Geotechnical Properties of Soil Sequence for Foundation ...B. Boring and Standard Penetration Testing (SPT) Two boreholes were drilled to depth 30.0m each with 250mm, 200mm and 150mm

Geotechnical Properties of Soil Sequence for

Foundation Design at Addo, Lagos, Nigeria *1Faseki, Oluyemi Emmanuel, 2Olatinpo, Olusegun Ayobami, 3Oladimeji, Akinsile Richard

1, 2, 3Department of Geology and Mineral Sciences, University of Ilorin, P.M.B. 1515,

Ilorin, Nigeria. [email protected], 2 [email protected], 3 [email protected]

Abstract – Geotechnical characterization was undertaken in a proposed construction site along Badore Road, Addo,

Lagos, Nigeria with the aim to unravelling the geo-stratigraphy and engineering properties of the shallow formations as

foundation material. Two boreholes were drilled upto the depth of 30.0m while Cone Penetration Tests were deployed

upto depth of 6.0m. Samples from boreholes were subjected to grain size and Atterberrg Limits tests. The results revealed

that the site is underlain essentially by soft silty sandy clay at the upper layer (0.0 – 5.0m) characterized by void ratio of

0.80 – 0.84, unit weight of 17.50 – 19.0KN/m3, friction angle of 6 – 80, natural water content of 22 – 30% and average SPT-

N of 2 which is indicative of poor foundation material. The middle layer is firm to stiff silty sandy clay (5.0 – 17.0m) with

void ratio of 0.84 – 0.86, cohesion values of 30 – 32KN/m2, unit weight of 19.0 – 20.50KN/m3, friction angle of 8 – 100 and

average SPT-N value of 8 typifying low foundation material. These layers are further underlain by medium dense silty

sand (17.0 – 30.0) with void ratio of 0.43 – 0.46, unit weight of 20.0 – 21.0KN/m3, friction angle of 32 -330 and SPT-N value

of 23 which is the most competent as foundation materials. The preponderance of soil with poor engineering properties at

the shallow foundation zones (0.0 – 3.0m) precludes the adoption of shallow foundation in the area. Pile installed to

competent layer at 20.0m within the dense silty sandy layer is recommended as foundation option for consideration for

the proposed structure.

Keywords: Standard Penetration Test, Pile Safe Working Load, Geotechnical, Cone Penetration Test, Addo, Lagos

Nigeria.

I. INTRODUCTION

Subsurface geotechnical investigation is an important aspect of design process since structures are sited on the earth

surface. This is particularly important in Lagos metropolis due to the incessant occurrence of failures of structures

and the great variability in the engineering properties of the Quartenary deposits within the area making depth to

foundation unclarified. Therefore, the designing and building of structures requires a thorough understanding of

properties of available soils and rocks that will constitute the foundation and other components of the structures.

A site investigation attempts to foresee and provide against difficulties that may arise during construction because of

ground and/or other local conditions [1]. This important aspect of building construction has however been neglected

in recent times and has therefore resulted the increase in the figures of failures of buildings throughout Nigeria [2,

3]. The design of a structure which is safe, durable and has low maintenance costs depends upon an adequate

understanding of the nature of the ground on which such building is located. Site characterization usually provides

subsurface information that assists engineers in the design of foundation of civil engineering structures.

MAYFEB Journal of Civil Engineering Vol 1 (2016) - Pages 12-25

12

Detailed understanding of the physical parameters which govern the behaviour of soil may result in substantial

savings by avoiding problematic foundation conditions and costly construction methods [4, 5]. Several workers have

reported the presence of clayed and peaty material with undesirable engineering properties in different part of Lagos

metropolis [6, 2, 3, 7, 8, 9]. This further emphasized the important of pre-construction sub soil and foundation

studies within the study area. Preliminary foundation investigation works are operations which include site

investigation and foundation engineering analysis. Exploration of the subsurface can be done using remote

geophysical methods and borings/penetration tests. The boring although expensive is more reliable and helps in

recovering samples for laboratory testing and evaluation [10]. Often used together with laboratory test is in-situ

testing which is very important in geotechnical engineering as simple laboratory tests may not be reliable while

more sophisticated laboratory testing can be time consuming [11].

