Rock Strength Evaluation for Tunnel Excavation by Hoek-Brown Equation Assessment Answer

For successful tunnel excavation along with construction, a thorough evaluation of the site's rock mass characteristics is essential. This evaluation of the rock mass characteristics is based on the data provided and the GSI computations.

Pont A, (Unbounded GSI)

Point A has excellent rock mass characteristics, according to the evaluation. If the GSI is indeed infinite, then the rock mass must be of very high grade. There are probably not any or very few geological faults or breaks. The granite at Location A is essentially in its natural state.

Implications: Tunnel excavation in Point A is predicted to be reasonably easy and safe.

The ground may need little reinforcement or support.

The rock probably has great stability and strength.

Point B (GSI - 3.08)

Despite being less ideal than Point A, the state of the rock mass at B is nonetheless acceptable. Some geological flaws or discontinuities are present, as indicated by the GSI of around 3.08, although the rock bulk is not significantly affected.

Conclusion: Excavation of a tunnel at Location B is possible, but extreme care should be used.

Moderate ground assistance may be required to maintain safety and stability.

The rock is predicted to be somewhat strong, and instability concerns may occur only in certain areas.

Analysis

To the center of the tunnel, the depth is estimated to be 20 meters. The conditions of rock masses may be affected by their depth. The stability of the tunnel and the behavior of the rock mass may be affected by the increased overburden pressure experienced by deeper tunnels.

Implications:

The in-situ stress conditions and the possibility for stress-induced deformations or rock explosions must be taken into account at this depth.

To evaluate the implications of depth on tunnel stability, more geotechnical studies, including stress measurements, may be necessary.

Selecting a GSI and a Set of Physical Parameters:

Using the Hoek-Brown equation, we shall determine the rock strength at locations A and B under Calga Place, Bronte. The Hoek-Brown equation connects geological parameters, such as the Geological Strength Index (GSI), to the rock's uniaxial compressive strength (UCS). The UCS must be determined at both A and B, and we must discuss the significance of these values.

The equation of Hoek-Brown is as follows:

UCS = σ3 * (m_i / s)^c

Where:

The rock strength (in MPa) is known as the uniaxial compressive strength (UCS).

The rock mass's GSI (Geological Strength Index) is represented by s, whereas c is another material constant and 3 is the minor primary stress (in MPa).

In light of this information:

• Tunnel centre depth (h) = 20 meters.
• The tunnel's diameter is 5 meters (D).
• A and B are one meter apart.
• Is50 = 1.0 MPa, point load index
• For point A: σ3 = 0 MPa, σ1 = 0.72 MPa
• For point B: σ3 = 0.435 MPa, σ1 = 0.99 MPa

Using the Is50 value, we must first determine the GSI of the rock masses at both locations:

GSI = (Is50 / σ3)^0.5

Determine the GSI at location A by:

Global Standardized Index (GSI_A) = (1.0 MPa / 0 MPa) ^0.5 = Infinity

Find the GSI at location B using the formula GSI_B = (1.0 MPa / 0.435 MPa)^0.5 = 3.08.

For the tunnel excavation project under Calga Place, Bronte, the GSI estimates have important ramifications. Let's talk about the ramifications and give reasons for picking GSI:

Point A with a High GSI

The infinite GSI at location A suggests that the quality of the rock mass is unparalleled. This indicates that the rock at point A is likely to be highly strong and stable, which is good news for anybody planning to dig a tunnel there.

To substantiate this claim, consider that the GSI value is a reflection of the quality of the rock mass as a whole, taking into consideration such things as joint spacing, orientation, and persistence. A GSI of infinity indicates that the rock mass contains nearly no geological flaws and is hence equal to entire rock. There is very little potential for instability, making this a perfect setting for tunneling.

At Point B, the GSI is Moderate

The lower GSI value (3.08) at location B suggests a lesser quality rock mass as compared to location A. when a result, it's possible that more reinforcement may be needed when the tunnel is dug at point B, where the rock is assumed to be weaker.

Argument in Favor: Although not very low, the GSI value at B indicates the presence of geological flaws or rock mass discontinuities that may cause the rock mass to be weakened. Tunnel stability may need technical interventions like reinforcing, bolting, or ground support systems at a modest GSI.

Decision on a GSI

The Is50 (point load index) value of 1.0 MPa was utilized to determine the GSI values employed in this study. Geotechnical engineers frequently resort to using Is50 as a proxy for GSI in the absence of hard data on the latter.

The Is50 test provides a quick and easy way to get a ballpark estimate of the GSI, which is a strong argument in its favor. It's not as precise as GSI measurements taken on-site, but it's good enough for quick evaluations. Until more extensive geological investigations are done, it is assumed that the Is50 value adequately represents the rock mass attributes.

As a result, point A is anticipated to have a highly strong and stable rock mass, which is good for tunnel excavation given the results. Additional technical measures may be necessary to maintain tunnel stability at Point B, which has a moderate GSI. These ramifications are only preliminary; further geological exploration is required to pin down precise UCS values, material constants, and ground support specifications for the tunnel.

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