Humid tropical expansive soils of Trinidad: Their geotechnical properties and areal distribution

Engineering Geology, 34 (1993) 27-44

27

Elsevier Science Publishers B.V., Amsterdam

Humid tropical expansive soils of Trinidad: Their geotechnical properties and areal distribution K.V. R a m a n a Fulbright Fellow, Dept. of Civil Engineering, Lehigh University, Beth&hem, PA 18015, USA (Received March 17, 1992; revised version accepted November 25, 1992)

ABSTRACT Expansive soils occur in abundance in the central and southern regions of Trinidad and contribute to extensive landsliding, and building and pavement damage. Since there is little documentation available in this country on these soils, this study is undertaken to develop the necessary data to cope up with the deleterious nature of these soils. Disturbed and undisturbed soil samples are obtained from a large number of sites covering both regions, and tested for grain size, classification, suction and potential swell measurements. Based on the results of these tests a modified plasticity chart and a multi-parameter criteria are presented to identify the expansive soils according to the degree of their expansivity. The results have shown that these soils exist in an overconsolidated state and their in situ water contents are high and are generally at plastic limit. As a result their potential swelling capability is apparently somewhat restrained. Further, a hazard map is presented showing areas of potential danger from expansive soils to light engineering structures.

Introduction Plastic clays exhibiting volume changes when subjected to moisture variations due to seasonal climatic conditions or artificial causes are termed expansive soils or more correctly, active soils. These soils, which lie above the water table, undergo shrinkage on drying and swelling on wetting. When swelling is restrained due to the presence of a specific structure, these soils exert swelling pressure, which is deleterious to the stability and efficient performance of that structure. Structures that are usually affected by this phenomenon are the lighter structures, such as single-story buildings, pavements and buried water pipes. Because the soil's activity is not a sudden dramatic phenomenon like other natural hazards, the only direct impact it causes, is on those who suffer from it directly. By the year 2000, the estimated damage attributed to expansive soil in the United States alone will be 4.5 billion dollars annually (Chen, 1988), so on a world-wide basis the cost of the total damage will be enormous. This explains why the expansive soil phenomenon is known as a silent natural hazard. 0013-7952/93/$06.00

Expansive soils are found in abundance in central and southern Trinidad, where several structures have been severely damaged. In addition, on slopes these soils have contributed to extensive landsliding, especially in central Trinidad. As such, these soils have become a serious problem. The geographical location of Trinidad is shown in the inset of Fig. 1. The causes of failures of structures on expansive soils are partly due to the inability to recognize the existence of such soils by design engineers and partly due to the lack of suitable knowledge required to cope with them. Despite the importance of the problem, there has been little research into expansive soils in Trinidad. This is critical because the government has plans to embark on low-cost housing and school building projects, and most of these projects are likely to be located in areas of expansive soils. Although there is an abundance of information on expansive soils in the literature, data cannot be universally applied because of the complexity of the behavior of these soils. The complexity comes especially from the drastic changes that can occur

© 1993 - - Elsevier Science Publishers B.V. All rights reserved.

28

K.V.

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NaparimaPeneplain

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GEOTECHNICAL PROPERTIES, AREAL DISTRIBUTION OF HUMID TROPICAL EXPANSIVE SOILS, TRINIDAD

in the soil-moisture condition. Thus, for an effective solution to the problem of building and pavement design on expansive soils, an understanding of the local conditions and the basic properties of the local soils is essential. It is, therefore, the aim of this study to develop the necessary information on Trinidad soils, which is presently lacking. Climate