In-situ testing provide direct information concerning the subsurface conditions, geo-stratigraphy and engineering

properties prior to design, bids and construction on the ground. Among in-situ testing methods is the Standard

Penetration Test (SPT) and Cone Penetration Test (CPT). SPT, with its ease of performance and extensive

correlation with parameters used in foundation design is the most popular insitu method used in evaluating the

allowable bearing stress for foundation of engineering structures. SPT data have been used in correlations for unit

weight, relative density, angle of internal friction and unconfined compressive strength [12]. [13] studied the

reliability of shallow foundation design using SPT approach. The results of reliability analysis showed that the

factor of safety approach can provide an impression of degree of conservatism that is often unrealistic and therefore

submitted that the reliability based approach using SPT-N provides rational design criteria, accounting for all key

sources of uncertainty in the foundation engineering process and should be the basis of design. [14] showed that the

SPT provides three numbers that can be used to evaluate soil properties through an analysis to illustrate how the

incremental blow counts may be used to obtain more information from the test. [15] carried out a review of the

applicability of SPT as a subsurface evaluation tool and submitted that it can provide much of the information

required during a site investigation as compared to other field techniques. CPT is often used to complement SPT due

to it fastness, repeatability, strong theoretical background and ability to generate detailed subsoil profile. Many

investigators [16, 17, 7, 18, 19, 20, 21, 22, 23] have successfully employed CPT in subsurface investigation for

engineering purposes. This study will therefore deploy SPT and CPT methods in x-raying the shallow formations

and estimating their allowable bearing capacity. The focus area is Addo, along Badore Road in Eti Osa Local

Government Area of Lagos State; the site is designated for the construction of four – storey residential buildings.

This study therefore evaluates the geo-stratigraphy and geo-engineering properties of the shallow formations

underlying the sites with a view to determining the suitability of the site for the intended construction purpose.


13

A. The Study Area

The focus location is Addo along Badore Road in Eti Osa Local Government area of Lagos State, Southwestern,

Nigeria. The site is designated for the construction of four – storey residential estate buildings. The area is located

within 6027’11.002”N/3023’44.9999”E (Fig. 1). Geologically, it falls extensively within the Eastern Dahomey Basin.

The Basin is a combination of inland/coastal/offshore sedimentary basin in the Gulf of Guinea [24].

Stratigraphically, the basin is divided into Abeokuta Formation, Ilaro Formation, Coastal Plain Sands and Recent

Alluvium sediments [25]. Deposition of Cretaceous sequence in the eastern Dahomey Basin began with the

Abeokuta Group, consisting of the Ise, Afowo and Araromi Formations [26]. The Ise Formation, the oldest,

unconformably overlies the basement complex and consists of conglomerates and sandstones at base and in turn

overlain by coarse to medium grained sands with interbedded kaolinite. Overlying the Ise Formation is the Afowo

Formation, which is composed of coarse to medium grained sandstones with variable but thick interbedded shales,

siltstones and claystone. The Araromi Formation overlies the Afowo Formation and is the youngest Cretaceous

sediment in the basin [26]. It is composed of fine to medium grained sandstone overlain by shales, siltstone with

interbedded limestone, marl and lignite.

The Ewekoro Formation, an extensive limestone body, overlies the Araromi Formation. The Ewekoro Formation is

overlain by the Akinbo Formation, which is made up of shale and clayey sequence. Overlying the Akinbo Formation

is Oshosun Formation which consists of greenish – grey or beige clay and shale with interbeds of sandstones. The

Ilaro Formation overlies conformably the Oshosun Formation and consists of massive, yellowish, poorly,

consolidated, cross-bedded sandstones. The Quaternary sequence in the eastern Dahomey basin is the Coastal Plain

Sands and recent littoral Alluvium [27] and consists of poorly sorted sands, silts and clay deposits with traces of peat

in parts. The sands are in parts crossbedded and show transitional to continental characteristics. The age is from

Oligocene to Recent. It directly underlies the study area and is composed of deposits which can be divided into the

littoral and lagoonal sediments of the coastal belt and the alluvial sediments of the major rivers. They consist

predominantly of unconsolidated sands, clays and mud with a varying proportion of vegetative matter (Fig. 2).


