DOI : 10.17577/IJERTV8IS110244
- Open Access
- Authors : Acidri Samuel , Kyakula Michael , Mugume Rodgers Bangi
- Paper ID : IJERTV8IS110244
- Volume & Issue : Volume 08, Issue 11 (November 2019)
- DOI : http://dx.doi.org/10.17577/IJERTV8IS110244
- Published (First Online): 30-11-2019
- ISSN (Online) : 2278-0181
- Publisher Name : IJERT
- License:
This work is licensed under a Creative Commons Attribution 4.0 International License
A Comparative Analysis of Foundations using Prescriptive Design and Static Loading Test Methods
(Case Study: The Karuma Interconnection Power Project in Uganda)
*Acidri Samuel1, Kyakula Michael2, and Mugume Rodgers Bangi3
1, 2, 3 Faculty of Engineering, Department of Civil and Building Engineering, Kyambogo University, Uganda
*Corresponding Author,
Abstract: Despite recent improvements in soil characterisation, geotechnical exploration and construction methodologies, 66.7% of overhead transmission line foundation design engineers use prescriptive design methods with applied traditional factors of safety between 2.5 to over 4.0, to design foundations in the face of uncertainties in ground conditions and design criteria, non-linear nature of the load-displacement response of foundations and the prescriptive designs tendency to give linear solutions. Hence, the use of full-scale foundations and static load tests to assess the overall response of foundations. A Ø900mm pile and 3 pad foundations along Ugandas 400 kV Karuma Interconnection Project were designed, constructed and tested under uplift, compression and lateral loads as per the respective failure modes. The results suggested that the maximum displacements were within 0.36-18.96% of the prescriptive 25 mm value for uplift, 3.32% of the prescriptive 25 mm value for compression, and 4.78% of the prescriptive 50 mm value for the lateral load test in conformity to IEC 61773 (1996) and COMESA/FDHS 293 (2007).
The foundations insitu load capacities from the hyperbolic graphs as per the Chin-Kondner extrapolation (1971), confirmed that the foundations could adequately resist the working loads at 100% and ultimate design loads at 130%, despite uncertainties of moderately aggressive chemical environment exposures as per BS EN 206 (2013) or soils with medium to high degrees of plasticity with low swell potentials.
Keywords: Foundations, Prescriptive Design, Static Load Tests (SLTs), Transmission Towers
- INTRODUCTION
-
- The Local Geology
The geology of Karuma Interconnection Project mainly consists of quaternary rock systems of laterite, alluvium, swamp and lacustrine deposits, Neoproterozoic rock systems of mudstones, shales, slates and phyllites, Mesoproterozoic rock system of mica schists, and Neoarchaean rock systems of metagabbro, porphyritic-granite, meta-dolerite, granodiorite, biotite-hornblende and banded gneiss.
- The Research Background
- The Local Geology
Foundation substructures are essential structural members that transmit and distribute different kinds of superstructure loads to the substrata below without exceeding the bearing capacity of the ground and preventing excessive or uneven settlements, and they are generally classified as shallow or spread and deep foundations [1-3]. Foundations must fulfil
both structural and geotechnical parameters, but due to uncertainties of the subsoil behaviour, most foundations are either statically or dynamically tested to verify conformity with the design load, but the latter is not commonly used in Southern Africa [4,5].
In Uganda, since 2013, load tests have been used to either prove the maximum capacity of the foundation and/or to verify the predicted design values and settlements under pressure on a foundation that is expendable to the main works; and at times on a working pile whilst limiting the maximum test load to less than 1.5 times the safe working load [5-7].
Foundation design entails that neither the foundation units collapse nor should they induce the overall shear failure of the supporting ground, lest the foundations post-construction settlement values exceed the permissible tolerances in the codes and specifications [8-11]. The design of foundations consists of proportioning the foundation, mitigating limit state conditions such as the ultimate limit state properties of loss of static equilibrium of the structure, failure by collapse or by fatigue; and/or the serviceability limit state properties of deflection, cracking, vibration, and deterioration of the foundation structure [3,12-14].
Uncertainties in geotechnical models and parameters and their effect have long been recognised [15-23]; and hence, to perform geotechnical and foundation designs using the prescriptive design approaches, conservative values of the uncertain soil parameters are often adopted along with an experience-calibrated factor of safety [11, 24]. In the quest to obtain a more rational design, many researchers such as Wu [25], Christian [26], Whitman [18], Phoon [27,28], Fenton [29], Najjar and Gilbert [30], Wang [31], and Zhang
[32] have turned to reliability-based designs such as foundation full-scale models, analyses and foundation tests, which creates a much more-realistic geotechnical and foundation design outcome and is better in quantifying the uncertainties in soil parameters than the prescriptive design approaches [24]. Therefore, since also soils can soften or harden upon shearing and have a much more complex response than perfect plasticity; the recent focus on using the realistic full-scale foundation models, provides important insights into the overall response of foundations [33-38]. -
- GEOTECHNICAL INVESTIGATIONS
-
- The Test Trial Pits and Borehole pits
Four test pits were carefully located at a 1m distance from the survey mark stones, and manually excavated to 1m x 0.5m x 3m dimensions for investigating the stratification of subsurface layers and ground water as per BS 5930: 1999+A2: 2010 and BS 6031: 2009. Obtained soil samples
3 = sampler correction
4 = borehole diameter correction FOS = factor of safety
Bowless (1982) approach based on Meyerhofs (1963) equations is given below:
2
were labelled and placed in air-tight plastic bags for
q = { N
(B+F3)
K
} for B > F
(7)
indicative laboratory soil tests.
