代做CVEN30010 Geotechnical Modelling and Design 2024帮做R编程

CVEN30010

Geotechnical Modelling and Design

(2024)

Brief for Design Project

(50 marks)

Overview

A new coal mine and a township are proposed to be built near the North Esk River in Tasmania. Launceston City Council (the client) has engaged your firm to provide a conceptual design for a reservoir system for the proposed mining site and nearby township. The reservoir will be located in the valley of the North Esk River to collect water from the upstream catchment and serve as the water source for the proposed mine  site and the township. You are required to carry out the following modelling and design tasks.

Project brief and site investigation data

A zoned earthfill embankment dam is proposed to be built to control the flow of water from the reservoir to the downstream locations. Two different types of soils (labelled as Soil  1, Soil 2) are available in large quantity from local sources, either of which can be used for construction of the shell of the dam. In addition, five other types of soil material (Soil 3, Soil 4, Soil 5, Soil 6 and Soil 7) are available at nearby borrow pits, some of which can be transported to the site for dam construction, if needed.

As part  of geotechnical  site investigation, three boreholes  (BH1 - BH3) were drilled near the proposed dam location. In addition, 8 other boreholes (BH4 - BH11) were drilled across the valley on the upstream side of the proposed dam location, since the valley slopes at that location are deemed to be susceptible to slope instability based on field reconnaissance study. The locations of the proposed dam and eleven boreholes are shown in Fig. 1. Each borehole was initially planned to be drilled up to a depth of 25 m from the ground surface. However, bedrock was encountered at a shallower depth in all boreholes, and drilling operations were terminated at the bedrock level.

Fig. 2 shows the bore log details of BH1-BH11 and Fig. 3 shows the elevation view of the critical valley slope where boreholes BH4-BH11 were drilled. Based on visual observations and laboratory tests, it was inferred that the site consisted of five distinct stratified layers, which are labelled as L1, L2, L3, L4 and L5. Note that L5 is the bedrock. From the eight bore logs spanning across the valley (BH4 - BH11), it is observed that L2 forms a lens near boreholes BH6 to BH9 (a lens is a soil layer that is confined within a small region, generally thick in the middle and thin at the edges). The reduced levels (RL)  of ground  levels  (GL)  at  different borehole  locations  and the  depths of ground water table (measured from GL) before dam construction are listed in Table 1.

Fig. 1 Locations of the proposed dam and the boreholes (Plan view: for illustration purpose; not to scale)

Fig. 2 Borehole logs

Fig. 3 Elevation view of the valley showing locations of boreholes BH4 - BH11 (for illustration purpose; not to scale)

Table 1 Reduced level (RL) of ground surface and depth of groundwater table (GWT) below ground surface before dam construction

Borehole	BH1	BH2	BH3	BH4	BH5	BH6	BH7	BH8	BH9	BH10	BH11
Ground Level (GL) (m)	181.5	178.1	182.0	197.5	191.4	187.8	179.3	179.5	190.3	201.8	207.3
Depth of GWT relative to GL (m) t	+2.7	-0.5	+3.1	+3.1	+2.1	+1.5	-0.3	-0.1	+2.3	+3.5	+4.1

† Positive signs indicate water table is below ground level

Several soil samples were collected and tested in a laboratory to determine their properties. These samples were collected from layers L1 to L4. Also,different soil samples of Soil 1 to Soil 7 were tested to check their suitability in design. Pycnometer tests showed that the specific gravity for various soil types varied within a very narrow range and may be assumed as 2.65 for all soil types. Sieve analyses and hydrometer analyses (where applicable) were conducted with the aim of determining the particle size distributions. Table 2 lists the results obtained from sieve analyses for all soil types. For soil types where % finer passing through the smallest sieve were larger than  12%, hydrometer analyses were carried out simultaneously using a set of ASTM 152H type hydrometers.

For each hydrometer analysis, 1000ml soil suspension was prepared using 60g of soils collected from the pan  and by  adding  Sodium Hexametaphosphate  (SHMP) powder in a  standard jar. The hydrometer readings were noted over a span of 24 hrs and the temperature readings were also recorded. These readings are listed in Table 3 for various soil types. The calibration curves for all hydrometers were He = 16.3 - 0.164R, where He  is the effective depth in cm. To obtain the hydrometer corrections, a hydrometer was initially inserted in ajar of distilled water. The hydrometer reading R for the lower meniscus of distilled water was observed to be 0.5 greater than the upper meniscus. Then the same hydrometer was inserted in ajar containing SHMP powder dissolved in water (at the same concentration level used in the actual soil suspension during tests), and the reading R was observed to increase by 1. Table 4 lists the temperature corrections recommended by the manufacturer of the hydrometer.

