integrated geological, geophysical and geotechnical assessment...

International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:17 No:06 1

171805-1801-4949- IJBAS-IJENS @ February 2018 IJENS I J E N S

Integrated Geological, Geophysical and Geotechnical

Assessment of Building Failure in Lagos;

A Case Study of Ogudu Lagos South Western Nigeria Emmanuel Oyem Ubido, Ogbonnaya Igwe, Bernadette Uche Ukah

Engineering unit, Department of Geology, University of Nigeria, Nsukka Corresponding Author

Emmanuel Oyem Ubido

Engineering unit

Department of Geology

University of Nigeria, Nsukka

Phone: +234-8069722240

Email:[email protected]

Abstract-- Incessant incidences of building collapse in Lagos

have continued unabated in recent times. Although, several

probable causes have been highlighted .Unfortunately, the

subsurface earth conditions have rarely been given any serious

consideration prior to construction exercise. Geophysical and

geotechnical laboratory test on soil samples were done on the

subsoil within the study area. Cone penetrometer test and a

standard penetration test were also conducted to identify the

depth to competent layer as a compliment for VES survey. The

VES identified three distinct geo-electric layers; the top soil,

peaty clayey sand, medium grained clayey sand and very stiff

clayey sand. The resistivity and thickness of each layer were in

the range of 145-351.34 Ωm and 3.1-4.3m; 49.1-97.2 Ωm and

10.7- 11.4m, 41.4-105.6Ωm respectively. The CPT and SPT tests

show that the subsurface around the area consist of materials of

very low shear strength (< 80 kgf/cm2) interpreted as peat/clay at

near surface to a depth of 11.50 m (being the maximum depth

probed by the CPT test) and 11.4 m for the SPT. The Plastic,

Liquid Limit, Plasticity Index, Natural moisture Content,

Maximum Dry Density, Optimum Moisture Content and

Unconfined Compressive Strength ranges are 20-40%, 40-

70%,20-30%,1.71-2.04g/cm3, 3.76-6.224% and 75– 90KN/m2

respectively. The existence of loose sand, peat and clay near at

the surface is capable of endangering building structures. The

result so far proved that the buildings in this area is on a shallow

foundation which is considered inappropriate, hence the

differential settlement of the structure.

Index Term-- I Distress building, Foundation, Settlement, Soil.

INTRODUCTION

The quality of the built environment, both natural and man-

made, depends on its management, that is, its process of

control and organization. Often, there are forces that cause

events that lead to unsafe built environment for water, land

and air inhabitants. While these forces are generally

categorized into natural and man-made, their resultant effects

are multifarious, calamitous and disastrous. These resultant

effects are generally called disasters. Disaster occurs in

different parts of the world at different times and in various

scales leaving behind various magnitude of loss to lives and

properties (Adedeji 2013). At other times man-made disasters

results from civil conflict like riot, unethical, non-professional

and careless endeavours like fire outbreak, damaged pipelines,

building collapse, chemical spill, road accident, food

poisoning, epidemic industrial disaster, crisis, deforestation,

war, environmental pollution and plane crash, among others.

Still yet, disasters may occur from natural forces like

earthquake, volcanic eruption acting negatively on man-made

inventions like buildings, boats, ships, cities and artificial

islands. Furthermore, the National Programme for Capacity

Building of Architects for Earthquake Risk Management

(NPCBAERM) in India (2009) saw building collapse as the

major issue in earthquake vulnerability and argued that

earthquakes are natural hazards but the disasters are man-

made. The programme pointed out that “earthquakes don’t

kill, unsafe buildings do” and “the Latur earthquake of 1993

caused large-scale collapse of non-engineered houses, due to

faulty design, weak construction material and poor

maintenance, non-compliance to seismic safety regulations in

engineered buildings lead to extensive collapses. In Nigeria,

building collapse is defined as a state of complete failure when

the structure has literally given way and most members have

either caved-in crumbled or buckled (Obiechina 2005).