14

Fig. 1: Map of the study area modified after [2]

Fig. 2: Map of surface geology and morphology of Lagos [7]


15

II. MATERIALS AND METHODS

B. Boring and Standard Penetration Testing (SPT)

Two boreholes were drilled to depth 30.0m each with 250mm, 200mm and 150mm diameter steel casings using a

Percussion motorized Shell and Auger rig employing light cable percussion boring techniques with a fully equipped

motorized Pilcon Wayfarer drilling rig. The position of borehole 1 and 2 coincide with that of CPT P1 and CPTP3

respectively. CPT P1 to P2 is approximately 30.0m apart on a straight line such that the distance between CPT P1 and

P7 is 180.0m. During drilling operations, disturbed soil samples were regularly taken at depth interval of 0.75m and

whenever a change of stratum is observed. All samples recovered from the borehole were examined, identified and

classified in the field. They were later taken to the laboratory for detailed investigations where a total of 10 and 12

samples were subjected to grain size distribution test by wet sieving and Atterberg limit tests respectively. Effort

was made to ensure that all strata encountered were tested appropriately. Standard Penetration Test (SPT) was also

carried out at 1.5m intervals in both cohesive and cohesion less soils with disturbed samples recovered from the SPT

sampling tool. In carrying out the SPT test, a 50mm diameter split spoon sampler is driven into the soil using a

63.5kg hammer with a 760mm drop, and the penetration resistance is expressed as the number of blows (SPT-N-

value) required obtaining a 300mm penetration below an initial 150mm penetration seating drive. The SPT-N-values

were corrected for borehole and dilatancy where necessary and all pertinent borehole data, penetration resistance,

and sample data were recorded on the boring log sheet (figs 3 – 4). In boreholes, SPT results are routinely used to

provide an estimate of density, consistency, unconfined compressive strength and shear strength parameters.

C. Cone Penetration Testing (CPT)

CPT is a means of ascertaining the resistance of the soil. Seven CPT tests were carried out to a depth approximately

6.0m. The tests were performed using a 2.5-Ton nominal capacity manually powered CPT machine. Penetration

resistance (qc) and the depth of penetration were recorded at each station (table 1). Most of the test reached refusal

before the anchors pulled out of the subsurface. The layer sequences were interpreted from the variation of the

values of the cone resistance with depth. On the basis of the expected resistance contrast between the various layers,

inflection points of the penetrometer curves were interpreted as the interface between the different lithologies or

density variation. The cone penetration test is economical and supplies continuous records with depth.

D. Classification Tests

A series of classification tests were carried out on the samples in strict compliance with relevant geotechnical

engineering standards including British standards [28, 29, 30]. Laboratory classification tests were conducted on a

number of soil samples to verify and improve on the field identification. These tests include natural moisture

content, unit weights, Atterberg limits (liquid and plastic) and grain size distribution.

E. Analysis

The void ratio, poisson ratio and modulus of elasticity were evaluated from the method of [31]. The angle of internal

friction was estimated from penetration tests based on [31, 32, 33, 34] methods.


16

Plasticity Index (PI) = LL – PL, where LL = liquid limit, PL = plastic limit

Liquidity Index (LI) = w – PL

PI

Where w = Natural water content, PL = plastic limit, PI = plasticity index.

Using the equation of ultimate bearing capacity for a driven pile;

Qf = qf x Ab + fs x As ……………………………………………………….(1)

where

Qf = ultimate load that can be applied at the top of the pile,

qf = ultimate bearing capacity of the stratum on which the pile is supported,

fs = the average shearing resistance of soil per unit area,

Ab = area of the pile at the base,

As = cylindrical surface area of the pile.

SPT-N method was employed using [35] equation for driven pile;

qf = 40 x N x D (limited by 400N)…………………………………………..(2)

B

fs = 2Na where

N is the SPT-N at the vicinity of the base of the pile,

Na is the average SPT-N value over the embedded depth of the pile.

Since the above equation is applicable for driven piles, the value of qf obtained was further multiplied by 0.33 while

that of fs was multiplied by 0.5 to derived corresponding values for bored piles.

Allowable bearing capacity:

qa = qu/F.S ……………………………………………………………………(3)

where F.S. is factor of safety = 2.5.

