NAMORA ILANGIT-2 (NIL-2) PRELIMINARY REPORT 1 of 13 PRELIMINARY GEOTECHNICAL RECOMMENDATION OF NAMORA ILANGIT-2 (NIL-2) WELLPAD SARULLA OPERATION LTD., NORTH SUMATERA 1. INTRODUCTION This preliminary report presents geotechnical recommendation of Namora I Langit-2 (NIL-2) which is focused on soil bearing capacity of the shallow foundation. This project is a part of Sarulla Operation Ltd. that is located at Tarutung, North Sumatera. Figure 1: The Location of NIL-2 2. GEOTECHNICAL INVESTIGATION In this project, site investigation were conducted by PT Tribina Wahana Cipta (12 May, 2014) and PT Medan Geotechnic and Structure Engineering (June, 2014). 2.1. FIELD INVESTIGATION Boreholes and field tests location could be seen at Appendix 1. 2.1.1 CONE PENETRATION TEST (CPT) The cone penetration test is the most common in-situ test and it is sometimes referred to as the Dutch Cone Test, or often abbreviated as CPT. CPT is conducted based on ASTM D3441. A typical cone is shown on Figure 2. This test method covers the determination of the end bearing and side friction. The components of penetration resistance are developed during the steady and slow penetration of a pointed rod into soil.
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NAMORA I LANGIT-2 (NIL-2)PRELIMINARY REPORT
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PRELIMINARY GEOTECHNICAL RECOMMENDATION OFNAMORA I LANGIT-2 (NIL-2) WELLPAD
SARULLA OPERATION LTD., NORTH SUMATERA
1. INTRODUCTION
This preliminary report presents geotechnical recommendation of Namora I Langit-2(NIL-2) which is focused on soil bearing capacity of the shallow foundation. This projectis a part of Sarulla Operation Ltd. that is located at Tarutung, North Sumatera.
Figure 1: The Location of NIL-2
2. GEOTECHNICAL INVESTIGATION
In this project, site investigation were conducted by PT Tribina Wahana Cipta(12 May, 2014) and PT Medan Geotechnic and Structure Engineering (June, 2014).
2.1. FIELD INVESTIGATION
Boreholes and field tests location could be seen at Appendix 1.
2.1.1 CONE PENETRATION TEST (CPT)
The cone penetration test is the most common in-situ test and it is sometimes referred toas the Dutch Cone Test, or often abbreviated as CPT. CPT is conducted based on ASTMD3441. A typical cone is shown on Figure 2. This test method covers the determinationof the end bearing and side friction. The components of penetration resistance aredeveloped during the steady and slow penetration of a pointed rod into soil.
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(A) (B)
Figure 2: A Typical Shape of Mechanical Cone (GEC, 1997)(A) Contracted, (B) Extended
The CPT supplies data on selected engineering properties of soil is intended to help withdesign and construction of earth works and the foundations for structures. CPT tests thesoil in place and does not obtain soil samples. The interpretation of the results from thistest method requires knowledge on the types of the penetrated soil.
Engineers often correlate the test result with laboratory or other types of field tests. Theaccuracy of such correlations will vary with the type of soil involved. Engineers usuallyrely on local experts to judge this accuracy.
Schmertmann (1978) classified soil type based on the friction ratio as the ratio of localfriction and cone resistant. Based on Schmertmann, clay fraction has the friction ratiomore than 2.0% and sand fraction has the friction ratio less than 2.0%.
Five Cone Penetration Tests (CPT) with a 2.5 ton capacity were conducted at the field. Ofthe five CPT, there are 2 points which is predicted to be in excavation area and 3 pointsin fill area. Table 1 shows the summary of CPT penetration depth and Table 2 shows thesummary of soil descriptions based on CPT. CPT’s result could be seen at Appendix 2.
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Table 1: CPT Penetration Depth
CPT IDCoordinate Depth of Penetration*
(m) LocationNorthing Easting
S-1 01o53’14.99” 099o01’22.09” 1.40 Near Well-1S-2 01o53’13.64” 099o01’23.10” 13.80 Barrack and OfficeS-3 01o53’14.40” 099o01’22.12” 3.80 Near Well-1S-4 01o53’14.50” 099o01’22.90” 15.80 Barrack and OfficeS-5 01o53’13.70” 099o01’23.90” 10.20 Barrack and Office
Note: *Depth of Penetration measured from existing ground surface
2.0 – 10.0 Clay/silt, very soft to soft consistency 4.0 – 6.0
10.0 – 14.0 Sand, loose 40.0
S-3 0.0 – 1.2 Sand, dense 125.0
S-4
0.0 – 2.0 Sand, loose 40.0
2.0 – 7.0 Clay/silt, very soft consistency 4.0
7.0 – 16.0 Sand, medium dense 50.0 – 100.0
S-5
0.0 – 3.0 Sand, medium dense 12.0
3.0 – 5.0 Clay/silt 4.0
5.0 – 10.4 Sand, loose to medium dense 12.0 – 110.0
2.1.2 TECHNICAL DRILLING AND STANDARD PENETRATION TEST (SPT)
Technical drillings were conducted to obtain drilling cores that will identify the soilclassification and record its drilling log. Standard Penetration Tests (SPT) were conductedwith 2.0 m interval at each technical drilling point.