Indeed, building collapse has become a common feature of

Nigeria cities. For instance, 57 people were buried as a

building collapsed on them in Ebutte Meta, Lagos on 18th

July, 2006 (Opara 2006). On 12th June, 1997 the collapse of

an unfinished three-storey building in Enugu killed 20 people

(News24/Africa 2009). In Lagos, a four-storey residential

building caved-in suddenly in July, 2006 killing 37 people and

leaving 50 survivors to be pulled out of the rubble

(News24/Africa 2009). The cases of building collapse in

Nigeria has reached a worrisome level in view of its alarming

loses. It has been the concern of numerous authors

(Oyewande1992; Weihen 1999; Chinwokwu 2000; Opara

2006 2007; Windapo 2006) to search for the causes of this

monster, in order to proffer adequate solution of prevention,

mitigation or preparedness. The Nigerian Institute of Building

said 84 buildings had collapsed in the past 20 years in Nigeria,

claiming more than 400 lives based on reported cases only

(News24/Africa 2009). Oyewande(1992) discovered that 50%

of building failure cases in Nigeria is attributed to design

faults, 40% to construction fault and 10% to product failures.

mailto:[email protected]



According to Chinwokwu (2000) and Windapo (2006) about

37% of these collapses are believed to be caused by

carelessness and greed on the part of construction professional

and 22% are traceable to design faults (Roddis 1993; Ameh et

al 2007; Oyedele et al., 2009). Also, about 40% of the reported

cases of collapse building are residential (Windapo 2006).

Uzokwe(2006) stated that the cause of a building failure is

almost always unique to the particular building in question.

However, he advanced some general reasons why buildings

may be susceptible to collapse which includes the quality of

the blocks used, the quality of the concrete used, poor

compaction and consolidation of foundation soil and weak

soil. According to Thisday newspaper (July 27th 2016) Lagos

State has the highest case of severe incidences of building

failures in Nigeria, resulting to loss of lives and material

resources. Lagos, population 21 million, is now one of the

biggest cities in the world, and its 3.2 percent growth rate has

forced development on land that can’t support multi-story

buildings (Olamide 2014; Ayininuola and Olalusi 2014). A

study by Littlejohn et al (1994) indicated that the greatest

element of risk in a building project lies within the

uncertainties in ground conditions. Series of research work

have been done on causes of building; foundation failure and

building collapse in Lagos only little was mentioned on the

possible implication of subsurface/subsoil geology on the

foundation failure. From the earlier research work done by

Oni (2010) and Oloke et al (2016), the occurrence of building

collapse in tends to be more in areas close to the lagoon or

swamp. (Fig1 and Table 1). The percentage of building

collapse from 1978 to May 2017 records 32.47% for areas

underlain by coastal sand and 67.53% account for littoral

alluvium. This study focused on investigating the implication

of the subsoil geology on the variation in the occurrence of

foundation failure of a residential building in Ogudu area of

Lagos. This was done by integrating Geophysical, and

Geotechnical techniques. The combine techniques were used

to delineate depth to bedrock in the building and also in

evaluating the competence of near surface formation as

foundation materials; to determine the soil type at a particular

depth as the well as the bearing capacity of the soil and

determine the stability and the integrity of the studied

building. The result from the above test will go along in

reducing the incidence of foundation failure since the soil

bearing capacity enables engineers in choosing the type of

foundation and the amount of load to place on it.

Fig. 1. Building Collapse in Lagos and no of occurrence from 1978 to 2017

0

1

2

3

4

5

6

7

8

9

1975 1980 1985 1990 1995 2000 2005 2010 2015 2020

No of Occurrences



Table I

Percent occurrences of Building Collapse in different local government in Lagos between 1978 and 2017

S/N Local government Occurrences Percentage

1 Agege 6 5.13

2 Ajeromi-Ifelodun 2 1.7

3 Alimosho 3 2.56

4 Apapa 0 0

5 Eti-Osa 7 5.98

6 Ifako-Ijaiye 0 0

7 Ikeja 13 11.11

8 Kosofe 2 1.7

9 Lagos Mainland 14 11.97

10 Lagos Island 32 27.35

11 Mushin 7 5.98

12 Oshodi-Isolo 9 7.69

13 Somolu 7 5.98

14 Surulere 15 12.82

15 Amuwo-Odofin 0 0

16 Ojo 0 0

Fig. 2 Geologic map of Lagos

DETAILS OF THE STUDIED BUILDING

The building is located within a fenced compound

with a storey building on a plot of land surrounded by existing

structure of low and rise building used for residential purpose.

The building has settled when view from the front. It was

founded on raft foundation and it is between one developed

site and one undeveloped site. The site has an undulating

surface and it is lower than the access road linking it. The age

of the building is around 15 years. This site was investigated

to identify the reasons for the sinking. The information

obtained can serve as a guide in the maintenance processes

needed to increase the safety factor of the building. Moreover,

delineating subsurface features responsible for these failures in

the area can assist in the future planning for the construction

of new buildings. The target is the general lithology and soil

bearing capacity of the immediate subsurface, which will



allow an interpretation of the stability of the subsurface

structures and its ability to support the proposed underlying

structure.

