evaluation of bearing capacity and settlement of...

Leonardo Electronic Journal of Practices and Technologies

ISSN 1583-1078

Issue 29, July-December 2016

p. 93-114

Engineering, Environment

Evaluation of bearing capacity and settlement of foundations

Bunyamin Anigilaje SALAHUDEEN 1* and Ja'afar Abubakar SADEEQ 2

1 Samaru College of Agriculture, Ahmadu Bello University, Zaria, Nigeria 2 Department of Civil Engineering, Ahmadu Bello University, Zaria, Nigeria

Emails: * [email protected]; [email protected] *Corresponding Author phone: +2348058565650

Received: February 12, 2016 / Accepted: December 8, 2016 / Published: December 30, 2016

Abstract

The standard penetration test (SPT) results (SPT N-values) used in this

study were corrected to the standard average energy of 60% (N60) before

using them to correlate soil properties and evaluate foundation settlement

characteristics and bearing capacity in the North Central zone of Nigeria.

Based on the corrected N-values, some geotechnical design parameters

including the allowable bearing pressure and elastic settlement of

foundations were predicted at varying applied foundation pressures of 50,

100, 200, 300 and 500 kN/m2 using conventional analytical models and

numerical modelling. The numerical analysis results using Plaxis 2D, a

finite element code, shows that Meyerhof’s and Peck’s et al.

analytical/empirical methods of estimating the allowable bearing pressure of

shallow foundations provide acceptable results. Results obtained show that

an average bearing capacity value of 150 – 350 kN/m2 can be used for

shallow foundations at embedment depth of 0.6 to 3.6 m in the North

Central zone. Based on recommendation of Eurocode 7 which allows a

maximum total settlement of 25 mm for serviceability limit state, it is

recommended that raft or deep foundations to be considered for applied

foundation pressures exceeding 300 kN/m2 in the North-Central zone to

avoid excessive settlement.

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mailto:[email protected]


Bunyamin Anigilaje SALAHUDEEN and Ja’afar Abubakar SADEEQ

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Keywords

Numerical modelling; Plaxis 2D; Finite element method; Standard

Penetration Test; Bearing capacity; Elastic settlement

Introduction

One of the most significant components of any structure is its foundation. Foundations

are integral to overall structural performance. They help in bearing and transmitting the

structural loads to the soil, reducing settlements (total and differential), preventing possible

movement of structures due to periodic shrinkage and swelling of subsoils, allow building

over water or water-logged grounds, resist uplifting or overturning forces due to wind, and

resist lateral forces due to soil movement and control water penetration and dampness. To

perform satisfactorily, foundations must have two main characteristics: they have to be safe

against overall shear failure in the soil that supports them and they must not undergo

excessive settlement [1].

Probably the most difficult of the problems that a soil engineer is asked to solve is the

accurate prediction of the settlement of a loaded foundation [2]. The problem is in two distinct

parts: the value of the total settlement that will occur and the rate at which this value will be

achieved. The design of shallow foundations is generally controlled by settlement rather than

bearing capacity [3]. As a consequence, settlement prediction is a major concern and is an

essential criterion in the design process of foundations. Consistent and accurate prediction of

settlement is yet to be achieved by the use of a variety of analytical methods [4].

Comparative studies of the available methods by engineers/researchers [5-8] indicate

inconsistent prediction of the magnitude of the calculated settlements. This may be attributed

to the fact that most of these methods are model driven, in which the form of the model has to

be selected in advance and the unknown model parameters are determined by minimising an

error function between model predictions and known historical values. Consequently, prior

knowledge regarding the relationship between model inputs and corresponding outputs is

needed. In case of settlement of shallow foundations on granular soils, such knowledge is not

yet entirely understood [3].

The finite element method can be particularly useful for identifying the patterns of

deformations and stress distribution during deformation and at the ultimate state. Because of

these capabilities of the finite element method, it is possible to model the construction method


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and investigate the behaviour of shallow footings and the surrounding soil throughout the

construction process, not just at the limit equilibrium conditions [9]. The finite element

method (FEM) allows modelling complicated non linear soil behaviour through a constitutive

model, various geometrics with different boundary conditions and interfaces. It can predict

the stresses, deformations and pore pressures of a specified soil profile [10].