The climate is of the hot humid, tropical marine type, a distinguishing feature is that the annual temperature range is small - - from 25°C in January and February to 33°C in the hotter months. Rarely does the temperature rise above 33°C. The dry season is between January and April, the wet season is from May to December, with a brief dry period in September and October. In the rainy season the humidity level rises to as high as 90%. Annual rainfall ranges from 1250 to 2750 mm. Formation of expansive soils The complex geology of the island has resulted in a diversity of soils and abrupt variations within short distances. Soils owe their nature largely to parent material under the combined influence of geomorphology and climate. Since geology and geomorphology had their influence on soil development in the island, the soils are both residual and transported. The residual soils abundantly occur in the central and southern ranges and in the dissected peneplains on either side of the central range (Fig. 1). The transported soils occupy areas of terraces and alluvial plains. The residual soils inherit the expansive nature from their parent materials which are predominantly clay shales, marls, clay stones (argillaceous sediments or rocks) and conditioned by the weathering process. Figure 2 shows the geological features of the island. Depending on the amount and type of the clay minerals present, different categories of soils exhibit different degrees of expansion. Young argillaceous material (younger than Mesozoic) is found to contain greater quantities of montmorillonite and hence is highly expansive (Snethen, 1975). However, older argillaceous materials could have been subjected to metamorphism, and other pro-

29

cesses of great pressure (either due to tectonic activity or greater overburden pressures), consolidation, cementation, etc. Due to the effect of diagenesis, the montmorillonite clay minerals would have been reduced to more stable clay minerals like kaolinite. Hence, those materials exhibit little or no expansive nature. Generally, the severity of the problem of expansive soils depends on the following factors: (a) soil type; (b) climatic conditions; and (c) drainage. Soil type

Lithologically the island can be divided into two zones: (a) a metamorphic zone (northern range), and (b) a sedimentary zone (central and southern ranges). Dissected peneplains, namely, the northern peneplain and the Naparima peneplain, situated on either side of the central range, form the most important landform unit concerning the problem of expansive soils. The residual soils occurring in these dissected peneplains are heavy clays and silty clays. These soils are derivatives of young sedimentary parent materials of clay shales, marls, clay stones etc., and contain significant amount of montmorillonite, up to about 40% (Taylor, 1987). Hence, they are prone to be expansive soils. These soils have been subjected to frequent uplift and burial due to tectonic activity resulting in overconsolidation. A generalized soil map of Trinidad is shown in Fig. 3. Climate

The amount of montmorillonite present in the soil is dependent on the parent material and diagenetic factors. The diagenetic factors that contribute to the formation of the montmorillonite clay material are: (a) High overburden pressure (because the parent material is situated at considerable depths) and temperature increase with depth. (b) Chemical changes produced by pore solutions. (c) The time period over which the above factors act. The strange fact is that these diagenetic factors which contribute to the formation of this mineral

30

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32

with passage of adequate geological time ultimately lead to the destruction of the mineral. That is why rocks older than Paleozoic are found to contain considerably less montmorillonite than rocks of Mesozoic or Cenozoic ages. The physical and chemical weathering of the argillaceous (i.e., containing clay matter) sediments and rocks produce changes in the expansive character of soils. The depth of weathering of a soil largely depends upon climate and topography. The important aspect of the physical weathering is cyclic wetting and drying of the soil. During the wet periods the clay mineral surfaces absorb water and lose it by evaporation during the dry periods. This results in the development of cracks disrupting the double-layer water. This process quite often may result in a reduction in the potential of the soil to volume change. Chemical weathering may lead to the destruction of mineral constituents and the formation of new minerals. These new minerals are more stable in the weathering environment. They have specific gravity lower than the original material and are hydrated resulting in an increase in soil volume (Snethen, 1975). The formation of montmorillonite occurs due to extreme disintegration of the parent minerals, strong hydration and restricted leaching. Restricted leaching allows certain cations (e.g., magnesium, calcium, sodium and iron) to accumulate which is necessary for this process (Chen, 1988). This is only possible in climates where there is enough water in the soil for hydration, without extensive rainfall that may remove these accumulated cations by leaching. Therefore, the interplay of the diagenetic factors and the weathering action determines the mineral content and the consequent expansivity of soils. However, the actual magnitude of the volume changes the soil finally undergoes is further moderated by two climate factors, namely: (a) rainfall, and (b) evapo-transpiration. In areas where evapo-transpiration is greater than the rainfall, the result is a moisture deficiency in the soil. In the rainy season the soil which is moisture-starved absorbs large quantities of water and swells substantially. Therefore, the initial water content of a soil is a prime factor controlling the magnitude of the volume change. This water content in the soil is controlled by climate and ground water regime. The effect of these climatic

