Engineering geology of basaltic residual soil in Leiqiong, southern China

Accepted Manuscript Engineering geology of basaltic residual soil in Leiqiong, southern China

X.W. Zhang, L.W. Kong, S. Yin, C. Chen PII: DOI: Reference:

S0013-7952(17)30175-8 doi: 10.1016/j.enggeo.2017.02.002 ENGEO 4482

To appear in:

Engineering Geology

Received date: Revised date: Accepted date:

9 July 2016 29 January 2017 2 February 2017

Please cite this article as: X.W. Zhang, L.W. Kong, S. Yin, C. Chen , Engineering geology of basaltic residual soil in Leiqiong, southern China. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Engeo(2017), doi: 10.1016/j.enggeo.2017.02.002

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Engineering geology of basaltic residual soil in Leiqiong, southern China X. W. Zhang*, L.W. Kong, S. Yin, C. Chen

*Corresponding Author: XianWei Zhang, Ph.D. Associate Professor of Geotechnical Engineering .

PT

All Author's Institution: State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and

RI

Soil Mechanics, Chinese Academy of Sciences , Wuhan, China.

SC

E–mail of the authors:

NU

Author 1: Xian Wei Zhang

Author 2: Ling Wei Kong

ED

E–mail of the authors: [email protected]

MA

E–mail of the authors: [email protected]

Author 3: Song Yin

Author 4: Cheng Chen

EP T

E–mail of the author: [email protected]

AC C

E–mail of the author: [email protected] Contact Address:

Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Xiaohongshan, Wuchang, Wuhan 430071, P. R. China Tel: +86–27–87199910 Fax: +86–27–87197386

ACCEPTED MANUSCRIPT Abstract: The physical, mechanical, chemical, mineralogical, and microstructural properties of basaltic residual soil obtained from Leiqiong area in southern China were examined. Emphasis was placed on the influences of minerals, oxides, and the soil fabric on engineering geological characteristics, particularly on strength and deformation properties.

PT

Distinctive properties, such as high strength and low compressibility relative to high void ratios, high liquid limits, and

RI

low natural densities, were explained. The soil fabric with the cement bond formed by iron oxides was considered as the main factor that caused to peculiar mechanical behavior, whereas mineralogy and climate processes also played

SC

important roles. Under the influence of repeated cycle of alternate wetting and drying, the residual soil generated a large

NU

collapsible deformation upon soaking, and its strength significantly decreased because of the derogation of cement bond,

MA

the formation of microcracks, and the weakening of microstructure failure zones.

ED

Key words: Residual soils; Engineering geological characteristics; Microstructure; Wetting –drying cycle; Iron oxide

c cohesion

AC C

c' effective cohesion

EP T

List of Symbols

Cc compression index d pore diameter

D particle diameter

δef free swelling ratio δcp swelling ratio with load 50 kPa δs linear shrinkage rate δv volume shrinkage rate

ACCEPTED MANUSCRIPT e void ratio e0 initial void ratio Gs specific gravity Ic collapse potential

PT

IL liquidity index

RI

Ip plasticity index= wL - wp K permeability coefficient

SC

Kr molecular silica-alumina ratio

NU

ρ density ρd dry density

MA

ρdmax maximu m dry density

ED

q u unconfined compressive strength S r saturation

φ internal friction angle

EP T

t time

w water content wL liquid limit

AC C

φ' effective internal friction angle

wp plastic limit wopt optimum water content

ACCEPTED MANUSCRIPT 1. Introduction Basaltic residual soil is a soil-like material derived fro m the in situ weather ing and decomposition of basalt. This soil is affected by tropical pedogenetic processes under hot and humid conditions. Basaltic residual soil is one of the most widely distributed groups of soil groups in the tropics , including Southern China (Tang, et al., 1992), Pacific and

PT

Caribbean islands (Pushparajah and Amin, 1977; Tuncer and Lohnes, 1977; Lohnes and Demirel, 1983; Moon and

RI

Jayawardane, 2004), Africa, and South America (Osinubi and Nwaiwu, 2008; Gutierrez et al., 2009). Majo r civ il engineering works have been conducted in these areas in recent years (de Vallejo, et al., 1981; Rahard jo, et al., 2004),

SC

and a full understanding of the engineering characteristics of basaltic residual soil, fro m the bed-rock to the top layer

NU

consisting of completely weathered soil, is required for the design and construction of foundations, railways, and slopes.

MA

For a long period, researchers in the field of residual soil mechanics have mostly focused on residual soil derived fro m gran ite (Agus et al., 2005; Kim and Kim, 2010; Rahardjo et al., 2012) and co mpacted residual soil (Indrawan et al.,

ED

2006; Rahardjo et al., 2011; Yan and Li, 2012; Chiu and Ng, 2014). Several features of basaltic residual soil are similar to, but not exact ly like those of granite residual soil, even in the same regions. Co mpared with granite residual soils,

EP T

basaltic residual soils are characterized by s maller grains, higher void ratio, and mo re hydrophilic minerals (Huat et al., 2012; Blight and Leong, 2012). Therefo re, current research result s regarding granite residual soil cannot be directly

AC C

applied to exp lain the distinctive properties of basaltic residual soil. Although several studies have exp lored the mechanical behavior of basaltic residual soil (Pushparajah and Amin, 1977; Tuncer and Lohnes , 1977; Moon and Jayawardane, 2004; Osinubi and Nwaiwu, 2008; Zhang et al., 2016), its mechanical p roperties remain unclear. For example, its physical property, such as high strength and low compressibility, are inconsistent with its mechanical property, such as high plasticity and small part icle size (Zhang et al., 2016). Basaltic residual soil exh ibits a peculiar relic structure in which soil g rains are arranged and oriented well, but is metastable and possibly conducive to collapsible settlement and disintegration upon wetting (Blight and Leong, 2012). Surprisingly, such fundamental geotechnical characteristics have not been satisfactorily exp lained. In addition, the properties of residual soil vary fro m

ACCEPTED MANUSCRIPT region to region because of its heterogeneous nature and highly variable weathering degree, wh ich are controlled by regional climat ic and topographic conditions as well as the nature of the bedrock (Rahard jo et al., 2004). Therefore, residual soil fro m d ifferent regions should be characterized individually to obtain an appropriate assessment of its engineering behavior. In Leiqiong area, Ch ina, the soils derived fro m basalt are frequently used as

PT

subgrades. Establishing the bearing capacity and deformation index values of th is type of soil is always difficu lt

RI

because of the peculiarity of its engineering properties, such as complex structural arrangements, high heterogeneity, and special sensitivity to water. Ho wever, a systematic investigation of the engineering geology of

SC

Leiqiong basaltic residual soil has not yet been conducted. Tang et al. (1992) summarized the geological

NU

characteristics of 20 series samples of basaltic residual soil fro m Leiqiong area, including their geological origin, as

MA

well as their physical and mechanical properties. The basaltic residual soil with a high void ratio and clay particle content was found to exhibit an exceptionally high strength. This peculiarity needs to be investigated further.

ED

The strength and deformation behavior of soil are influenced by the arrangement (fabric) o f particles and determined by their mineralogical and chemical properties (Mitchell and Soga, 2005). Recent researchers

EP T

suggested that the microstructural, mineralogical, and chemical features of the constituent particles are necessary for the basic understanding of the engineering geological characteristics of soil developed in a tropical environ ment

AC C

(Madu, 1977; Ola, 1980; Benatti and Miguel, 2013; Otalvaro et al., 2016). However, in co mparison with other tropical reg ions, the basaltic residual soil in Leiq iong area has not been fully investigated in terms of engineering geological characteristics.

The current study investigates engineering geological characteristics of soil derived fro m basalt in Leiqiong area, Southern China, to exp lore the mechanis m that underlies its peculiar mechanical behavior. Such objective differs fro m those of previous research. Soil fabric is considered to be the main factor in determining engineering geological characteristics. The geochemical environment, and the associated mineralogical and chemical constitutions are also analyzed. This paper is organized as follows. First, the sampling apparatus and test

ACCEPTED MANUSCRIPT procedures are described in detail. Next, the experimental results for the physical and mechanical properties, as well as microstructural, mineralogical, and chemical features, are presented. Finally, the relationship between the engineering geological characteristics and microscopic features of the soil is discussed based on the experimental findings. A microscopic mechanism is proposed to explain the peculiar mechanical behavior.

