PO 3122B 08 Final Interpretative Report

GEOTECHNICAL INVESTIGATION

FOR NORTH STAR PLANT

AT RAS AL DORAH, KHASAB

SULTANATE OF OMAN

REPORT : PO/3122B/08/02 7th SEPTEMBER 2008

REPORT ISSUE STATUS

<01> 9-Aug-08 Draft – Interpretative AA KC NS <02> 7-Sept-08 Final – Interpretative AA KC NS

Issue Date Description Prepared Checked Approved MOTT MACDONALD & CO. LLC FUGRO MIDDLE EAST & PARTNERS LLC

P.O. BOX 587 P.O. BOX 1334,

P. C. 112, RUWI P.C. 112 RUWI

SULTANATE OF OMAN SULTANATE OF OMAN

GEOTECHNICAL INVESTIGATION

FOR NORTH STAR PLANT

AT RAS AL DORAH, KHASAB

SULTANATE OF OMAN

CONTENTS PAGE SUMMARY (i) 1.0 INTRODUCTION 1 2.0 PROPOSED DEVELOPMENT 1 3.0 LABORATORY TESTING 2 4.0 SUBSURFACE CONDITIONS 3 5.0 ENGINEERING DISCUSSIONS AND RECOMMEDNATIONS 4 6.0 REPORT LIMITATIONS 8

PLATES

PLATE 1 LOCALITY PLAN

PLATE 2 SITE PLAN

Report: PO/3122B/08/02 Page i

SUMMARY

A geotechnical investigation, comprising seven (7) exploratory boreholes, six (6) trial pits, six (6)

electrical resistivity tests and a single (1) down-hole seismic testing, was performed as instructed by

Mott MacDonald & Company LLC in connection with the proposed North Star Gas Plant at Ras Al

Dorah, in Musandam peninsula, Sultanate of Oman. Geotechnical fieldwork was undertaken in

stages, between 8th June and 29th July 2008, and again on 6th August 2008 and a subsequent

programme of laboratory testing as performed on representative soil and rock samples recovered

from the site.

On the basis of the geotechnical data obtained, recommended bearing pressures for square and strip

footing, and raft foundations have been provided for the proposed structures, with consideration of

bearing capacity and limiting settlement criteria.

Excavation of the near surface soils and weathered rock has been considered. The corrosive

potential of the soil with respect to appropriate concrete mix design has also been provided.

The groundwater level was encountered during fieldwork at depths of between 0.6m (TP-H) and

14.7m (BH-10) below existing ground level.

The recommendations given in this report are based on the findings of the boreholes, trial pits and the

results of the laboratory tests being representative of the site as a whole.

Report: PO/3122B/08/02 Page 1

1.0 INTRODUCTION

Oman Gas Company SAOC (OGC) intends to build a new onshore oil and gas processing

plant (North Star Plant), to process the oil and gas fluids from West Bukha field, at Ras Al

Dorah, near Khasab, Musandam Peninsula, Sultanate of Oman. Mott MacDonald & Company

LLC (MMCI) is the engineering consultant for the project. Fugro Middle East & Partners LLC

(Fugro) was commissioned by MMC to perform a geotechnical investigation in connection

with the project. The investigation has been based on a Fugro proposal (ref: R/04/218R/08)

dated 3rd May 2008, which was confirmed by MMC through their email dated 7th May 2008

and authorised Local Purchase Order (Ref No.: 0345) dated 10th May 2008.

The site location is indicated on the locality plan, Plate 1. Field test locations, relative to the

proposed site layout, are indicated on the site plan, Plate 2. Fieldwork for the investigation

was performed in stages, between 8th June and 29th July 2008 and again on 6th August 2008.

This report (Ref.: PO-3122B-08) presents the interpretation of the data collected. A

description of the proposed development, summary of laboratory tests and subsurface

conditions are given in this report. The site description, regional geology, borehole and trial pit

logs, detailed description of field and laboratory test procedures and results are contained in

the Factual Report (Ref: PO-3122A-08) and are not repeated in this report. The factual report

is being submitted independently of this report.

2.0 PROPOSED DEVELOPMENT

It is understood that the proposed development will consist of new oil and gas processing

plant, which mainly includes construction of oil/gas processing/treatment plant, crude oil &

LPG storage tanks, compressors, substation and associated structures.

The distribution of boreholes among the proposed structures is given in Table 1, below.