III. RESULTS AND DISCUSSION

1). Soil Stratigraphy

Results of boring and Standard Penetration Test (SPT) are presented as borehole lithologic logs (Figs 2 and 3) while

the classification tests results are summarized in Tables 2 and 3. A careful interpretation of the results revealed the

occurrence of three geotechnical layers from ground surface to about 30.0m depth which consists essentially of soft

silty sandy clay (0.0 – 5.0m), firm to stiff silty sandy clay/peat (5.0 – 17.0m) and medium dense silty sand (17.0 –

30.0m) which exemplify alluvial sediments of creek environments. The first layer is soft silty sandy clay with

thickness ranging between 4.50 – 5.0m. The second layer composed of firm to stiff sandy clay with thickness

approximately 11.50 – 12.0m sandwiched with peaty materials. The third layer is medium dense light grey silty

sand. It is pertinent to mention that only the third layer is competent as foundation material.


17

2). Penetration Tests

The cone penetrometer depth probed varied from 4.75 – 6.0m with resistance values range of 2-102kg/cm2 (table 1).

The topsoil to approximately 2.50m have very low resistance of 2kg/cm2 which is undesirable as foundation material

while soil at depths approximately 2.50 – 4.25m have resistance ranging from 2 – 38kg/cm2 interpreted to be very

low foundation material since it is <40kg/m2 pointing to their probable unsuitability as foundation materials for the

proposed structure. At depths 5.0 – 6.0m, the silty sandy clay has resistance range of 56 – 102kg/m2 which typify

low to medium foundation material. The Standard Penetration Test revealed that the upper layer of silty sandy clay

has SPT-N value of 1 interpreted to be very soft and therefore unsuitable as foundation material. The middle layer of

silty sandy clay with peat intercalations has an average SPT-N value range of 7 – 12 interpreted as firm to stiff clay

which is indicative of low foundation material thereby corroboting the result of cone penetration test. The presence

of the fibrous peat intercalations within the essentially silty sandy clay deposit in the middle layer is suggestive of

very low strength characteristic due to its extremely soft nature which made collection of its undisturbed sample

impossible, hence these strata are considered unsuitable as foundation material. The lower layer of silty sand has

SPT-N value range of 17 – 33 interpreted to be medium dense pinpointing it relative suitability as foundation

material. The layer is the most competent for foundation consideration.

3). Classification Tests

The soils (table 2) are generally classified as poorly graded sand since they all have their coefficient of uniformity

less than 6 except for sample Bad2 B2 which is well graded [36, 37]. Also, with natural water content (22 -68%), the

silty sandy clay at depths approximately 0.0 – 17.0m is indicative of low to high compressibility and settlement

potential under imposed load. The plasticity index (table 3) range of 16 – 57% are indicative of low to high plasticity

of the essentially clayed material [38] and is therefore classified as low to medium foundation material. Also, the

liquidity index of the clay which ranges from 0.15 – 0.70 is quite low and is suggestive of a clay with little or no

potential for liquefaction under sudden load [39].


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TABLE 1: Cone penetration test data

Cone Readings(kg/cm2)

Depth(m) P1 P2 P3 P4 P5 P6 P7

0.25 2 2 2 2 2 2 2

0.50 2 2 2 2 2 2 2

0.75 2 2 2 2 2 2 2

1.00 2 2 2 2 2 2 2

1.25 2 2 2 2 2 2 2

1.50 2 2 2 2 2 2 2

1.75 2 2 2 2 2 2 2

2.00 2 2 2 2 2 2 2

2.25 2 2 2 2 2 2 2

2.50 2 2 2 2 2 2 2

2.75 2 10 6 2 2 2 2

3.00 2 10 4 2 4 2 2

3.25 2 10 10 8 8 2 2

3.50 2 14 15 10 12 2 2

3.75 2 20 28 14 12 2 2

4.00 2 20 30 26 16 2 2

4.25 18 26 38 34 26 22 2

4.50 80 44 50 46 34 72 36

4.75 100 60 72 56 42 84 58

5.00 - 70 100 67 56 92 72

5.25 - 80 - 84 66 - 88

5.50 - 95 - 92 75 - -

5.75 - - - 96 - - -

6.00 - - - 102 - - -


19

30.00

24.00

27.00

21.00

18.00

15.00

12.00

9.00

6.00

3.00

0.00

Depth(m) Sample N0 Legend(S.P.T)Values“N”

Strata Description Thickness(m)

1

2

3

4

5

67

8

9

1011

12

13

1415

1616

17

1819

20

21

2223

24

25

26

27

28

29

3031

32

33

34

35

36

37

38

3940

41

11.50

26

End of borehole

Soft dark grey Silty Sandy Clay

31

.............