PT

2. Location and geology of sample site

RI

The Leiqiong area, wh ich includes the Leizhou Peninsula and the northern part of Hainan Island, is the largest province of exposed basalt in Southern China. The basalt in this area was formed when lava flow intercalated with

SC

pyroclastics, which were main ly tholeiitic with several alkali and transitional series metals (Ho et al., 2000). Basalt

NU

started forming around the Middle Pleistocene to the Late Pleistocene (0 .7 Ma to 0.1 Ma) (Tang et al., 1992). Basalts in

MA

this area can be div ided according to the formation phase as follo ws; the Middle Pleistocene Shimaoling Format ion (Q 2s ) and the Late Pleistocene Huguangyan Formation (Q3h ) (Zhang, 2009). A detailed description of the geology of this area

ED

was provided by Ho et al. (2000). In this area, basaltic residual soil with depths ranging from 2 m to 20 m is located mainly over tableland and the volcanic dome and has a total surface area of 4815 km2 .

EP T

Climate plays an important role in the development of residual soil with distinctly different characteristics as a consequence of altered temperature and precip itation patterns. The climate in Leiq iong area is hot and humid, with no

AC C

pronounced dry season, which dramat ically increases weathering rate. Temperature varies minimally throughout the year, with an annual average temperature of 23 °C and an average annual rainfall of 1400 mm to 1700 mm. The studied samples were collected fro m typical residual soil (Q 4 el ) fro m Sh imaoling Format ion (Q2s ) in Longmen Town (lat itude 110° 02′ 03″ N, longitude 21° 41′ 53″ E) in China, at depths varying between 1.5 and 2.5 m (Fig.1). Unweathered basalt at around 20.0 m was also obtained to examine the influence of weathering on chemical transformations. The basalt weathering profiles based on four borehole data are presented in Fig. 2. In the profiles of the residual soil obtained from basalt, five degrees of weathering were classified according to the intensity to which the weathering was reduced with increasing depth, and the uppermost portion was considered as colluvial soil.

ACCEPTED MANUSCRIPT Basaltic residual soil can be easily destroyed by any disturbance during the sampling process. Thus, a sampling technique via open hand-dug pit, which is most suitable at shallow depths for completely weathered material, was adopted (Huat et al., 2012). The samp ling procedure is described as follows. Soil p linths (width: 150 mm, height: 400 mm) were excavated. Soil colu mns (diameter: 100 mm, height: 300 mm) were then cut using a

PT

sharp-edged cutter, placed in samp le bo x tubes, tightened by tape, sealed by wax, and then transported to the

RI

laboratory for relevant tests. 3. Test procedures

SC

3.1 Physical and mechanical tests

NU

Soil colo r can provide potential information regard ing the Fe o xide mineral, and thus, can indirectly exp lain

MA

the elementary pedogenetic process (Schwertmann, 1993). In this study, soil color was determined using the Munsell soil co lor charts (Munsell Color Co mpany, 1975), and part icle size distribution was determined using the

ED

pipette method (Day, 1965). The sands were separated via wet sieving, and silt and clay fractions were segregated as proposed by Jackson et al. (1950). Furthermo re, three test conditions of the pipette method, namely, addit ion

EP T

NaOH solution (20 mL 0.5 mo l/L NaOH solution per 30 g soil), u ltrasonic dispersion, and removing free iron oxides using dithionite–citrate–bicarbonate (DCB) solution (detail in Mehra and Jackson, 1960; Jackson, 1965),

AC C

were adopted to study the stability of the particles. The phys ical and mechanical p roperties of basaltic residual soil, including specific g ravity, Atterberg limit, permeability, consolidation, and shear strength, were measured according to the standard GB/T 50123 (Nat ional Standard for the People’s Republic of China , 1999). The shear strength parameters of the soil were evaluated by unconfined compressive strength test and standard triaxial consolidated undrained and drained shear tests under natural water content and saturated conditions. For the saturated samples, saturation was initially achieved within the triaxial cell through the circulation of carbon dio xide via the pore voids, and thus, saturation was later facilitated under back p ressure. In addit ion, high-level heavy compaction has been commonly used in this region; such compaction is driven by the energy derived from

ACCEPTED MANUSCRIPT a 4.5 kg hammer falling through 45.7 cm unto five layers of soil in a mold with a capacity of 1000 cm3 , in accordance with ASTM D 1557-02 (2006). The shrinkage factors of soil, including shrinkage limit, volu metric shrinkage, and linear shrinkage, were determined according to ASTM D 4943-08. The porous nature of residual soil makes them susceptible to collapse caused by under load wetting. To investigate

PT

their collapsible behavior, a quantitative parameter for evaluating collapsibility, namely, collapse potential Ic, was

RI

suggested according to ASTM D 5333–92 (2002). Load-wet odometer tests were performed using the undisturbed samples at stepwise consolidation pressure values of 5, 12.5, 25, 50, 100, 200, 400, 800 and 1600 kPa. After equilibriu m

SC

was achieved under these loads, the samples were soaked after apply ing a stress at 200 kPa and additional deformat ions

collapse is calculated at any stress level as follows:

e 100% , [1] 1  e0

MA

Ic 

NU

(collapse) were measured until co mp lete stabilizat ion. Collapse potential, wh ich is the relative magnitude of soil

ED

where ⊿e is the change in void ratio caused by wetting and e0 is the initial void ratio.

EP T

The climatic zones where residual soil occurs are frequently characterized by alternating wet and dry seasons. The influences of cyclic wetting and drying on the collapsible behavior and strength property of residual soil were studied.

AC C

Cyclically wetted and dried specimens were prepared as fo llo ws : specimens with a diameter of 61.8 mm and a height of 40 mm for the co llapse test and those with a d iameter 50.0 mm and a height of 100 mm fo r the unconfined co mpressive strength test were obtained. These specimens were kept in artificial climate chest at 40 °C. A mass change of less than 0.02 g/h indicated the end of the drying process. Subsequently, the dried specimens were sprayed with d istilled water every 2 h, and the wetting process was completed when the specimens could no longer absorb water. These processes constituted one wetting and drying cycle, and cycles of alternate wetting and dry ing were repeated. The specimens were subjected to one and two W–D cycles before they were subjected to experiment. 3.2 Mineralogical and chemical tests

ACCEPTED MANUSCRIPT The mineral identification was achieved via X-ray diffract ion (XRD) and differential thermal analysis (DTA); a semiquantitative estimation of the mineral was performed using a modified method sugg ested by Biscaye (1965). Loss on ignition (LOI) tests generally measure the mo lecular water content in soil minerals (Nishida, 1998), and are conducted in accordance with BS-1377 (1990). The pH was measured using an FG2–FiveGo pH meter

PT

manufactured by Toledo International (Toledo, OH). Organic matter content, cation exchange capacity (CEC),

RI

soluble salts and carbonate content were determined using the potassium dichro mate digestion method, ammoniu m acetate method, electrical conductivity method, and volumet ric measurement, respectively. These methods are

SC

described in detail in Sparks et al. (1996) and Pansu and Gautheyrou (2006). The total and external specific surface

NU

areas were respectively measured using the ethylene glycol monoethyl ether method (present ed in detail in Carter et

MA

al. 1965) and NOVA1000e BET surface area analyzer produced by FEI Co., which utilized nitrogen adsorption. 3.3 Microstructure tests

ED

Microstructure test is particularly useful for assessing the arrangement and size of part icles and was proposed as an important factor in determin ing the strength of soil (Mitchell and Soga, 2005). The microstructure of the

EP T

specimens was analyzed via scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). The dry specimens for SEM and MIP examination were prepared v ia vacuum freeze–drying technique (Ahmed et al.,