Table 2 Mechanical/sieve analysis results for each soil type

Table 3 Hydrometer analysis results for various soil types

Table 4 Temperature correction factors provided by the hydrometer manufacturer

Temperature (°C)	16	17	18	19	20	21	22	23	24	25	26
Temperature  correction, CT	-0.9	-0.7	-0.5	-0.3	0.0	+0.2	+0.4	+0.7	+1.0	+1.3	+1.65

Several laboratory tests were carried out on reconstituted soil samples to determine their properties. Based on the test results, the soil strength properties are summarised in Table 5. Saturated unit weights of these soil specimens are also provided for your reference. Results from constant head and falling head permeameter tests on different soil types are listed in Tables 6 and 7 respectively.

Table 5 Soil properties obtained from laboratory test data

Table 6 Constant head permeameter test results

Table 7 Falling head permeameter test results

Task 1. Analysis of seepage through embankment dam (20 marks)

According to the client’s requirement, the maximum seepage from the upstream to the downstream side must be less than 1 m3/day per metre length of the dam. To cater to potential tourism as well as dam maintenance requirements in the near future,a two-lane road is proposed to be built on the crest of the dam. A dam shell slope of 2.5:1 (2.5 Horizontal :  1 Vertical) is suggested to be adopted in the preliminary design (see Fig. 4). The client approved the adoption of SEEP/W for seepage analysis. The normal water level on the upstream side near the dam is expected to be 7 mnear BH2. However, during wet season, the flood water level is expected to rise by 2 m. While selecting the height of the dam, a minimum freeboard of 1 m should be considered, and an additional 0.5 m should be considered for potential wave action. In addition, while determining the overall height of the dam, an extra 5% of the height should be considered to accommodate long term settlement.

Fig. 4 Cross-section of the proposed embankment dam

For constructability purpose, the embankment needs to be built by soil compaction. Based on the construction  requirements,  gravimetric  water  content  (GWC)  of  18%  has  been  suggested  to  be maintained for all materials required during embankment construction. The natural GWCs and natural bulk unit weights (in the borrow pits) for each of these soil types are listed in Table 8. The unit costs (including transportation costs, if any) for these soil types are also included in the table. It is to be noted that the saturated unit weights of different soil types (Soil 1 to Soil 7) listed earlier in Table 5 were estimated based on the void ratio of these soils after compaction.

Table 8 Natural GWCs, natural bulk unit weights and associated costs for different soil types

(including construction and transportation costs)

Material	Natural GWC (%)	Bulk unit weight in natural state (kN/m3)	Unit cost ($/m3 of soil in natural state)
Soil 1	15	16.5	395
Soil 2	18	15.9	405
Soil 3	14	17	575
Soil 4	16	16.2	487
Soil 5	15	16	501
Soil 6	17	17	755
Soil 7	16	17.5	693

Your dam design should satisfy the client’s requirements. You also need to consider other aspects (e.g., constructability, budget) to produce an economic and safe design. In this regard, the following tasks are outlined to assist your design:

(1)   Apply your engineering judgement to choose the appropriate material for the shell of the dam. Provide justifications showing relevant calculations and figures if necessary. (3 marks)

(2)   Please  consider  appropriate  dam  dimensions  based  on  the  client’s  requirements  and  design considerations. Obtain the maximum seepage rate flowing from the upstream to the downstream side considering ahomogenous dam consisting of shell material only. In your report, the total head and seepage velocity distribution through the dam should be shown. Also, provide evidence of your model boundary conditions. (3 marks)

(3)   If your calculated seepage did not meet the requirement, propose your design to reduce seepage. Based on your judgement, select a material that may be needed to be carried to the site from the available materials for constructing a core (along with trench if necessary). For constructability purpose, only one type of material is recommended to be used in  core (and trench). You are recommended to find available design guidelines and other reliable references to support your chosen core geometry. (4 marks)