all
[ ] d 4F2 B
qall
= [ N K
F1 d
] for B F4
(8)
- The Dynamic Penetration Light Test
The DPL test was used in the determination of the soils
K = [1 + 0.33D
]
]
d
d
B
1.33 (9)
insitu resistance to the dynamic penetration of a cone, and in determining the soils bearing capacity () and resistance values using the Dutch formula below. Thus, the 10 values were interpreted to give the respective granular and fine- grained soil consistencies as per BS EN ISO 22476-2: 2005
+ A1: 2011 and DIN 4094: 1990.
Where:
qall = allowable bearing capacity N = corrected SPT N55 values N55 = adjusted N-values
B = foundation width/breadth
F1 = 0.05; F2 = 0.08; F3 = 0.3; and F4 = 1.2
D = foundation depth
q = = [ ] = [ E ] x [ M ] (1)
(+ )
A x H
(M+P)
Meyerhofs (1963) equations are given below:
r = Mgh = [gh x 10] =
(2)
d A e
0.1
qa = 0.73 N RD
x Sa
(B 1.2m) (10)
1
1
Where:
q = 0.48 N R
(B + 0.3) S
2
2
(B > 1.2m) (11)
= soils bearing capacity/dynamic point resistance
a D2 B a
= soils unit point resistance; and = Mgh
= cone area (A = 0.001m2); = penetration depth
= mass of hammer (M = 10.252 kg)
RD1 RD2
= 1 + 0.2 (Df) 1.2 for = 0 (12)
B
= 1 + 0.1 (Df) 1.2 for = 0 (13)
B
= total assembly mass where the DPL assembly mass is 6.714 kg, and each rod is 2.86 kg
10 = blows per 10 c penetration
Where:
qa = qall = allowable bearing capacity
= penetration rate = 0.110
N = corrected SPT N55 values
RD1 and RD2 = Meyerhofs depth reduction factors
- The Standard Penetration Test
The SPT test was conducted in accordance with BS EN ISO 22476-3:2005 and ASTM D1586-99: 1999 to compute
allowable bearing capacities from the corrected SPT N55 values using the Terzaghis formula (1967) and Bowless (1982) approach based on Meyerhofs (1963) equations.
The Terzaghis (1967) formulae is given below:
N55 = CN x N x 1 x 2 x 3 x 4 (3)
1
= Internal angle of soil friction in degrees
B = foundation width/breadth (in metres) Df = foundation depth
= Allowable settlement limit of 25 mm
- Indicator Laboratory Soil Tests
The obtained disturbed and undisturbed soil samples were tested for moisture content [39], particle size distribution [40,41], liquid limit [42], plastic limit and plasticity index [43], linear shrinkage limit [43], pH [44,45], sulphate and
CN =
p
o
o
(
po
2
) for 0.4 CN
1.7 (4)
chloride content [44,46], bulk density and unit weight [43,47], specific gravity [43,48], direct shear [49] and one-
q = 5.14 x qu =
5.14 x 13.1 x N55] (5)
dimensional consolidation [50,51].
{
{
ult 2 [ 2
=
=
q
q
qult
all FS
= 5.14 x cu = 5.14 x 13.1 x N 55} (6)
FS 2 x FS
- Static Load Foundation Tests
Static Load Tests are the most reliable and fundamental
Where:
qall = allowable bearing capacity
CN = adjustments for overburden pressure
p
p
p o = overburden pressure
forms of in-situ loading tests, considered as the bench-mark of foundation performance and used for validating load capacities and design assumptions of the foundation regarding the axial compression or tension resistance, or its deflected shape under a lateral load. They involve the
o
= reference overburden pressure (95.76 kPa)
measurement of foundation head displacements in response
N = corrected SPT N55 values
1 = ErErb (hammer efficiency correction)
Er = average energy ratio Erb = standard energy ratio 2 = rod length correction
to a physically applied test load until its failure point to replicate the long-term sustained load conditions [5,6,52]. They are standardised by ASTM D1143 for static axial compressive test; ASTM D3689 for static axial tensile (or uplift) test, and ASTM D3966 for lateral load test [6,53-56].
- The Static Axial Tensile Load Test
As standardised by ASTM D3689/D3689M-07 (2013) and IEC 61773 (1996), the Static Axial Tensile load test was used for verifying the behaviour of vertical or batter tension foundations like those of overhead transmission lines with respect to their tensile capacity and axial stiffness, so as to
= ( ) + (14)
= Df (15)
= Df (16)
Where:
= unit weight; and = effective unit weight
provide the most reliable relationship between the static
tensile load applied axially to a foundation and the resulting
= 1 [ + ( )] for ( )
axial movements. Hence, the obtained information was used in assessing the foundation shafts side shear resistance distribution, amount of end-bearing developed and the long- term load-deflection behaviour. It was also used to determine if the foundation had an ultimate static capacity and a deflection at service load satisfactory to support the specified foundation or superstructure [53,57].