AC C

1974; M itchell and Soga, 2005) to min imize changes of soil microstructure caused by shrinkage. A Quanta 250 scanning electron microscope was used to observe the microstruct ure of soil and selected interesting areas were analyzed using an X-ray energy dispersive system (EDS) attached to the scanning electron microscopy. The pore size distribution of soil was determined using a Pore Master 33 mercury injection apparatus and th e tests were conducted in accordance with BS-7951 (1992). 4. Laboratory test results 4.1 Particle size and composition After adding NaOH solution to Leiqiong basaltic residual soil, the content of soil particles with size D < 0.08

ACCEPTED MANUSCRIPT mm increased and reached 70% (Fig. 3). However, when ultrasonic dispersion was subsequently applied, no obvious alteration in particle composition was observed. In samples deprived of free iron oxides, the content of particles with size D < 0.02 mm increased markedly, and the increase was more evident in smaller particles. For example, after removing free iron oxides, the content of particles with size D < 0.002 mm increased from 42% to 80%. The

PT

experimental results show that free iron oxides can enhance the aggregation of soil particles. The range may vary

RI

because of the characteristics exhibited by iron oxides in basaltic residual soil (Zhang et al., 2016). These results indicate that the aggregations in Leiqiong basaltic residual soil are associated with the presence of free iron oxide, given

SC

that the removal of such oxides resulted in the disappearance of soil particles, which consequently increased clay–size

NU

content.

MA

4.2 Physical properties

The color of moist samples fro m a depth of 1.5 m to 8.0 m in the field changed from red (10 R 4/8) to yellowish

ED

red (5 YR 4/6), based on the Munsell Co lor System. Subtle differences in soil color were observed at different depths, which were primarily attributed to the variation in clay mineral composition and iron oxides (Schwertmann, 1993).

EP T

Several indices of the physical properties of basaltic residual soil in Leiq iong area and other tropical areas are summarized in Table 1. Leiqiong residual soil exhib ited similar physical features with other tropical soils. It has high

AC C

specific gravity with a maximu m value of 3.14 based on experimental data by Gutierrez et al. (2009). The high values of specific gravity possibly resulted fro m the accumu lation of iron. The natural void rat io of the soil was higher than 1.0 in all the statistics and even reached as high as 2.06. These findings imp ly that basaltic residual soil has a porous structure, and consequently, it has low bulk density and high permeability coefficient (Table 2). Rahardjo et al. (2004) explained that the porous structure of residual soil was caused by the considerable leach ing of minerals fro m the soil during the weathering process, during which water and air replaced the soluble mineral, thereby resulting in high void rat io. Hence, although its water content is high, the natural residual soil is usually unsaturated. For examp le, the water content w of

ACCEPTED MANUSCRIPT Leiqiong residual soil is 38.09%, but its saturation S r is 78.48%. The basaltic residual soil is frequently regarded as clayey soil. However, its permeability coefficient (10–6 – 10–5 cm/s) is higher because of its porous structure. Leiqiong basaltic soil exhibited physical features similar to those of basaltic residual soil, but a significant difference in their position in the plasticity chart was found, as shown in Fig. 4. Un like that of soil in other areas,

PT

the plasticity index of Leiqiong basaltic residual soil was mostly located above the A -line, and thus, it could be

RI

classified as clay and silt with high plasticity. This result may be attributed to the high clay particle content, as shown in the results of particle size analysis.

SC

For Leiqiong basaltic residual soil with natural water content, the swelling was low, whereas the shrinkage

NU

potential was high. For examp le, its free swell rat io δe f was less than 50%, the swelling rat io δcp with a load 50 kPa

MA

was nearly zero, and the volume shrinkage rate δv was mo re than 10%. Furthermore, soaking and disturbance increased the shrinkage capacity of the soil. The volu me shrin kage rate increased fro m 10.1% in natural soil (w = w)

ED

to 27.6% in remolded soil (w = wL ), as shown in Table 2. 4.3 Mechanical properties

EP T

Through a heavy compaction test, deionized water was added to two sets of identical air-d ried samp les to observe the change in water content. The results of the co mpaction test indicated that the maximu m d ry density

AC C

ρdmax of basaltic residual soil was 1.47 g/cm3 , with an optimu m water content wopt of 21.0% (Fig. 5). In contrast to soil derived fro m granite and amphibolite, wh ich have been used to form successful impervious layers in water-retain ing embankments (Ogunsanwo, 1989), Leiq iong basaltic residual soil is hard to compact (e.g. wopt > 20% and ρdmax < 1.5 g/cm3 ) because of rich iron–bearing minerals (Gidigasu, 2012), even in the presence of high compaction energy. Therefore, Leiqiong basaltic residual soil is unsuitable as a filler material for dam embankments or as road subgrades. The strength characteristics of Leiqiong basaltic residual soil were determined via unconfined compressive strength test, CD and CU tests. Figs 6 and 7 depict a series of stress–strain curves of the samples under undrained

ACCEPTED MANUSCRIPT compression. The curves of natural soils showed a single peak. After the peak was reached, the stress showed an intensive drop and the failure strain was s maller (appro ximately 4% to 5%). Th is finding indicates that the failure of cement bond results in the softening of soil response. In addit ion, basaltic residual soil demonstrated relatively superior shear strength properties except for the individual parameter of the soil in Hawaii, as shown in Table 3. The average

cu

PT

value of q u of the studied soil is approximately 87.7 kPa, and the effective cohesion c'cu and the internal friction angle φ' are 58.3 kPa and 21.25 °, respectively. The high value o f strength index of the soil under undrained co mpression were

RI

also observed in Table 3. However, once the soil was subjected to W –D cycles, the strength parameters exh ibited a

SC

significant decrease. For examp le, the unconfined compressive strength was reduced from 93 kPa to 17.6 kPa in the

NU

natural soil after t wo W–D cycles, as shown in Fig. 6. Furthermore, the influence of saturation on the strength

MA

parameters was main ly reflected by the decrease in cohesion, rather than in the internal frict ion angle, by comparing strength parameters of the basaltic residual soil at different water contents.

ED

An interesting characteristic of Leiq iong basaltic residual soil is the significant decrease in strength caused by saturation. The unconfined compressive strength curves of saturated soil lack a definite yield point compared with that

EP T

of natural soil. A similar pattern was also observed in the results of the CU test (Fig. 7). Moreover, c'cu decreased by 18.2% after saturating, whereas φ' decreased by only 2.7%. Th is result demonstrates that the influence of saturation on

AC C

strength parameters mainly reflects a decrease in cohesion, rather than in the internal friction angle. Consequently, Leiqiong basaltic residual soil demonstrates a peculiar hydraulic behavior and also presents sensitivity to wetting or soaking. This behavior is possibly attributed to the weakening of the bonds between particles when soil is saturated and pores are filled with water, thereby decreasing shear strength. The influence of water condit ions on the mechanical behavior of residual soil is not only reflected in the strength of soil, but also in its deformation. Fig. 8 shows the compression curves and soaking -induced deformation (collapsible deformation) measured under a vertical stress of 200 kPa, and the void ratios are normalized relat ive to the initial void ratio e0 . The figure also shows that the collapsible deformat ion of natural soil is negligib le (e/e0 = 0.03). However,

ACCEPTED MANUSCRIPT collapsible deformat ion demonstrated a marked increase caused by remold ing and the W –D cycles, which increased with the nu mber of wetting–drying cycles. The degree of deformat ion by soaking increased fro m 0.15 to 0.18 after two cycles of alternate wetting and dry ing. The potential values of c ollapse were obtained at different states, and the degree of the collapse was classified, as shown in Table 4. The results showed that Ic notably

PT

increased fro m 1.75% in the natural soil to 7.35% after t wo W–D cycles and the degree of co llapse transformed

RI

fro m slight collapse to moderately severe collapse. The experimental results indicated that the cyclic wetting and drying process increased the degree of collapse. Examp les of this category of climate effects are also observed in

SC

compacted residual soil (Rao and Revanasiddappa, 2006)

NU

4.4 Mineral composition

MA

A strong peak for kaolinite was observed fro m the XRD patterns of Leiqiong basaltic residual soil (Fig. 9), thereby suggesting that the most weatherable mineral, such as feldspar, were weathered well int o clay minerals.