The high level of demands for housing in Nigeria due to the growing population and

migration of people to urban areas require alternative construction methods that provide fast,

safe and affordable quality housing for the citizens [11-13]. The aim of this study is to explore

numerical modelling method that better represents soil constitutive behaviour to develop an

improved approximation of foundation soil bearing capacity and settlement and compare the

results with those of empirical methods. Also, there is a need to investigate and determine the

most appropriate methods to Nigerian soil peculiarities and distinctions based on SPT results,

being the most common and economical geotechnical field test used in Nigeria. The study

focused on the prediction of foundation soil bearing capacity and settlement based on

Standard Penetration Test (SPT) N-values using empirical/analytical (deterministic) models

and numerical modelling in the North-Central zone (i.e. Benue, Federal Capital Territory,

Kogi, Kwara, Nassarawa, Niger and Plateau States) of the Federal Republic of Nigeria.

The objectives of this research were to estimate the bearing capacity and settlement of

foundation soils from measured penetration resistance in terms of the SPT corrected N-values

at varying depths, to evaluate design equations for foundation soil bearing capacity and

settlements using different constitutive models based on SPT results, to model foundation

settlement numerically using PLAXIS 2D software and to compare the results of

empirical/analytical methods with those of numerical analysis.

Research Methodology

The research made use of standard penetration test (SPT) data (using Donut hammer

type) collected from 592 test holes (5328 data set) distributed over the study area.

Computations were done based on the average that reliably represents each State and the

average of the States was used for the North-Central zone. Bearing capacity and foundation

settlement estimations were made at depths of 0.6, 2.1, 3.6, 5.1, 6.6, 8.1, 9.6, 11.1 and 12.6 m

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and settlement was determined at varying applied foundation pressures of 50, 100, 200, 300

and 500 kN/m2.

Based on the analytical methods, bearing capacity and foundation settlement were

evaluated using some carefully selected models listed in Table A1 and A2, respectively (see

Appendix). On the other hand, numerical analysis of foundation settlement and bearing

capacity were performed using a non-linear finite element analysis with a finite element code,

Plaxis 2D, which is a software for deformation analysis and modelling of geotechnical

problems.

The input data in Plaxis are index, elastic and strength parameters obtained from the

processed SPT N-values. The software portfolio includes simulation of soil and soil-structure

interaction. Plaxis 2D is an axisymmetric finite element package used for two-dimensional

analysis of deformation and stability in geotechnical engineering. It uses advanced soil

constitutive models for the simulation of the non-linear, time dependent and anisotropic

behaviour of soils and rocks. Plaxis 2D portfolio models the structure, the soil and the

interaction between the structure and the soil. Soil layers and foundation structure parameters

are inputted into Plaxis and the construction stages, loads and boundary conditions are defined

in an already defined geometry cross-section containing the soil model then Plaxis

automatically generates the unstructured 2D finite element meshes with options of global and

local mesh refinements. Using its calculation facilities, Plaxis 2D undergoes a calculation

process and presents the calculation and model outputs which can be accessed in animation

and/or numerical forms. The input data in numerical modelling are index, elastic and strength

parameters obtained from the processed SPT N-values [14]. The steps involved in developing

the numerical model can be depicted by the chart shown in Figure 1.

Figure 1. Chart depicting the steps involved in developing the numerical models


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Results and Discussion

The results of this study were computed using the most common and conventional

empirical methods in the literature based on the average values of input data obtained from

the available field information at the time of the study. It is pertinent to state that result

presented herein can approximately represent soil conditions in the North-Central zone of

Nigeria considered. The elastic settlement and allowable bearing capacity results of the

empirical/analytical methods were compared with those of numerical modelling output using

Plaxis.

Corrected N-values (N60)

According to Bezgin [15] a correction to average energy ratio of 60% (N60) is required

to SPT N-values because of the greater confinement caused by the increasing overburden

pressure. The correction factors used in the study are those proposed by Das [1] to standardize

the field penetration number as a function of the input driving energy and its dissipation

around the sampler into the surrounding soil. The variation of N60 with depth of test is shown

in Figure 2.

Figure 2. Variation of corrected N-values with boring depth

N60 increased with depth having the highest value of 89.25 on the average in the

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North-Central zone. The increase in N60 value with depth is due to increased overburden. This

confirms the conclusions of Salahudeen et al. [13] that the soils in the northern part of Nigeria

are crystalline in nature (which has higher N60 values compared with sedimentary formations)

from the basement complex. N60 values are needed for more accurate design analyses and

have less variability or scatter due to the test method.

