Geotechnical Engineering: Site Investigation And Construction Methods

Site Investigation in Geotechnical Engineering

In most cases, a typical Geotechnical engineering projects starts with the definition of the material properties, followed by a site investigation of the soil, fault distribution, rock, and bedrock properties on and beneath the site area to determine the engineering properties which include how the soil will interact with the proposed construction (Allen & Iano, 2017). Site investigation help the project engineers to understand the site, These include the assessment of risk to property, human, and the environment from the natural hazards which include landslides, earthquakes, soil liquefaction, sinkholes, rock falls and debris flows (Allen & Iano, 2017).

Geotechnical engineers role is to design the earthworks, type of foundation and pavement subgrades required to build the structure (Bahrami, et al., 2012). Foundation designs depend on the type of the structure such as the high rise buildings, commercial structures, small structures and bridges (Rose, et al., 2012). There are different types of foundations that are built above the structure such as retaining walls, shallow and deep foundations (Bahrami, et al., 2012). Earthworks include tunnels, reservoirs, embankments, channels, dikes and levees sanitary landfills and hazardous wastes.

Cargo transporting company in Melbourne has approached Maestro Company which is our geotechnical company to carry out an assessment on their site in Melbourne on geotechnical aspects to determine the effectiveness of the soil for construction since the site was used before as a storage area of petrochemical company, which means there has been oil spillage that have happened on site for a long period of time which might contaminate the soil structure and condition.

Scope of work

The building project will consist of A concrete industrial building with plan dimensions of 15 m × 40 m. The building is to have a mat concrete foundation with a thickness of 500 mm, with a finished RL of 14.0 m. The walls are load bearing and there are some internal columns carrying the weight of the roof and other elements.

• A steel water tank having an external diameter of 13 m and an effective height of 8 m, with its base at RL 17 m (top of the footing).
• There is an existing one-storey double brick building on the site. The RL of the slab on the ground is 17.5 m (top of the slab on ground).

A Site Plan is attached below, showing the location of the tank, the car park (RL 17.5 m, finished surfacing) the proposed and existing buildings. The Structural Engineer for the project has determined the proposed building loads and they are as follows:

• Total vertical loads: 250 MN per meter run (NOTE: the weight of foundation is included.)
• Total moment in each direction: 500 MN.m
• Total horizontal loads: 50 MN.

Consistency of the soil layers based on SPT N-values

Standard Penetration Test (SPT) is a test done on site to determine the geotechnical engineering properties of the soil. The [procedure is mostly described in ISO 22476-3, Australian Standards AS 1289.6.3.1, and ASTM D1586 (Lovisa, et al., 2010). It is the most common test used worldwide in geotechnical engineering (Boly, et al., 2012).

Construction Methods

Soil consistency is the ability of the soil materials to hold itself by resisting any form of pressure that may lead to the deformation or rupture of the soil (Chotia, et al., 2012). It is measured for dry soil and wet moist samples. When it comes to the wet soil sample, it is expressed as both plasticity and thickness (Long & Vietnam, 2010).

The boreholes are usually drilled deeper and the test repeated again. During this process, errors might occur per interval due to the poor state of soil recovery (Clayton, et al., 2012). The number of the hammer strikes it takes for a tube to dig deep or penetrate the 2^nd and 3^rd 6 inch depth is referred to as “standard penetration resistance” or referred to as ‘N-value (Clayton, et al., 2012).

For 3 or 4 increments of six inches each

Three increments: we get the sum of the last 2 increments in SPT N value. For 4 increments, the same procedure is repeated (Chotia, et al., 2012).

Correction to SPT N Value

N_measured = Raw SPT value obtained from the field test

N₆₀ = corrected N values corresponds to 60% of energy efficiency

N₆₀  = C_E C_B C_S C_R N_measured

Factor	Term	Equivalent Variable	Correction
Energy Ration	CE =ER/60	Donut hammer Automatic hammer Safety hammer	0.5 -1.0 0.8 – 1.5 0.7 – 1.2
Borehole diameter	CB	65mm to 155mm 150mm 200mm	1.0 1.05 1.15
Sampling method	CS	Non Standard sampler standard sampler	1.1  – 1.3 1.0
Rod length	CR	3m – 4m 4m – 6m 6m – 10m   >10m	0.75 0.85 0.95 1.00

