curvesobtained at the ˆnal condition of suction before the directshear tests. As shown in this ˆgure, the suction rangedfrom 40 kPa to 0 kPa for a degree of saturation from40z to 70z. Thi carried out the measurements of thevolumetric water content and the suction for the naturalMasado slope at the Mt. Gagara observation site (Thi,2005). Figure 6 shows the relationship between the volu-metric water content and the suction measured at the Mt.Gagara observation site when the degree of saturation in-creased. As shown in this ˆgure, the range in measuredsuction was at most 6 kPa (－60 cm head) for a degree ofsaturation of approximately 40z (the volumetric watercontent was 0.2). Although the suction in the laboratorytests seemed larger than the measured suction whendegree of saturation Sr was less than 50z, the range insuction was almost the same for Sr larger than 50z.

Results of Direct Shear Tests and Strength ParametersFigures 7(a)–(f) and 8(a)–(f) show plots of the shear

stress and the changes in pore pressure against the defor-mation obtained from the direct shear tests for sampleswith adjusted void ratios of 0.70 and 0.90, respectively.As shown in these ˆgures, the pore pressure decreasedwith the displacements, apart from when the degree ofsaturation was larger than 80z for both e＝0.70 and0.90. Accordingly, it is considered that in these tests,although drainage was permitted from the upper porousstone, the shear was made under a constant water contentcondition, when the degree of saturation was not largerthan 80z. However, when the degree of saturation waslarger than 80z, the suction in the sample was almostzero and the condition was close to a drained condition.

As shown in Figs. 7(a)–(f) and 8(a)–f) , the larger thesuction in the shear, the more the shear stress was mobi-lized. Figures 9(a), (b), and (c) are the failure envelopesof the total stress for void ratios after consolidation of0.7, 0.8, 0.9, and 1.0 and for degrees of saturation of 50,60, 70, and 80z, which were obtained by correcting theshear strengths from the direct shear tests on the void ra-tios and the degrees of saturation. As illustrated in theseˆgures, the shear strength of each void ratio and thedegree of saturation show a linear relationship with the

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Fig. 7. Stress-displacement and suction of Masado samples during shear (e＝0.68–0.70)

502 TSUCHIDA ET AL.

normal stress, thereby enabling the determination of thecohesion and the internal friction angle. Additionally, theapparent cohesion for the total stress is larger with asmaller void ratio and a larger degree of saturation.Figure 10 shows a plot of the apparent internal frictionangle against the void ratio after consolidation. As shownin this ˆgure, the internal friction angle (9) of Masado isnot dependent on the degree of saturation and is mostlydetermined by the void ratio, as in the following equa-tion:

qd＝52.7－19.2e (3)

Figure 11 shows a plot of apparent cohesion cd againstthe void ratio after consolidation. As shown in thisˆgure, cd develops as an almost linear function of the voidratio at diŠerent degrees of saturation and decreases asthe degree of saturation increases. The value for cd (kPa)is given by the following equation:

cd＝27.5－0.146Sr－14.2e (4)

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Fig. 8. Suction of Masado samples during shear (e＝0.80–0.90)


It can be observed that the apparent cohesion decreasesby more than 50z when the degree of saturation variesfrom 40z to 100z for all ranges of void ratios. The ap-pearance of cd for sandy Masado is mainly attributed tosuction in unsaturated conditions, while in almost satu-rated conditions, the small apparent cohesion seems to beattributed to the eŠect of the reduction in the internalfriction angle with particle crushing in very low normalstress ranges.

CALIBRATION TESTS ON qd IN LWDCPTS ANDSTRENGTH PARAMETERS

Testing Method and Correction of Overburden StressIn order to develop strength correlations between qd

with the LWDCPT device and the shear strengthparameters, a series of LWDCPTs was performed in thecalibration chamber with void ratios ranging from 0.60 to1.10 and degrees of saturation ranging from 50z to 90z.

A typical calibration test arrangement is shown in Fig.