FOUNDATION DETAILS OF THE STUDIED

BUILDING

The type of foundation is stepped continuous footing

(wall footing). The depth of the foundation of the building is

about 0.7m from the existing ground level. It is laid over 0.2m

thickness of plain cement concrete and 0.4m thick

sandcushion. The foundation details of the distressed building

are shown in Fig.3 and 4.

Fig. 3. Foundation details

Fig. 4. Distressed Building in the Study Area

1.4 Topography and drainage of the Study Area

The study area, Ogudu, Lagos, lies within the western part of

the Dahomey Basin, between Latitude 6037’19”N and

6033’87”N and Longitude 3021’00”E and 3025’00”E West of

Lagos State.relatively undulating lowland with topography of

low-lying coastal beaches, extensive inland lagoons and

depressions and marsh and mangrove wet land at elevation of

0 to 2m and upland areas with moderately drained soils and an

elevations of 10-14m (Fig. 5). The major river in the state

include estuary of the Ogun River, Adiyan, and Osse. All

discharges into Lagos Lagoon, in Ogudu area, the dominant

drainage system is basically lagoons which connect to the

main ocean (Fig. 6)



Fig.5. (A) Elevation Map of the Study Area.

Fig. 5. (B) Elevation Map of the Study area in 3D

Geomorphology and Geology of the study area The study area has a tropical wet and dry season that lies on a

tropical monsoon climate. Thus it experiences two raining

seasons, with the heaviest rain falling from April to July and

not so heavy rainy season in October and November.

However, a short rain drops is prevalent in August and



September and a longer dry season from December to March.

WWIS (2012) report revealed that monthly rainfall between

May and July averaged over 400 mm (16 in), while in August

and September it fell to 200 mm (7.9 in) and in December

reached as low as 25 mm (0.98 in). The main dry season

which is accompanied by harmattan winds from the Sahara

Desert, is between December and early February and can be

quite strong. The highest maximum temperature ever recorded

in Lagos was 37.3°C (99.1°F), and the minimum was 13.9°C

(57.0°F). According to Nwajide (2013) two main vegetation

types recognisable in the area; swamp forest in the coastal belt

and dry lowland rain forest in the vegetated area. The swamp

forests in the state are a combination of mangrove forest and

coastal vegetation developed under the brackish conditions of

the coastal areas and the swamp of the freshwater lagoons and

estuaries. In this area, accessibility and construction would be

challenging due to the swampy nature of the terrain.

The geology of studied area and its environs is

covered by the sedimentary rocks of the Dahomey basin which

includes the coastal and continental shelf sedimentary

sequence west of Benin City, and expanding westward

through Republic of Benin, Togo and slightly into south-

eastern Ghana. During deposition, tertiary and cretaceous

sedimentation was partially separated from the Niger Delta

Basin to the east of the Okitipupa Ridge. Lagos belongs to the

Coastal Plain Sand formation which consists of loose sediment

ranging from silt, clay and fine to coarse grained sand.

According to the works of Omatsola and Adegoke (1981);

Kogbe (1976); Jones and Hockey (1964); Reyment (1965) and

Ogbe (1972), the Formations recognized in the Nigerian part

of the basin according to lithologic unit of Formational rank

are: the Abeokuta Group (comprising of Ise, Afowo and

Araromi Formations); others are the Ewekoro, Akinbo,

Oshoshun, Ilaro Formations and the coastal plain sands. The

geologic period and age dates back to Quaternary and recent

Oligocence to Pleistocene. Geologically, Ogudu also falls

within the zone of coastal creek and lagoons developed by

barrier beaches associated with intercalations of sand and

peat/clay deposits.The site on which the investigation was

carried out forms part of the lagoon environment, which has

encroached the southern parts of Lagos. Its general subsoil

condition is therefore expected to compose mainly of Peat

/clay overlying other competence clay, is fine to coarse

grained sand referred to as the coastal plain sand (Fig. 6). The

investigated area is close to an old lagoonal river channel. It is

a swampy environment bordered on the north by an open

lateritic soil.

