CC001: The Soil Inspector Pty Ltd Site Classification Report

Assignment Overview

Objectives

• To report on the existing site conditions and soil profile.

• To drill and log an appropriate number of boreholes/pits.

• To classify the site in accordance with AS 2870 – Residential Slabs and Footings.

• To provide estimated foundation bearing capacities at various depths.

• To recommend minimum founding depths for the footings.

• To make relevant observations and recommendations pertaining to drainage, construction and/or footing performance.

3. Site Description

A site with existing sheds and fencing. The slope was approximately 5° from North West to South East. Vegetation consisted of 3 medium height trees. Topography was even and drainage was fair. There was no groundwater at the time of testing.

3.1 Sub-Surface Profile

The sub-surface consisted of a clayey silt stratum and a silty clay (see borelogs).

4. Regional Geology

Geological Survey of Victoria maps (1:63,360) and information published by the Department of Primary Industries on the GeoVic online geological mapping site (1:250,000) show the site is underlain by:

• DEVONIAN (Dlh)

In general, the anticipated subsurface conditions have been encountered during this site investigation and are considered to be consistent with the geological map.

5. Climatic Zone

This climatic zone of the area with reference to AS2870-2011 Table 2.5 and Figure D1 and D2 is:

• Climatic Zone 1 – Depth of design suction change (Hs) is 1.5m

6. Classification

This site is classified with reference to footing design and construction as per AS 2870-2011:

• CLASS M – All proposed new footings are required to be engineered designed with consideration to:

  • Moderately reactive clay or silt sites, which may experience moderate ground movement from moisture changes.

This classification was determined by taking into consideration the area geology, soil profile encountered and any potential abnormal moisture conditions. This report will be valid for a maximum period of up to five years. During this period, if the site has been modified or a change in AS2870-2011 has taken place, this site classification will need to be revised.

In accordance with AS2870-2011 Table 2.3, it is estimated that the characteristic surface movement (Ys) will be in the range of:

• 20 to 40mm in the natural soils under normal moisture conditions

The Ys value will increase if site cuts are conducted and the clay stratum is exposed (refer page 114, Table C 2.3).

This investigation is based on a limited geotechnical assessment. Should the subsurface conditions encountered during construction vary from those described above, The Soil Inspector Pty Ltd must be advised of these variations to provide comment or inspect the site where necessary.

The use of standard footings as presented in AS2870-2011 is only applicable to building with a loading and a construction style similar to that of a residential dwelling as described in Section 3.1 of AS2870-2011.

7. Findings and Observations Re. Footings

7.1 Existing Footings

No existing footings were encountered.

7.2 Proposed

Considering the conditions observed on site, the following is recommended as the preferred footing system:

Founding Depth:
To satisfy the footing requirements and based on the present conditions of this site, it will be necessary to either remove the trees from around the proposed building or alternatively adopt an engineer-designed deepened footing system.

Pier & Beam Footing System:
If pier footings are required for this site, we recommend an engineer-designed pier footing system founded at least into the stiff–very stiff clay stratum with the total depth to be specified by the design engineer.

7.2.1 Strip, Stump and Pad Footings

The proposed footing construction can utilise strip, stump and pad footings. These must be founded at least 100mm into the natural stiff silty clay stratum as follows:

Minimum founding depths to achieve an allowable bearing capacity of 100 kPa beneath strips/pads:

• Borehole 1: Depth 0.6 m — Founding Material: Silty clay

• Borehole 2: Depth 0.7 m — Founding Material: Silty clay

The proposed strip, stump and pad footings can be founded at the recommended founding depths and proportioned to an allowable bearing capacity of 100 kPa.

7.2.2 Stiffened Raft Slab

Only if normal moisture conditions can be achieved and maintained can the following founding depths be used. After stripping all vegetation and proof-rolling, a slab on ground may be constructed directly on to the natural ground surface. All slab beams must be founded at least on to the stiff silty clay stratum.

Minimum founding depths to achieve an allowable bearing capacity of 50 kPa beneath slabs:

• Borehole 1: Depth 0.3 m — Founding Material: Silty clay

• Borehole 2: Depth 0.4 m — Founding Material: Silty clay

Note: The founding depths listed above are only applicable to the location tested and are only to be used if normal moisture conditions can be achieved and maintained.

7.2.3 General

Notwithstanding the above, the footing system must comply in all respects with either:

1. AS 2870-2011 Residential Slabs and Footings Construction.

2. Building Code of Australia.

3. AS1684.2-2010 Residential Timber-framed Construction.

All excavations should be carried out in accordance with the requirements of the BCA.

Retaining Walls:
Retaining walls are to be constructed in accordance with AS4678-2002 Earth-retaining structures.

If the height of the retaining walls is equal to or greater than 1.5m, then further investigation including laboratory testing will be required. Please refer to Tables 1.1 & 2.1 in the above standard.