K.V. R A M A N A

factors is best explained by the Thornthwaite Moisture Index (TMI) (Thornthwaite, 1948). Figure 4 shows the soil-humidity map with moisture index values and possible heave conditions under covered areas. High relative humidity levels in Trinidad indeed keep the moisture levels high and hence the swelling capacity of the soils is somewhat restrained. But there are areas having strong dry periods as well, which deplete the soil of its moisture. Therefore, the swelling phenomenon is very explicit and the numerous failures of buildings are a witness to this.

Drainage Drainage is also an important factor in the behavior of expansive soils. Drainage has two components, external drainage and internal drainage. The external drainage is determined by the run-off and the internal drainage is determined by the rate at which the water infiltrates into the ground (seepage). In case of the former, the slope of the terrain is the controlling factor. This, in turn, depends on the landform or the topography. The island of Trinidad has five physiographic divisions and each has different topographic features. Poor surface drainage results in the accumulation of water or ponding effect which can provide a source of moisture for expansive soils to absorb and swell. Poor surface drainage is a frequent problem for building foundations, pavements, etc. on expansive soils. Internal drainage is classified as: (a) free drainage; (b) imperfect drainage; and (c) impeded drainage, depending on the ease with which the water seeps into the ground. This again depends largely on the soil type. Expansive soils occur mostly in flat areas but they are also found on moderate slopes. In central Trinidad the predominant soil types are heavy clays and silty clays and almost all of them are characterized by imperfect or impeded drainage. These soils are generally saturated in the wet season and desiccated during the dry season. The mottled subsoil provides an indication of the depth of desiccation. The depth of desiccation is the depth to which evaporation influences the soilmoisture profile. It is an indication of the depth to which expansive soils are active.

GEOTECHNICAL PROPERTIES, AREAL DISTRIBUTION OF HUMID TROPICAL EXPANSIVE SOILS, TRINIDAD

Legend

i Climate Moisture ISoil humidity Index Humid Continuously moist (no dry months Humid Weak dry season (I month)

40

EguilibrJ.um Suction (pF)

Active Depth (m)

Moisture regime Heave condition l:mostly unaffected 2.significant 3 edge lift dominant

2

1-2

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2-2.5

1-2

l.small 2.present 3.edge lift

10-20

2.5-3 0

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Moist subhumid Marked dry season (2-3 months)

33

3.edge lift

Moist subhumid Intense dry season (4-6 m o n t h s )

0-10

3.0-3.5

3-4

l.central moisture accumulation 2.small

3.centre lift if water table is deep. i, soil moisture variation between the edge aDd centre of covered areas 2, effect of cyclic moisture change around the edge 3 possible heave pattern NOTE: Moisture index values a r e approximate and can vary 10-20 units from year to year Dry period : Monthly rainfall 50-100mm or less. Evaporation exceeds rainfall. Wet period Fig. 4. Soil humidity map.

: Monthly

rainfall

100mm or more. Rainfall exceeds

evaporation.

KV RAMANA

34

Major expansive soil groups

piing was done using Shelby tubes and the samples were properly sealed and stored in a temperaturecontrolled room. Though it was recognized that the sampling should be done during the dry season, when the soil would have the lowest moisture and give maximum swell, it was not possible to do this in all cases, because soils were too hard for undisturbed sampling. Disturbed and undisturbed soil samples are collected from open pits excavated for this purpose at several sites covering both the central and southern regions (Fig. 6).