Fig. 6. Geologic Map of the Study Area



METHOD OF STUDY

Geotechnical survey

Cone Penetration tests were conducted at two points within the

studied area. The tests were carried out to a depth of 15m. The

Dutch Static Penetration measures the resistance of

penetration into soils using a 60o steel cone with an area of

10.2 cm2. The cone penetrometer test is a means of

ascertaining the resistance of the soil. The layer sequences are

interpreted from the variation of the values of the cone

resistance with depth. The test is conducted by securing the

winch frame to the ground by means of anchors. These

anchors supplied the essential power to push the cone into the

ground. The cone and the tube were pushed together into the

ground for 20 to 25 cm; the cone is pushed ahead of the tube

for 3.5 cm at a uniform rate of about 2 cm/sec. The resistance

to the penetration of the cone registered on the pressure gauge

connected to the pressure capsule is recorded. The tube was

then pushed down and the procedure described above was

repeated. From the series of recorded gauge readings, cone

resistance was plotted against depth. Undisturbed samples

were taken at appropriate intervals, using a specially designed

60.5 mm internal diameter U – Type sampler. The sampler is

fitted with a cutter at the open end and a waste barrel at the

other end. A round steel ball in the driving head of the sampler

permits the escape of air and water as the sample enters the

tube. The diameter of the sample tube is 25 mm and lined with

60.5 mm plastic tube. The samples are trimmed to the desired

length and usually 15 cm covered in a plastic tube. An

identification label is attached. The numbers of blows required

to drive the sample 15 cm into the ground is recorded. The in

situ Standard Penetration Test (SPT) was carried out, usually

in the non-cohesive strata. The standard penetration test

consists of driving a thick walled 50 mm diameter steel tube

into the sand at the bottom of each borehole by means of a

63.5 kg hammer dropping 75cm. The number of blows

required to drive the tube 30 cm after an initial penetration of

15 cm is recorded as the SPT number. The SPT number can be

used as anempirical measure of the compactness of the sand.

All laboratory tests (grain-size distribution, consistency,

compaction, consolidation and shear strength) on the selected

samples were in accordance with specifications in the British

Standards BS: 5930 (1981), 8004 (1986), 1337 (1990) and

American Society for Testing and Materials ASTM;

Designation 2487 (2011) for soil classification and civil

engineering testing purposes.

Computation of bearing capacity values was based on the

Mayerhoff (1956) theory, which employed volume

compressibility and penetration testing data with some

assumed dimensions.

Geophysical survey

Two vertical electrical soundings were conducted within the

study area using an ABEM-SAS 1000 Terrameter.

Schlumberger array was employed with electrode separations

(AB) ranging from 2 to 300 m. The location of each sounding

station was recorded with the aid of GPS. The soundings were

performed parallel to the traverse lines and the apparent

resistivity values were calculated. The apparent resistivity

measurements at each station were plotted against electrode

spacing (AB/2) on bi-logarithmic graph sheets. The curves

were inspected to determine the number and nature of the

layering. Partial curve matching was carried out for the

quantitative interpretation of the curves. The results of the

curve matching (layer resistivity and thicknesses) were fed

into the computer as a starting model in an iterative forward

modeling technique using WINRESIST version 1.0 (Vander

Velper 1988). From the interpretation results (layer resistivity

and thicknesses), geoelectric sections along directions (N-S

and E-W) were produced, and results were also used to

generate maps.

RESULTS AND DISCUSSION

Geotechnical

Plasticity Result

The natural moisture contents values range between 25% and

30% (Table 2). These values indicate Clay of medium

plasticity with low moisture content on Casagrande Plasticity

Chart (Figure 7). This shows that the moisture content of the

soil in the area is relatively low at its natural state. Moisture

variation is generally determined by intensity of rain, depth of

collection of sample and texture of the soil .The soil is in the

A-6 group of AASHTO soil classification system. While the

Maximum Dry Density ranged between 1.71g/cm3 and

2.04g/cm3 with an average of 1.85g/cm3. The results showed

that Liquid Limits (LL) ranged from 40% to 70%, Plastic

Limits (PL) ranges from 25% to 40%. The Plasticity Index

(PI) range was 20 – 30%, respectively between 8.5m and

10.00m indicates medium compressibility. Sowers and

Sowers (1970) noted that P1>31 should be considered high

and which indicates high content of expansive clay. On the

basis of LL and PI values, the samples are classified as

inorganic clays of medium plasticity. Hence it shows a poor

engineering property; since the higher the plastic index of a

soil, the less competence of a soil as foundation material.

From the result the building failure observe as sinking may

have caused by foundation soils that made up incompetent

materials (clay) that could compress on imposing loads by

differential settlement.