Table 1: STP Corrections Source: (Eswaraiah, et al., 2011)

Note: it is important that the ER value should be measured according to the Soil engineering property in regard to the insitu testing

Soil	Consistency/ Density	N
SANDS	Very Loose Loose Medium dense Dense Very dense	0 -4 5 – 10 11 – 30 31 – 50 >50
COHESIVE SOIL	Very soft Firm Stiff Very stiff Hard	0 -2 2– 8 9 – 15 15 –30 >30

Cross section of the ground layers of the site

Soil profile sample

The construction methods that should be adopted in the construction

This construction is more involving in the deep ground and therefore the existence of groundwater is required to be considered that can lead to limit the working space. In every deep ground construction, methods of soil supports, sub-soil condition, and the layout requirement of the building must be considered to design for the method of works (Allen & Iano, 2017). Provision of a retaining structure such as the diaphragm wall or sheet piles wall, any is needed during excavation at the sides of the walls. This method is most effective method that can be used to construct ordinary basement (Kato, et al., 2012).

Use of lattice beams

A series of steel trusses is put in place to span between the top of opposite diagram walls. The retaining walls act as propped cantilevers (Bahrami, et al., 2012). After the construction of internal slab, the trusses can be removed since the slab will support the lateral forces from soil (Boly, et al., 2012).

Standard Penetration Test (SPT)

Using Anchors in the ground

The ground anchors are used to provide stabilization of the walls as the works is in process, since the diagram walls are directly exposed to earth pressure during the excavation stages (Chotia, et al., 2012). The method is suitable for basement of very large spans, without the intermediates floors as lateral support (Clayton, et al., 2012).

The top-down method

It involves the construction of floor slab  as support, where after the perimeter diaphragm walls have been constructed, the ground floor slab and beams are cast providing tip edge lateral support to the walls (Das & Shukla, 2013). An opening in the slab provides access for labors and material as the excavation continue to the lower stages. The process continues until the attaining of the required depth (Eswaraiah, et al., 2011).A general layout of the retaining walls around the proposed building.

Retaining walls

This is a structure designed using either concrete insitu, precast concrete, stone blocks in order to resist the lateral pressure coming from the soil to prevent it from collapsing or rapturing (Clayton, et al., 2012). The lateral pressure can also be as a results of the liquid pressure, sand, earth filling and any other granular material used to fill the wall on its back side after it has been constructed (Clayton, et al., 2012).

In most cases, these walls are normally used as the transporting structures when construction is happening on top of the hill or a raised ground, bridge wing walls, masonry dams, abutments (Das & Shukla, 2013). The type of material to be used in the construction of the retaining wall depends on the site condition, height of the wall to be constructed and the type of material to be retained (Clayton, et al., 2012).

The primary role of a retaining wall is to hold the earth back in its position without instances of soil instability where there is sliding, overturning or structural failure. Earth fill, water table, and surcharge are important in retaining the wall design (Eswaraiah, et al., 2011). However, disaster may occur when the soil can bear the compressional force that acts upon it due to excess axial load or shear force causing a lot pressure which might force the soil to tip off (Kravchenko & Robertson, 2011).

Therefore, its use in the construction projects is always very crucial since it helps in maintaining the ecological condition of the landscape hence avoiding more cut and fill process which might be very expensive (Eswaraiah, et al., 2011). In addition, the project design will try and blend in with the environment by maintain the gradient at some point. Some of the retaining wall that will be used in the proposed project include:

Soil Consistency

Gravity retaining wall

Gravity retaining wall depends on its own weight, whereby it only stands alone. This retaining wall is always massive (Eswaraiah, et al., 2011). To design this retaining wall, it is important to take in certain consideration such as sliding, overturning forces and bearings which should be tested to ensure its suitability (Eswaraiah, et al., 2011). Mostly used where there is a lot of pressure applied.  

Sliding of retaining wall              Overturning retaining wall               Global stability of retaining wall 

Pile retaining wall

Piles are normally driven deep into the earth surface so that the forces acting from the top of the wall are prevented from collapsing the wall over instead it is held back in position (Eswaraiah, et al., 2011). It uses counter forces by holding back the force acting downwards and balancing it with the upward resistance hence preventing the wall from tipping off (Halpern, et al., 2014). Pile retaining wall can be used in a permanent works or temporary works since they provide high stiffness retaining elements which have got large excavation depths which cause no disturbance to the surrounding structures (Kato, et al., 2012).