.............

.............

.............

.............

.............

.............

.............

.............

.............

.............

.............

.............

.............

.............

.............

-------....---------------------------------...--------------------------------....---------------------------------.....-------

.------.-----------------.......----------------------------.....-----------------Y----------.......---------------......------------------------------.....--------------------------------------.......Yy.......................yy----------------------------------------yy...------------------

1 V

9

9

17

21

12

13.50

7

Dense light grey Silty Sand

Firm to stiff dark grey Silty Sandy Clay with tracesof Peat between 5.25m and 12.75m

1

5.0

Fig. 3: Log of borehole 1


20

30.00

24.00

27.00

21.00

18.00

15.00

12.00

9.00

6.00

3.00

0.00

Depth(m) Sample N0 Legend(S.P.T)Values“N”

Strata Description Thickness(m)

1

2

3

4

5

67

8

9

1011

12

13

1415

1616

17

1819

20

21

2223

24

25

26

27

28

29

3031

32

33

34

35

36

37

38

3940

41

12.0

25

End of borehole

Soft dark grey Silty Sandy Clay

32

.............

.............

.............

.............

.............

.............

.............

.............

.............

.............

.............

.............

.............

.............

.............

.............

-------....---------------------------------...--------------------------------....---------------------------------.....-------

.------.-----------------.......----------------------------.....-----------------Y----------.......---------------......------------------------------.....--------------------------------------.......Yy.......................yy----------------------------------------yy...----------------------------------

1 V

10

9

17

22

12

13.50

6

Dense light grey Silty Sand

Firm to stiff dark grey Silty Sandy Clay with tracesof Peat between 5.25m and 12.75m

1

4.50

Fig. 4: Log of borehole 2


21

TABLE 2: Grain size distribution results parameters

Serial

No

Sample

Designation

Depth

(m)

Effective

size (d10)

Effective

size (d30)

Effective

size (d60)

Coefficient of

Uniformity (Cu)

Coefficient of

Curvature (Cc)

Remark

1 Bad1 B1 16.50 0.18 0.28 0.40 2.22 1.09 SP

2 Bad2 B1 18.00 0.15 0.30 0.50 3.33 1.20

SP

3 Bad3 B1 21.00 0.10 0.30 0.50 5.00 1.80

SP

4 Bad4 B1 24.00 0.15 0.18 0.22 1.6 1.0 SP

5 Bad5 B1 27.00 0.15 0.20 0.50 3.3 0.5 SP

6 Bad1 B2 16.50 0.20 0.40 0.70 3.5 1.1 SP

7 Bad2 B2 17.25 0.20 0.60 1.50 7.5 1.2 SW

8 Bad3 B2 22.50 0.10 0.30 0.50 5.0 1.8 SP

9 Bad4B2 24.00 0.15 0.30 0.40 2.7 1.5 SP

10 Bad5 B2 25.50 0.15 0.25 0.40 2.7 1.0 SP

TABLE 3: Atterberg Limit Test Results

S/N Sample No Depth (m) Natural Water

Content

(%)

Liquid Limit

(%)

Plastic Limit

(%)

Plasticity

Index(%)

Liquidity Index

(%)

1 Bad6 B1 0.75 28 50 18 32 0.31

2 Bad7 B1 1.50 30 48 16 32 0.50

3 Bad8 B1 5.25 68 84 30 54 0.70

4 Bad9 B1 12.00 40 49 20 29 0.69

5 Bad10 B1 13.50 39 86 31 55 0.15

6 Bad11 B1 15.00 27 51 20 31 0.23

7 Bad6 B2 1.50 31 46 16 30 0.50

8 Bad7 B2 4.50 30 51 16 35 0.40

9 Bad8 B2 5.50 63 86 29 57 0.60

10 Bad9 B2 13.50 22 45 15 30 0.23

11 Bad10 B2 15.00 30 53 22 31 0.26

12 Bad11 B2 15.75 31 49 19 30 0.40

V. GEOTECHNICAL ENGINEERING PARAMETERS


22

The summary of geotechnical engineering parameters derived from integration of boring, penetration and laboratory

tests results is presented in table 4. The void ratio of 0.82 – 0.85 for the upper and middle silty sandy clay layer is

considered high pointing to a very poor foundation material that will be prone to high initial settlement under load.