- The Static Axial Compressive Load Test
The Static Axial Compressive load test measured the axial deflections of vertical or inclined foundations when loaded in static axial compression in order to confirm the foundations structural and geotechnical reliability and to predict its settlement rate. The load is thus, increased in stages until the proposed working load and a certain factor of safety is reached, and then unloading the load until the rise or rebound has substantially ceased. The foundation may be tested in three cycles, whereby the first cycle is to 150% of foundations Design Load (DL), the second cycle test is to 200% of DL and the third cycle tests the foundation to its ultimate load, defined as 250% to 300% of its DL [53,54]. Since the procedure leading up to 300% of the Design Load is very time consuming, the commonest method used stops at the first cycle and is limited to between 100% to 130% of the design load [5,6,57].
- The Lateral Load Test
- The Static Axial Tensile Load Test
- The Test Trial Pits and Borehole pits
As per ASTM D3966/D3966M-07 (2013), the Lateral load test was conducted to measure the lateral deflection of a vertical or inclined foundation when subjected to lateral loading, with the results used in characterising the variation of pile-soil interaction properties such as the coefficient of horizontal subgrade reaction, and estimation of bending stresses and lateral deflection over the piles length for use in its structural design [53,56].
-
- RESULTS AND DISCUSSIONS
-
-
- The Test Trial Pits and Borehole pits
Ground water tables were encountered above the base of footings in the swampy locations of KL 30 and AP 104/5 at 0.3m and 1.14m levels respectively; and ground water tables were encountered below the base of footings in locations AP 108/15 and AP 108/20 at levels of below 10m and 4m respectively below existing ground level [58]. The ground water table results in Table 1, were used in computing the soils effective unit weight (), unit surcharge magnitude () and corrections for water table effects on bearing capacities. Also, the insitu soil profile descriptions were used as a precursor assessment to the final soil grading and classifications as shown in Table 2 below.
= for ( > )
Table 1: The insitu ground water table levels
Site Depth (m) Pits WT (m) FFL (m) 1 (PS) 10.0 BHP 10.0 2.75 2 (GS) 3.0 TP 4.0 3.50 3 (WL) 10.0 BHP 1.14 4.50 4 (PL) 20.0 BHP 0.30 12.8 Note: 1 = AP 108/15; 2 = AP 108/20; 3 = KL 30; 4 = AP 104/5;
PS = Poor Soil; GS = Good Soil; WL = Waterlogged Location; PL = Pile Location; WT = Water Table; FFL = Foundations Formation Level; BHP = Borehole Pit; TP = Trial Pit
Table 2: The insitu soil strata descriptions
Site Depth (m) Insitu soil strata Soil Classification 1 (PS) 2.45-4.2 Grey-dense clayey sand Silty sand (SM) 2 (GS) 0.1-3.5 Brownish-orange laterite* with duricrust
Clayey sand with gravel (SC)
3 (WL) 4.5-6.5 Moist reddish brown, mottled grey, hard gravelly-clays
Gravelly clays of intermediate plasticity (CI) 4 (PL) 12.7-15 Slightly moist greyish brown, medium- dense clayey sand
Clayey sand (SC) Note: 1 = AP 108/15; 2 = AP 108/20; 3 KL 30; 4 = AP 104/5;
PS = Poor Soil; GS = Good Soil; WL = Waterlogged Location; PL = Pile Location
- *Medium-dense gravelly-sand
- SC and SM = Using the USCS soil classification system
- CI = Using the BS 5930 classification system
- The Dynamic Penetration Light (DPL)
The DPLs 10 readings of 10 to 54 under the respective penetration rates () , corresponded to granular soils of
medium-dense consistency mainly coarse-grained sandy soils as shown in Tables 3 and 4 below [59,60]. DPL tests were done to determine the blows per 10cm penetrations (10), consistency descriptions, computations of unit point ( ) resistance, and dynamic point () resistance/soil bearing capacity as per Eq. (1) and (2).
Depth (m) (kg) e (m/blow) (MPa) (MPa) 1.0 10.252 10 0.010 4.90 2.7 2.0 10.252 54 0.002 26.46 13.2 3.0 10.252 13 0.008 6.37 2.9 3.5 10.252 14 0.007 6.86 2.9 Depth (m) (kg) e (m/blow) (MPa) (MPa) 1.0 10.252 10 0.010 4.90 2.7 2.0 10.252 54 0.002 26.46 13.2 3.0 10.252 13 0.008 6.37 2.9 3.5 10.252 14 0.007 6.86 2.9 Table 3: The DPL result summary for AP 108/20
Note:
= mass of hammer; = blows per 10 cm penetration e = penetration rate (m per blow); = unit point resistance
= dynamic point resistance/soil bearing capacity
Table 4: Granular-soil consistency from DPL test
Blows, N10 Consistency description Blows, N10 Consistency description Less than 1 Very Loose 7 – 83 Medium Dense 1 – 7 Loose Over 83 Dense Source: Nilsson (2012) - The Standard Penetration Testing
The SPT N-value of fine-grained soils at KL 30 of 100, corresponded to a hard soil consistency, whereas the SPT N- values of coarse-grained soils at locations AP 108/15 and AP 104/5 were 24 and 27 respectively, corresponding to medium-dense soil consistencies as shown in Tables 5 to 7. The SPT results were used in determining the soils preliminary consistency descriptions using the N-values and parameters for bearing capacity analysis [58,61] as shown in Eq. (3) to (13).