ED

The peak for gibbsite was weak, which suggested that most aluminum was crystallized well in soil materials. The results of the XRD analysis indicate that the original mineral of the studied samples is quartz that may be produced

EP T

fro m quartz tholeiites (Ho et al., 2000). The clay minerals, wh ich comprise up to 87.4% of the soil, are kaolinite, gibbsite, illite, and a small amount of s mectite (Tab le 5). In addit ion, Leiqiong basaltic residual soil contains

AC C

hematite, wh ich is a high iron-bearing mineral. The major clay minerals of basaltic residual soil are g ibbsite and kaolinite (Tab le 5), suggesting that the presence of clay mineral is most probably attributed to weathering, even if the mineral compositions of soil fro m different tropical areas vary as a result of the differences in the weathering degree, components, and contents of the parent rock. DTA result also indicated that the main mineral in Leiqiong basaltic residual soil is kaolinite because this mineral exh ibited a large endothermic peak at 502.5 °C, and crystallized kaolinite demonstrated an extremely sharp exothermic peak at 970 °C on the DTA curve (Fig.10). 4.5 Microstructure observation and pore characteristics

ACCEPTED MANUSCRIPT A different view o f the structural surface of basalt and its derived soil is shown in Fig.11. Fig. 11a shows the structure of the unweathered basalt characterized by an intact stoma that is a typical characteristic of vesicular basalt. basalt. The basaltic residual soil has a skeleton structure, in which the aggregates formed by particles exhib it face-to-face and edge-to-face contact (Figs. 11b and 11c). Large dissolved pores formed during weathering and

PT

pedogenesis are visible (Figs. 11b and 11d), and the number of t iny pores distributed on the particle surface can be

RI

detected under high magnification (Fig. 11d). In addition, a significant bonding between particles or aggregates is observed. An example of such bonding of free iron o xides in soil was provided by Zhang et al. (2016), whose study

SC

suggested that the free iron o xides functioned mostly as a form of cladding in the basaltic residual soil. After the

NU

removal of free iron o xides using the DCB method, the mo rphology of Leiqiong basaltic residual soil changed dramat ically, as observed in Fig. 11e. The dense aggregates in the basaltic residual soil, in fact, are formed by numerous

MA

flaky p lates that are cemented together. These well-formed six-sided flaky plates with size ranging fro m 0.1μm to 3 μm

ED

are most probably kaolinite, based on the XRD and DTA results. These results indicate that free iron o xides can interact with clay minerals to form stable aggregates that resist dispersion, enhance the structural strength of soil and weaken

EP T

swelling–shrinkage capacity.

As mentioned earlier, repeated alternate wetting and drying can change the physical and mechanical properties of

AC C

residual soil. In contrast to the SEM micrograph of natural soil, the intact structure became looser after two W –D cycles and large aggregations gradually broke and then transformed into considerably smaller grain s. In addition, repeated cycles of alternate wetting and drying cause an increase in the nu mber of pores and the growth of microcracks as shown in Fig. 11f. A qualitative analysis of the pore characteristics of basaltic residual soil was obtained using the SEM . The pore types of Leiqiong basaltic residual soil include dissolved pores, a small nu mber of irregular intergranular pores, and a large nu mber of intragranular pores inside aggregations. A quantitative analysis of the corresponding pores is illustrated in Figs. 12 and 13. The pore size d istribution curves of the soil exh ibit a single peak at a diameter o f 0.01 < d < 0.1 μm

ACCEPTED MANUSCRIPT and the corresponding cumulative pore volu me curves present a steeper slope. These results indicate that intragranular pores are the main pore types found in Leiqiong basaltic residual soil. These pores, with d < 0.1 μm, comprise appro ximately 70.0% of the total pore volu me of soil, whereas dissolved pores with d > 10 μm co mp rise less than 21.0%. In addition, the comparison between the pore size distributions of the natural samples and the

PT

remolded samp les, indicates that the curve in the remo lded state exh ibits mult iple peaks and that the volumes of

RI

pores with d > 10 μm are relatively large. This interesting point is mainly attribut ed to the different microstructure alterations caused by remold ing, e.g., the increase in pore size caused by structure degradation, which agrees well

SC

with the observation in Fig. 8.

NU

Furthermore, the MIP results illustrate that Leiqiong basaltic residual soil has a high cumulative pore volu me

MA

and a small range of pore size distribution, wh ich is in good agreement with the laboratory test results on the low dry density, the high void ratio and permeability coefficient (Tab les 1 and 2). Fro m these results, and those of the

ED

SEM analysis, the characteristics of deformation and the strength of basaltic residual soil at the macro scale are attributed to their structural feature and the relative the characteristics of pores and particle associations at the

4.6 Chemical properties

EP T

micro scale.

AC C

A detailed assessment of the data confirms that the values of the chemical properties of all the specimens fro m three sites are highly similar (Table 6). Leiq iong basaltic residual soil contains less soluble salts, selenite, and carbonate, which implies that they are not the main cementing substance that causes the bonding action among particles. A pH of 6.5 indicates that Leiq iong basaltic residual soil is acidic then confirming the typical characteristic of the tropical soil acid ity as observed by other researchers (West and Dumbleton, 1970; Gid igasu, 2012). The values of the organic matter content range between 0.1% and 0.4%, suggesting that the soil only contains a lower proportion of organic matter in contrast to the soil derived fro m the sedimentary rocks (Gid igasu,

ACCEPTED MANUSCRIPT 2012). In addition, the CEC values and the specific surface area of the soil are high because of the high contents of clay minerals and free iron oxides (Zhang et al., 2016). The chemical co mpositions of typical basalt and its derived soil fro m different tropical areas are shown in Tab le 7. The LOI o f Leiqiong basaltic residual soil is ext remely h igh, but that of fresh basalt is low because most of the water is

PT

contained in the crystal structure of the clay minera ls rather than in the rock minerals. The silicon dio xide in weathered

RI

soil is generally low (Gidigasu, 2012), but all the studied samples present significant amounts of this compound, probably in the form of kaolinite, which appears in significant amounts in the silicate clay mineral in the studied

SC

residual soil. Other co mmon chemical constituents of basaltic residual soil include TiO2 , MnO, CaO, Mg O, Na 2 O, and

NU

K2 O; however, these compounds are present in min imal amounts (0% to 1%) except for TiO2 . In addition, a

MA

characteristic of basaltic residual soil is the higher proportion of sesquioxides of iron (Fe 2 O3 ) and alu minu m (A l2 O3 ) relative to other chemical co mponents. Gidigasu (2012) suggested that the silica to sesquioxide ratio Kr can be used to

ED

determine the extent of weathering and to differentiate soil types. The values of Kr < 2 are typical of o xide-rich ferrallit ic soil, ferrisols, and some ferruginous soil. Hence, Leiqiong basaltic residual soil is classified as ferrallitic soils

EP T

based on the Kr values (see Table 7) and residual soil proposed by Duchaufour (1982). Furthermo re, in contrast to the residual soil, fresh basalt has low iron and aluminu m contents and relatively high silicon content, and Kr > 2, as shown

AC C

in Table 7. These results imp ly that the transition fro m basalt to residual soil involves the chemical alterat ion of the bases (e.g. K, Na, Ca and Mg). Silica is released and leached, whereas Fe and Al remain and are accumulated. Free metal o xides in soil and their hydrates cement clay particles together, thereby significantly affecting various physicochemical propert ies (Zhang et al., 2016). Experimental data fro m different sites in Leiqiong show that the major components of the free o xides of basaltic residual soil are free iron o xides and free alu minu m o xides, whereas free silicon oxide content is relatively low (Table 8). The spectra of the area scanned via EDS analyses and the precision results of Leiq iong basaltic residual soil are illustrated in Fig. 14 and Table 9, respectively. The relat ive content of oxygen is the highest in these soils, those of

ACCEPTED MANUSCRIPT silicon, carbon, alu minu m, and iron are the second highest; and those of other elements are relatively low. The iron in Leiq iong basaltic residual soil is unevenly distributed, which implies that iron functioned as the cementation points during the bonding of particles and aggregates. The EDS results are consistent with those of the chemical composition analysis.