The expansive soils may be classified into calcareous and non-calcareous types (Fig. 5). The Talparo series is the most prevalent of the noncalcareous type, occurring in both the central and southern regions, whereas the Tarouba series is the most widely occurring soil of the calcareous type. Clay shales possess a high percentage of montmorillonite content and hence, are bound to be highly expansive types.

Soil sampling and testing

Soil tests

Sampling It should be noted here that the evaluation of swell behavior of a soil using undisturbed soil samples and specialized swell tests is a difficult and expensive process for practicing engineers and small builders. Therefore, there is need for simple routine tests to be performed on remoulded samples to achieve the same purpose. But then the question arises as to how the behavior of the natural soil fabric can be related to the destructed

The expansive soil types of Trinidad consist of shales, marls, clays and clay alluvium. To cover most of these soils encountered in the central and southern regions, several locations were selected for soil sampling and laboratory testing. Disturbed and undisturbed soil samples were obtained from open pits at a depth of 1 m at which the foundations of light structures are commonly laid. Sam-

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Fig. 5. Major expansivesoil groups.

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GEOTECHNICAL PROPERTIES, AREAL DISTRIBUTION OF HUMID TROPICAL EXPANSIVE SOILS, TRINIDAD

35

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soil fabric. It is well established in the literature that the Atterberg limits, like liquid limit, can be used to reflect the swell behavior of a soil (Holtz and Gibbs, 1956). Since both of them depend on the amount of water adsorbed they are presumed as related to each other. Therefore, an attempt is made to correlate the swell potential of various soils investigated with their index properties. In the recent past a considerable progress has been made in an attempt to characterize the effect of moisture on soil volume changes using soil suction (Aitchinson and Woodburn, 1969; Snethen and Johnson, 1977; McKeen and Nielson, 1978). Soil suction is that property of soil that indicates the intensity with which an unsaturated soil attracts water. Thus, it represents the potential for water uptake of unsaturated soils. Filter paper technique is developed for measuring soil suction using undisturbed soil samples. This method is found to be the most convenient and economical procedure. However, the soil suction method is excluded from the discussion for the present study, because

it is still in a developing stage and hence not widely used to be treated as a routine test. The types of tests performed for the present analysis include grain-size distribution, Atterberg limits, in s i t u water content and density, compaction characteristics, suction tests and consolidometer swell tests. There are different procedures for swell tests in vogue. The procedure adopted in the present study is described. Table 1 gives the description of the soils tested and their locations, while Table 2 gives their engineering properties determined according to British Standard test procedures. Potential swell test

The potential swell may be defined as the percent vertical increase in height of a soil sample laterally confined, when saturated with water from the initial condition of moisture, density and overburden pressure. The potential swell of a natural ground is termed the potential heave. The potential swell of an undisturbed or a

36

K.V

RAMANA

TABLE 1 Soil types studied No.

Location

Symbol

Soil series

Parent material

Classification Textural

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Barrackpore Brasso-Venado Caparo Caigual Chaguanas Couva Gran Couva Guayaguayare Mayaro Moruga Pleasantville Poole Princess Town Rio Claro Sangre Grande Tabaquite Talparo

BKP BRS CPR CGL CHG CVA GCV GUY MYR MGA PVL POO PRT RIO SNG TBQ TPR

239 674/L 335 39 33 231 575/68/L 75 77 78/L 278/L 35/77 474/L 474/L 233 177 177

Clay

4

Clay alluvium Clay Calcareous silt stone Silty clay Clay alluvium Silty clay loam Clay alluvium Silty clay loam Clay alluvium Silty clay loam Clay alluvium Clay loam Mixed shale Clay Clay shale Silty clay loam Clay shale Clay Volcanic mud Silty clay loam Clay shale Clay Clay alluvium Clay Clay shale Clay Marl Silty loam Clay alluvium Clay Clay shale Clay Clay shale Clay

Unified CH CH CI CI MI CI CH CL CH CL CH CH CH CI CH CH CH

82 52 40 35 35 36 69 39 70 35 50 57 72 22 49 62 60

NOTE: All soils contain Montmorillonite, Illite and Kaolinite clay minerals in varying percentages

TABLE 2 Soil properties No.