Table 5: The insitu SPT result summaries
Site SPT N-value
Corrected N55 Soil consistency Soil Classification 1 (PS) 24 19 Medium- dense Silty Sand (SM) 3 (WL) 100 77 Hard Gravelly clays of intermediate plasticity (CI)
4 (PL) 27 20 Medium- dense Clayey Sand (SC) Note: 1 = AP 108/15; 2 = AP 108/20; 3 = KL 30; 4 = AP 104/5;
PS = Poor Soil; GS = Good Soil; WL = Waterlogged; PL = Pile
- SC and SM = USCS soil classification system
- CI = BS 5930 soil classification system
Table 6: Consistency table for coarse-grained soils
S/No Consistency Description SPT N-values 1 Very Loose Less than 4 2 Loose 4 to 10 3 Medium-Dense / Compact 10 to 30 4 Dense 30 to 50 5 Very Dense Over 50 Source: BS 5930: 1999 + A2: 2010 Description Unconfined Compressive Strength (kPa) SPT N-value
Very soft Less than 25 Less than 2 Soft 25 to 50 2 to 5 Firm 50 to 100 5 to 10 Stiff 100 to 200 10 to 20 Very stiff 200 to 380 20 to 40 Hard Over 380 Over 40 Source: BS 5930: 1999 + A2: 2010 Description Unconfined Compressive Strength (kPa) SPT N-value
Very soft Less than 25 Less than 2 Soft 25 to 50 2 to 5 Firm 50 to 100 5 to 10 Stiff 100 to 200 10 to 20 Very stiff 200 to 380 20 to 40 Hard Over 380 Over 40 Source: BS 5930: 1999 + A2: 2010 Table 7: Consistency table for fine-grained soils
- The Soil Resistivity Testing
The resistivity values showed that the soils at locations KL 30, AP 108/20 and AP 104/5 were essentially non- corrosive, whereas the soil at AP 108/15 was highly corrosive as shown in Tables 8 and 9 below. The soil resistivity test was used as a preliminary and non-conclusive test to provide generalised insitu environmental exposure conditions which may lead to steel depassivation and corrosion, and affect the structural design of the reinforced concrete foundations. Thus, for a more conclusive study, chemical tests in section 3.11, were deemed necessary regardless of the level of corrosiveness encountered [61,62].
Table 8: The insitu soil resistivity test results
Site Average soil resistivity () Soil corrosiveness description (See Table 9 below) 1 (PS) 29.845 Highly corrosive 2 (GS) 1201.472 Essentially non-corrosive 3 (WL) 220.5 Essentially non-corrosive 4 (PL) 285.192 Essentially non-corrosive Table 9: Resistivity explanations
S/No Soil Resistivity (m) Soil Corrosiveness 1 Greater than 200 Essentially non-corrosive 2 100-200 Mildly corrosive 3 50-100 Moderately corrosive 4 30-50 Corrosive 5 10-30 Highly corrosive 6 Less than 10 Extremely corrosive Source: Roberge (2000) - The Vane Shear Testing
No vane shear tests (VST) were conducted in the borehole pits as the clay soils encountered were of a stiff to very stiff consistency and not fitting the criteria for tests as per the requirements of ASTM D2573/D2573M-18 (2018). The VST method is not applicable for unsaturated or non-plastic silts, sands, gravels or other high permeability soils which dilate, collapse and generate pore pressures [58,61,63,64].
- The Specific Gravity Testing
The specific gravity () values showed that the soils at location AP 108/15 were sand with silty particles, AP 108/20 had gravelly soil with clay mineral compositions, KL 30 had clay with gravel particles, and AP 104/5 had sand with clay compositions as shown in Tables 10 and 11. These descriptions are fairly comparable but inconclusive to the final descriptions by the USCS and BS 5930 soil classification systems as discussed in section 3.9. Thus, higher values give higher load bearing capacities since an increase in increases the soils shear strength parameters and its suitability as a construction material [65-67].