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Fig. 9. Failure envelope obtained by direct shear test

Fig. 10. Reference void ratio of reconstituted Masado soils and inter-nal friction angle

Fig. 11. Reference void ratio of Masado and apparent cohesion

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12(a). Acrylic cylinders, each 29 cm in diameter and 11cm in height, were fastened with nuts and bolts, and aporous plate was sealed onto the bottom cylinder. Thenumber of acrylic cylinders used for each test varied fromtwo to six, and some tests were conducted by applyingsurcharge weights. Prior to each test, the water requiredfor a single cylinder was calculated based on the test con-ditions. Masado soil, with a maximum particle size of 2mm, was mixed with the calculated amount of water andimmediately placed into 4 consecutive layers, ˆlling thecylinders. Light compaction was done to each layer,avoiding particle crushing. This procedure was repeatedto make the chamber in each test. After preparing the re-

quired number of cylinders, the LWDCPT device wasmounted on the top cylinder. Two to three trials ofLWDCPTs were performed for each preparation, andspecimens were taken from each cylinder for water con-tent and void ratio measurements in order to compare thevalues before and after the tests. The suction was meas-ured at a depth of 60 cm by a tensiometer. Figure 12(a)also shows how the calibration tests conducted with sur-charge loads were done to examine the eŠect of overbur-den in determining the cone resistance.

Calibration test data are illustrated in Fig. 12(b). Allthese tests were conducted without surcharge weights.The variations in cone resistance along the soil chamberwere given with corresponding void ratios and degrees ofsaturation measured after the test in each cylinder. Com-paratively low values of cone resistance were recorded athigh degrees of saturation. Moreover, high void ratiosimplicate low cone resistance.

Figure 13 illustrates the variation in cone resistancewith overburden stress under diŠerent degrees of satura-tion for void ratios of 0.70–0.80. It is observed that thevariation in cone resistance with overburden stress showsvery similar rates, irrespective of the void ratios or thedegrees of saturation, and the increase in qd with an in-

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Fig. 12(a). Calibration test of LWDCPT

Fig. 12(b). Result of calibration test

Fig. 13. EŠect of overburden stress on cone resistance qd

Fig. 14. Initial void ratios, degree of saturation, and qd5 (reconstitutedMasado)

Fig. 15(a). Internal friction angle qd vs. qd5

Fig. 15(b). Apparent cohesion vs. qd5


crease in overburden stress of 1 kPa was found to be ap-proximately 0.01 MPa. To correct the eŠect of overbur-den stress, a cone resistance under an overburden stressof 5 kPa, qd5, should be used by adjusting qd, as in thefollowing equation:

qd5＝qd－0.01×(gt･z－5) (5)

where qd5 and qd are the penetration resistance in MPa, gt

is the unit weight of the soil in kN/m3, and z is the depthin m.

Penetration Resistance vs. Void Ratio Relationship atDiŠerent Degrees of Saturation

Figure 14 shows the relationship between correctedcone resistance qd5 and the entire range of void ratios and

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Fig. 16. Recorded rainfall and observed degree of saturation(Mt. Gagara observation site in Hiroshima University)

506 TSUCHIDA ET AL.

degrees of saturation. From Fig. 13, the void ratios aregiven as follows:

e＝1.19－0.084･ln (qd5)－0.0074Sr (6)

where qd5 is given by Eq. (5) in MPa and Sr is the degreeof saturation in percentage.

Using Eqs. (3), (4), and (6), internal friction angle qd

(9) and apparent cohesion cd (kPa) of the Masado soil aregiven as

qd＝29.9＋1.61･ln (qd5)＋0.142･Sr (7)cd＝10.6＋1.19･ln (qd5)＋0.041･Sr (8)

Equations (7) and (8) show that when we know the in situdegree of saturation and the penetration resistancethrough LWDCPTs, the strength parameters of a Masa-do slope can be obtained.

Figures 15(a) and (b) show the relationship among qd,cd, qd5, and Sr. As Eqs. (7) and (8) were developedthrough laboratory shear tests and penetration tests in asmall calibration chamber, the applicability of the equa-tions must be conˆrmed by êld data.

FIELD INVESTIGATION BY LWDCPTS ANDAPPLICABILITY OF CALIBRATION OBTAINEDIN LABORATORY TESTS

Results of LWDCPTs Before and After RainIn order to apply the laboratory ˆndings to actual

slopes in the êld, a series of in situ LWDCPTs was con-ducted at the northern slope of the Mt. Gagara observa-tion site, where Hiroshima University installed its êldobservation facility in 2000 and has carried out continu-ous measurements since then of the volumetric groun-dwater content, the ground suction, and the rainfall (Thiet al., 2003, 2004, 2008).