Table II

Geotechnical Results on Plasticity test carried out in the Study Location

Sample Description Bh1 Bh2

Liquid Limit 40 70

Plastic Limit 20 40

Plasticity Index 20 30

Natural Moisture Content 25 30

Soil Description Medium Plasticity Clay Medium Plasticity Clay

Consistency Index 0.75 0.83

Liquidity Index 0.69 0.17

Flow Index 2.86 1.46

Toughness Index 2.86 0.83

Group Symbol CL CL

Specific Gravity g/cm3 2.47 2.46

MDD g/cm3 2.047 1.71

OMC % 6.24 3.76

Permeability 3.16 X 10-7 1.33x10-6

Fig. 7. Plasticity Chart

Table 2 and Figure 7 show the particle size distribution

statistics for the non-plastic soil materials and classification

according to the unified soil classification system. Figs. 7 and

Table 2 presented the particle size distribution curves for the

cohesionless soil materials at various depth intervals. Sieve

analysis carried out on selected sand samples encountered

between the 16.00m and 21.00m depth in the borehole

revealed that the sand is predominantly medium to fine

grained and non plastic. The soil samples classified based on

Unified Soil Classification System and falls within the Well-

Graded Sands. The results of the particle size distribution are

summarized in Table 2.

Table II

Results of Particle Size Distribution for the Studied Area.

SAMPLE DESCRIPTION BH1 (16M) BH2 (21M)

D10 0.19 0.20

D30 0.26 0.31

D60 0.5 0.62

CC 0.71 0.78

CU 1.1 1.0

%MEDIUM 55.83 52.54

% COARSE 40.49 43.19

% FINES 0.74 1.32



Fig. 7. (A) Particle size Distribution curve for BH1

Fig. 7. (B) Particle size distribution for BH2

3.1.2 Cone penetrometer test

The graphs of the cone penetrometer reading are presented as

penetration rate against depth in Figure 8. The depth

penetrated by the penetrometer test is about 11.2 m. The

readings show significantly low cone resistance of about 80

(kgf/cm2) which indicates peat material (Table3). The linear

nature of the graph shows constant penetration as the

subsurface materials offer no resistance to the driven cone.

The result on the CPT test indicates that the depth range of

12.5m penetrated is unfit for erecting the foundation of most

structures due to its poor shear strength. This was replicated in

the other CPT conducted in the area.

0

10

20

30

40

50

60

70

80

90

100

0.001 0.01 0.1 1 10 100

Pe

rce

nta

ge P

assi

ng

Sieve Size (mm)

Particle Size Distribution Curve

0

10

20

30

40

50

60

70

80

90

100

0.001 0.01 0.1 1 10 100

Pe

rce

nta

ge P

assi

ng

Sieve Size (mm)