PT

5. Discussion

RI

Fro m the results mentioned above, the Leiqiong basaltic residual s oils possess peculiar properties. This type of soils with a high content of clay part icle is generally associated with a higher void rat io in co mparison with that

SC

of other residual soils; however, it can exh ibit so metimes acceptable strength characteristics, which poses

NU

difficulties in predicting its mechanical behavior. Therefore, in this section, an explanation behind the peculiar

MA

properties of the Leiqiong basaltic residual soils is discussed with a focus on its microstructural features, the mineralogical and chemical co mponents involving free iron o xides in the periodical W -D cycles. The basaltic

ED

residual soil in Leiqiong area is classified as ferric luvisols (FAO/UNESCO System of So il Classification), eluvial soil (Co mmittee on Tropical Soils of ISSMFE, 1985), ferrallit ic soil (Duchaufour, 1982), or o xisols (USDA So il

EP T

Taxonomy, 1975), which is also locally known as “basalt-derived residual laterite” (Zhang et al., 2016). Basalt ic residual soil exh ibits typical features that are frequently rich in kaolinite and iron o xides, as shown in the XRD,

AC C

EDS, DTA, and chemical test results. Kaolin ite, wh ich co mprises nearly 50% of minerals in Leiqiong basaltic residual soil, is the lo w-activ ity and nonswelling clay mineral. In addition, appro ximately 3% of clay mineral in the residual soil is smect ite. Th is clay mineral is prone to shrinkage and swelling depending on water content (Mitchell and Soga, 2005). Although residual soil contains less smectite, this soil still demonstrates a strong shrinkage phenomenon because of its porous structure, as shown in Fig. 11d. Consequently, a low level of expansive minerals in basaltic residual soil is considered to exert only limited influence on soil properties. Kaolinite is an inactive clay with min imal effects on the format ion and strength of aggregations (Peterson 1946), whereas sesquioxides (i.e., iron o xide and alu minu m o xide) are likely to cement aggregations to improve

ACCEPTED MANUSCRIPT soil strength. Furthermore, iron o xides in Leiq iong basaltic residual soil co mmon ly exist in free form (iron freeness is 50.2% to 58.0%, as shown in Table 7). Although free iron o xides are a secondary chemical co mposition, they function as an important cementing material in the soil structure framework (Deshpande et al., 1968). This important role is attributed to the strong interparticle attractive forces generated between the positive charges of free iron oxides and the

PT

negative charges on the surface of clay minerals under acidic condition (Davidtz and Su mner, 1965). The geotechnical

RI

significance of the free iron o xides has been investigated in our previous study (Zhang et al., 2014, 2016). We have shown that free iron o xides in Leiqiong basaltic residual soil that coat the clayey constituents and binds them into

SC

coarser aggregations, suppress their normal behavioral characteristics, whereas aggregations provide the soil with a

NU

skeleton structure. This remarkable finding reflects that basaltic residual soil, the bond formed by the free iron o xides is

MA

an important contributing factor in achieving desirab le engineering properties. In addit ion, cementation by sesquioxides are particularly sensitive to climate changes over time. As weathering progresses, cementation resulting fro m the

ED

increase in sesquioxides causes the formation of large aggregates (Tuncer and Lohnes, 1977). Ho wever, repeated alternate wetting and drying can destroy the bond between particles (Fig. 11f) and result in a decrease in strength (Fig. 6)

EP T

and in severe collapse (Fig. 8). Under such circumstances, the extent of weakening increases with the wetting rate and the number of W–D cycles, as shown in Fig.6. Th is phenomenon is possibly an indirect response to the formation of

1992).

AC C

microcracks and the weakening of microstructure failure zones under wetting or drying conditions (Kay and Dexter

A general consensus among experts states that the properties of residual soil are strongly influenced by both macroand microstructures (Tuncer and Lohnes, 1977; Hürlimann et al., 2001; Zhang et al., 2016). SEM and MIP analyses show that the microstructure of residual soil is a porous skeleton structure, because of the numerous tiny pores that can generate strong suction activity to increase the capacity of soil to absorb water. Me anwhile, the cementation of natural basaltic residual soil, wh ich results in the formation of water–stable soil aggregates, is the main factor that prevents collapsible deformat ion. Consequently, the degree of collapse in this natural residual soil under water conditions is not

ACCEPTED MANUSCRIPT severe, as shown in Table 3. Nevertheless, under the influence of repeated alternate wetting and dry ing, such as during a perennially hot and humid weather, the bond action between particles in the fabric units appears to breakdown, resulting in generation of microcracks . Th is effect does not only weaken soil structure but also provide passage for water penetration, which further results in soil softening and a decrease of the strength. Therefore, the

PT

engineering geological characteristics of basaltic residual soil with climat ic in fluences are proven via structural

RI

degradation. 6. Conclusion

SC

A tropical residual soil from Leiqiong area in China derived from basalt was studied to assess its engineering

NU

geological characteristics namely, physical, mechanical, mineralogical, chemical, and microstructural properties.

MA

The physical properties of Leiqiong basaltic residual soil are inadequate for engineering applications (e.g., high void ratio, water content, and clay particle content). However, its mechanical properties are excellent (e.g.,

ED

high strength and low compressibility). In addition, the properties of basaltic residual soil are sensitive to soaking and climate processes. This type of soil exhibits considerable strength in its natural s tate but generates large

EP T

collapsible deformation upon soaking. Its strength significantly decreases after periodical W –D cycles. This research provides an explanation for the mechanism of the distinctive properties of basaltic residual soil

AC C

from various aspects. Although mineralogy is not the principal factor that controls the engineering properties of Leiqiong basaltic residual soil, it is an important contributing factor that is closely related to particle size and its associated fabric. This experiment demonstrates that the bond formed by iron oxides in the soil plays a fundamental role as the main factor that enhances the aggregation of soil particles , which causes changes in their behaviors, such as enhanced strength, restrained swelling, and collapse. Nevertheless, under the influence of repeated cycles of alternate wetting and drying, microcracks can develop because the bonding action between particle units breaks down. This effect does not only weaken soil structure but also provide passage for water penetration, which further results in soil softening and a decrease in soil strength.

ACCEPTED MANUSCRIPT Leiqiong basaltic residual soils exhibit material composition and engineering geological characteristics similar to those of other widely distributed tropical residual s oil, and thus, we believe that the work presented in this paper will be helpful in explaining the peculiar properties of a basaltic residual soil in general. Acknowledgements

PT

The present study had the financial support of the National Natural Science Foundation of China (Grant No.

RI

41472292; 41372314; 41672293), the Natural Science Foundation of Hubei Province, China (Grant No. 2011CDB406), and the Key Laboratory of Geotechnical and Underground Engineering at Tongji University, Ministry of Education,

SC

China (Grant No. KLE–TJGE– B1103).

NU

References

MA

Ahmed, S., Lovell, C.W., Diamond, S., 1974. Pore sizes and strength of compacted clay. J. Geotech. Geoenviron. 100(4), 407–425.

ED

Agus, S. S., Leong, E. C., Rahardjo, H., 2005. Estimating permeability functions of Singa pore residual soils. Eng. Geol. 78(1), 119–133.

EP T

ASTM D 1557–02, 2006. Standard test methods for laboratory compaction characteristics of soil using modified effort. Annual Book of ASTM Standards, American Society for Testing and Materials, West Conshohocke n, PA.

AC C

ASTM D 2487–11, 2011. Standard practice for classification of soils for engineering purposes (unified soil classification system). Annual Book of ASTM Standards, American Society for Testing and Materials, West Conshohocken, PA.