If higher allowable bearing capacities are required, these may be obtained from the table supplied. All footings can be proportioned to the allowable bearing capacities and the recommended founding depths supplied as part of this report.

7.2.4 Allowable Maximum Bearing Pressures

Borehole 1:

• 0.3 m — 150 kPa

• 1.0 m — 170 kPa

• 1.5 m — 200 kPa

• 2.0 m — 220 kPa

Borehole 2:

• 0.4 m — 70 kPa

• 0.7 m — 120 kPa

• 1.4 m — 150 kPa

• 2.0 m — 170 kPa

8. Site Observations / Recommendations

Excavation and Footings

At the time of construction, excavations should be carefully checked for the presence and variation in the depth of fill material and in the consistency of foundation soils. If there is any doubt about the natural ground or bearing capacity of founding soils at the time of excavations, The Soil Inspector Pty Ltd is to be contacted to verify conditions.

If any fill material encountered during excavations is deeper than is listed in this report as found at the borehole locations, the site classification of this report may need to be revised accordingly. The design and depth of footings may need to be altered if this occurs.

The texture of soils could cause the sides of unsupported trenches to collapse. If groundwater enters the trenches or excavations, the risk of collapse is amplified. The use of shoring or trench shields is recommended for excavations deeper than 1.0m.

Existing and/or adjacent footings, services and easements should be considered in the design and construction of all footings.
Any new or existing site cuts, batters and retaining walls should also be considered in the design and construction of all footings.

Drainage

Drainage systems are to be installed as per AS2870 Section 5.2 & 5.6.3 ‘Drainage Requirements’.
All plumbing and drainage systems are to be designed and installed in accordance with AS3500 ‘Plumbing and Drainage’.

Vegetation

Trees and large shrubs on this site or adjacent properties close to the proposed structure should be removed or suitably engineered design footing should be adopted.

Trees and large shrubs should not be planted or allowed to exist closer to the structure than 1.0 times their mature height. This distance must be increased to 1.5–2.0 times the mature height where groups of trees exist on or adjacent to the site or bedrock exists close to the surface.

Future landscaping and tree planting on this site or neighbouring properties may impact the performance of existing or proposed footings. Overwatering of garden beds close to the building and its footings may soften or add to ground movement of the founding soils.

Reference should be made to Appendix H and CH from AS2870 ‘Guide to Design of Footing for Trees’ for all footing designs for this site.

Articulation

Full height openings or articulation joints to masonry should be provided as per AS4773 ‘Masonry in Small Buildings’ or to the design engineer’s specifications.

Assessment Summary and Approach: Understanding the Soil Classification Report

Part 1: Assessment Requirements – Overview and Key Pointers

The assessment titled “CC001: The Soil Inspector Pty Ltd Site Classification Report” required students to conduct a professional site classification and soil investigation aligned with the Australian Standard AS 2870 – Residential Slabs and Footings. The main goal was to assess soil properties, classify the site, and recommend suitable footing systems based on subsurface conditions.

Key assessment objectives included:

1. Reporting on existing site conditions and soil profile, including slope, vegetation, drainage, and topography.

2. Drilling and logging boreholes/pits to determine soil types and stratification.

3. Classifying the site as per AS 2870 to identify the level of soil reactivity and corresponding footing requirements.

4. Estimating foundation bearing capacities at various depths based on borehole data.

5. Recommending minimum footing founding depths suitable for the soil conditions.

6. Making observations and recommendations regarding site drainage, excavation, vegetation management, and construction practices to ensure footing stability and performance.

The assessment simulated a real-world geotechnical report requiring both technical accuracy and professional presentation. Students were expected to demonstrate their ability to apply geotechnical engineering principles, interpret borehole data, and prepare recommendations aligned with relevant standards such as:

• AS 2870-2011: Residential Slabs and Footings

• AS 1684.2-2010: Residential Timber-Framed Construction

• AS 4678-2002: Earth-Retaining Structures

• AS 3500: Plumbing and Drainage

Part 2: Academic Mentor’s Step-by-Step Guidance

To help the student successfully complete this assessment, the academic mentor provided structured and systematic guidance throughout each section of the report. The process focused on analytical thinking, technical writing, and referencing standards correctly.

Step 1: Understanding the Objective and Context

The mentor began by explaining the purpose of a soil classification report to determine how soil and site conditions affect building foundations. The student was guided to review the assessment brief carefully and identify the expected deliverables: a comprehensive site report with soil profile analysis, classification, and footing recommendations.

• The mentor emphasized reviewing AS 2870 and its definitions of site classifications (A, S, M, H, E, P).

• The student was advised to interpret the relationship between soil reactivity and foundation performance.

Step 2: Site Description and Observations

The next step involved writing a clear and factual description of the site. The mentor demonstrated how to organize field data into a professional structure.

The student was guided to:

• Describe topography (slope direction and degree).

• Note vegetation (trees, shrubs) and their influence on moisture variation.

