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Proceedings of Pile 2013, June 2-4th 2013
QUANTIFYING THE EFFECT OF LIMITED SITE INVESTIGATION ON THE COST
OF BUILDING CONSTRUCTION WITH PILE FOUNDATION
A. Arsyad, A. Djamaluddin, A. Thaha. T. Harianto, L. Samang
ABSTRACT: It is widely accepted that soil investigation conducted in construction projects is limited. The scope of
soil investigation is dictated by budget allocation rather than the proper need of characterizing soil properties. The
budget allocated for soil investigation ranges from 0.1% to3% of the construction budget. Poor soil investigation can
result in foundation failure leading to construction delay or even structural damage with live loss. Otherwise, it can
cause over-design of foundation with consequence of high project cost. This paper investigated the effect of limited
soil investigation on the cost of foundation work in a building construction project. Case study was undertaken on the
project of Engineering Faculty New Campus in Gowa South Sulawesi Indonesia, located in the sedimentary deposit of
River Jeneberang. To characterize the soil properties of the Geology Building with the area of 200×116 m2, soil
investigations were conducted with limited amount, consisting of 4 CPTs and 2 SPTs. The limited site investigation
was found to increase additional cost of pile foundation, accounted for 21% of the total pile foundation cost.
Therefore, Geostatistical methods including Kriging and Inverse Distance were employed to estimate bearing stratum
of pile foundation with limited site investigation. The use of Geostatistical methods was found to reduce the additional
cost to be 7.14% and 6.21% for Kriging and Inverse Distance respectively. The addition of 3 CPTs into the existing
site investigation conducted was also found to reduce additional cost of pile foundation to be 4.93%. The results
suggested that sufficient amount of site investigation with the use of Geostatistical method could enhance the design
of pile foundation and reduce unpredicted cost of pile foundation during construction.
Keywords: Soil Variability, Pile Foundation, Limited Soil Investigation, Construction Cost.
INTRODUCTION
Over the last 30 years, site investigations (SI) have
been undertaken in construction projects within limited
amount (Institute of Civil Engineers 1991). This is due to
budget allocated for SI is often minimized in order to
reduce preliminary cost of project, rather than being
allocated to characterize properly soil properties. It is
well understood that ground risk is one of the highest
risk in design and construction of civil engineering and
building project (Littlejohn et al, 1994). Thus, limited SI
will increase the risk, either the risk foundation/structural
design failure or the risk of additional cost for repair and
additional construction with the consequence of over
budget (ASFE 1996). There is no clear guidance of
determining the scope of SI for building and civil
engineering projects. Several countries have established
such code of determining the scope of SI, including
Malaysia, Singapore, Hong Kong, Taiwan, and China
(Moh, 2004). In contrast, Indonesia has no such
guidance of proper amount of SI. This has resulted in
determining the amount of SI on the basis of common
practice and personal experience of engineer.
This study investigates the effect of limited SI on
cost of building construction project, particularly on the
cost of pile foundation. Case study was undertaken on
limited SI performed in the New Campus Project of
Faculty of Engineering, Hasanuddin University in Gowa
South Sulawesi. The study evaluates its effect on the
project cost. Later, in the final section of this paper, the
study proposes a number of geostatistical methods and
additional SI to enhance the reliability of SI and design
of pile foundation.
CASE STUDY
The project of Engineering Faculty New Campus,
Hasanuddin University’s is located about 19 km
southeast of Makassar, the capital of South of Sulawesi.
The project has been constructing several buildings over
the area of 630×330 m2, including Faculty and Center of
Technology, Civil Engineering, Architecture, Electrical
Engineering,
Mechanical
Engineering,
Geology
Engineering, and Naval Engineering Buildings. The
object of the study is Geology Building, whose pile
foundations have been constructed since 2011 and
recently its structure is under construction. The Geology
Building stands on the area of 200×116 m2.