Particle Size Distribution Curve



Table III

CPT Results from the Study Area

CPT1 CPT2

QC Depth Const. BC. QC Depth Const. BC

0 0 0 0

35 -0.25 2.7

94.50 20 -0.25 2.7

54.00

20 -0.5 2.7

54.00 10 -0.5 2.7

27.00

15 -0.75 2.7

40.50 10 -0.75 2.7

27.00

10 -1 2.7

27.00 15 -1 2.7

40.50

15 -1.25 2.7

40.50 10 -1.25 2.7

27.00

25 -1.5 2.7

67.50 15 -1.5 2.7

40.50

20 -1.75 2.7

54.00 20 -1.75 2.7

54.00

25 -2 2.7

67.50 15 -2 2.7

40.50

25 -2.25 2.7

67.50 25 -2.25 2.7

67.50

35 -2.5 2.7

94.50 30 -2.5 2.7

81.00

25 -2.75 2.7

67.50 40 -2.75 2.7

108.00

25 -3 2.7 67.5 10 -3 2.7 27

10 -3.25 2.7 27 25 -3.25 2.7 67.5

25 -3.5 2.7 67.5 15 -3.5 2.7 40.5

20 -3.75 2.7 54 15 -3.75 2.7 40.5

35 -4 2.7 94.5 30 -4 2.7 81

50 -4.25 2.7 135 45 -4.25 2.7 121.5

40 -4.5 2.7 108 35 -4.5 2.7 94.5

10 -4.75 2.7 27 25 -4.75 2.7 67.5

10 -5 2.7 27 15 -5 2.7 40.5

15 -5.25 2.7 40.5 10 -5.25 2.7 27

5 -5.5 2.7 13.5 15 -5.5 2.7 40.5

5 -5.75 2.7 13.5 20 -5.75 2.7 54

10 -6 2.7 27 10 -6 2.7 27

5 -6.25 2.7 13.5 15 -6.25 2.7 40.5

10 -6.5 2.7 27 5 -6.5 2.7 13.5

5 -6.75 2.7 13.5 10 -6.75 2.7 27

10 -7 2.7 27 10 -7 2.7 27

5 -7.25 2.7 13.5 10 -7.25 2.7 27

10 -7.5 2.7 27 10 -7.5 2.7 27

15 -7.75 2.7 40.5 20 -7.75 2.7 54

10 -8 2.7 27 10 -8 2.7 27

5 -8.25 2.7 13.5 10 -8.25 2.7 27

10 -8.5 2.7 27 20 -8.5 2.7 54

15 -8.75 2.7 40.5 30 -8.75 2.7 81

10 -9 2.7 27 20 -9 2.7 54

5 -9.25 2.7 13.5 25 -9.25 2.7 67.5

10 -9.5 2.7 27 40 -9.5 2.7 108

5 -9.75 2.7 13.5 30 -9.75 2.7 81



10 -10 2.7 27 50 -10 2.7 135

5 -10.25 2.7 13.5 40 -10.25 2.7 108

15 -10.5 2.7 40.5 30 -10.5 2.7 81

5 -10.75 2.7 13.5 20 -10.75 2.7 54

10 -11 2.7 27 30 -11 2.7 81

20 -11.25 2.7 54 25 -11.25 2.7 67.5

25 -11.5 2.7 67.5 40 -11.5 2.7 108

40 -11.75 2.7 108 50 -11.75 2.7 135

70 -12 2.7 189 30 -12 2.7 81

80 -12.25 2.7 216 50 -12.25 2.7 135

Table IV

Geotechnical engineering data from Dutch cone probes.

Test Location Depth Of Penetration (M) Average Cone Resistance

Analysis (Kgf/Cm2)

Remarks

P1 12.5 80 Dark grey, stiff medium

Grained clayey sand

P2 13 85 Dark grey, stiff medium

Grained clayey sand

Fig. 8(A). Cone penetration graph for pit1

-13

-12

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

0 10 20 30 40 50 60 70 80 90 100 110 120

Dep

th (

m)

Cone resistance (Kg/cm2)



Fig. 8(B). Cone Penetration Graph for pit 2

3.1.3 Standard penetration test

A summary of the borehole log derived from the SPT is

represented in Table 5 Figure 9. During the percussion drilling

exercise peat material was encountered within the depth of 0

to 8.5 m (Figure 9). This material is dark to brownish dark and

soft in texture. This region is attributed with poor geotechnical

properties, low shear strength and high compressibility

potential. This region is not suggested for erecting foundation

of most structures, Figure 9. Below this material is clayey

sand material encountered at a depth range of 8.5 to 11.50 m.

The material present at this depth is classified as moderate and

good geotechnical properties. This region has moderate to

high shear strength and low compressibility potential. This

region might be fit for sizable structures. At the depth ranges

of 11.50 to 27.50m very stiff clayey sand was encountered but

with varying texture. The texture varies from loose, medium

and dense clayey sand and compact (derived from the number

of blows). The material at this depth is associated with good

geotechnical properties, high shear strength and low

compressibility potential. This depth range is deemed fit to

erect the foundation of most structures, but the thickness of the

overburden to be removed may be expensive.

Table V

Geotechnical engineering data from Standard Penetration Test.

Depth (m) Subsoil Encountered Description Laboratory Analysis Description/Remark Stratum Thickness (m)

0.00to -4.50

Dark grey, debris i.e broken glass,

Nylon. lateritic sand

Nil

4.50

-4.50 to 8.50 Dark grey, stiff, fine- medium

grained peaty Clayey sand.

Partially Compressible.

4.00

-8.50 to -11.50 Dark grey stiff, medium grained

Clayey sand.

Low compressibility

2.00

-11.50-27.50 Dark grey, very stiff medium

Grained clayey sand

Very low Compressibility 16.0

-27.50-30.00 Dark grey medium-grained sandy

Clay

Predominantly fine to medium

grained sand but slightly plastic

2.50

-13

-12

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

0 10 20 30 40 50 60 70 80 90 100 110 120

Dep

th (

m)

Cone resistance (Kg/cm2)