Bearing capacity

Based on field test results, the bearing capacities of shallow foundations are

determined in terms of the allowable bearing pressures while those of deep foundations (piles)

are given in terms of the ultimate bearing capacity. This is because settlement (service limit)

controls the allowable bearing capacity in design of shallow foundations while the ultimate

limit (shear failure) usually controls the allowable bearing capacity in deep foundations

design [13]. For the allowable bearing pressures of shallow foundations, footing plan

dimensions of 2 m by 2 m by 0.4 m for length, breadth and depth, were respectively assumed

with a safety factor of 3. Variations of allowable bearing capacity of shallow foundations and

bearing capacity of piles with boring depth are shown in Figures 3 and 4, respectively.

Figure 3. Variation of allowable bearing pressure with depth


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Figure 4. Variation of bearing capacity of pile with depth

Based on the method proposed by Meyerhof [16] and Plaxis, foundation pressures in

the range of 150 – 350 kN/m2 were proposed for use in the North-Central zone at shallow

depths (depths in the range of 0.6 - 3.6 m).

In evaluating the bearing capacity of piles, assumed dimensions of 0.3 m by 0.3 m

cross-sectional area with embedment length of 10 m were used. Although the results

according to Briaud et al. [21] are high compared to those proposed by Meyerhof [22], based

on 60 pile case histories and SPT borehole results comprising 43 full scale pile load tests and

17 dynamic tests with Case Pile Wave Analysis Program (CAPWAP) collected from 18

sources reporting data from 26 sites and from 7 countries, Shariatmadari et al. [23] reported

up to 10,505 kN/m2 pile bearing capacity at 25 m depth. Maximum pile bearing capacity

values of 3748.50 and 10006.36 kN/m2 were obtained for the North-Central zone using

Meyerhof [22] and Briaud et al. [21] methods respectively, at foundation embedment depth of

12.6 m.

Elastic settlement of foundations

For the elastic settlement of shallow foundations, plan dimensions of 2 m by 2 m by

0.4 m for length, breadth and depth were respectively assumed. Variations of elastic

settlement of foundations with embedment depth for various applied pressures are shown in

Figures 5 - 9.

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Figure 5. Variation of elastic settlement with depth for 50 kN/m2 foundation pressure



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The figures show the different empirical/analytical models commonly used in

computing elastic settlement of shallow foundations. The N60 values indicate that settlement

values will be low due to high N60 values in the region as confirmed in the elastic settlement

results. The recorded trend is consistent with observations reported by Rasin [24] and

Salahudeen et al. [13].

The numerical analysis results of soil body deformation, stress distribution and

settlement respectively at collapse of the soil body for the North-Central zone at 0.6 and 12.6

m depths of embedment are shown in Figures 10 - 15.


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Figure 10. Numerical analysis mesh showing deformation of the soil body at collapse at 0.6 m

embedment depth in the North-Central zone

Figure 11. Numerical analysis result of stress distribution up to the collapse of the soil body at

0.6 m embedment depth in the North-Central zone

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Figure 12. Numerical analysis result of settlement up to the collapse of the soil body at 0.6 m


Figure 13. Numerical analysis mesh showing deformation of the soil body at collapse at 12.6

m embedment depth in the North-Central zone


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Figure 14. Numerical analysis result of stress distribution up to the collapse of the soil body at

12.6 m embedment depth in the North-Central zone

Figure 15. Numerical analysis result of settlement up to the collapse of the soil body at 12.6 m


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A comparison among the analytical methods used in this study with the results of

numerical modelling show that the methods proposed by Schmertmann et al. [29], Burland

and Burbidge [31], Canadian Foundation Engineering Manual (CFEM) [34] as well as that of

Mayne and Poulos [36] gave good estimations of foundation settlement.

Total settlement of piles

Variation of settlement of pile (bored) under a vertical working load with depth based

on methods proposed by Vesic [38] and that of Das [1] is shown in Figure 16.

Figure 16. Variation of total settlement of piles with depth

The total settlement is the sum of elastic settlement of pile, settlement of pile caused

by the load at the pile tip and settlement of pile caused by the load transmitted along the pile

shaft. The wide margin between the methods used is an indication that the settlements need to

be modelled in order to come up with an appropriate method that will be more suitable for

Nigerian soils. However, it is pertinent to state that numerical modelling of piles was not

included in the scope of the study carried out.