For designing pile foundation of the Geology
Building, the project characterized the soil properties
with 4 cone penetration tests (CPTs) and 2 standard
penetration tests (SPT-borehole). The amount of those
Sis was suggested to be adequate to cover all
variabilities of the soil. CPTs consist of S42, S43, S44,
S45 while SPT-boreholes are B17 and B18. Figure 1
presents the distribution of CPT and SPT data across the
field. It can be seen that SI sampling is combination of 2
CPTs and one SPT-borehole for one building area.
Figure 2 presents the data of CPTs and SPT across
the Geology Building. Due to pile foundation is endbearing pile supported by bearing stratum, the CPTs and
SPTs must identify the level where bearing stratum lying.
It was found that the bearing stratum varies from the
shallowest (-5.80 m) to the deepest one (-12.20 m). The
SPTs shows -9.25 m and -10.25 m. The bearing stratum
consists of dark grey sand and fine gravel with some
breccias. As we can see in Figure 2, the bearing stratum
is located under 6 meters yellowish brown silty clay
layer with low plasticity. On the top of silty clay, there
are 2 meter reddish brown clay. The stratigraphy
indicates such transported sediments compacted and
deposited overlying volcanic rock of Batu RappeCindako formation. The sediment is agented by River
Jeneberang, a main river located 1.5 km south from The
Geology Building.
High variability of bearing stratum levels, as seen
Figure 2, make determining planned pile length quite
difficult. That is why the contractor ordered concrete
pile (PC Spun type) within various lengths, such as 10 m,
11 m and 12 m as bottom pile and 6 m as top pile. The
10 m pile can be planted in the northern side of the
building due to the S-42 and B-17 indicated 5.8 meter
and 9.25 m depth of the bearing stratum. 10 m pile was
considered more than enough to cover the northern side.
Otherwise, 12 m pile into the southern side is due to the
S-43 and S-44 showing the bearing stratum at around
10.25 meters. In the south-eastern site, the bearing
stratum is very deep (12.2 m). Therefore, the contractor
planned 12 + 6 m piles to be planted there.
Difference between Designed Pile Length and
Driven Pile Length
As the driving of piles into the ground was
undertaken, the length of pile needed to reach bearing
stratum different than the designed pile length. As a
consequence, the contractor had to order more additional
piles. For instance, nearby Building Geology B, the
contractor estimated that bearing stratum at -10.4 meter
from the CPT S 44b and planted 12 meters pile. Yet, this
estimation was not valid as the driven pile over 14 m.
The pile driving needed several additional 6 m piles,
and the pile constructed became 12 + 6 meter. Such
inaccurate estimation of the bearing stratum was also
found in the eastern part of planned Geology Building B.
The CPT S45 indicates the bearing stratum at 12.20
meters, but the driven pile is around 8-9 meters.
S 42
B 17
Geology A
S 45
B 18
Geology B
S 43
S 44
Figure 1. Planned Geology Building and Distributions of CPTs and SPT-Boreholes
Figure 2. CPTs and SPT-Boreholes on Planned Geology Building.
estimated as the difference between planned and It is
It is shown by Figure 3, the difference between
designed pile and driven pile is so significant. The
project must extend the length of pile driven. The piles
need extension is about 35% of the total piles. The
bearing stratum seems to be underestimated. It is around
17% of the total piles, the pile had to be cut since the
bearing stratum was overestimated. Only 48% of the
total piles were well estimated length. If the cost of pile
extension was quantified, in which the construction cost
of pile with 500 mm diameter is assumed to be 1.5
million rupiahs per meter, it was found that the extension
of pile has led to additional cost, about 21% of the total
budget allocated (Table 1).
Table 1. Estimated and Real Cost of Pile Foundations
Configurations
of
pile
foundations
Estimated
Cost
(in
million
rupiahs)
Real Cost
(in million
rupiahs)
Additional
cost
(in
million
rupiahs)
45× 10 m
36×12 m
675
648
828
756
153
108
7×8 m
8×6 m
84
72
84
126
0
54
1.479
100%
1.794
121%
315
21%
Total
Percentage
existing CPTs and SPTs to achieve more accurate
estimated bearing stratum.