I 2 3 4 5 6 7 8 9 I0 I1 12 13 14 15 16 17

Location

Barrackpore Brasso-Venado Caparo Caigual Chaguanas Couva Gran Couva Guayaguayare Mayaro Moruga Pleasantville Poole Princess Town Rio Claro Sangre Grande Tabaquite Talparo

LL = Liquid Limit. PI = Plasticity Index. SL = Shrinkage Limit.

LL (%)

84 79 55 56 51 60 93 47 74 43 94 81 83 70 76 106 73

PI (%)

56 46 32 33 23 33 65 23 48 26 71 48 58 44 44 67 48

SI (%)

22 18 21 13 13 19 21 19 12 25 16 19 14 24 13 19 16

Activity

In situ density

(kg/m 3)

0.69 0.88 0.79 0.94 0.67 0.91 0.94 0.59 0.69 0.73 1.43 0.83 0.79 1.99 0.89 1.08 0.80

1741 1816 1975 1921 1997 1887 1818 1766 1796 1995 1647 1749 1927 1726 1766 1711 1889

Natural moisture content

Optimum moisture content

(%)

(%)

39 34 25 23 14 25 33 16 36 15 31 37 37 39 38 43 32

27 23 20 18 16 18 30 20 23 15 28 23 25 26 15 31 25

GEOTECHN1CAL PROPERTIES, AREAL DISTRIBUTION OF HUMID TROPICAL EXPANSIVE SOILS, TRINIDAD

remoulded soil sample is usually measured in a consolidometer after subjecting the sample to appropriate overburden pressure and inundating it with water, noting the increase in height of the sample. Since there are many versions of this test, the procedure used in this investigation is described below. As little is known before hand about the swell behavior and the ground-water condition, the swell-test procedure recommended by the U.S. Army Corps of Engineers (1968) is adopted. It is reported, however, that this test is carried out on undisturbed soil samples using a fixed ring oedometer. The soil sample is first loaded to its in situ overburden pressure. When compression of the sample is completed then the specimen is unloaded to zero load and then saturated by inundating it with distilled water and the expansion is measured using dial gauges reading to 0.0001" until the dial reading showed a constant value (primary swell). On an average, these readings took a minimum of 48 hours. The test procedure is illustrated in Fig. 7. The potential swell is calculated using the rela-

37

tion: S % = A H / H = [AH/(H o - Hi) ] x 100

The test procedure just described, more or less simulates a typical sequence of construction operations in Trinidad. The measured swell values are plotted against ( L L - W ) values and the resulting curve is shown in Fig. 8. The curve is represented by the equation: SO/o = 0.20expO.O6(CC -

w)

where: L L = liquid limit percent; W= in situ moisture content. The frequency diagram rating the potential swell in terms of low, medium, high and very high is also shown in the same figure. Analysis of soil test results

The results of soil tests are analyzed with a view to correlate the soil properties with the potential swell.

D~ ~ , ~ l o a d i n g D~ X:: C: .,-4

~H

"~ve

o

o~

H

--~erbu~:~l pressure

4~

Ho=original sample height ~H,= compression under m overburden pressure ~H== specimen expansion after saturation Fig. 7. Swell test p r o c e d u r e .

Log. of applied pressure

~

38

Kv. RAMANA

40-

20

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2o

olo U~ v

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Fig. 8. Measured potential swell.