LWDCPTs were conducted at six selected locations(Points a-8, b-7, c-5, d-9, e-2, and e-5) on June 5–8, 2005and on July 12, 2005. Figure 16 shows the recorded rain-fall and the changes in the degrees of saturation at thesite, which were calculated from the data on the volumet-ric water content measured at diŠerent depths in theslope. As shown in this ˆgure, the rainfall received duringthe ˆrst week of July was 275 mm and that received dur-ing July 9–12 was 110 mm. The sudden increase in thedegree of saturation was observed on the day of the heavyrainfall, July 2. The ˆrst LWDCPT was carried out aftercontinuous ˆne weather, and the average degree of satu-ration was approximately 65z. The second penetrationtest was carried out after the rainfall from July 1 to July11, and the degree of saturation at the site had increasedby 5–30z depending on the depth (the average increasewas 15z). With regard to the depth, the water content ata depth of 50 cm responded quickly to the rain, but theeŠect of the rain disappeared within a few days. It seemsthat the increase in the water content was clearly observedat a depth of 1 m, and a slight increase was seen at adepth of 1.95 m when the second LWDCPT was carriedout. Furthermore, as the degree of saturation is depend-ent on the topographical conditions and the void ratio of

the soil, it may not be the same at all locations orthroughout a certain depth.

Figure 17 shows the qd with depth measured during ˆneweather conditions and after rainfall at the site. Asshown in the ˆgure, decreases in the penetrationresistance due to the partial saturation of the soil wereclearly observed for all six testing points. The decrease inqd was signiˆcant from the ground surface to a depth of2.0 m. Figure 18 shows a comparison of qd values fordepths of less than 2 m. As shown in this ˆgure, qd

decreased at most of the measurement points and theaverage rate of decrease was 72z. According to Fig. 14,the qd value decreases approximately 50z with every 10z increase in the degree of saturation. Considering thatthe average increase in the degree of saturation was 15z,the decrease in the qd values seems small. However, thediŠerence may be due to the variability of the increase inthe degree of saturation at the various locations.

Shear Strength of Undisturbed Samples of MasadoA series of LWDCPT and laboratory shear tests on un-

disturbed samples taken from a depth of 50 mm was car-ried out at three points, namely, b-7, c-5, and e-2, at theMt. Gagara observation site. The results of theLWDCPTs at the three points are shown in Fig. 17.

As pointed out earlier, undisturbed sampling of Masa-do soil is di‹cult because of its sandy nature, whichcauses the sample to collapse easily (Nakayama et al.,1988; Yagi et al., 1988). Therefore, care must be takenwhen extracting undisturbed samples from naturalslopes. In this study, the nail sampling method was usedto minimize the disturbance to the soil specimens. Thenail sampling method is a type of block sampling in whicha small conˆnement is made to the core of the Masadosoil to prevent it from becoming loose, by putting a

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Fig. 17. qd5–depth relationship before and after rainfall(Mt. Gagara observation site in Hiroshima University)

Fig. 18. qd5 before and after rainfall


square-shaped plate with 18 holes on the ground surfaceand driving 18 nails into the ground through the holes. Inthis case, the nails were 150 mm in length with a diameterhead of 5.2 mm, and the conˆning plate size was 150 mm

×150 mm and 10 mm thick. The core conˆned by theplate and 18 nails was taken by excavating the surround-ing soil.

Figure 19(a) shows a plot of the stress and the verticaldisplacement (volume change) against the shear deforma-tion curves obtained from the direct shear tests on threeundisturbed samples taken from a depth of 50 cm atPoint b-7. The initial void ratios of the three undisturbedsamples were 1.084, 0.720, and 0.909, and the averagevalues for the void ratio and the degree of saturation were0.904z and 66.9z, respectively. As shown in this ˆgure,when sv＝19.6 kPa, the shear stress has a clear peak at ashear deformation of d＝0.54 mm, while the residualshear stress mobilized at a larger deformation and wasalmost half of the peak stress. For the other normalstresses, the shear stress increased with the deformation,and the stress-deformation relations were similar to thoseof the reconstituted Masado soil shown in Fig. 7. It isknown that the strength in direct shear tests is aŠected bythe grain size of the soil sample, because for specimenscontaining a larger grain size, the development of theshear band in the shear test is more restricted by themechanically determined gap of the shear box. There-fore, the peak strength of sv＝19.6 kPa may be due to the

508

Fig. 19(a). Shear stress and vertical deformation versus shear defor-mation curve in direct shear test (Point b-7)

Fig. 19(b). Failure envelope in direct shear tests (Point b-7)

Fig. 20(a). Shear stress and vertical deformation versus shear defor-mation curve in direct shear tests (Point c-5)

Fig. 20(b). Failure envelope of direct shear tests (Point c-5)

508 TSUCHIDA ET AL.

existence of large grain soil near the shear surface in thesample. Figure 19(b) shows the failure envelope of Masa-do at point b-7. The internal friction angle and the appar-ent cohesion obtained by the peak value were 43.09and12.0 kPa, respectively, while those obtained by the resid-ual strength were 42.09and 8.0 kPa.