Geostatistical Method
Kriging and Inverse Distance methods were
employed to estimate the contour of bearing stratum.
Kriging (1951), is a geostatistical estimator used to
estimate a value of a random field at empty soil data,
whereas inverse distance is multivariate interpolation for
a known scattered set of soil data points. In this study, by
using Kriging and later compared with Inverse Distance
Methods, we estimated bearing stratum depths at
locations where CPTs and SPTs do not exist. It can be
seen in Figure 4 that the bearing stratum contours
generated using Kriging Method is different than that
generated using Inverse Distance Method. Kriging
Method indicates one spot in the site with the depth over
-12.5 m while Inverse Distance shows two spots with the
depth over -12.5 m.
12.5
12
S 42
S 45
11.5
11
10.5
10
B 17
B 18
9.5
9
8.5
S 43
8
7.5
S 44
7
6.5
6
5.5
(a)
12.5
S 42
S 45
12
11.5
11
10.5
10
B 17
B 18
9.5
9
8.5
S 43
8
S 44
7.5
7
Figure 3. Difference between Planned Pile Lengths and
Planted Pile Lengths
6.5
6
5.5
ENHANCING
(b)
Figure 4. Bearing Stratum Contours drawn using (a)
Kriging and (b) Inverse Distance Methods.
In order to achieve a reliable estimation of bearing
stratum of pile foundation designed for Geology
Building, two techniques can be implemented. First is
the use of Geostatistical method including Kriging and
Inverse Distance methods to draw bearing stratum
contours. Second is the addition of more 3 CPTs into the
These contours were evaluated by comparing them
with the bearing stratum contours generated using the
data of driven piles during the construction. The “real”
bearing stratum contour indicates two spots with depth
about -15 m lying at the center, northern side and
western side of the field (Figure 5). It was also observed
PROPOSED
METHOD
OF
ESTIMATED BEARING STRATUM
that the contour generated using Inverse Distance shows
more similarity to the real bearing stratum. However,
both Kriging and Inverse Distance methods infer such
lack of accurate estimation particularly the maximum
depth of bearing stratum. In this case, either Kriging or
Inverse Distance suggested -12.5 m as maximum depth
while the existing bearing stratum suggested -15 m.
15
14.5
14
13.5
13
12.5
12
11.5
11
10.5
10
9.5
9
8.5
8
7.5
7
6.5
6
Figure 5. Bearing Stratum Contour Generated based
on Data of Driven Piles during the Construction.
Figure 6 presents the comparison between planned
pile and planted pile using Kriging and Inverse Distance.
Both Kriging and Inverse Distance are able to reduce the
gap between the planned (designed) and planted
(constructed) pile. The Inverse Distance shows more
significant effect of reducing the gap compared to the
Kriging method.
Addition of more 3 CPTs
Even though Geostatistical methods was performed,
the gap between designed pile and constructed pile
lengths was still found. Therefore, we added more 3
CPTs and evaluated its effect on the estimation of
bearing stratum under the Geology Building. In this way,
3 CPTs were added to the existing 4 CPTs and 2 SPTs.
The additional CPTs were located at the west and south
corners (SA-1, SA-2) and the centers (SA-3) of the
Building Geology A (Figure 7). SA-1 identifies the
bearing stratum at -9 m, while SA-2 indicates the bearing
stratum at -8.8 and SA-3 at -13 m. The additional CPTs
appears to enhance the contour of bearing stratum
(Figure 8). The contour becomes more similar to the real
bearing stratum contour. The additional CPTs also can
improve the designed pile lengths and its gap to the
constructed pile lengths lessen. As we can see in Figure
9, the gap between the designed and constructed pile
lengths becoming smaller.