Correlation of properties Plasticity

The index properties of the soils tested are presented in Fig. 9. The plot shows the general relationship between the clay content and the Atterberg limits. It is obvious from the figure that the higher the clay content, the higher are the liquid limit, plasticity index and shrinkage index. This is in conformity with the expected trend. Expansive soils are highplasticity soils and as such exhibit high values of liquid limit and plasticity index, and low values of shrinkage limit. Clearly there is a boundary line separating the clay loams and the silty clays. Obviously clay loams are relatively less plastic than silty clays, and hence are expected to have lower swelling capacity. This fact is clearly seen in the plasticity chart in Fig. 10. All soils plotting above the A-line indicate high-plasticity behavior. The clay loams plot in the CL and CI zones, and

the clays plot in the CH zone. Sometimes nonexpanding clays such as clays with a high organic content may also indicate high values of liquid limit but they can be differentiated from expanding clays by the low values of plasticity index.

Index properties The percent swell of all soils so measured are correlated with the index properties in Fig. 1 1. Despite some scatter in the results, the correlation exhibits a broadly nonlinear trend with LL, PI and SI. The potential swell is found to increase with increasing values of these indices. This shows that the potential swell can be predicted using these indices and the relation shown. Initial water content

The potential swell is found to increase with increasing values of LL and decrease with increasing initial water content. It must be noted that all

GEOTECHNICALPROPERTIES,AREALDISTRIBUTIONOF HUMIDTROPICALEXPANSIVESOILS,TRINIDAD

i0(

0 0

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" plastic limit

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+ shrinkage limit

§ 4o

20

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0 20

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1~__.__,1~ 60

80

clay content %

39

does not produce any more shrinkage, these limits are identified and marked in the diagram. Now it becomes obvious that between the two limits the shrinkage is greater than swelling and beyond these limits swelling controls the soil behavior. When an expansive ground is covered by a structure, the moisture under the structure gradually increases until it reaches equilibrium moisture. This means that under the structure the soil tends to gradually swell as a long-term phenomenon. However, around the building the ground is exposed to the climatic effects, and shrinkage of the ground in the dry season increases the differential movement of the ground. Since the shrinkage of soil is aided by the weight of the structure its effect on the structure becomes more severe than swelling which has to act against it. Therefore, in the arid and semi-arid regions it may be expected that shrinkage rather than swelling produces the worst effect on the stability of the structure because the water content will usually lie between the shrinkage limit and the plastic limit. In the case of humid and semi-humid regions, because the water contents are generally around the plastic limit swelling controls the soil behavior. Since further swelling is going to be small the damage to the structure will be less severe.

Fig. 9. Clay content and Atterberg limits.

A multi-parameter assessment

silty clays tested exist at natural water contents slightly above the plastic limit and hence show small magnitudes of swell when wetted. The clay loams on the other hand have moisture contents well below the plastic limit but their potential for swell is quite limited by low clay contents and low values of Atterberg'limits. The initial moisture content plays an important role in the swell-shrink behavior of an expansive soil. Between the dry state and the state of saturation of an expansive soil, Popescu (1980) observed that the swell-shrink magnitudes vary as depicted in Fig. 12. For water contents less than the shrinkage limit, the rate of shrinkage becomes zero. Assuming that at plastic limit the soil is sufficiently wet to cause not much further swelling and at shrinkage limit further reduction in water content

A single parameter for identification of expansive soils may not prove adequate. Therefore, it is more desirable to apply at least three parameters at the same time which are well-correlated with the swell. Table 3 presents the prominent indicators for the local soils. Hazard map of Trinidad

In spite of the severity of the problem of expansive soils, there is little documentation in the country on this phenomena. Therefore, this report is an attempt to compile one such map based on the soil investigations carried out and the data collected from various soil testing organizations. A hazard map provides the basic information about the distribution of expansive soils covering

40

K.V. RAMANA

SWELLING non-swelling

low

med.

n~plastic slight

lOw

reed.

high

V. high

PLASTICITY

I

CL

high

I

I

CI

CH

v. high

ol~I-I///

60_

(PI):O.81(LL)-I3 ~P'~

/

/

u 40 -

/ /

A-LINE

/

20/

0

0

20

40

,

,

60

80

I i00

liquid limit % Fig. 10. Modified plasticity chart. TABLE 3 Potential swell classification Potential swell Parameter