Fig. 9. Result from Standard Penetration Test

3.2 Geophysical Result

The plots of the results of the apparent resistivity of the

geophysical studies carried out in the study area shows that

the VES curves in this area are predominantly KH-type curve

(ρ1 < ρ2 > ρ3 < ρ4) (Fig. 10a and b). The VES interpretation

results are presented in Table 6. Two geo-electric sections

were drawn along approximately W- E and S – N directions

(Fig. 10a and b). Three subsurface strata were delineated. The

topsoil is composed of decomposed organic materials,

vegetable remains, and pockets of exotic sand filling materials

of 3.1 to 4.3 m thick sandy clay/silty sand/mud/peat with

resistivity values varying from 145 - 351 Ωm. Underlying the

upper layer is a peaty clayey layer having thickness and

resistivity values ranging from 10.7 to 11.4 m and 49.1 to 97.2

Ωm respectively. This layer is unfavourable for foundation of

engineering structure along this traverse. Beneath the second

layer is 24.4 to 26.1 m thick medium grained clayey sand with

a variable resistivity values from 41.4 to 105.6 Ωm. The fourth

horizon is the dark grained clayey sand (with layer resistivity

values of 41.4- 63.1 Ωm) identified between the depths of

44.00 to 51.9 m.

Table VI

The results of the geophysical survey done in the study area

VES

Location

Layer Apparent Resistivity ρ

(ohm-m)

Thickness (m) Depth(m) Lithological

Description

Curve

type

1 1 351 4.3 4.3 Top soil k-type

2 49.1 7.1 11.4

3 99.3 14.6 26.1

4 41.4 25.9 51.9

2 1 145.4 3.1 3.1 Top soil K-type

2 97.2 7.6 10.7

3 105.6 13.7 24.4

4 63.1 19.6 44.0



Fig. 10. (A) Resistivity Graph

Fig. 10(B). Resistivity Graph

3.3 Correlation of resistivity section with CPT and

SPT

Figure 11 represents the juxtaposition of the results from

all surveys for better correlation of the results. The CPT has

indicated materials with low shear strength within the depth

range of 0 to 12 m identified to be composed of peaty clayey

sand. The SPT has also revealed this depth range to be

composed of peat while the resistivity result section has shown

material with low resistivity value (1.0 to 49 Ωm) prevalent to

be peat/clay material. The SPT revealed that the subsurface is

composed of peat within the depth range of 0 to 8.5 m while

the resistivity section indicates this depth to be composed of

low resistivity value material indicating peaty clayey sand.

The presence of sandy clay material is revealed within the

depth range of 11.50 to 27 m on the SPT and the resistivity

section has indicated this material to be with resistivity range

of 49.1 to 97.2 Ωm. Sand material is identified at 27 to 30 m

on the SPT and represented by relatively high resistivity value



range of 99.3 to 105.6 Ωm on the resistivity section. The result

of the borehole data at the SE portion of the study area

correlates well with the second half of resistivity tomography

sections towards the SE portion of the study area. However,

in addition to this, the resistivity tomography has shown that

the peat/clay material is much thicker within the central

portion (30 to 60 m) of which there was no SPT or CPT

information. This clearly shows the need for the integration of

resistivity tomography into various engineering site

investigation for proper evaluation of soil integrity.

Fig. 11. Correlation between the VES and CPT

3.4 Shear Strength Properties

Shear Strength Parameters The strength and

consolidation of foundation soils play major roles in

construction projects. These engineering parameters determine

the ability to carry weights, and support buildings and roads.

The nature of shearing resistance of a soil offers the

opportunity to analyze its stability problems such as bearing

capacity. In Table 7, the cohesive strength (Cu) ranges from 75

to 90 kN/m2, while the undrained angle of internal friction (∅ )

varies between 9 and 11o. The soils have Cu value far greater

than zero U which is characteristic of normally consolidated

clay, with the values indicative of clayey sand.

Table VII

The shear strength parameters of the studied soils are summarized

Sample Cohesive strength (Cu ) kN/m2 undrained angle of internal friction (∅ )

1 90 11

2 75 9

3.5 Compressibility and Bearing Capacity Properties in

foundation design

Settlement and bearing capacity are generally the critical

issues. The process in which reduction in volume would take

place by expulsion of water under long term structural loads

on the sampled is summarized in Table 8. The rate at which

the studied soil undergoes compression (Cv), ranges between

0.18 and 0.34 m2 /Yr. This shows that the soils in both

locations are highly compressible by the building which is

imposed on it, meaning that the building is underlain by an

incompetent soil layer which is susceptible to settlement.

While the amount of settlement as related to the coefficient of

volume change (mv) varies between 0.19 and 0.32 m2 /MN.

The consolidation parameters were based on pressure

increment of 25–50 kN/m, 50–100 kN/m, 100-200kN/m and

200–400kN/m with the time for consolidation to occur

predicted in years. It is evident that both rate and amount of

settlement of the studied soils would vary with confining

pressure. Establishing any structure on such a soil layer with

compressible fines content could result to differential

settlement (Sridharan and Nagaraj 2012). The estimated

bearing capacities for shallow and deep foundations envisaged

for civil structures in the area are summarized in Table 8.