Figure 20(a) shows a plot of the stress and the volumechange against the shear deformation curves obtainedfrom the direct shear tests on three undisturbed samplestaken from a depth of 50 cm at point c-5. The initial voidratios of the four undisturbed samples were 0.839, 0.788,0.771, and 0.864, and the average values for the void ra-tio and the degree of saturation were 0.816 and 75.4z,respectively. For all the samples, the shear stress showed

a clear peak, and the residual stresses for the larger strainwere 40–60z of the peak stresses. Of the three pointstested at this site, the diŠerence between the undisturbedsamples and the reconstituted samples was most promi-nent at Point c-5. Figure 20(b) shows the failure envelopeof Masado at Point c-5, as well as the strength parametersfor the peak strength and the residual strength. In thisˆgure, the internal friction angles are almost the same,but there is a large diŠerence in the apparent cohesion.Whether this large diŠerence is attributed to the structureof the undisturbed Masado sample or to some experimen-tal problem is not resolved in this study.

Figure 21(a) shows a plot of the stress and the volumechange against the shear deformation curves obtained

509

Fig. 21(a). Shear stress and vertical deformation versus shear defor-mation curve in direct shear tests (Point e-2)

Fig. 21(b). Failure envelope in direct shear tests (Point e-2)

Fig. 22(a). Predicted and measured internal friction angles (Mt.Gagara, Point b-7)

Fig. 22(b). Predicted and measured apparent cohesions (Mt. Gagara,Point b-7)


from the direct shear tests on four undisturbed samplestaken from a depth of 50 cm at Point e-2. The initial voidratios of the four undisturbed samples were 1.495, 1.401,1.483, and 1.378, and the average values of the void ratioand the degree of saturation were 1.439 and 38.1z, re-spectively. The void ratio was much higher than that ob-tained at the other points. One reason for the diŠerencewas that the undisturbed samples taken from Point e-2contained more ˆne particle soil. The average ˆnes con-tent of the four samples was 12.1z; this was more thanthree times larger than the samples from the other twopoints. Figure 21(b) shows the failure envelope of Masa-do at Point e-2, as well as the strength parameters for the

peak strength and the residual strength. As shown in thisˆgure, the internal friction angles for the peak strengthand the residual strength were almost the same, while theapparent cohesion for the peak strength was larger by 10kPa than that for the residual strength.

Comparison between Predicted and Measured ShearStrength Parameters

The internal friction angles and the apparent cohesionsat the three points were predicted using qd from Fig. 17and Eqs. (5), (7), and (8), where the degrees of saturationat the points were predicted using êld observations. Thepredicted values were compared with those measured inthe laboratory shear tests on undisturbed samples takenfrom a depth of 50 cm. In the prediction from qd, thedata before and after the rainfall were used. For themeasured values from the direct shear tests, the strengthparameters obtained in the residual state were also plot-ted. This is because the residual strength would be less

510

Fig. 23(a). Predicted and measured internal friction angles (Mt.Gagara, Point c-5)

Fig. 23(b). Predicted and measured apparent cohesions (Mt. Gagara,Point c-5)

Fig. 24(a). Predicted and measured internal friction angles (Mt.Gagara, Point e-2)

Fig. 24(b). Predicted and measured apparent cohesions (Mt. Gagara,Point e-2)

510 TSUCHIDA ET AL.

aŠected by a mechanical deˆciency of the direct sheartests for samples containing particles of a large grain size.

Figures 22, 23, and 24, show comparisons between thepredicted and the measured shear strength parameters atPoints b-7, c-5, and e-2, respectively. As shown in Figs.22(a) and (b), the measured internal friction angles atPoint b-7 were larger than the predicted values, while themeasured apparent cohesions were almost the same as thepredicted values. For the data at Point b-7, the diŠerencebetween the disturbed and the undisturbed samples waslarge, and the variability of the sample void ratio was alsolarge. As mentioned earlier, one of the reasons seems tobe the existence of a large grain size in the undisturbedsample, but more studies will be necessary in order tofully understand the reason for this diŠerence. At Pointc95, as shown in Figs. 23(a) and (b), the measured inter-nal friction angles and the apparent cohesions of thedirect shear tests obtained at the residual strength were

close to the predicted values. However, in the same wayas for Point b-7, the apparent cohesions obtained at thepeak strength were larger than the predicted values. AtPoint e-2, as shown in Figs. 24(a) and (b), the measuredinternal friction angles and apparent cohesions obtainedat the residual strength were close to the predicted values,and the cohesions obtained by the peak strength werealmost twice the predicted values.