f = S = (qu) = (12 x N55) (20)
s u 2 2
q = S = [ (12 x N55) ] (21)
b u
2
Table 10: Insitu specific gravity summaries
Site Gs (Mg/m3) Generalised soil type Soil Classifications [USCS & BS 5390] 1 (PS) 2.370 to 2.777
Sand with silty particles Silty sand (SM) 2 (GS) 2.380 to 2.483
Gravelly soil with clay mineral compositions Clayey sand with gravel (SC) 3 (WL) 2.453 to 2.739
Clay with gravel particles Gravelly clays of intermediate plasticity (CI) 4 (PL) 2.640 to 2.73
Sand with clay composition Clayey sand (SC) Note: 1 = AP 108/15; 2 = AP 108/20; 3 = KL 30; 4 = AP 104/5;
PS = Poor Soil; GS = Good Soil; WL = Waterlogged; PL = Pile
- SC and SM = Unified Soil Classification System (USCS)
- CI = BS 5930 classification system
Table 11: Specific gravities of some soils
= tan = [1 ( sin tan )] (22)
= (23)
Where:
fs = shaft skin resistance; and = Adhesion factor
= average undrained shear strength
= unconfined compressive strength
= end bearing resistance; and = friction angle
, Nq & N = Terzaghis bearing capacity factors
= effective overburden pressures
= effective internal friction angle from SPT test
= lateral earth pressure coefficient
Table 12: Insitu direct shear results for AP 104/5
Depth (m) Width (m) Clay and silt (%) from PSD* Bulk Density (Mg/m3) c (kPa) (°) (kPa) 5.20 1 43.8% 1.790 12.3 21 342 10.40 1 63.2% 1.745 18.5 15 348 Note: * PSD = Particle size distribution or soil grading = Internal angle of soil friction in degrees
= Foundations allowable bearing capacity
= Cohesion of the soil
3.8 The One-Dimensional Consolidation Testing
S/No Type of Soil Specific gravity (Gs) range (Mg/m3) 1 Gravel 2.65 – 2.68 2 Quartz Sands 2.64 – 2.66 3 Silty 2.67 – 2.73 4 Clay 2.70 – 2.90 5 Chalk 2.60 – 2.75 6 Loess 2.65 – 2.73 7 Peat / Organic soils 1.30 – 1.90 (Less than 2.0)
Clay soil mineral compositions 8 K-Feldspars (1) 2.54 – 2.57 9 Montmorillonite (2) 2.35 – 2.70 10 Illite (2) 2.6 – 3.0 11 Kaolinite (2) 2.6 – 2.68 12 Biotite (1) 2.8 – 3.2 References: (1) Lambe and Whitman, 1969; (2) Mitchell, 1993
Source: Das (2016); Das and Sobhan (2018)
S/No Type of Soil Specific gravity (Gs) range (Mg/m3) 1 Gravel 2.65 – 2.68 2 Quartz Sands 2.64 – 2.66 3 Silty 2.67 – 2.73 4 Clay 2.70 – 2.90 5 Chalk 2.60 – 2.75 6 Loess 2.65 – 2.73 7 Peat / Organic soils 1.30 – 1.90 (Less than 2.0)
Clay soil mineral compositions 8 K-Feldspars (1) 2.54 – 2.57 9 Montmorillonite (2) 2.35 – 2.70 10 Illite (2) 2.6 – 3.0 11 Kaolinite (2) 2.6 – 2.68 12 Biotite (1) 2.8 – 3.2 References: (1) Lambe and Whitman, 1969; (2) Mitchell, 1993
Source: Das (2016); Das and Sobhan (2018)
The consolidation value of 0.0042 cm2/s in the range
- The Direct Shear Testing
- The Test Trial Pits and Borehole pits
-
The direct shear test showed that the friction angle () value was greater than that of cohesion () (. > ) at 5.2m depth, and friction angle () was less than cohesion () (. < ) at 10.4m depth, despite both soil strata being classified as clayey-sand soils. This showed an increased clay and silt composition in the bottom soil strata since they induce the sand with increased interlocking behaviour/cohesion [63,68]. The obtained results of cohesion () and angle of internal friction () were used in computing the soils bearing capacities, pile skin and end-bearing resistances using the lateral earth pressure coefficient. Equations (17) to (19) are bearing capacity formulae for Terzaghi, and Meyerhofs vertical and inclined loads respectively whereas Eq. (20) and (21), and Eq. (22) and (23) are for predominantly clay and sandy soils respectively.
qu = cNcSc + 0.5tBNS + tDfNq (17)
of 0.00032 to 0.0032 cm2/s corresponded to a medium consolidation category, typical of 15-25% clays of the low plastic clay (CL) as per USCS system; whereas, the values of 0.187 m2/MN in the range of 0.25-0.125 (Table 15) and 0.1-0.3 (Table 16), corresponded to stiff or firm clays of consolidated lake deposits or lacustrine/swampy soils of
medium compressibility properties of 0.05 to 0.15 compression index () [63,69].