Low

Medium

High

Very high

Liquid limit (%) Shrinkage Index (%) Optimum moisture content (%) Matric suctin (pF) Potential swell (%)

< 50 < 30 < 16 <3 <1

50-70 30-50 16-22 3-3.5 1-2

70-90 50-70 22-28 3.5-4 2-5

> 90 > 70 > 28 >4 >5

NOTE: Shrinkage Index = Liquid limit- Shrinkage limit.

different regions. It also delineates areas o f different degrees o f severity o f expansiveness. It w o u l d f o r m a useful guide for p r e l i m i n a r y assessm e n t o f the expansive soil site. To f o r m a m o r e

c o m p r e h e n s i v e picture o f the site c o n d i t i o n s this m a p s h o u l d be used in c o n j u n c t i o n with local soil investigation r e p o r t s a n d soil b o r i n g data. It s h o u l d be n o t e d t h a t the m a p p r o v i d e s only

GEOTECHNICAL PROPERTIES, AREAL DISTRIBUTION OF HUMID TROPICAL EXPANSIVE SOILS, TRINIDAD

I

10

i

i

41

i

i

LL = 57.54 (S) 0.288

PI = 34.289(S) 0"415

correlation coefft. = 0.926 opt&,

correlation coefft. = 0.923

I

SI = 34.712(S)0"5~.. 30

correlation coefft. = ~ 0

70

/ -~

np

90

"°/S.

~8o ~-

.,.4 6O

"

"~ 7 0

,. 5C (n

mYR

.,.-i ,--4 o S~

o~

4G w a ~ '°

60

4(

'

Kv 3¢

50

3(

P

)mGA

40 0

I

I

2

4

2( "/ 0

I 2 potential

i 4 swell %

6

0

2

4

6

Fig. 11. Correlationof potential swell with index properties.

a general and rough estimation of the severity of the problem. However, the existence of pockets of highly expansive soils within areas of low expansive descriptions, and vice versa cannot be overruled. Soil testing alone can detect these variations. Besides, even the low rated areas may exhibit differential heave if they are subjected to extreme variations in the soil-moisture condition due to unforseen climatic conditions, man-made or caused by the presence of the structure. These limitations in the use of the map should be appreciated. Basis for the map For the preparation of the hazard map, the basis applied for delineating expansive soil areas and their qualitative categorization consists of: (a) geological and soil maps; (b) geological age and

lithology; (c) soil types responsible for the actual failures of buildings and pavements; (d) topography, soil humidity and drainage; (e) data collected from soil investigation reports; and (f) field sampiing and laboratory testing. The description of local expansive soils, their areal distribution, etc. have been discussed in the earlier sections of this report. Sites surveyed for soil sampling, soil tests performed and the procedures of identification and classification are also discussed. Based on the above studies, in conjunction with the gathered data, the expansive soils prevalent in the central and southern regions are divided into four categories of potential swell as follows: (a) low; (b) medium; (c) high; and (d) very high. The areal distribution of these categories of expansive soils are marked in the map presented in Fig. 13.

42

K.V. RAMAN-% I

1

I

I

I

I

12 curve

10

( ~ 1

shrinkage limit - - ~ I swelling

~I

2rig ~rve

shrinkage

swelling

I

CONTROLLING FACIDRS

g6 I/I

~4

plastic li.tit

.fi

~2 I 010

12

I

I

i

14 16 18 Initial moisture content %

I

I

20

22

Fig. 12. Initial water content and swell-shrink behavior (modified after Popescu, 1980). Absolute values of swelling and shrinkage are super imposed for comparison of their relative magnitudes.