These values were based on the Mayerhoff (1956) theory.



3.6 SHALLOW FOUNDATIONS The result of the investigation as calculated based on

Meyerhof’s formulae for Strips or Square/Circular

foundations irrespective of the foundation width at various

depths range gave the following Allowable Bearing Pressure

values for the subsoil condition in its present natural state as

follows:

qa = 2.7Ckd kN/m2 ~ Ckd / 40 kg/cm2

Where Ckd = cone resistance in kg/cm2 and qa = allowable

bearing pressure in kN/m2

Table VIII Bearing capacity for Shallow and Deep foundation

Depth Range (m) Allowable Bearing Pressure

(qa) in kN/m2

- 0.50 2.7kN/m2

- 1`.00 2.7kN/m2

- 2.00 5.4kN/m2

-3.00 13.5kN/m2

-4.00 67.5kN/m2

The allowable bearing is low to mobilize the building for the

shallow foundation that was adopted (Table 8).

3.7 DEEP FOUNDATIONS

The results of the borehole tests revealed very stiff clayey

sand at 11.50m depths into which end bearing Piles could be

terminated. Some form of deep foundations such as deep

reinforced concrete columns of medium to big size cross-

sectional area can be used to transmit the column loads from

the building to terminate within the medium dense to very

dense indicated to occur below 11.50m depths. Such

foundations should be capable of mobilizing high safe

working loads sufficient for the building to stop the building

from further settlement. The following piles working loads

in Table 9 are quoted as a guide based on data obtained from

SPT results for bored cast-in-place piles:

Table IX

Table Showing Safe Working Load

Pile Type Pile Length (m) Safe working Load (kN)

300mm Bored cast-in-place pile 11 210



Settlement of the building on set of piles with the above

quoted safe working loads (SWL) and using 3.0 as a factor of

safety are expected to be minimal in view of the fact that the

piles will terminate within the medium dense to very dense

Sand. Also, spacing of piles or number of piles used per

column and pattern must ensure that groups are not heavily

loaded.

4. CONCLUSION AND RECOMMENDATION

Early detection and discovery of the causes of building

collapse via the mandatory, periodic or conditional structural

integrity assessment go a long way in preventing incessant

building collapse and eliminate the attendant loss of life and

other properties.Geophysical and geotechnical investigations

have been carried out to probe the subsoil conditions of

abuilding at risk in Ogudu area of Lagos in order to establish

its engineering characteristics and the appropriate foundation

suitable for the site. The occurrence of low resistivity,

incompetent peat and organic clay within the depth range of 1

to 9 m implies that shallow foundation will was not be suitable

for thebuilding, the imposed load resulted to differential

settlement of the building. From the subsoil strata revealed in

the completed borehole, considering the storey building and

the subsoil/water conditions as revealed in the completed

borehole, CPT results and Laboratory analyses of selected

subsoil samples and the Shallow Foundation in form of Rafts

that was adopted was not appropriate for the building.

Meanwhile, ancillary structures such as the gate house could

be placed on Shallow Strip footings up to 0.50m depth while

the generator house could be placed on Raft footings up to

0.75m depth below the existing ground level in view of the

induced vibration it will impose on the ground.

Deep foundation involving piling through the

incompetent shallow layers to the competent sand with a pile

depth of 11.5m depth is recommended. Adequate drainage

system should be provided for surface runoff and to eliminate

surface water infiltrations around the building foundation. If

pile was not adopted for the building, the settlement may

continue as a result of materials that underlie the site, the

immediate solution may be that the occupant may have to

vacate the building, to introduce underpinning pile to the

11.50m to prevent it from further settlement. Before this, Non

destructive test; the building integrity must be ascertain before

the underpinning. The mere fact that if the subject property

failed the mandatory structural integrity test, it would be

demolished would compel the client, the builder and the

contractors to do their job right. In order to ensure

implementation and compliance; the structural integrity



assessment report must be backed up by law, making it a

statutory requirement before and after occupation. The law

must also recommend that every property, whether private or

public property and for any purpose must be managed by

professional property manager, that is, the estate surveyors

and valuers.

ACKNOWLEDGEMENT

The authors are grateful to God who made the work a success

and also acknowledge the contributions of the Engineering

Geological group, University of Nigeria Nsukka towards the

success of this work.

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