Standard Penetration Test is required for the information on the consistency or density ofthe soil. Test should be carried out in accordance with the ASTM D1586-67/84. Figure 3shows a typical standard penetration test procedure.
With this project, PT MGS Engineering conducted six boreholes with 30.0 m of depth.Recovered cores from technical drilling, called coring, are identified to get soilclassification and stratification and it is recorded in a drilling log. Coring result issummarized in a borlog (Appendix 3) to indicate the soil stratification of the project site.Table 4 shows a summary of soil description based on the borlog.
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Figure 3: A Typical Procedure of Standard Penetration Test (Manual Pondasi Tiang, GEC)
Table 3: Summary of Borehole’s Depth, UDS, Ground Water Level, and Location
BoreholeID
Coordinate DrillingDepth(m)
UDS*(nos)
GWL**(m) Location
Northing Easting
BH – I 01o53’15.28” 099o01’20.52” 30.45 - - Mud PitBH – II 01o53’14.92” 099o01’22.00” 30.00 - -19.00 Near Well-1BH – III 01o53’16.50” 099o01’24.07” 30.00 - -16.00 Cementing AreaBH – IV 01o53’15.76” 099o01’23.59” 30.00 - -14.00 Near Well-8BH – V 01o53’13.76” 099o01’26.36” 30.00 - -16.00 Production Test PitBH – VI 01o53’17.68” 099o01’23.52” 30.00 - -7.00 EPC Separator
Note: *UDS = Undisturbed Sample**GWL = Ground Water Level
Based on observation during technical drilling, ground water level is found at -7.00 to-19.00 m of depth from existing ground level (see Table 3).
The soil stratification condition and soil characteristics were obtained from drilling log.Simplified soil stratification is presented in Figure 4.
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Table 4: Soil Description Based on Borlog
BorholeID
DepthSoil Description
NSPT
(m) (blow/ft)
BH - I
0.00 - 20.00 Medium SAND, white, brightly grey, non plasticity, lowmoisture content 60 - >60
15.00 - 19.00 Clayey SAND, grey, non plasticity, high moisture content 5 - 21
19.00 - 28.00 CLAY, grey, brown, high plasticity, medium moisturecontent 25 - >60
28.00 -30.00 SAND, grey, non plasticity, low moisture content >60
BH - VI
0.00 - 24.00 Clayey SAND, grey, non plasticity, high moisture content 2 - 27
24.00 - 30.00 Clayey SAND, yellow, non plasticity, high moisturecontent 11 - 27
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0 10 20 30
852.
50
857.
50
862.
50
867.
50 0.00
40.0
080
.00
120.
0020
0.00
240.
00
A.2
BH-V
200
4060
4020BH
-IV0
6040
BH-II
060
20
Clay
Sand
0 10 20 30
0 10 20 30
842.
50
837.
50
832.
50
827.
50
822.
50
817.
50
812.
50
847.
50
Clay
Clay
Clay
Sand
Sand
Sand
Sand Sa
ndy
Clay
Sand
y cl
aySa
ndy
Clay
Sand
yCl
aySa
ndSa
ndSa
nd
LEGE
ND
Fig
ure
4.
Soi
l Str
atifi
catio
n at
Cro
ss S
ection
A.2
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ANALYSIS METHOD
2.2. BEARING CAPACITY OF SHALLOW FOUNDATION
A. Terzaghi’s Bearing Capacity Formulas
Bearing capacity of shallow foundation is calculated based on limit equilibrium methodproposed by Terzaghi (1943). His method includes the following assumptions:
The depth of the footing is less than or equal to its width (D ≤ B). The bottom of the footing is sufficiently rough that no sliding occurs between the
footing and the soil. The soil beneath the footing is a homogeneous semi-infinite mass (i.e., the soil
extends for a great distance below the footing and the soil properties are uniformthroughout).