Conclusions This paper is an attempt to compile the basic information that is necessary for identifying expansive soils, the degree of severity of their expansiveness, and their areal distribution. Based on the results of this investigation the following conclusions are drawn: (1) The expansive soils of this country are identified and classified into calcareous and noncalcareous types. (2) These soils are found to exist in an overconsolidated state. The in situ moisture contents are high and are close to the plastic limit on account of high relative humidity levels. This indicates that the soil's swelling capability is under restraint. (3) A modified plasticity chart and a multiparameter criteria are presented for assessing the swell behavior according to the degree of expansiv-

ity, i.e., low, medium, high, and very high expansive types. (4) Structures such as small buildings, pavements, buried pipelines, etc. crack due to soil heave. The severity of the damage depends on the degree of expansivity of the soil. A hazard map showing the areal distribution of high and very high expansive soils is presented. It is hoped that this map will be a useful tool for planners and engineers in their efforts to achieve better land use planning.

Acknowledgements The author wishes to express his appreciation and thanks to the National Institute of Higher Education Research Science and Technology (NIHERST) of Trinidad and Tobago and the University of the West Indies, St. Augustine

43

G E O T E C H N I C A L PROPERTIES, A R E A L D I S T R I B U T I O N O F H U M I D T R O P I C A L EXPANSIVE SOILS, T R I N I D A D

I

J

11

12

I

13

14

I5 #

NORTHERN i

REGION

!

+ N

$CA LE

i

2,

J

25 CENTRAL REGION

I : I~,O~O

.35

36

C A T E G O R I E S OF EXPANSIVE SOILS

,,,,,,,,,

VERY

HIGH

16

HIGH

53

~6

SOUTHERN REGION 65

71

75

76

Fig. 13. Areal distribution and intensity of expansive soils in the island of Trinidad.

Campus, Trinidad this s t u d y .

for the support

received for

References

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tion in West Coastal Area of Trinidad - - the role of geology. Tech. Rep., Inst. Marine Affairs, Trinidad. Erol, A.O. and Dhowian, A., 1990. Swell behavior of arid climate shales from Saudi Arabia. Eng. Geol., 23: 243-254. Gromko, G.J., 1974. Review of expansive soils. ASCE J. Geotech. Eng. Div., 100(GT6): 667-687. Holtz, W.G. and Gibbs, H.J., 1956. Engineering properties of expansive clays. Proc. ASCE, 120. Land Capability Survey, 1971. Ministry of Agriculture, Gov. Printery, Port-of-Spain, Trinidad, Vols. 4, 5 and 6. MacPherson, J., 1963. Caribbean Lands. Longman Caribbean Publishers, Trinidad. McDowell, C., 1956. Interrelationship of load, volume change and layer thickness of soils to the behavior of engineering structures. Proc. Highways Res. Board, 35: 754-772. McKeen, R.G., 1976. Design and construction of airport

44

pavements on expansive soils. U.S. Dep. Transportation, Washington, D.C., Res. Rep. FAA-RD-76-66. McKeen, R.G. and Nielson, J.P., 1978. Characterization of expansive soils for airport pavement design. U.S. Dep. Transportation, Washington, D.C., Res. Rep. FAA-RD78-59. Popescu, M., 1980. Behavior of expansive soils with a crumb structure. ASCE 4th Int. Conf. Expansive Soils, Denver, Colo. Snethen, D.R., 1975. A review of engineering experience with expansive soils in highway subgrades. U.S. Dep. Transportation, Washington, D.C., Res. Rep. FHWA-RD-75-48.

K.V, RAMANA

Snethen, D.R. and Johnson, L.D., 1977. Characterization of expansive soil subgrades using soil suction data. Proc. Conf. Transportation Res. Board, Washington, D.C. Suter, H.H., 1960. The general and economic geology of Trinidad. H.M. Stationery Office, London. Taylor, L.O., 1987. Parameters for the landslide prone clays of Central and Southern Trinidad. Proc. Seminar on Landslides. Assoc. Eng. Trinidad and Tobago. Thornthwaite, C.W., 1948. An approach toward a rational classification of climate. Geogr. Rev., 38: 55-94. U.S. Army Engineer District, Fort Worth, 1968. Investigations for Building Foundations in Expansive Clays. Vols. 1 and 2.