Table 13: Insitu consolidation results for AP 104/5
Test Depth (m) eo b cv mv po (-) (Mg/m3) (cm2/s) (m2/MN) (kPa) 10.4 – 10.7 0.752 1.745 0.0042 0.187 201 Where: eo = Initial void ratio; b = Initial bulk density; cv = Coefficient of consolidation
mv = Coefficient of volume compressibility;
po = Pre-consolidation pressure
Range cm2/s
Category Typical material Soil classification (USCS) < 0.000032 Very Low – – 0.000032 to 0.00032
Low >25% Clay Medium plasticity clays (CL-CH), and volcanic silt (MH)
0.00032 to 0.0032
Medium 15-25% Clay
Low plasticity clay/mud (CL) 0.0032 to 0.032
High <15% Silt Organic silt (OL) > 0.032 Very High – – Range cm2/s
Category Typical material Soil classification (USCS) < 0.000032 Very Low – – 0.000032 to 0.00032
Low >25% Clay Medium plasticity clays (CL-CH), and volcanic silt (MH)
0.00032 to 0.0032
Medium 15-25% Clay
Low plasticity clay/mud (CL) 0.0032 to 0.032
High <15% Silt Organic silt (OL) > 0.032 Very High – – Table 14: Coefficient of consolidation
q = cN S d
+ q N S d
+ 0.5BN S d
(18)
u c c c
o q q q
qu = cNcdcic + qoNqdqiq + 0.5BNdi (19)
Range cm2/s
Category Typical material Soil classification Table 17: Insitu soil grading summaries (USCS)
Note: Formation Soil 1 m2/year = 5/15768 cm2/s; = coefficient of consolidation Site Soil Grading Level Classification
Source: George et al. (2006), and Carter and Bentley (2016) AP 108/15 Poorly-graded 2.75m SM (USCS)
[Adapted from Holtz and Kovacs (1981)]AP 108/20 Well-graded 3.50m SC (USCS)
Table 15: Coefficient of volume compressibility
(m2/MN) Soil type 10.0 – 2.0 Peat 2.0 – 0.25 Plastic clay (normally consolidated alluvial clays)
0.25 – 0.125 Stiff clay 0.125 – 0.0625 Hard clay (boulder clays) Note: = Coefficient of volume compressibility Source: Smith (2014)
Table 16: Compression index and descriptions
Category of compression Soil Material Indicated m2/MN <0.05 0.025 Very Low compression Hard over-consolidated glacial till, hard clay & stiff weathered rocks.
0.05 to 0.1
0.025 to 0.05
Low compression Stiff glacial till/boulder clays, marls, very stiff tropical residual clays.
0.1 to 0.3
0.05 to 0.15
Medium compression Firm clays of consolidated swampy or lake/ lacustrine deposits, glacial outwash clays, weathered marls, firm glacial till, normally consolidated clays at depth, firm tropical residual clays.
0.3 to 1.5
0.15 to 0.75
High compression Poorly consolidated alluvial clays, estuarine deposits & sensitive clay
>1.5 0.75 to 5+
Very High compression Highly organic alluvial clays, and peats. Where: = compression index; = coeff. of vol. compressibility;
= coefficient of compressibility
= ( ) = [( ) ]
+ ( + )
Source: Carter & Bentley (2016) [Adapted from Bowles (1997)]
-
-
-
-
- The Particle Size Distribution (Soil grading)
The particle size analysis showed that the soils at the formation levels of the locations AP 108/15 were poorly graded silty sand (SM), AP 108/20 were well graded clayey sand with gravel (SC), AP 104/5 were poorly graded clayey sand (SC) using the USCS classification system; whereas, the soils at KL 30 were gap graded gravelly clay (CI) as per the BS 5930 classification system. The gradations and soil classifications confirmed the previously inconclusive descriptions from the Test trial pit and borehole soil strata, parts of the preliminary soil consistency descriptions by DPL, specific gravity soil generalisations, material type identifications using the consolidations and values, and the general soil type descriptions under plasticity index (PI) interpretations as discussed in sections 3.1, 3.2, 3.6, 3.8
and 3.10 respectively [58,61,63].
AP 04/5 Poorly-graded ~12.80m SC (USCS)
KL 30 Gap-graded 4.50m CI (BS 5930)
Note: USCS = Unified Soil Classification System; BS 5930 = British Soil Classification System
- The Plasticity Index Interpretations
The Atterberg tests showed that the soils at location AP 108/15 had PI values in range of 7-17, corresponding to medium-plastic soils of cohesive silty-sand type, whereas AP 108/20, KL 30 and AP 104/5 had soils with PI values greater than 17 (>17), corresponding to high plastic soils of cohesive clay type. Meanwhile, all the above locations had Liquid Limit (LL) values less than 50 (<50), corresponding to fine- grained soils with low swell potentials. The PI and LL test value interpretations were used in complementing the final classification and grading descriptions of the fine-grained soils as discussed in section 3.9 above [58,68].
Table 18: Insitu Atterberg limit summaries
Site Atterberg Limits FFL (m)
PI value range (From Table 19) LL PL PI 1 (PS) 24.7 12.5 12.20 2.75 7 – 17 2 (GS) 44.8 21.4 23.4 3.50 > 17 3 (WL) 38.4 16.6 21.8 4.50 > 17 4 (PL) 34.5 14.8 19.7 ~12.80 > 17 Note: 1 = AP 108/15; 2 = AP 108/20; 3 = KL 30; 4 = AP 104/5; PS =
Poor Soil; GS = Good Soil; WL = Waterlogged Location; PL = Pile Location; FFL = Foundations Formation Level
Table 19: PI interpretations and cohesiveness
PI Degree of Plasticity Degree of Cohesiveness Soil Type 0 Non-Plastic Non-cohesive Sand < 7 Low Plastic Partly cohesive Silt 7-17 Medium Plastic Cohesive Silty-Sand > 17 High Plastic Cohesive Clay Source: Surendra and Sanjeev (2017) Table 20: Atterberg Limits and Swell Potential
Liquid Limit (LL) Plasticity Index (PI) Swell Potential (SP) < 50 < 25 Low 50 – 60 25 – 35 Marginal > 60 > 35 High Source: Pitts (1984); Kalantari (1991) - The Chemical Analysis Tests
- The Particle Size Distribution (Soil grading)
-
-
-
The chemical tests were done as a conclusive test following the preliminary soil resistivity test in section 3.4 above, to determine the insitu pH, and presence of corrosion- causing sulphates and chlorides, in describing the insitu environmental exposure conditions as summarised in the Tables 21 and 22 below.