(4)   Improve your design considering long-term dam stability against potential seepage hazards (e.g., internal erosion, piping). You may need to design a zoned earthfill dam comprising of shell, core, drains/filters and/or transition zones. Please provide justifications behind the choice of materials for each component of the dam and refer to engineering standards/available guidelines wherever possible. You need to show the necessary details of the steps involved in your design. Estimate the seepage from upstream to downstream for your chosen dam section. Also, include relevant model outputs wherever needed to support your design. (6 marks)

(5)   Considering a unit length of dam cross-section (e.g., considering 1 m length perpendicular to the plane shown in Fig. 4), estimate the cost per unit length for your designed dam cross-section. During your design process, you may need tooptimise by minimising the cost and come up with a safe and economic design while adhering to relevant engineering practices/guidelines. (4 marks)

Task 2. Slope stability analysis of embankment dam (11 marks)

Slope stability analysis of the upstream and downstream slopes of the embankment dam (see Fig. 4) is needed to be carried out. A minimum Factor of safety (FoS) of 1.4 is recommended. The client has approved the use of SLOPE/W for assessing slope (in)stability.

(1)   Based  on  the  final  design  from  Task   1,  please  analyse  the  stability  of both  upstream  and downstream dam slopes (see Fig. 4) using Bishop’s method in SLOPE/W. You need to consider the effect of seepage during your stability analyses. Based on your analyses, comment on the stability of the dam  slopes. Also,  compare the FoS obtained for the two  slopes and provide justifications. (3 marks)

(2)   To   benchmark    your   calculated    results,   please    use    spreadsheet/hand   calculations    and AutoCAD/graph paper to estimate the FoS for both upstream and downstream dam slopes. In your calculations, you can directly consider the critical  slip  surface for each  slope obtained using SLOPE/W. Logical assumptions can be made to simplify your calculations, if needed. Compare the results with those obtained using SLOPE/W and discuss the reasons if you find any differences between the obtained FoS using the two methods. (6 marks)

(3)   Based on your analysis, please recommend whether the shell slopes can be deemed to be safe? Please discuss whether your stability analyses are adequate. If not, what other considerations should be included in your modelling to analyse slope stability. (2 marks)

Task 3. Slope stability assessment of critical valley slope (14 marks)

It was identified that the valley slopes located near BH4 – BH11 (see Fig. 3) represent the most critical section of the valley from a slope (in)stability point of view. Therefore, stability analyses of these existing slopes against sliding considering an empty and a full reservoir are deemed necessary. The client suggests a minimum factor of safety (FoS) of 1.4 to be considered for design purpose.

(1)   You are required to generate a geological ground model (2D cross-section of soil layers) for your client to represent the conditions at the reservoir site. You can draw it by hand or by using third- party software, such as AutoCAD, Excel or GeoStudio. However, you are required to make it presentable by providing necessary and clear labels. The RL at the eight borehole locations and the depth of GWT before building the dam have already been provided in Table 1. (3 marks)

(2)   You are required to model both slopes (left and right) on the two sides of the reservoir/valley along the cross-section BH4 – BH11 using SLOPE/W (see Fig. 3). Find the critical slip surfaces and the minimum FoS against sliding using Bishop’s method for both slopes considering both empty and full reservoir conditions. When the water reservoir is full, the maximum water level in the reservoir is likely to be the same as the flood level mentioned earlier. Please provide a summary of the slope stability results. The soil unit weight and shear strength properties have already been mentioned in Table 5. (4 marks)

(3)   If slopes are deemed unstable, refer to Table  9 for different stabilisation methods that can be implemented at the site. Analyse the suitability of each of these methods and propose one method to stabilise the slope at critical locations. Make a brief justification for the method you proposed. Your suggested method should be practical and reasonable, and you should justify why your design is optimal. (3 marks)

(4)   Considering the tourism business for the township development, a two-lane road is planned to be built along the reservoir on the right slope (see Fig. 3). Choose an appropriate road location and road width; and select a suitable cut and/or fill method for road construction. Assess whether any stability issues are likely to happen after road construction and suggest remedial measures if needed. During your analysis, you need to consider both the self-weight of the road pavement and the vehicle axle loads from traffic. The road is likely to have a 125 mm thick surface course, 250 mm thick base course and 275 mm thick sub-base. You can assume unit weights of course, base and sub-base as 22 kN/m3. To simplify vehicle axle loads, you may consider a uniform. pressure of 5 kPa on the road surface for small vehicles (like cars) and a uniform. pressure of 12 kPa for large vehicles (like trucks). Include all relevant load calculations in your report. (4 marks)