"Hard rock has a compressive strength of over 30MPa, is strong and durable, and is an ideal choice for engineering structures; soft rock has high porosity and is easy to deform, requiring special treatment to ensure stability. Understand their differences to ensure engineering safety."

Rocks on Earth can be divided into different types, one of the most common classifications is soft rock and hard rock. These two types of rocks have significant differences in properties, composition and uses. The following will introduce the differences and characteristics of soft rock and hard rock in detail.

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1. Division

The main difference between hard rock and soft rock is their compressive strength, porosity and structural tightness.

‌Hard rock‌: Generally speaking, the uniaxial compressive strength of hard rock is usually greater than 60MPa, and the saturated uniaxial ultimate compressive strength is greater than 30MPa when not weathered. Hard rocks include granite, diorite, basalt, gneiss, quartzite, marble, etc.

Soft rocks: In geology, soft rocks usually refer to loose, loose, soft, and weak rock layers with low strength, high porosity, poor cementation, significant influence of structural surface cutting and weathering, or containing a large amount of expansive clay minerals. This type of rock is mostly mudstone, shale, siltstone, and mud sandstone, etc., with a uniaxial compressive strength of less than 25 MPa. The International Society for Rock Mechanics defines soft rock as a type of rock with a uniaxial compressive strength (σc) between 0.5 and 25 MPa

II. Differences

(I) Mechanical properties

Compressive strength

The compressive strength of hard rock is usually greater than 30 MPa, and the specific range is between 30 MPa and 60 MPa. According to different classification standards, the compressive strength range of hard rock varies. For example, some standards call rocks with uniaxial saturated compressive strength greater than 60 MPa "hard rock", rocks with uniaxial saturated compressive strength between 30 MPa and 60 MPa "relatively hard rock", and rocks with less than 30 MPa "soft rock" ‌1.

The compressive strength of soft rock ranges from 5 to 30 MPa ‌‌12. According to different classification standards, soft rock can be further subdivided into relatively soft rock (compressive strength of 15 to 30 MPa), soft rock (compressive strength of 5 to 15 MPa) and very soft rock (compressive strength less than 5 MPa) ‌3.

The engineering properties of soft rock include unconfined compressive strength less than or equal to 300 kg/cm2, poor water stability, softening coefficient not greater than 0.6, low deformation modulus and significant rheological effect ‌5. These characteristics require special attention to the stability and deformation of soft rock in engineering applications.

Hard rock is widely used in engineering, mainly for structural parts that bear greater pressure, such as foundations, tunnel linings, etc. Due to its high strength and good physical properties, hard rock plays an important role in civil engineering.

Tensile strength and shear strength

Hard rock has a relatively high tensile strength. The strong bond between its internal mineral particles and the dense structure of the rock enable it to withstand certain stresses when subjected to tensile forces. For example, the tensile strength of basalt is generally between 10 and 30 MPa. In rock slope engineering, hard rock can better resist the tensile stress caused by the deadweight of the mountain and other external forces, reducing the occurrence of slope cracks.

The tensile strength of soft rock is very low, because its mineral composition and structural characteristics make it easy for mineral particles to separate when subjected to tensile forces. The shear strength of soft rock is also low, and when subjected to shear forces, it is easy to slide along the weak surfaces inside the rock (such as bedding, cracks, etc.). For example, when mudstone is subjected to a certain shear force, it is easy to shear failure due to the properties of its clay mineral composition and weak particle bonding.

(II) Physical properties

Porosity

The porosity of hard rocks generally ranges from 1% to 30%. The specific value depends on the type of rock and the conditions of formation. For example, the porosity of sandstone is generally 10% to 25%.

Porosity refers to the ratio of the volume of pores in the rock to the total volume of the rock. The calculation formula is: φ = Vp/Vt × 100%, where φ is the porosity, Vp is the volume of pores in the rock, and Vt is the total volume of the rock.

The porosity of soft rocks is usually between 10% and 40%. Porosity refers to the percentage of the total volume of pores in the filler to the total volume of the filler, which directly affects the permeability, strength and stability of the filler.

Porosity has a significant impact on the properties of soft rocks:

Permeability and drainage: When the porosity is low, the filler is dense, and the permeability and drainage are poor; when the porosity is high, the permeability and drainage are good, but the stability and bearing capacity will be reduced. 1.

Strength and stability: A porosity between 15% and 25% is an ideal choice, which can ensure both permeability and drainage, as well as stability and bearing capacity.

III. Characteristics

(I) Hard rock

The main characteristics of hard rock include high strength, corrosion resistance, good toughness and high compressive strength. Hard rock usually has high mechanical strength, and its uniaxial saturated compressive strength is usually greater than 30MPa, and can even reach hundreds to thousands of kilograms per square centimeter. This high strength makes hard rock highly stable and durable in engineering applications.

(II) Soft rock

Soft rock is a type of rock composed of fine-grained particles, and its main characteristics are high water content, soft texture and easy deformation. Soft rock is widely distributed on the earth's surface, including mudstone, claystone, shale, sandstone, etc.

Soft rock has special engineering properties in engineering construction. Due to its soft and deformable characteristics, soft rock needs special treatment in the fields of tunnels, water conservancy projects, foundation treatment, etc. to ensure the safety and stability of the project. For example, soft rock is the key research object in geological surveys of water conservancy and hydropower projects because of its low shear and compressive strength, easy to soften and disintegrate when exposed to water, and prone to pipe gushing and erosion under the action of high water head for a long time.