Summarizing the comparisons shown in Figs. 22, 23,and 24, although the number of points was not su‹cient,it seems that the prediction proposed in this studyproduces good results or yields slightly underestimatedstrength parameters. The prediction method in this studyis based on laboratory shear tests on reconstituted Masa-do soil. As shown in Figs. 22, 23, and 24, some of themeasured strength parameters of the undisturbed sampleswere much larger than the predicted values. It seems thatthis diŠerence is attributed to the existence of a largergrain size or, as mentioned by Murata and Yasufuku(1987), the eŠects of some additional structures formed


by natural weathering processes at each point. To takethese eŠects into account in the predictions, further stu-dies will be necessary. However, from a practical point ofview, the proposed method is surely useful for estimatingthe strength parameters of natural Masado soil.

CONCLUSIONS

Masado is a residual sandy soil of heavily weatheredgranite, and in the natural slopes of Masado, landslidedisasters induced by torrential rainfall are common. Onemajor problem associated with the geotechnical approachfor natural slopes is that there has been little informationon the geotechnical conditions of natural slopes. This isbecause of the extreme di‹culty of conducting ground in-vestigations on the steep slopes, which are covered withvegetation. In this study, a lightweight dynamic conepenetration test (LWDCPT) is introduced for a geo-technical survey of natural Masado slopes. A method isnewly developed to predict the shear strength parametersof natural Masado slopes from the penetration resistancemeasured by the LWDCPTs and the in-situ degree ofsaturation. A series of laboratory shear tests and calibra-tion chamber tests of LWDCPT was carried out. Theapplicability of the proposed method was examinedthrough êld tests and laboratory direct shear tests on un-disturbed samples. The main conclusions are summarizedas follows:1) The internal friction angle and the apparent cohesion

of reconstituted Masado soil were found to be fairlyclosely related to the void ratio and the degree of satu-ration. Empirical equations were developed to calcu-late the internal friction angle and the apparent cohe-sion from the void ratio of Masado at a known degreeof saturation.

2) Laboratory calibration tests revealed a fairly good re-lation between qd and the void ratios of reconstitutedMasado under diŠerent degrees of saturation. An em-pirical equation was developed to calculate the voidratio from cone resistance data at a known degree ofsaturation.

3) The following equations for estimating the shearstrength parameters from the data on qd and thedegree of saturation were proposed on the basis of thecorrelations developed between void ratio qd and in-ternal friction angle qd (9) and between void ratio qd

and apparent cohesion cd (kPa):

qd＝29.9＋1.61･ln (qd5)＋0.142･Sr

cd＝10.6＋1.19･ln (qd5)＋0.041･Sr

where qd5 (MPa) is the penetration resistance in theLWDCPTs, corrected for an overburden stress of 5kPa, and Sr is the degree of saturation in percentage.

4) A series of LWDCPTs was carried out at the Mt.Gagara observation site at Hiroshima University. Adecrease in the cone resistance was observed in the in-situ tests performed after rainfall, which correspond-ed to the results of the laboratory calibration tests onreconstituted Masado soil.

5) The internal friction angle and the apparent cohesionat three points at the Mt. Gagara observation site werepredicted using the qd in the LWDCPTs and theproposed equations, where the degrees of saturationat the three points were predicted from êld observa-tions or directly measured. Although the number ofdata points is not su‹cient, it seems that the strengthparameters predicted by the proposed method in thisstudy produced good results or slightly underesti-mated the strength.

6) Some strength parameters measured by the directshear tests on undisturbed samples were much largerthan the predicted values. It seems that the existenceof a larger grain size in the samples or the eŠects of ad-ditional structures formed by natural weathering proc-esses at each point are the reason for the diŠerences.To consider these eŠects in predictions, further studieswill be necessary. However, from a practical point ofview, the proposed method is surely useful for es-timating the strength parameters of natural Masadosoil.

REFERENCES

1) Aboshhi, H. and Sokobiki, H. (1972): Studies on the slope failurein weathered granite, Proc. of the 7th Annual Meeting of JSSMFE,pp. 507–510 (in Japanese).