ASTM D 4943–08. 2008. Standard test methods for shrinkage factors of soils by the wax method. Annual Book of ASTM Standards, American Society for Testing and Materials, West Conshohocken, PA. ASTM D 5333–92, 2002. Standard test method for measurement of collapse potential of soils. Annual Book of ASTM Standards, American Society for Testing and Materials, West Conshohocken, PA. Benatti, J. C. B., Miguel, M. G., 2013. A proposal of structural models for colluvial and lateritic soil profile from

ACCEPTED MANUSCRIPT southwestern Brazil on the basis of their collapsible behavior. Eng. Geol. 153, 1–11. Biscaye, P. I., 1965. Mineralogy and sedimentation on recent deep sea clay in the Atlantic Ocean and adjacent seas and oceans. Geol. Soc. Am. Bull. 76: 803–832. Blight, G. E., Leong, E. C., 2012. Mechanics of residual soils, the second edition. The Netherlands: A. A. Balkema

RI

British Standard Institution, 1992. Laboratory testing. BS: 7591 BSI, London.

PT

Publishers.

British Standard Institution, 1990. Methods of test for soils for civil engineering purposes. Part 3. Chemical and

SC

electro-chemical tests. BS: 1377 BSI, London.

NU

Carter, D. L., Heilman, M. D., Gonzalez, C. L., 1965. Ethyleneglycol monoethyl ether for determining surface area of

MA

silicateminerals. Soil Sci. 100, 356–360.

Casagrande, A., 1948. Classification and identification of soils. T. Am. Soci. Civil Eng. 113(1): 901–930.

ED

Chiu, C. F., Ng, C. W., 2014. Relationships between chemical weathering indices and physical and mechanical properties of decomposed granite. Eng. Geol. 179, 76–89.

EP T

Committee on Tropical Soils of ISSMFE, 1985. Peculiarities of geotechnical behavior of tropical lateritic and saprolitic soils: progress report (1982-1985). São Paulo-SP, Brazil, Edile.

AC C

Day, P. R., 1965. Particle fractionation and particle size analysis. In Methods of soil analysis. Part 1. Edited by C.A. Black. American Society of Agronomy Inc., Madison, Wis., Agronomy, No. 9, pp. 545–567. Davidtz, J. C., Sumner, M. E., 1965. Blocked charges on clay minerals in sub –tropical soils. J. Soil Sci. 16(2), 270–274. Deshpande, T. L., Greenland, D. J., Quirk, J. P., 1964. Role of iron oxides in the bonding of soil particles. Nature 201:107–108 Dexter, A. R., Kroesbergen, B., Kuipers, H., 1984. Some mechanical properties of aggregates of top soils from the Ijsselmeer polders. II: Remoulded soil aggregates and the effects of wetting and drying cycles. Neth. J. Agr. Sci. 32, 215–227.

ACCEPTED MANUSCRIPT De Vallejo, L. G., Salas, J. J., Jimenez, S. L., 1981. Engineering geology of the tropical volcanic soils of La Laguna, Tenerife. Eng. Geol. 17(1), 1–17. Duchaufour, P., 1982. Pedology: pedogensis and classification. London. George Allen and Unwin Ltd. FAO., 1974. Soil Map of the World, 1–10. Food and Agriculture Organization of the United Nations and UNESCO,

PT

Paris 1: 5,000,000.

RI

Gidigasu, M. D., 2012. Laterite soil engineering: pedogenesis and engineering principles (Vol. 9). Elsevier. Gutierrez, N. H., Nóbrega, M. T., Vilar, O. M., 2009. Influence of the microstructure in the collapse of a residual clayey

SC

tropical soil. B. Eng. Geol. Environ. 68, 107–116.

MA

area, southern China. J. Asian Earth Sci. 18(3), 307–324.

NU

Ho, K. S., Chen, J. C., Juang, W. S., 2000. Geochronology and geochemistry of late Cenozoic basalts from the Leiqiong

Huat, B. B. K., Gue, S. S. Ali, F. H., 2004. Tropical Residual Soils Engineering. London: A. A. Balkema Publishers.

ED

Huat, B. B. K., Toll, D. G., Prasad, A., 2012. Handbook of tropical residual soils engineering. CRC Press. Hürlimann, M., Ledesma, A., Martı, J., 2001. Characterisation of a volcanic residual soil and its implication s for large

EP T

landslide phenomena: application to Tenerife, Canary Islands. Eng. Geol. 59(1), 115–132. Indrawan, I. G. B., Rahardjo, H., Leong, E. C., 2006. Effects of coarse–grained materials on properties of residual soil.

AC C

Eng. Geol. 82(3), 154–164.

Jackson, M. L., Whittig, L. D., Pennington, A. S., 1950. Segregation procedure for the mineralogical analysis of soils. Soil Sci. Soc. Am. J. 14: 77–81.

Jackson, M. L., 1965. Free oxides, hydroxides, and amorphous aluminosilicates. Methods of soil analysis: part 1, physical and mineralogical properties, including statistics of measurement and sampling. American Society of Agronomy, Madison, pp 578–601. Kay, B. D., Dexter, A. R., 1992. The influence of dispersible clay and wetting/drying cycles on the tensile stre ngth of a red–brown earth. Aust. J. Soil Res. 30, 297–310.

ACCEPTED MANUSCRIPT Kim, C. K., Kim, T. H., 2010. Behavior of unsaturated weathered residual granite soil with initial water contents. Eng. Geol. 113(1), 1–10. Lao, G. L., 1988. Distribution survey of soils in the Hainan Island and their physic–mechanical properties. Rock Soil Mech. 9(4): 51–61.

PT

Lohnes, R. A., Demirel, T., 1983. Geotechnical properties of residual tropical soils. Proc. ASCE Convention, Huston,

RI

Texas, 150–166.

Madu, R. M., 1977. An investigation into the geotechnical and engineering properties of some laterites of Eastern

SC

Nigeria. Eng. Geol. 11(2), 101–125.

MA

sodium bicarbonate. Clays Clay Miner. 7: 317–329.

NU

Mehra, O. P., Jackson, M. L., 1960. Iron oxide removal from soils and clays by dithionite –citrate system buffered with

Ministry of Construction, 1999. Standard for soil test method, GB/T 50123–1999. Ministry of Construction, Beijing.

ED

Mitchell, J. K., Soga, K., 2005. Fundamentals of soil behavior, 3rd Ed. New Jersey, John Wiley and Sons. Moon, V., Jayawardane, J., 2004. Geomechanical and geochemical changes during early stages of weathering of

EP T

Karamu Basalt, New Zealand. Eng. Geol. 74, 57–72.

Morton, S. R., 2000. Factors influencing shear strength and slope failures in basalt soils of north eastern New South

AC C

Wales. In ISRM International Symposium. International Society for Rock Mechanics. Munsell Color Commpany Inc., 1975. Munsell soil color charts. Munsell Collor Company, Inc., Baltimore, Md. Nishida, K., 1998. Peculiarities of properties and problematic behaviour of residual soils. In Proceedings of International Symposium on Problematic Soils, Sendai, Tohoku, Japan (pp. 865–884). Ola, S. A., 1980. Mineralogical properties of some Nigerian residual soils in relation with building problems. Eng. Geol. 15(1–2), 1–13. Ogunsanwo, O., 1989. Some geotechnical properties of two laterite soils compacted at different energies. Eng. Geol. 26(3), 261–269.

ACCEPTED MANUSCRIPT Osinubi, K. J., Nwaiwu, C. M. O., 2008. Desiccation–induced shrinkage in compacted lateritic soils. Geotech. Geol. Eng. 26, 603–611. Otalvaro, I. F., Neto, M. P. C., Delage, P., Caicedo, B., 2016. Relationship between soil structure and water retention properties in a residual compacted soil. Eng. Geol. 205, 73–80.

PT

Pansu, M., Gautheyrou, J., 2006. Handbook of soil analysis– mineralogical, organic and inorganic methods. Springer,

RI

Heidelberg, Berlin.

Peterson, J. B., 1946. The role of clay minerals in the formation of soil s tructure. Soil Sci. 61, 247–256.

SC

Pushparajah, E., Amin, L. L., 1977. Soils under Hevea in peninsular Malaysia and their management. Rubber Research

NU

Inst. of Malaysia, Kuala Lumpur.

by weathering. Eng. Geol. 73(1), 157–169.