• Comment on drainage conditions and presence of groundwater.

• Record surface conditions like sheds or fencing.

The mentor explained how these features influence soil moisture and footing movement, reinforcing the importance of accurate field observation.

Step 3: Subsurface Profile and Borehole Data Interpretation

Under this step, the mentor explained how to analyze borehole logs to identify soil strata, texture, and consistency.

The student learned to:

• Interpret clayey silt and silty clay strata from borehole data.

• Recognize how different layers influence bearing capacity and moisture reactivity.

• Relate borehole findings to geological maps (GeoVic and Geological Survey of Victoria).

This step taught the student to compare observed conditions with regional geology, ensuring that field data were consistent with mapped geological formations (Devonian Dlh unit in this case).

Step 4: Determining Climatic Zone

The mentor guided the student in identifying the Climatic Zone (Zone 1) using AS 2870 Table 2.5 and Figures D1–D2, which determine the depth of design suction change (Hs).

The student learned that climate affects soil moisture variations and, therefore, ground movement potential. For this site, an Hs of 1.5 m was noted, influencing footing design depth and moisture control recommendations.

Step 5: Site Classification (AS 2870-2011)

Here, the mentor focused on the core analytical part classifying the site.

The student learned how to:

• Assess moderate soil reactivity based on the presence of silty clay.

• Identify the site as Class M, indicating moderately reactive clay or silt.

• Estimate characteristic surface movement (Ys) between 20–40 mm under normal conditions.

• Understand how site modifications (e.g., cuts or vegetation removal) could alter Ys values.

The mentor ensured the student correctly referenced AS 2870 and explained the engineering rationale behind choosing Class M.

Step 6: Foundation and Footing Recommendations

Once the classification was established, the mentor guided the student through footing design recommendations:

• Founding Depths: Determining minimum depths to achieve 100 kPa bearing capacity for strip, stump, and pad footings.

• Stiffened Raft Slab: Advising that slab-on-ground systems may be used if normal moisture conditions are maintained.

• Pier & Beam Systems: Recommending deepened piers for stability in moderately reactive clay.

The student was instructed to tabulate the bearing capacities at different depths and correlate them with borehole data, ensuring clarity and professional presentation.

Step 7: Compliance with Australian Standards

The mentor emphasized the importance of compliance documentation. Students were guided to reference the applicable Australian Standards for:

• Excavation practices (BCA requirements)

• Timber-framed construction (AS 1684.2)

• Retaining wall design (AS 4678)

• Plumbing and drainage (AS 3500)

This reinforced the integration of regulatory knowledge into technical reporting.

Step 8: Site Observations and Practical Recommendations

In this section, the mentor encouraged the student to provide practical, field-based recommendations for construction:

• Ensure proper drainage installation to prevent soil movement.

• Manage vegetation proximity: trees should not be within 1.0–2.0 times their mature height from structures.

• Implement articulation joints in masonry to allow for minor soil movement.

• Monitor for groundwater infiltration and use trench shields during excavation.

The student learned how to transform technical data into actionable advice for builders and engineers.

Part 3: Final Outcome and Learning Achievements

Through this structured mentoring process, the student successfully produced a comprehensive and professional site classification report that addressed all assessment criteria.

Final Deliverables Included:

• Clear and factual site description

• Accurate borehole interpretation

• Correct AS 2870 classification (Class M)

• Calculated foundation bearing capacities

• Detailed footing system recommendations

• Contextual compliance with building and drainage standards

• Professional presentation consistent with engineering reporting standards

Part 4: Learning Objectives Achieved

By completing this assessment under guided mentorship, the student achieved the following learning outcomes:

1. Applied Geotechnical Knowledge:
   Understood soil behavior, reactivity, and how subsurface conditions affect structural foundations.

2. Technical Data Interpretation:
   Learned to interpret borehole logs, soil profiles, and geological maps effectively.

3. Regulatory Compliance:
   Gained familiarity with multiple Australian Standards (AS 2870, AS 1684.2, AS 4678, AS 3500, and BCA).

4. Analytical Problem Solving:
   Developed analytical reasoning to determine appropriate footing systems for variable soil conditions.

5. Professional Report Writing:
   Learned to structure technical findings clearly and concisely using professional geotechnical terminology.

6. Practical Construction Awareness:
   Recognized the impact of environmental and site management factors (vegetation, drainage, slope) on footing performance.

7. Critical Thinking and Decision-Making:
   Acquired the ability to reassess site conditions during construction and adapt recommendations accordingly.

Conclusion

This assessment provided a valuable bridge between theoretical soil mechanics and practical geotechnical engineering. The student, guided by the academic mentor, learned to produce a site classification report that reflects real-world industry standards. By systematically interpreting field data, referencing relevant codes, and formulating engineering recommendations, the student achieved both technical proficiency and professional confidence—successfully meeting all learning and assessment objectives.

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