Table 1: Distribution of boreholes & trial pits and proposed structures

BH/TP location

Original Ground Level

Finished Ground Level (Fill

Thickness, ‘m’) Name of the Structure BH 1 +4.2 +15 (10.8) Near Gate No.02 BH 4 +5.2 +15 (9.8) Fire Water Tank BH 6 +8.4 +15 (6.6) Near 30” Export Line BH 7 +40.4 +50 (9.6) Crude Oil Storage Tanks (44m dia & 20m Ht)BH 8 +2.3 +15 (12.7) Switch Yard BH 10 +17.2 +15 (-2.2, cut) Sulphur Recovery Unit BH 12 +7.2 +15 (7.8) Control Building

TP D +5.6 +15 (9.4) Utility Road TP E +3.9 +15 (11.1) Sulphur Pit TP G +4.2 +15 (10.8) Diesel Storage Tank TP H +1.1 +15 (13.9) Switchyard TP I +1.4 +15 (13.6) Power House TP J +5.5 +3 (-2.5, cut) Utility Road

3.0 LABORATORY TESTING

A programme of laboratory testing was performed on selected soil, rock and groundwater

samples obtained during the fieldwork. Details of the tests undertaken and quantities are

given below in Table 2. In general, laboratory testing has been performed in accordance with

the international Standards.

Laboratory test results are presented in the factual report, Appendix B. Table 2 below

provides summary of the laboratory tests performed for the project.

Table 2:- Summary of Laboratory Tests Performed

Type of Test Test Specification Quantity

Grading (by wet sieving) BS 1377, Part 2, Cl 9.2 25

Atterberg Limits BS 1377, Part 2, Cl 4 & 5 8

Dry density/moisture content relationship BS 1377, Part 4 5

Point Load Index Tests ISRM 8

Unconfined compression test on rock

samples

Method based on ASTM D 2938 –

1995

12

Sulphate, chloride and pH of

soil/weathered rock and groundwater

samples

BS 1377, Part 3, Cl 5, 7 & 9 15


The properties of engineering parameters of the materials, as determined from the laboratory

tests are summarized in Table 3 below.

Table 3:- Summary of Engineering Parameters of soil deposits encountered at site

Type of Test Unit No. of tests Range Average value

Moisture Content (cemented rock/ cobbles) % 20 0.1 – 15.1 2.1

Liquid Limit % 7 17 – 47 30.5

Plastic Limit % 4 19 – 28 22.2

Plasticity Index - 4 12 – 19 16

Gravel % 27 1 – 92 55.8

Sand % 27 5 – 73 29.1 Particle Sizes

Silt/Clay % 27 1 – 82 15.1

OMC % 5 4 – 6.2 5.3 Density/Moisture content relationship MDD Mg/m3 5 2.25 – 2.30 2.27

Sulphate Content asSO3 g/l 15 0.02 – 1.84 0.76

Chloride Content % 15 <0.01 – 14.18 3.60

pH value - 15 7.8 – 9.6 8.6 Unconfined compressive strength (cemented formation/ cobble)

MPa 12 1.38 – 140.71 92.8

Point Strength Index (cemented formation/ cobble)

MPa 8 0.08 – 4.36 1.62

4.0 SUBSURFACE CONDITIONS

The boreholes and trial pit logs are presented in Appendix, on Plates A4.1.1 to A4.7.2 and

A5.1.1 to A5.6.1, respectively.

The borehole observations and test results indicate variable subsurface conditions, possibly

due to the long distance between the boreholes, difference in ground elevations and also due

to the fact that the site is located within the transition zone between terrestrial and marine

environments.

The existing ground elevations at the investigated locations are varying between 1.1m and

40.4m MSL. On the basis of the relative positions of the boreholes and subsurface conditions

revealed, have been described separately with respect to depths and summarized as follows.


Boreholes BHs 1, 7 & 8, revealed following geological horizons:

• From the ground level to depths of between 3.5m to 7.5m: Medium dense, locally

dense to very dense, silty, very sandy GRAVEL/gravelly SAND, except, a thin layer of

soft sandy SILT is revealed between 1m & 2m depths in BH8

• Below this to boreholes termination depths of 20m and 25m below the existing

ground level: Very dense, silty, sandy GRAVEL

*Boreholes BHs 1 & 8 locally revealed cemented conglomerate cap rock, thickness of 1m

Boreholes BHs 4, 6, 10 & 12 revealed dense to very dense, silty, very sandy GRAVEL/ silty

SAND (BH12 locally revealed cemented conglomeratic Sandstone cap rock, thickness of

2.5m), to boreholes completion depth of 20m below the existing ground level. Except, very

stiff, cemented sandy SILT revealed in boreholes BH12 & BH6, below depths of 9m & 16m,

respectively.