MA

Rahardjo, H., Aung, K. K., Leong, E. C., Rezaur, R. B., 2004. Characteristics of residual soils in Singapore as formed

ED

Rahardjo, H., Satyanaga, A., Leong, E. C., Ng, Y. S., Pang, H. T. C., 2012. Variability of residual soil properties. Eng. Geol. 141, 124–140.

EP T

Rahardjo, H., Melinda, F., Leong, E. C., Rezaur, R. B., 2011. Stiffness of a compacted residual soil. Eng. Geol. 120(1), 60–67.

AC C

Rao, S. M., Revanasiddappa, K., 2006. Influence of cyclic wetting drying on collapse behaviour of compacted residual soil. Geotech. Geol. Eng. 24(3), 725-734. Schwertmann, U., 1993. Relations between iron oxides, soil color, and soil formation. In: Bigham, J.M., Ciolkosz, E.J. (Eds.), Soil Color, Special Pub., vol. 31. Soil Science Society of America, Madison, WI, pp. 51–70. Sparks, D. L., Page, A. L., Loeppert, P. A., Soltanpour, P. N., Tabatabai, M. A., John ston, C. T., Sumner, M. E., 1996. Methods of soil analysis Part 3: chemical methods. Soil Science Society of America and American Society of Agronomy, Madison, WI. Tang, D. X., Wang, Q., Zhang, Q. Y., Li, B. H., 1992. Engineering geological study of residu al laterite soil of basalt in

ACCEPTED MANUSCRIPT Leiqiong region. J. Changchun Univ. Earth Sci., 22(3), 317–323. Tuncer, R. E., Lohnes, R. A., 1977. An engineering classification of certain basalt derived lateritic soils. Eng. Geol. 11, 319–339. USDA Soil Taxonomy, 1975. A basic system of soil classification for making and interpreting soil surveys. Agriculture

PT

Handbook, Vol. 436, US Department of Agriculture, Washington, DC.

RI

Wang, C. S., Lei, X. W., Meng, Q. S., Yang, X., 2012. Testing study on strength of red –brown soil of weathered basalt soil in Guizhou. Geo. Invest. Surv. 1, 12–19.

SC

West, G., Dumbleton, M. J., 1970. The mineralogy of tropical weathering illustrated by some west Malaysian soils. Q. J.

NU

Eng. Geol. Hydroge. 3(1), 25–40.

MA

Yan, W. M., Li, X. S., 2012. Mechanical response of a medium–fine–grained decomposed granite in Hong Kong. Eng. Geol. 129, 1–8.

ED

Zhang, S., 2009. Geological formation names of China (1866–2000). Springer. Zhang, X. W., Kong, L. W., Li, J., 2014. An investigation of alterations in Zhanjiang clay p roperties due to atmospheric

EP T

oxidation. Géotechnique 64(12), 1003–1009.

Zhang, X. W., Kong, L. W., Cui, X. L., Yin, S., 2016. Occurrence characteristics of free iron oxides in soil

AC C

microstructure: evidence from XRD, SEM and EDS. B. Eng. Geol. Environ. 75(4), 1493–1503.

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC C

EP T

ED

Figure 1

RI

PT

ACCEPTED MANUSCRIPT

AC C

EP T

ED

MA

NU

SC

Figure 2

PT

ACCEPTED MANUSCRIPT

AC C

EP T

ED

MA

NU

SC

RI

Figure 3

PT

ACCEPTED MANUSCRIPT

AC C

EP T

ED

MA

NU

SC

RI

Figure 4

PT

ACCEPTED MANUSCRIPT

AC C

EP T

ED

MA

NU

SC

RI

Figure 5

PT

ACCEPTED MANUSCRIPT

AC C

EP T

ED

MA

NU

SC

RI

Figure 6

ACCEPTED MANUSCRIPT

AC C

EP T

ED

MA

NU

SC

RI

PT

Figure 7

PT

ACCEPTED MANUSCRIPT

AC C

EP T

ED

MA

NU

SC

RI

Figure 8

PT

ACCEPTED MANUSCRIPT

AC C

EP T

ED

MA

NU

SC

RI

Figure 9

ACCEPTED MANUSCRIPT

AC C

EP T

ED

MA

NU

SC

RI

PT

Figure 10

AC C

EP T

ED

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Figure 11

PT

ACCEPTED MANUSCRIPT

AC C

EP T

ED

MA

NU

SC

RI

Figure 12

PT

ACCEPTED MANUSCRIPT

AC C

EP T

ED

MA

NU

SC

RI

Figure 13

SC

RI

PT

ACCEPTED MANUSCRIPT

AC C

EP T

ED

MA

NU

Figure 14

ACCEPTED MANUSCRIPT Figure captions: Fig. 1. Geological map showing the simp lified d istribution of the basalt in Leiqiong area, China and soil sampling location. Fig. 2. Geological section of the selected site.

PT

Fig. 3. Grain size distribution curves for the basaltic residual soil with different treated methods.

RI

Fig. 4. Casagrande plasticity chart for the basaltic residual soils. Date fro m Gu izhou, New South Wales, Northern Hainan Island and Brazil, referred fro m Wang, et al, 2012; Morton, 2000; Lao, 1988 and Gutierrez et al., 2009,

SC

respectively.

NU

Fig. 5. Compaction curves of the basaltic residual soil using the standard heavy compaction test.

MA

Fig. 6. Curves of unconfined compressive strength test of the natural and saturated basaltic residual soils and the soils after different W–D cycles.

ED

Fig.7 Undrained stress–strain behaviour of the natural and saturated soils under isotropic consolidation from CU tests Fig.8 Collapsible behavior of the natural and remolded soils and the soils for different W–D cycles.

EP T

Fig. 9 X-ray diffraction patterns of the basaltic residual soil, where Qt z: quart z; Ill: illite; Mnt: smectite; Kln: kaolinite; Gbs: Gibbsite; Hem: hematite.

AC C

Fig. 10 Differential thermal curves of clay minerals in the basaltic residual soil. Fig.11 SEM micrographs of soil surfaces, a: unweathered basalt; b to d: basaltic residual soils; e: basaltic residual soil with free iron o xides removed; f: basaltic residual soil after t wo W–D cycles. Magnification, a, c and f: 2000; b: 800; d: 5000; e: 10000. The area I and II were tested areas for EDS. Fig. 12 Pore size distribution curves of the basaltic residual soil. Fig.13 Cumulative pore volume curves of the basaltic residual soil. Fig.14 EDS spectra of two areas on the surface of the basaltic residual soil. Spectras a and b corresponds to areas in Figs. 11c and 11d, respectively.

ACCEPTED MANUSCRIPT Table 1 Physical properties of basaltic residual soils

Dry 3 density ρd (g/cm )

Specific gravity Gs

Initial void ratio e0

Liquid limit wL (%)

Plastic wp (%)

38.09

1.64

1.18

2.80

1.37

58.01

31.10

Leiqiong area, China b

34.3

—

—

—

1.36

55.2

—

Leiqiong area, China c

30.4

—

—

—

1.24

56.6

—

Guizhou, China d

40.1

1.71

—

2.77

75.9

47.2

New South Wales, Australia e;

37.2

—

1.34

PT

—

—

—

53.9

33.6

23

—

1.4

3.0

—

45–49

—

Brazil g

29.5–32.4

1.33–1.66

1.01–1.27

3.0–3.14

1.43–2.06

57–73

39–49

Hawaiih

22.82–52.2

—

1.17–1.48

2.7–3.0

1.02–1.90

40–72.6

35.4–54

11.7–40.6

—

50–84

29–44

31–85

26–52

Segamat, Malaysia

Kuantan, Malaysia i

i

17.3–54.2

NU

f

—

2.54–2.9

—

8 2.62–2.8

—

Note: a Result are the average of 10 groups. Data from

6 b

Tang, et al., 1960; c Lao, 1988; d Wang, et al.,

2012; e Morton, 2000; f Lohnes and Demirel, 1983; g Gutierrez, et al., 2009; h Tuncer and Lohnes, 1977; i

AC C

EP T

Huat et al., 2012.