The 4 results showed that the soils at locations AP 108/15, AP 108/20 and AP 104/5 had values 3000 mg/kg ( 0.3% by weight) corresponding to XA1 exposure condition of slightly aggressive chemical environments; whereas, the KL 30 ground water had values of 4 > 600 ppm, corresponding to XA2 exposure condition of moderately aggressive chemical environment as shown in Tables 21 and 22 below.
The pH values for AP 108/15 and AP 108/20 were 5.38 and 5.22 respectively, corresponding to XA2 exposure condition of moderately aggressive chemical environment, whereas KL 30 and AP 104/5 had pH values of 6.27 and 6.10 respectively, corresponding to XA1 exposure condition of slightly aggressive chemical environments. Thus, the chemical analysis and pH conditions were reconciled to provide XA2 exposure conditions of moderately aggressive chemical environment for all locations as per BS EN 206 (2013).
Sulphate Resistant Cements (SRC) of strength class 42.5N and a 3.5% limited C3A (chloro-aluminate) content were used under moderate water-cement ratios of 0.40 to 0.50 in order to inhibit the effects of chlorides forming insoluble chloro-aluminates (C3A) upon combining with the Tricalcium Aluminate (3CaO·Al2O3) in concrete.
Table 21: Insitu chemical test results
Site content (% by weight) content (g/l) pH value Sample type 1 (PS) 0.05% (500 ppm)
0.007 (7 ppm)
5.38 Soil 2 (GS) 0.06% (600 ppm)
0009 (9 ppm)
5.22 Soil 3 (WL) 0.0686% (686 ppm)
10 (10,000 ppm)
6.27 Ground water 4 (PL) 0 0.021 (21 ppm)
6.10 Soil Where: 1 g/L = 1000 ppm; and 1 ppm = 1 mg/L = 0.001 g/L Table 22: Measured SO4 results interpretations
strength values within the 104.2-123.36% range of the 25 MPa design value, and 28-day test strength values in the range of 155.12-211.08% of the 25 MPa design value. The compressive test values were used to confirm and provide assurance to the foundations design concrete strength value of 25 MPa using 42.5N SRC as a remedy to the sulphate and chloride attacks, as discussed in section 3.11.
Table 23: Insitu compressive concrete cube results
Site Tested Cube strength values [For a 28-day Design Strength (DS) of 25 MPa] 7-day strength 28-day strength Results (MPa) % of DS Results (MPa) % of DS 1 (PS) 29.81 119.24% 43.31 173.24% 2 (GS) 26.05 104.20% 38.78 155.12% 3 (WL) 27.71 110.84% 40.08 160.32% 4 (PL) 30.84 123.36% 52.77 211.08% NB: 1 = AP 108/15; 2 = AP 108/20; 3 = KL 30; 4 = AP 104/5;
PS = Poor Soil; GS = Good Soil; WL = Waterlogged Location; PL = Pile Location
3.13 Static Load Tests
The insitu static load tests were done to determine the insitu displacement and load capacity values of the foundations under tension/uplift, compression, and lateral load tests in conformity to IEC 61773 [57] as shown in Tables 24 to 26.
The static load test results showed that the location AP 104/5 exhibited maximum displacement valus of 0.09 mm, -0.83 mm and 2.39 mm under tension, compression and lateral loading respectively; while locations AP 108/15, AP 108/20 and KL 30 exhibited maximum tension displacement values of 0.83 mm, 0.19 mm and 4.74 mm respectively. The limiting reference displacement values were 25 mm for both tension and compression loading tests, and 50 mm for the lateral loading test. The static load results were used in determining the slope (1) of the hyperbolic model graphs empirical line equation using Eq. (24) and (25) below, and in the calculation of the insitu foundations load capacity () using Chin- Kondner extrapolation (1971) as per Eq. (26) below.