Cohesion value of 32KN/m2 is also considered low while an angle of internal friction of 90 for the middle layer of

firm clay is attributable to the presence of silt in the essentially clayed deposit. The lower layer of dense silty sand

with low void ratio of 0.45 and relatively high angle of internal friction of 330 is considered the most competent

foundation material exemplified by it average SPT-N value of 32.

TABLE 4: Summary of Engineering Properties

Str

atu

m D

epth

Ran

ge(m

)

Th

ick

nes

s(m

Geotechnical Engineering Parameters

Typ

e of

Soi

l

Ave

rage

CP

T V

alu

e (k

g/cm

2 )

Ave

rage

SP

T V

alu

e

Voi

d R

atio

Coh

esio

n (

kN

/m2 )

Un

it W

eigh

t(k

N/m

3 )

An

gle

of I

nte

rnal

Fri

ctio

n

Est

imat

ed

mod

u

Ela

stic

ity(

kN

/m2 )

Poi

sson

Rat

io

0.0 – 5.0 5.0 Soft Silty

Sandy

Clay

7 2 0.82 - 18.50 7 3,225 0.5

5.0 – 17.0 11.50 Firm Silty

Sandy

Clay

33 8 0.85 32 19.50 9 5,344 0.5

17.0 – 30.0 8.50 Dense

Silty Sand

- 23 0.45 - 20.50 33 25,972 0.3

TABLE 5: Pile Safe Working Load

Pile Type Driven Pile Bored Pile

Diameter (mm) Founding Depth (m) Ultimate Pile

Capacity

(KN)

Safe Working Load

(KN)

Ultimate Pile

Capacity

(KN)

Safe Working Load

(KN)

300 20.0 898 359 368 147

400 20.0 1408 563 561 225

500 20.0 2024 810 790 316

600 20.0 2753 1101 1056 422


23

The relatively resistive sandy deposits is comparable to the sandy deposits reported as occurring from 35.0 – 60.0m

in several parts of southwestern Lagos by [40, 41, 42] as medium coarse grained in texture, characterized by SPT-N

values of 25 to 50 indicating its competence as foundation soils. Since pile is usually installed to at least 5 times pile

diameter into the bearing stratum, pile foundation installed at 20.0m within the dense silty sandy layer is considered

appropriate for foundation of engineering structure in the area, this is in tandem with [2, 18] which established the

presence of mechanically competent layer at 16.0m in southwestern part of the study area. Such pile installed at that

depth could generate anticipated safe working loads of 359.0kN – 1101.0kN and 147.0kN – 422.0kN under driven

and bored piles respectively at assumed diameters of 300.0 – 600.0mm (table 5).

VI. CONCLUSIONS

Penetration and geotechnical laboratory tests has revealed that the study area is underlain predominantly by soft silty

sandy clay (0.0 – 5.0m) at the upper layer underlain by firm to stiff silty sandy clay layer (5.0 – 17.0m) with peat

intercalations at 5.25 and 12.75m. Beneath these layers is medium dense silty sandy layer (17.0 – 30.0m) which is

considered the most competent foundation layers. From engineering assessment, the upper and middle silty sandy

clay layers with poor geotechnical properties have the potential to settle appreciably under foundation loads and are

therefore considered inappropriate for support of the proposed structure. The preponderance of soil with poor

geotechnical properties at the shallow foundation zones precludes the adoption of shallow foundation in the area.

Hence, pile installed to at least 20.0m within the medium dense silty sandy layer is recommended for the proposed

structure.

ACKNOWLEDGMENTS

The authors are grateful to Optimal Gems Resources Limited, Ikeja for providing equipment for the fieldwork.

REFERENCES

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Documents

Geotechnical Properties of Soil Sequence for Foundation ...B. Boring and Standard Penetration Testing (SPT) Two boreholes were drilled to depth 30.0m each with 250mm, 200mm and 150mm