Over the excavated depths of between 2.7m and 3.0m, trial pits TPs D, E, H, I & J from the

ground level revealed silty, very sandy GRAVEL, occasionally gravelly SAND. However, trial

pit TPG, from the ground level revealed very silty, very gravelly SAND to 1m depth, overlying

slightly gravelly, slightly sandy SILT to the trial pit excavated depth of 3m below the existing

ground level. The presence of Silt layer, locally, is possibly due to repetition of accumulation

of fines in low lying ground, which transported by floods over the years.

A graphical representation of SPT ‘N’ values versus elevation profile for each borehole

location is presented on Plates 3.1 to 3.7 of the factual report (PO/3122A/08).

The results of dynamic shear modulii obtained by performing Downhole seismic test are

presented on Plate 7 of the factual report (PO/3122A/08), are used in the designing of

machine foundations such as reciprocating pumps, turbines etc, where cyclic or dynamic

loads are expected. The results of seismic test performed at BH8 location indicate presence

of granular material; with generally increase in relative density with depth.

For more detailed, location and depth specific descriptions, reference should be made to the

boreholes and trial pit logs presented in Appendix A of the Factual Report (PO/3122A/08).

5.0 ENGINEERING DISCUSSIONS AND RECOMMEDNATIONS

The purpose of the investigation was to evaluate the subsurface conditions with respect to

the design of foundations for the proposed development. As mentioned in Section 2, the

proposed development will mainly include a new oil and gas processing facility, crude oil &

LPG storage tanks, compressors and associated structures.


At the time of writing this report, detailed information regarding details of loading and layout of

the structures had not been made available, and therefore only a general discussion can be

undertaken.

Further, due to access restrictions limited number of boreholes were mainly spread at the

structures only on the northern half and western parts of the site, along the foot of the jebel,

and also considering the distance between boreholes and results of this preliminary

investigation, it is recommended to conduct a detailed subsurface investigation to include

more boreholes at the proposed structures, before finalising detailed design of the

foundations.

5.1 General Foundation Considerations

On the basis of the subsurface conditions summarized in Section 4 and finished ground levels

provided by MMC (given in Table 1), for all the structures it is considered that shallow foundations

set on fill materials are considered appropriate for the proposed developments, however, for

some structures locally cutting is required and the foundations will be set on natural soil

formation.

5.2 Shallow Foundations

Allowable bearing pressures were calculated in accordance with classical bearing capacity

theory, considering bearing capacity factors recommended by Vesic (1975). Settlement

estimates for layered, predominantly sandy soils, were based on the Schmertmann method

(1978).

As indicated in Table 1, the thickness of fill material at the borehole locations is varying

between 6.6m (BH6) and 12.7m (BH8), except at BH10, cutting of 2.2m is required to reach

finished ground level. Therefore, foundations for all the structures will be set on fill material.

For selection and placing of fill material, guidelines provided in section 5.4 below, should be

followed as a minimum.

For the foundation analysis the relative density of fill material is considered as ‘medium

dense’ (with SPT value as 25). For the proposed structures, the maximum allowable bearing

pressures of shallow square and strip foundations of various sizes (1.0m to 3.0m), and

foundation depths that can be recommended are presented below in Table 4.


Table 4: Maximum Recommended Allowable Bearing Pressure (Qallow in kPa) of shallow

square and strip footings of various widths ‘W’ for the proposed structures

Allowable Bearing Capacity (kPa) for Various Foundation Widths (m)

For Structures at Borehole Nos.

Foundation depth below

FGL (m)

Foundation

Type W= 1.5 W= 2.0 W= 2.5 W= 3.0

Square 300 300 275 250 BHs 1, 4, 6, 8, 10 & 12

1.5m Strip 250 250 225 200

Table 5: Allowable bearing pressure (kPa) for mat foundations

For Structures at Borehole Nos. Foundation depth

below FGL (m)

Allowable bearing

pressure (kPa)

BHs 1, 4, 6, 8, 10 & 12 1.5m 150

FGL – Finished Ground Level

It should be noted that the recommended maximum bearing pressures presented in this

report are for static loading conditions only, and make no allowance for any simultaneous

horizontal load or vertical load eccentricity. Where foundations are to be subjected to cyclic or

dynamic forces, such as under reciprocating machinery or turbines, an appropriate dynamic

load reduction factor should be applied.

At the quoted bearing pressures given above it is estimated that the settlements for

square/strip footings will be about 25mm and for the raft foundations on the order of 30mm to

35mm.