2) Athapaththu A. M. R. G., Tsuchida, T., Suga, K., Nakai, S. andTakeuchi J. (2007a): Evaluation of in-situ strength of Masadoslopes, Journal of Japanese Society of Civil Engineers-C, 63(3),848–861.

3) Athapaththu, A.M.R.G., Tsuchida, T., Suga, K. and Kano, S.(2007b): A lightweight dynamic cone penetrometer for evaluationof natural Masado slopes, Journal of Japanese Society of Civil En-gineers-C, 63(2), 403–416.

4) Cassan, M. (1988): Les essays in situ en mechanique des sols, reali-zation et interpretation, 2nd ed. Eyrolles, Paris, 146–151.

5) Chaigneau L., Gourves R. and Boissier D. (2000): Compaction con-trol with a dynamic cone penetrometer, Proc. of InternationalWorkshop on Compaction of Soils, Granulates and Powders,Innsbruck, 103–109.

6) Dissanayake, A. K. (2002): A Study on the initiation of rain in-duced landslides in decomposed granite slopes, PhD Thesis,Hiroshima University, Japan, 2002.

7) Fredlund, D. G. and Rahardjo, H. (1993): Soil Mechanics for Un-saturated Soils, John Wiley and Sons. Inc., 217–248.

8) Hiroshima Prefecture. (2010): http://www.mlit.go.jp/river/sabo/link20.htm

9) Japanese Geotechnical Society (1995): Japanese Standards for Geo-technical and Geoenvironmental Investigation Methods—Stan-dards and Explanations—, 208–209 (in Japanese).

10) Japanese Geotechnical Society (2000): Japanese Standards for Geo-technical and Geoenvironmental Laboratory Testing Methods—-Standards and Explanations—, Chapter 4 direct shear test,563–597.

11) Kato, S., Yoshimura, Y. and Shinkai, H. (2002): EŠect of suctionon strength and deformation of unsaturated Masado soil specimenin unconˆned compression test, Proceedings of 37th Annual Meet-ing of Japanese Geotechnical Society, 949–950 (in Japanese).

12) Langton, D. D. (1999): The PANDA-lightweight penetrometer forsoil investigation and monitoring material compaction, Ground En-gineering, September, 33–37.

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14) Murata, H. and Moriwaki, T. (1990): Natural Disasters and Geo-technical Problems in Chugoku Area, Chapter 2. Geotechnical en-gineering problems in Chugoku area, Tsuchi to Kiso, Japan Geo-technical Society, 38(3), 49–50 (in Japanese).

15) Nakayama, Y., Nishida, K., Aoyama, K. and Sawa, K. (1988):Sampling and testing method of undisturbed Masado, Proc. ofSymp. on Weathered Residual Soil, Jpn. Geotech. Soc., 117–120(in Japanese).

16) Nishida, K. (1981): Masado, Technical note, Tsuchi to Kiso,Japanese Geotechnical Society. 29(5), 51–52 (in Japanese).

17) Onitsuka, K., Yoshitake, S. and Nanri, M. (1985): Mechanicalproperties and strength anisotropy of decomposed granite soil,Soils and Foundations, 25(2), 14–30.

18) Thi, H., Takeo, M., Yasushi, S., Seiji, K. and Dissanayake, A. K.(2003): The change in suction and water content in natural Masadoslopes due to rainfall, The International Symposium on Ground-water Problems Related to Geo-environment, Okayama, Japan,

181–187.19) Thi, H., Sasaki, Y. and Kano, S. (2004): Groundwater behavior in

natural Masado slopes and a simpliêd method to calculate ground-water level during rainfall, Annual Symposium of Japanese Geo-technical, Society, 49, 315–322 (in Japanese).

20) Thi, H. (2005): Study on the mechanism of slope instability inducedby rainfall in decomposed granite alope, PhD Thesis, HiroshimaUniversity (in Japanese).

21) Thi, H., Sasaki, Y., and Kano, S. (2008): Study on rainwater in-ˆltration in subsoil of sandy slopes during rainfall by êld monitor-ing, Proceedings of the 2nd International Conference on Geo-technical Engineering for Disaster Mitigation and Rehabilitation(GEDMAR08), 509–515.

22) Yagi, N., Enoki, A. and Yatabe, R. (1988): Shear characteristic ofundisturbed Masado, Proc. of Symp. on Weathered Residual Soil,Jpn. Geotech. Soc., 155–160 (in Japanese).

Documents

Estimation of In-situ Shear Strength Parameters of Weathered Granitic (Masado) Slopes Using Lightweight Dynamic Cone Penetrometer