The shear strength of the soil is described by the formula s = c + ’ tan . The general shear mode of failure governs. No consolidation of the soil occurs (i.e., settlement of the footing is due only to
the shearing and lateral movement of the soil). The footing is very rigid in comparison to the soil.
Terzaghi developed his theory for continuous footings, which he then extended it tosquare and round footings by adding empirical coefficients. These formulas, written interms of net pressures, are as follow:
For square footings : qu’ = 1.3 c Nc + ‘D (Nq – 1) + 0.4 B N
For continuous footings : qu’ = c Nc + ‘D (Nq – 1) + 0.5 B N
For circular footings : qu’ = 1.3 c Nc + ‘D (Nq – 1) + 0.3 B N
where:qu’ : net ultimate bearing capacityc : soil cohesion (use = su when analyzing undrained conditions)D’ : effective stress at depth D below the ground surface (D’ = D if depth to
groundwater table is greater than D) : soil unit weightD : depth of footing below ground surfaceB : width (or diameter) of footingNc, Nq, N : bearing capacity factors = f()
(use = 0 when analyzing undrained conditions). These bearing capacityfactors are presented in tabular form in Table 5.
B. Brinch Hansen’s Bearing Capacity Formulas
The formula developed in Denmark by Brinch Hansen (1970) reflect the theoretical andexperimental findings from the mentioned sources and others; and is an excellentalternative to Terzaghi. It produces more accurate bearing values and it applies to amuch broader range of loading and geometry conditions. Brinch Hansen retainedTerzaghi’s basic format and added the following additional factors:sc, sq, s = shape factorsdc, dq, d = depth factorsic, iq, i = load inclination factorsbc, bq, b = base inclination factorsgc, gq, g = ground inclination factors
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For soils with > 0, the basic formula written in terms of net bearing pressure is:qu’ = c Nc sc dc ic bc gc + ‘D (Nq sq dq iq bq gq – 1) + 0.5 B N s d i b g
For clays that fail in undrained conditions ( soils), Brinch Hansen felt it was moreappropriate to use additive constants instead of factors, giving:
Dac
ac
ac
ac
acuu σ')gbids(1s5.14'q
Terzaghi’s formulas consider only vertical loads acting on a footing with a horizontal basewith a level ground surface, whereas Brinch Hansen’s inclination factors allow any or allof these to vary. The notation for these factors is shown in Figure 5. In Figure 5, ismeasured around the base of footing and is zero when the load acts perpendicular to thebase. All angles are expressed in degrees.
Figure 4: Notation for Brinch Hansen’s Load Inclination
Shape FactorsBrinch Hansen also considered a broader range of footing shapes and defined them in hiss factors.
There are three sets of shape factors: one to be used when the load is perpendicular tothe base of the footing, one for loads inclined in the plane of the B dimension, and onefor loads inclined in the plane of the L dimension. These factors are listed in Table 6.
It should be noted to use extra attention when using Brinch Hansen’s formula withrectangular footings that have the load inclined in the plane of the L dimension. In suchcases, it is not immediately clear whether the bearing capacity failure would occur in theplane of the B dimension or in the plane of the L dimension, so both possibilities shouldbe checked. When checking for failure in the B direction, use the factors with the Bsubscript; for failure in the L direction, use those with the L subscript. Results with thelowest bearing capacity is the controlled bearing capacity.
For continuous footings (B/L ∞), , , and become equal to 1 and becomes
equal to 0, regardless of the direction of the load. This means that the s factors can beignored when analyzing continuous footings.
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Table 5: Bearing Capacity Factors
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Depth FactorsUnlike Terzaghi, Brinch Hansen has no limitations on the depth of the footings. Thismethod could even be used for deep foundations. The depth of the footing is consideredin the following depth factors:dc = 1 + 0.4 kdc
a = 0.4 kdq = 1 + 2 k tan (1 - sin )2
d = 1.0For relatively shallow footings (D/B ≤ 1), use k = D/B. For deeper footings, use k = tan-1
(D/B) with the tan-1 term expressed in radians. Note that this will cause a discontinuity atD/B = 1.
Table 6: Shape Factors for Brinch Hansen’s Bearing Capaties Formulas
Load Inclination FactorsThe load inclination factors are for loads that do not act perpendicular to the base of thefooting, but still act through its centroid. The variable P refers to the component of theload that acts perpendicular to the bottom of the footing, and V refers to the componentthat acts parallel to the bottom. The load inclination factors are:
Ac
V1i c for V/Ac < 1
0i c for V/Ac ≥ 1
u
ac sA
V10.50.5i
for V/Ac ≤ 1
5
q φcotAcP
V0.51i
≥ 0
5
γ φcotAcP
V0.71i
≥ 0
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where:A = base area of footingc = cohesionsu = undrained shear strengthIf the load acts perpendicular to the base of the footing, the i factors may be neglected.