Conclusions

The study considered N-values corrected to the standard average energy of 60% (N60)


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as input data in analytical/empirical and numerical models used to predict foundation

settlement and bearing capacity in the North-Central zone of Nigeria for footing of 2 m by 2

m by 0.4 m size and varying pressures of 50, 100, 200, 300 and 500 kN/m2. Based on the

results of this study, the following conclusions can be taken:

÷ Allowable bearing pressures of 150-350 kN/m2 at depths between 0.6 and 3.6 m obtained

using the Meyerhof method are adequate for North-Central soils. The values are very

close with those of numerical analysis using Plaxis 2D.

÷ The maximum elastic settlement values respectively for all the applied foundation

pressures show that the soils in the North-Central zone are on the average, less

susceptibility to compression.

÷ It was observed that settlement increased with increased value of applied foundation

pressure. Settlements of footings embedded at depths in the range 0.6-3.6 m and pressures

above 300 kN/m2 exceeded the limiting value of 25 mm value of allowable total

settlement stipulated by Eurocode 7.

÷ A comparison of the fifteen empirical/analytical methods considered in this study, showed

that the Schmertmann et al. [29], Burland and Burbidge [31], Canadian Foundation

Engineering Manual (CFEM) [34] as well as the Mayne and Poulos [36] methods gave

good estimations of foundation settlement.

÷ Plaxis 2D tend to overestimate the elastic settlement of footings for embedment depths up

to 2 m and at applied foundation pressure greater than 300 kN/m2.

Recommendations

Based on the results of this study, the following are hereby recommended for the

North-Central zone of Nigeria:

÷ Foundations should be placed at a minimum depth of 1.0 m.

÷ Deep foundations should be considered for applied foundation loads exceeding 300 kN/m2

to avoid excessive settlement.

÷ Results of the study can be used as first approximation of foundation bearing capacity and

settlement but does not preclude the use of site specific data.

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Acknowledgements

The authors wish to acknowledge the assistance of the Management of In-depth

Engineering Limited, Kaduna, Nigeria that provided all standard penetration test data used for

the study. For the assistance of Dr. M. Jalili of Islamic Azad University, Semnan, Iran with

respect to training on the use of Plaxis software.

Appendix

Table A1. Empirical/analytical models for soil bearing capacity analysis Property Model Reference

Corrected N-value Seed et al. (1985) andSkempton (1986) Teng (1969)

Meyerhof (1974)

Peck etal. (1974)

Bowles (1996)

Allowable bearing pressure of shallow foundations

Terzaghi et al. (1996)

Meyerhof (1976) Bearing capacity of piles

Briaud et al. (1985)

Total settlement of piles

*Vesic (1977) **Das (2011)


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Table A2. Empirical/analytical models for elastic settlement analysis S/N Model Reference

1 Janbu et al. (1956)

2 Terzaghi and Peck (1967)

3 Schmertmann (1970)

4 Schultze and Sherif (1973)

5 Meyerhof (1974)

6 Schmertmann et al. (1978)

7

Timoshenko and Goodier (1982)

8 Burland and Burbidge (1985)

9 Bowles (1987)

10 Anagnostropolous et al. (1991)

Can. Found. Eng. Man.

(CFEM) (1992) 11

12 Papadopoulos (1992)

13 Terzaghi et al. (1996)

14 Mayne and Poulos (1999)

15 Anderson et al. (2007)

Abbreviations

(N1)60 = N60 correction for overburden pressure

B = Width of foundation (m)

BR = Reference width = 0.3 m

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Df = Depth of embedment (m)

Es = Appropriate value of elastic modulus of soil (kN/m2)

Es = Elastic modulus of soil

H = Thickness of the compressible layer (m)

L = Length of foundation (m)

N = Measured penetration number (N-value)

N60 = Corrected standard penetration number for field conditions

N60(a) = Adjusted N60 value

Pa = Atmospheric pressure = 100 kN/m2

q = Applied foundation pressure (kN/m2)

q = Net effective pressure applied at the level of the foundation (kN/m2)

qn= Net pressure on the foundation (kN/m2)

Se = Elastic settlement (mm)

Se(1) = Elastic settlement of piles

Se(2) = Settlement of pile caused by the load at the pile tip

Se(3) = Settlement of pile caused by the load transmitted along the pile shaft

μS = Poisson’s ratio of soil

σ10 = Effective overburden pressure in kN/m2

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