Subsurface Investigations for Stable Excavation Support Using Nailing

Introduction

Subsurface investigations are critical for designing and implementing stable excavation support systems using the nailing method. These investigations aim to define and stabilize subsurface conditions, determine soil disturbance levels, and assess the lateral and spatial extent of soil at the nailing site. Typical subsurface studies include:

  • In-situ tests to characterize soil and rock properties

  • Sampling of soil or rock for visual classification and laboratory testing

  • Geological stratigraphy assessment

  • Identification and monitoring of groundwater conditions

Soil and Rock Properties

Laboratory and in-situ tests help determine soil classification, index parameters, strength, compressibility, and corrosion potential. Groundwater conditions are especially important for nailing projects; a high groundwater table can complicate and increase the cost of installation.

In-Situ Testing and Sampling

Exploratory drilling usually accompanies soil sampling and local tests essential for nailing wall design. Drilling provides:

  • SPT blow counts (“N”) useful for soil classification and stratigraphy

  • Disturbed and undisturbed soil samples

  • Groundwater level observations

The type, number, location, and depth of drillings depend on the project phase, availability of geotechnical data, subsurface variability, and other constraints. For nailing walls longer than 30 meters, drilling is recommended every 30 to 60 meters along the wall centerline.

Drilling is performed both in front and behind the proposed nailing wall. Behind the wall, drillings should be spaced 1 to 1.5 times the wall height, with intervals around 45 meters. For sloped backfills, the drilling distance increases to about 1.5 to 2 times the wall height. Drillings in front of the wall should be located at a distance greater than 0.75 times the wall height, with about 60 meters spacing.

Drilling depth depends on the wall stability mechanisms and subsurface factors affecting the wall’s performance. Typically, drilling extends at least one full excavation height below the excavation bottom. Presence of soft, highly compressible soils, or deep saturated layers may require deeper drilling, especially in seismic areas prone to liquefaction. If rock is encountered, at least a 3-meter core (two 1.5-meter segments) should be extracted to examine rock nature and continuity.

Table of Common Geotechnical In-Situ Processes and Tests

Activity Standards (ASTM, FHWA) Suitable for Not Suitable for Data Obtained
Soil sample protection and transport ASTM D4220-95 All soils N/A Samples
Thin-walled sampling ASTM D1587-00 Clay, silt Sand, gravel Undisturbed samples
Subsurface exploration (soil & rock) ASTM D5434-97 All soils N/A Variable
Standard Penetration Test (SPT) ASTM D1568-99, ASTM D6066-96el Sand, silt – Stratigraphy, SPT N-values, relative density, groundwater, samples
Cone Penetration Test (CPT) ASTM D5778-95 Sand, silt, clay Gravel, rubble Stratigraphy, soil type, strength, relative density, lateral pressure coefficient (K0), pore pressure
Vane Shear Test (VST) ASTM D2573-94 Soft to medium clay Sand, gravel Undrained shear strength
Pressuremeter Test (PMT) ASTM D4719-00 Soft clay, dense sand, soft rock Loose sand, silt Soil type, strength, K0, OCR, compressibility, soil modulus
Dilatometer Test (DMT) ASTM D6635-01 Sand, clay Gravel Soil type, K0, OCR, undrained shear strength, modulus

Standard Penetration Test (SPT)

The SPT is widely used for local evaluation of nailing wall soils. It provides the number of blows (Nmeas) needed to drive a split-spoon sampler 300 mm into soil at test depth. Several correlations relate SPT N-values to soil engineering properties, useful in feasibility and design.

SPT also helps obtain disturbed soil samples from 1.5 to 3 meters depth. Sampling should continue in soft or heterogeneous layers to better characterize soil conditions. SPT offers a good estimate of relative density for cohesionless soils (Table 2) and can evaluate strength of fine-grained soils despite limitations (Table 3).

Table 2: Relative Density of Cohesionless Soils by SPT N-values

  • Very Loose: 0–4 blows/300 mm

  • Loose: 5–10

  • Medium Dense: 11–30

  • Dense: 31–50

  • Very Dense: >50

Table 3: Strength of Fine-Grained Soils by SPT N-values

  • Very Soft: 0–1 blows/300 mm

  • Soft: 2–4

  • Medium Stiff: 5–8

  • Stiff: 9–15

  • Very Stiff: 16–30

  • Hard: 31–60

  • Very Hard: >60

Corrected N-values (N60) account for energy efficiency, borehole diameter, sampling method, and rod length. Normalized values (N1)60 adjust for effective overburden pressure.

Sampling

SPT samples are disturbed and suitable for classification, grain size, moisture content, Atterberg limits, unit weight, organic content, and unconfined compressive strength tests. For shear strength and compressibility testing, undisturbed samples from thin-walled samplers (e.g., Shelby tubes) should be used to avoid disturbance effects.

Other In-Situ Testing Techniques

SPT is less reliable for evaluating undrained shear strength (Su) of fine-grained soils. Alternative in-situ tests include CPT, VST, DMT, and PMT:

  • CPT: Rapid, continuous profiling for soil stratigraphy, soil type, strength, relative density, K0, and pore pressure. More economical and faster than SPT, though no sample recovery. Less applicable in gravel or rubble soils.

  • VST: Measures undrained shear strength in soft to medium clays.

  • PMT: Provides soil strength, K0, OCR, compressibility, and modulus.

  • DMT: Used in sands and clays for strength and deformation parameters.

Trial Pits

Trial pits provide direct visual and stability evaluation of excavation faces without support. Useful in flat or sloped terrains, typically 6–8 meters long and 2–2.5 meters deep, parallel and in front of the proposed wall. Left open for 3–4 days for stability monitoring.

Stratigraphy

Once sufficient drilling and in-situ test data are collected, soil stratigraphy maps should be prepared to identify critical subsurface variations affecting wall design and construction, such as layer thickness, continuity, and disturbance potential.