Cantilever retaining walls

They are always constructed using reinforced concrete. Cantilever retaining wall consist of a slab base and a thin stem (Eswaraiah, et al., 2011). The base has got two parts referred to as the toe and the heel. The heel is located at the base under the backfill. This wall in most cases uses less concrete than the retaining walls, but it needs keen concentration during design and construction. This wall is usually 25 feet in height and can either be pre cast in factories or formed insitu (Halpern, et al., 2014).

Anchored retaining walls

Where there is high retaining walls, deep wires are driven deep along the earth sideways after which the ends are anchored with concrete to keep the wires and soil in position (Halpern, et al., 2014). They are referred to as tie backs. They are used when the spaces required to construct the retaining wall is small or a thinner retaining wall is needed (Kato, et al., 2012). They are deemed to be more effective on the loose soil compared to the solid rocks. Anchored retaining walls have been employed in the construction of highways whereby they keep the soil lumps and rock from falling into the highway (Kato, et al., 2012).

Condition of stability of Retaining walls

The stability of retaining walls can be achieved by certain requirements or conditions such as

• The wall should be structurally sound to resist any form of pressure.
• The wall should be proportionate to avoid instances of being overturn by lateral forces.
• The wall should be safe from sliding
• The weight of the wall and the resultant force of the earth causing the pressure should not cause stress on its foundation to a value exceeding the bearing capacity of the soil(Lovisa, et al., 2010).
• Long retaining walls should be constructed with expansion joints positioned at 6 – 9m apart(Kravchenko & Robertson, 2011).
• The backfilling materials used to fill the back of the retaining walls should not have the ability of retaining water.
• Water pressure could be relieved by providing water holes.

Retaining Wall Designs

Loads on Retaining Walls

Major loads that act on retaining walls are as follows.

• The walls self-weight
• Vertical earth pressure
• Lateral earth pressure
• Horizontal water pressure
• Horizontal live load surcharge
• Vertical live load
• Buoyancy due to water table,

Pedestrian access from the car park to this building

Preliminary design of a retaining wall          

The purpose of this is provide clear properties and aspects that can be during the detailing design of the retaining wall (Das & Shukla, 2013).

Theses aspects to be put in place are

The location of the retaining wall. Put into consideration the existing site features and the show how they impact the impact the retaining wall (Kato, et al., 2012). The location should cater for the drainage patterns, which one can specify the type of drainage incorporated

Specify the geometry of the retaining wall. Calculate the height of the retaining wall. Specify the thickness and shape to be used. Incorporate the effect of surcharges and some cases seepage (Kato, et al., 2012).

Suggest the structural required to be used in the design. This should provide the bearing capacity of the soil, moment from resultant forces (Allen & Iano, 2017). Specify if geo-grids are being used to come with the total wall envelope (Kravchenko & Robertson, 2011).

With all this specified will be used by the structural engineering to come up with the structural design and some cases of a concrete wall provide reinforcement at the most economical (Lovisa, et al., 2010)

The retaining structure employed between proposed building and water tank

The retaining structure that should be employed between the proposed building and the water tank is the pile retaining wall. This retaining walls provide high stiffness retaining elements giving them more strength due to the deep piles penetrated into the soil (Das & Shukla, 2013). Due to their strength, Pile retaining walls will be able to resist the lateral pressures that will be coming from the steel water tanks compression pressure that results into the lateral pressures. (Halpern, et al., 2014) The resistance makes the wall to remain steady and not topple over to the proposed concrete block walled building (Halpern, et al., 2014).

The retaining structure employed between proposed and existing building plus proposed building and planned car park

The retaining structure that should be employed between the proposed and the existing building as well as the proposed building and the planned car park will be cantilever retaining wall. This type of retaining wall is mostly used as a boundary in separating two building structures (Kato, et al., 2012). Also used when a sloppy land is being levelled to carry out construction, where by it is used at the upper side of the slope to retain the soil in construction before construction commence. The heel makes the retaining wall to have extra strength when used in earthworks (Kravchenko & Robertson, 2011).