For Crude Oil Storage tanks at borehole BH7 location, the allowable bearing pressure for the

raft is dependent on the settlements allowed. Recommended bearing pressures for different

settlements are given below:

- Allowable bearing pressure of 140kPa for a settlement of 45mm




When excavating to foundation level care should be taken not to disturb the underlying strata.

Any disturbed material, gypsum soils or “soft spots” encountered at foundation level should be

removed and replaced with properly compacted engineering fill. Prior to foundation

construction, the soil at foundation level should be compacted to 95% of maximum dry density

as determined from a modified Proctor test. The compacted surface should then be protected

by immediately covering with a layer of concrete blinding.


5.3 Earthworks

Excavation of the upper granular deposits present across the site should be within the

operating capacity of typical hydraulic excavators. However it is considered that excavation

into underlying locally present cemented formations may require the use of suitable rock

breaking equipment.

Un-retained temporary excavations should be suitably supported, or battered back to a

maximum slope of 2horizontal to 1 vertical.

5.4 Guidelines for Selection and Placement of Fill Materials

The granular materials encountered at the site are considered suitable for structural fill, with

exclusion of larger diameter (more than 75mm) particles.

The following comments and recommendations are given as guidelines for selection and

placing of fill material below any of the foundations or load bearing ground floor slabs. It is

recommended that the material used should meet, or exceed, the following criteria:-

1. Type of soil : Inorganic and free draining with no more than trace amounts

of organic matter, gypsum or other salts.

2. Particle size : 50mm max.

3. Fines content : 15% max.

4. Gradation : Well graded

5. Liquid Limit : 25 max.

The fill should be placed in loose layers not more than 225mm thick and each layer should be

compacted with suitable equipment to not less than 95% of its maximum dry density as

determined from the modified Proctor test.

It is recommended that confirmation of compliance with compaction requirements be made by

field density testing at a frequency of about one test per 300 to 500m2 per layer.

5.5 Chemical Attack on Buried Concrete:

The soil samples tested had soluble sulphate contents (determined as SO3 in 2:1 aqueous

extracts) of between 0.02 and 0.78 grams per litre, with water sample concentrations of 1.04

and 1.84 grams per litre. The soil samples tested had chloride contents of between <0.01%

and 0.58%, with water sample concentrations of 6.56 and 14.18 grams per litre. The pH value

of the samples tested varied between 7.8 and 9.6.


For guidance of suitable concrete design it is recommended that reference should be made to

CIRIA Special Publication C577 (Guide to construction of reinforced concrete in the Arabian

Peninsula, published by the Construction Industry Research and Information Association,

London 2002). This publication considers both sulphate and chloride contamination of soil

and groundwater in various exposure conditions and is not restricted to just to sub-surface

concrete.

It is considered that this site would be classed as having significant contamination with both

sulphate and chloride salts. On this basis it is considered that Exposure Condition d (iii)

would apply, for which it is recommended that a moderate sulphate resisting cement such as

an ASTM type V or ASTM type II cement confirming to ASTM-C150 be used to in a mix

meeting the following criteria.

Minimum cement content for 20mm aggregates 320 to 400kg/m3

Maximum water cement ratio 0.42

Minimum cover to reinforcement 40mm to 50mm

In addition it is recommended that the outer surface of the concrete be protected with a

waterproof coating or membrane. However, prior to finalising and adopting a mix design,

appropriate consultation with the structural engineer is advised.

5.6 Construction Supervision

It is recommended that an experienced engineer, or supervisor, should be present on the site to

oversee all earthworks and foundation construction activities. In particular, it is recommended

that all foundation excavations are inspected by a suitably qualified engineer, prior to foundation

construction, to ensure that the contact surface is properly prepared, and that exposed

subsurface conditions are consistent with design assumptions.

6.0 REPORT LIMITATIONS

The boreholes and trial pit logs and related information depict subsurface conditions only at

the specific locations and times where sampling was conducted. The lines designating the

changes between soil / rock layers represent approximate boundaries. The transition

between materials may be gradual or may occur between recovered samples.

PLATES

LOCALITY PLAN PLATE 1

SITE PLAN PLATE 2

Project: North Star Plant TITLE: Locality Plan

Location: Ras Al Dorah, Khasab JOB REF : PO/3122/08 Sultanate of Oman

DATE: 09-Aug-08 CLIENT: Mott MacDonald & Co. LLC PLATE No: 1

Site Region

PO 3122B 08 Final Interpretative Report

Documents

Transcript of PO 3122B 08 Final Interpretative Report