Base Inclination FactorsThe vast majority of footings are built with horizontal bases. However, if the applied loadis inclined at a large angle from the vertical, it may be better to incline the base of thefooting to the same angle so the applied load acts perpendicular to the base. The inclinedbase factors are:
147
α1b c
147
αb a
c
φ tanα0.0349q eb
φ tanα0.0349γ eb
If the base of the footing is level, which is the usual case, all of the b factors may beignored.
Ground Inclination FactorsFootings located near the top of a slope will have a lower bearing capacity than those onground level. Brinch Hansen’s ground inclination factors, presented below, account forthis. However, there are also other matters to consider when placing footings nearslopes.
147
β1g c
147
βg a
c
5γq β0.5tan1gg
If the ground surface is level ( = 0), the factors may be ignored.
Bearing Capacity FactorsBrinch Hansen recommended the use of Prandtl’s formulas for computing Nc and Nq:
φ/2)(45taneN 2q
φ tanπ
φtan
1NN q
c
for > 0
5.14Nc for = 0
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Most engineers also accept these formulas or others that produce very similar results.However, there is much more disagreement regarding the proper value of N. Relativelysmall changes in the geometry of the failure surface below the footing can createsignificant differences in N, especially in soils with high friction angles. Brinch Hansenrecommended the following formula:
φ tan1)(N1.5N qγ
Brinch Hansen’s bearing capacity factors are also presented in a tabular form of Table 5.
Groundwater EffectsRegardless of the method used to calculate the ultimate bearing capacity, we also mustconsider the possible influence of a shallow groundwater table. Although it is customaryto perform bearing capacity analysis using the saturated strength of the soil (to obtainworst-case value of c and ), a shallow groundwater table will further diminish thestrength of the soil. This additional loss is the result of pore water pressure and thecorresponding reduction in effective stress.
When exploring the subsurface conditions, current location of the groundwater table andworst-case (highest) location that might reasonably be expected during the life of theproposed structure should be determined. The depth from the ground surface to thegroundwater table is Dw. If Dw < B + D, the groundwater table will affect the bearingcapacity because the effective stress in the shear zone is reduced (Meyerhof, 1955). Insoils with > 0, this reduction causes the soil strength to diminish.
Groundwater conditions may be divided into three cases, as described below and shownin Figure 6.
Case 1 : Dw ≤ D Case 2 : D < Dw < D + B Case 3 : D + B ≤ Dw
Figure 5: Three Groundwater Cases for Bearing Capacity Analysis
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The effective unit weight is the value that when multiplied by the appropriate soilthickness will give the effective stress. It varies between the buoyant unit weight, b, andthe unit weight, , which depends on the position of the groundwater table. Calculationsare as follow:
For case 1:’ = b = sat - w
For case 2:
B
DD1γγγ' w
w
For case 3 (no groundwater correction is necessary):’ =
In case 1, the second term in the bearing capacity formulas is also affected, but theappropriate correction will be implicit in the calculation of σD’.
4. BEARING CAPACITY OF NIL-1 WELLPAD
Bearing capacity analysis of shallow foundation is calculated by using FTGBC computerprogram (Coduto, 1994). Analysis is conducted by using data that obtained from siteinvestigation. There are three methods that used to analyze the bearing capacity ofshallow foundation, which are Terzaghi, Meyerhoff, and Brinch Hansen. The safety factorof 3.0 is used to obtained allowable bearing capacity.
Table 7 shows the allowable bearing capacity per meter square of CPT and technicaldrilling.
Table 7: Summary of Allowable Bearing Capacity of Shallow Foundation
CPT and Allowable Bearing Capacity
LocationBorhole (ton/m2)
ID Terzaghi Meyerhoff Brinch Hansen
S-1 39.61 56.29 19.01 Near Well-1S-2 3.11 3.81 1.75 Barrack and OfficeS-3 26.60 37.11 13.35 Near Well-1S-4 3.65 4.55 2.05 Barrack and OfficeS-5 3.65 4.55 2.05 Barrack and OfficeBH-I 48.66 69.87 22.79 Mud PitBH-II 35.37 49.99 17.20 Near Well-1BH-III 26.60 37.11 13.35 Cementing AreaBH-IV 5.36 6.79 3.01 Near Well-8BH-V 39.61 56.29 19.01 Production Test PitBH-VI 3.89 4.76 2.19 EPC Separator