Table 2 shows the effect of using geostatistical
method and additional SI on the discrepancy between
estimated cost and real cost. The additional CPTs was
found to improve the reliability of designed pile lengths
and estimated cost of the pile foundation. The project
just bears the additional cost, accounted for 4.93% of the
total project budget. On the other hand, the use of
Geostatistical method can reduce additional cost to be
7.14% and 6.21% for Kriging and Inverse Distance
respectively.
16
14
10
8
6
SA-3
planned pile
4
planted pile
2
SA-1
100
103
91
94
97
82
85
No. Pile
88
73
76
79
64
67
70
55
58
SA-2
61
46
49
52
37
40
43
28
31
34
19
22
25
10
13
16
1
4
7
0
(a)
Figure 7. Additional 3 CPTs in the Geology Building.
16
14
13
12
12.5
10
12
11.5
8
11
6
4
planned pile
10.5
planted pile
10
9.5
2
9
97
100
7.5
103
88
91
94
79
82
85
70
73
76
61
64
8
67
52
55
No. Pile
58
43
46
49
34
37
40
25
28
8.5
31
16
19
22
7
10
13
1
0
4
Length (m)
Length (m)
12
(b)
Figure 6. Comparison between Designed Pile
Lengths and Constructed Pile Lengths Generated by (a)
Kriging and (b) Inverse Distance
7
6.5
6
5.5
Figure 8. Bearing Stratum Contours Generated using
Existing 4 CPTs and 2 SPTs and additional 3 CPTs.
ACKNOWLEDGMENTS
The authors would like to thank to the Project
Implemented Unit (PIU) of New Campus of Engineering
Faculty Hasanuddin University for sharing the data of
the project.
16
14
Length (m)
12
10
8
6
planned pile
4
planted pile
2
94
97
100
103
85
88
91
76
79
73
No. Pile
82
64
67
70
52
55
58
61
43
46
49
31
34
37
40
22
25
28
13
16
19
4
7
10
1
0
Figure 8. Gap between Designed and Constructed
Pile Lengths due to the additional 3 CPTs.
Table 2. Estimated and Real Cost due the use of
Geostatistical Method and additional CPTs
Estimated
Cost
(in
million
rupiahs)
1,665.9
Real Cost
(in
million
rupiahs)
Additional
cost (in million
rupiahs)
1,794
128.1 (7.14%)
Inverse
Distance
1,682.55
1,794
111.45
(6.21%)
Additional
CPTs
1,705.0
1,794
88.5 (4.93%)
Kriging
CONCLUSSIONS
1.
2.
3.
4.
Limited SI can cause additional cost of pile
foundation. For the case of pile foundation at
Geology Building, the project must afford 21%
additional cost due to the SI with 4 CPTs and 2
SPTs is too limited to characterize properly
bearing stratum of the field.
The additional cost can be reduced by
performing Geostatistical methods such as
Kriging and Inverse Distance to estimate bearing
stratum contours.
The Inverse Distance seems to be more effective
to estimate bearing stratum contours of pile
foundation with limited SI.
By adding 3 CPTs into existing 4 CPTs and 2
SPTs, the estimated bearing stratum can be
enhanced. Therefore, the additional cost, which
is caused by inaccurate estimated bearing
stratum, can be minimized, accounted for 4.93%
of the total cost of pile foundation.
REFERENCES
ASFE. (1996). Case Histories of Professional Liability
Losses: ASFE Case Histories, ASFE: Professional
Firms Practicing in the Geosciences, Maryland, USA.
Institution of Civil Engineers. 1991. Inadequate site
investigation, Thomas Telford, London.
Krige D G. (1951). "A Statistical Approach to Some
Mine Valuations and Allied Problems at the
Witwatersrand," unpublished Master's Thesis,
University of Witwatersrand.
Littlejohn, G. S., Cole, K. W., and Mellors, T. W. (1994).
"Without Site Investigation Ground is a Hazard."
Proceeding of Institution of Civil Engineers, 102.,
pp.72-78.
Moh, Z.C. (2004). Site investigation and geotechnical
failures. International Conference on Structural and
Foundation Failures, Singapore.