= + (Line of best fit) (24)
Site Measured SO4 (ppm) Limiting values of SO4 (ppm)* Exposure condition* 1 (PS) 500 2000 and 3000 (soil) XA1 2 (GS) 600 2000 and 3000 (soil) XA1 3 (WL) 686 > 600 and 3000 (water) XA2 4 (PL) 0 2000 and 3000 (soil) XA1 NB: 1 = AP 108/15; 2 = AP 108/20; 3 = KL 30; 4 = AP 104/5; PS = Poor Soil; GS = Good Soil; WL = Waterlogged Location; PL = Pile Location
* Source: BS EN 203 (2013)
Site Measured SO4 (ppm) Limiting values of SO4 (ppm)* Exposure condition* 1 (PS) 500 2000 and 3000 (soil) XA1 2 (GS) 600 2000 and 3000 (soil) XA1 3 (WL) 686 > 600 and 3000 (water) XA2 4 (PL) 0 2000 and 3000 (soil) XA1 NB: 1 = AP 108/15; 2 = AP 108/20; 3 = KL 30; 4 = AP 104/5; PS = Poor Soil; GS = Good Soil; WL = Waterlogged Location; PL = Pile Location
* Source: BS EN 203 (2013)
Slope 1
= ( = + ) (25)
1
C
C
Insitu Load Capacity Rc =
1
(in kN) (26)
3.12 Concrete cube compressive strength Tests
The concrete cube compressive strength test was done as per BS EN 12390-1 (2012) and BS EN 12390-2 (2009), to
determine the 7-day and 28-day strengths as a confirmatory quality control test of the 25 MPa design strength using 42.5N Sulphate Resistant Cement as shown in Table 23 below.
The results showed that all the locations had 7-day test
The actual insitu load capacities of the test-foundations under static load methods were from 105.29% to 249.14% fraction of the prescriptive design values, which reaffirmed the conclusion that the load capacity results of the insitu-tested full-scale foundations exceeded the prescriptively designed load capacity values, as shown in Table 26.
Fig.1: Insitu static axial tension/uplift load test
Fig.2: Insitu static axial compression load test
Fig.3: Insitu static lateral load test
Table 24: Insitu static load test summaries
Site Location Insitu Maximum Displacements (mm) IEC 61773 (1996) Limit-value (mm)
T C L T C L AP 108/15 0.83 – – 25 25 50 AP 108/20 0.19 – – KL 30 4.74 – – AP 104/5 0.09 -0.83 2.39 Where:
T = Static Axial Tension/Uplift Loading Test C = Static Axial Compression Loading Test L = Static Lateral Loading Test
Table 25: Slope readings for insitu static load tests
Site Location Graph Line Slopes (x 10-3) Tension Test Compression Test Lateral Test AP 108/15 0.9996 – – AP 108/20 1.3568 – – KL 30 0.8755 – – AP 104/5 0.7223 0.5019 5.2591 Site Load Capacities Tension Test (kN) Compression Test (kN) Lateral Test (kN) Insitu Load UDL Insitu Load UDL Insitu Load UDL 1 (PS) 1000.4 945.36 – – – – 2 (GS) 737.03 594.45 – – – – 3 (WL) 1142.2 962.30 – – – – Site Load Capacities Tension Test (kN) Compression Test (kN) Lateral Test (kN) Insitu Load UDL Insitu Load UDL Insitu Load UDL 1 (PS) 1000.4 945.36 – – – – 2 (GS) 737.03 594.45 – – – – 3 (WL) 1142.2 962.30 – – – – Table 26: Insitu foundation load capacities
4 (PL) 1384.5 555.69 1992.43 1077.1 190.15 180.6 Note: 1 = AP 108/15; 2 = AP 108/20; 3 = KL 30; 4 = AP 104/5; PS = Poor Soil; GS = Good Soil; WL = Waterlogged Location; PL = Pile Location; UDL = Ultimate Design Load
/tr>
4 (PL) 1384.5 555.69 1992.43 1077.1 190.15 180.6 Note: 1 = AP 108/15; 2 = AP 108/20; 3 = KL 30; 4 = AP 104/5; PS = Poor Soil; GS = Good Soil; WL = Waterlogged Location; PL = Pile Location; UDL = Ultimate Design Load
Fig.4: AP 108/15- Hyperbolic graph in uplift
Fig.5: AP 108/20- Hyperbolic graph for uplift
Fig.6: KL 30- Hyperbolic graph for uplift
Fig.7: AP 104/5- Hyperbolic graph for uplift
Fig.8: AP 104/5- Hyperbolic graph for compression
Fig.9: AP 104/5- Hyperbolic graph for lateral
-
- CONCLUSION
Based on the reviews of the experimental results, the following conclusions were reached:
The insitu load capacities of the test foundations under static load were 105.29% to 249.14% fraction of the theoretical load capacities, confirming that the prescriptive design approaches use equations and methods governing a linear-elastic boundary in the design value extrapolations instead of the more-realistic plastic and non-linear approach. The maximum insitu displacement values from static load tests differed significantly from that of the prescriptive design and technical specifications by being 0.36% to 18.96% fraction of the 25 mm prescriptive limit under uplift and 3.32% fraction of the prescriptive 25 mm under compression and 4.78% fraction of the prescriptive 50 mm limit under lateral test. These displacements were less than 19% of the prescriptive values as verified by the insitu static load tests of
the foundations.
Due to the acidic soils and ground water, the use of 42.5N Sulphate Resistant Cement (SRC) led to very high compressive strength provided for the concrete foundations due to lack of a low grade 32.5N SRC in Uganda. This led to an overdesign in the concretes compressive strength ranging from 104.2% to 123.36% of the design strength at 7 day and 155.12% to 211.08% at 28 days.
- ACKNOWLEDGMENTS
The author would like to thank Sinohydro Corporation Ltd, Kalpataru Power Transmission Ltd, and GOPA- International Energy Consultants for their contributions towards the research at the Karuma Interconnection Project in Uganda.
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