ED

Hawaii

a

MA

Leiqiong area, China

w

SC

Density ρ 3 (g/cm )

RI

Moisture content (%)

Location

ACCEPTED MANUSCRIPT

Table 2 Permeability and swelling-shrinkage properties of basaltic residual soils

K

(cm×10 /s) By laboratory test

ratio δef (%)

Swelling ratio with load 50 kPa δcp (%)

Natural soil (w = w) Linear shrinkage rate δs (%)

situ 77.70

35.47

Nearly 0

5.71

—

—

<40.0

Nearly 0

66 –67

—

12

—

shrinkage rate δv (%)

Linear shrinkage rate δs (%)

V

s

(

2

1.5 –3.5

5.0–5.5

23.0 –38.0

2

1.47

5.53

—

—

SC NU

Volume

15.36

Note: a Result is the average of 10 groups. Data from b Tang, et al., 1960; c Lao, 1988.

MA

Remolded soil (w= w

10.06

RI

5.83

ED

China c

injection tests in

Free swell

EP T

b

By water

AC C

China a

China

coefficient

-6

PT

Permeability

ACCEPTED MANUSCRIPT Table 3 Mechanical properties of basaltic residual soils Unconfined Compressio Location

compressive

strength qu (kPa)

n index Cc Natural state Leiqiong area, China

Shear strength parameters based on triaxial consolidation undrained (CU) and drained shear (CD) test Natural state

Saturated

ccu

state

(kPa)

Saturated state

φcu (°)

c′cu (kPa)

φ′cu (°)

cscu (kPa)

58.3

21.25

32.5

0.52

87.69

68.00

43.30

21.55

a

—

—

—

39

34.5

Leiqiong area, China b

—

—

—

30.8

18.7

Guizhou, China c

—

—

—

69.01

19.66

Hawaii d

—

—

—

22 –40

45–58

Hawaii e

—

—

—

7 –50

27–52

Leiqiong area, China

D E

A M

U N

C A

E C

20.66 —

—

—

63.15

15.23

—

—

—

—

Data from a Tang, et al., 1960; b Lao, 1988; c Wang, et al., 2012; d Lohnes and Demirel, 1983; e Tuncer and Lohnes, 1977.

T P

(°)

I R

C S —

T P

φscu

c′ scu (kPa)

φ′scu (°)

cscd (kPa)

φscd (°)

47.7

20.67

48.9

19.7

ACCEPTED MANUSCRIPT

Table 4 Collapse potential and collapse degree of natural and remolded soils and samples after different W-D cycles

a

Sample state

e/e0 at 200 kPa

Collapse potential I c /%

Degree of Collapse a

Natural

0.03

1.75

Slight

Remolded

0.06

3.45

After 1 W-D cycle

0.15

6.15

After 2 W-D cycle

0.18

7.35

I R

T P

C A

E C

C S

U N

Degree of collapse is according to ASTM D 5333-92.

D E

T P

Moderate

A M

Moderately severe Moderately severe

ACCEPTED MANUSCRIPT

T P

I R

C S

U N

D E

T P

C A

E C

A M

ACCEPTED MANUSCRIPT

Table 5

Mineral composition (%)

Location Leiqiong area, China (depth 1.5-2.0 m) Leiqiong area, China (depth 2.0-2.5 m) Leiqiong area, China (depth 2.0-2.5 m) Leiqiong area, China a Leiqiong area, China Hawaii

Mineral composition of basaltic residual soil

b

c

Quartz

Illite

smectite

Kaolinite

Gibbsite

Hematite

4.8

9.8

2.9

59.9

15.6

7

6.6

8.9

3.5

61.6

7.0

7.2

2.1

61

1–10 (4.5)

Trace

Trace

0–10 (3)

Trace

I R 12.6

6.8

18.5

4.2

70 –85 (74.5)

0–15 (9.5)

0–5 (1.5)

70 –80 (77)

5–15 (10)

Trace

C S

U N

A M Trace

T P

Main mineral are Kaolinite, Gibbsite and Hematite

D E

Data from a Tang, et al., 1960 ; b Lao, 1988; c Tuncer and Lohnes, 1977. Note: The values in the brackets are average.

T P

C A

E C

ACCEPTED MANUSCRIPT Table 6 Chemical analysis results of basaltic residual soil

Organic

Cation exchange

content (%)

capacity (CEC)

salts

(meq/100 g)

(%)

5.0

0.34

14.63

0.02

Leiqiong area, China

a

5.2

0.41

13.33

Leiqiong area, China

b

5.62

0.13

13.01

Soil pH

Location

Leiqiong area, China

Soluble

Selenite (as

Carbonate (as

Total specific

External

calcium

calcium

surface area

specific

sulfate)

carbonate)

content (%)

content (%)

0.13

0.06

0.07

0.23

0.27

0.02

0.29

0.14

U N

D E

T P

C A

E C

A M

(m /g)

T P

I R

C S

Data from a Tang, et al., 1960 ; b Lao, 1988.

2

surface area (m 2/g)

129

56.22

104.4

—

112.3

—

ACCEPTED MANUSCRIPT

Table 7 Chemical composition and LOI of basaltic residual soil Relative contents (%)

Resudal soil

33.00

17.85

a, China

Fresh basalt

60.30

4.90

a, China a

Resudal soil

37.38

16.52

a, China b

Resudal soil

37.39

16.77

Fresh basalt

47

a

0.9 8 6.6 0 0.9 1 1.7

13 (Fe 2O3+FeO) b

TiO2

MnO

30.52

1.75

0.11

16.45

0.40

Trace

26.88

2.49

0.18

25.54

3.5

14

—

c

O 0.2 1

3.2 5

0.0 2

0.18

0.0

—

2 12

Na2O

K2O

(LOI) (%)

0.17

0.09

0.05

15.25

1.3

1.30

1.40

1.00

5.2

0.16

0.07

0.15

—

1.7

0.21

0.04

0.09

—

1.7

8

3

—

—

Mor

4.36

AC C

EP T

ED

MA

Data from Tang, et al., 1960 ; Lao, 1988; West and Dumbleton, 1970. Kr=S/R, S= Molecular SiO2, R (=Molecular sesquioxides) = Molecular Al2O3 + Molecular Fe2O3.

48

Kr

MgO

PT

a, China

O

Al 2O3

Loss on ignition

RI

Fe2O3

Ca

SC

SiO2

Fe

NU

Samples

ACCEPTED MANUSCRIPT

Table 8 Free metal oxide content of basaltic residual soil Free oxide content (%)

Leiqiong area, China

Free oxide 10.69

Leiqiong area, China a

8.75

Leiqiong area, China b

iron

11.06 a

Iron activation

56.77

9.54

Free aluminum oxide 4.36

1.36

Total oxide 16.41

4.34

2.82

15.91

50.2

—

3.43

2.43

16.52

58

—

b

free

Iron freeness (%)

Free silicon oxide

PT

Location

AC C

EP T

ED

MA

NU

SC

RI

Data from Tang, et al., 1960; Lao, 1988. Iron freeness is the ratio of the percent content of free iron oxide to total iron; Iron activation is the content ratio of amorphous iron oxide content to free iron oxide.

49

ACCEPTED MANUSCRIPT Table 9 Energy dispersive spectrometer (EDS) results of two test areas of Leiqiong basaltic residual soil.

Atomic percent (%)

Area I

Area II

Area I

Area II

C

7.00

7.14

10.61

10.86

O

64.06

63.61

72.89

72.61

Al

11.31

10.95

7.63

7.41

Si

9.57

9.58

6.20

6.23

Ti

0.70

0.82

0.27

0.31

Fe

7.36

7.90

2.40

PT

Mass percent (%)

2.58

AC C

EP T

ED

MA

NU

SC

RI

Elements

50

AC C

EP T

ED

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

51

ACCEPTED MANUSCRIPT Highlights Engineering geological characteristics of the residual soil are studied from various aspects.



Geotechnical properties of basaltic residual soil in different areas are summarized and analyzed.



Effect of climate processes on mechanical properties of residual soil is investigated.



The bond formed by iron oxides in residual soil is the main factor for increasing strength.



Mechanism of the peculiar mechanical behavior of basaltic residual soil is proposed.

AC C

EP T

ED

MA

NU

SC

RI

PT



52