The retaining structure employed between proposed building and car park

The retaining structure that should be employed between the proposed building and the planned car park is the gravity retaining wall. This retaining wall makes it easier to have steps that will help pedestrians walk towards the proposed building after coming from the car park area, due to its strong self-weight, it will be able to handle the heavy traffic that will be experience in that areas (Boly, et al., 2012). Since the gravity retaining wall will cover a distance of more than 30m, expansion joints will be introduced in the wall to allow for expansion in order to relieve excess pressure that might be experience hence avoiding cracking or rapture (Halpern, et al., 2014).

Factor of safety of the large mat foundation proposed for the building A factor of safety to range from 2.5 -3.0. The footing is 4m below the ground surface. It is above the sandy soil layer (Halpern, et al., 2014).Maximum Settlement of the footing

Circular tank footing

The footing will be designed as a rigid footing. You need to consider it as axisymmetric. The design depends on the SPT perform (Long & Vietnam, 2010).

The depth of the upper layer of which is Settlement of the footing

The requirement is to find the allowable bearing capacity that the footing can settlement to a maximum 50mm (Rose, et al., 2012). The depth of the base of the mat to the bed, H= 7- 1 = 6m

Recommendations

The settlement of the rafter footing is expected to be greater than the allowable because the settlement gotten from the applying the factor of safety on the ultimate bearing capacity produced settlement of 244.3mm which exceeds the allowable 50mm. recommendation of using pile foundation for the new building which will transfer the loads directly to the bedrock. For the retaining wall recommend concrete walls which weep holes.

References

Allen, E. and Iano, J., 2017. The architect’s studio companion: Rules of thumb for preliminary design. John Wiley & Sons.

Bahrami, M., Bazzaz, D.H. and Sajjadi, S.M., 2012. Innovation and improvements in project implementation and management; using FMEA technique. Procedia-Social and Behavioral Sciences, 41, pp.418-425.

Boly, M., Garrido, M.I., Gosseries, O., Bruno, M.A., Boveroux, P., Schnakers, C., Massimini, M., Litvak, V., Laureys, S. and Friston, K., 2011. Preserved feedforward but impaired top-down processes in the vegetative state. Science, 332(6031), pp.858-862.

Chotia, A., Neyenhuis, B., Moses, S.A., Yan, B., Covey, J.P., Foss-Feig, M., Rey, A.M., Jin, D.S. and Ye, J., 2012. Long-lived dipolar molecules and Feshbach molecules in a 3D optical lattice. Physical review letters, 108(8), p.080405.

Clayton, C.R., Woods, R.I., Bond, A.J. and Milititsky, J., 2014. Earth pressure and earth-retaining structures. CRC Press.

Eswaraiah, V., Aravind, S.S.J. and Ramaprabhu, S., 2011. Top down method for synthesis of highly conducting graphene by exfoliation of graphite oxide using focused solar radiation. Journal of Materials Chemistry, 21(19), pp.6800-6803.

Das, B.M. and Shukla, S.K., 2013. Earth anchors. J. Ross Publishing.

Halpern, Y., Choi, Y., Horng, S. and Sontag, D., 2014. Using anchors to estimate clinical state without labeled data. In AMIA Annual Symposium Proceedings (Vol. 2014, p. 606). American Medical Informatics Association.

Kato, H., Onda, Y. and Teramage, M., 2012. Depth distribution of 137Cs, 134Cs, and 131I in soil profile after Fukushima Dai-ichi Nuclear Power Plant Accident. Journal of environmental radioactivity, 111, pp.59-64.

Kravchenko, A.N. and Robertson, G.P., 2011. Whole-profile soil carbon stocks: The danger of assuming too much from analyses of too little. Soil Science Society of America Journal, 75(1), pp.235-240.

Long, P.D. and Vietnam, V.W., 2010. Piled raft—a cost-effective foundation method for high-rises. Geotechnical Engineering, 41(1), p.149.

Lovisa, J., Shukla, S.K. and Sivakugan, N., 2010. Behaviour of prestressed geotextile-reinforced sand bed supporting a loaded circular footing. Geotextiles and Geomembranes, 28(1), pp.23-32.

Rose, P., Boguslawski, M. and Denz, C., 2012. Nonlinear lattice structures based on families of complex nondiffracting beams. New Journal of Physics, 14(3), p.033018.