Resilient modulus prediction of soft low-plasticity Piedmont residual soil using dynamic cone penetrometer

Dynamic cone penetrometer（DCP） has been used for decades to estimate the shear strength and stiffness properties of the subgrade soils. There are several empirical correlations in the literature to predict the resilient modulus values at only a specific stress state from DCP data, corresponding to the predefined thicknesses of pavement layers（a 50 mm asphalt wearing course, a 100 mm asphalt binder course and a200 mm aggregate base course）. In this study, field-measured DCP data were utilized to estimate the resilient modulus of low-plasticity subgrade Piedmont residual soil. Piedmont residual soils are in-place weathered soils from igneous and metamorphic rocks, as opposed to transported or compacted soils.Hence the existing empirical correlations might not be applicable for these soils. An experimental program was conducted incorporating field DCP and laboratory resilient modulus tests on "undisturbed" soil specimens. The DCP tests were carried out at various locations in four test sections to evaluate subgrade stiffness variation laterally and with depth. Laboratory resilient modulus test results were analyzed in the context of the mechanistic-empirical pavement design guide（MEPDG） recommended universal constitutive model. A new approach for predicting the resilient modulus from DCP by estimating MEPDG constitutive model coefficients（k;,k;and k;） was developed through statistical analyses. The new model is capable of not only taking into account the in situ soil condition on the basis of field measurements,but also representing the resilient modulus at any stress state which addresses a limitation with existing empirical DCP models and its applicability for a specific case. Validation of the model is demonstrated by using data that were not used for model development, as well as data reported in the literature.

Application of automated cone penetrometer for railway investigation using correlations with DCPI and deflection modulus

Portable in-situ devices have been used for characterizing low accessible field,such as the railway subgrade.In this study,the automated cone penetrometer(ACP)was designed for the application on the railway subgrade.ACP is composed of the cone tip,driving rod,and hydraulic hammer system.The hydraulic motor lifts and drops the 294.3 N hammer from a height of 200 mm such that the potential energy of 58.9 N m impacts the driving rod.The N-value(NACP)from the ACP test was compared with the dynamic cone penetration index(DCPI)from the dynamic cone penetrometer(DCP)test.The test results show that the NACP and DCPI profiles show opposite trend owing to the inverse concept of the unit.From the correlation of DCPI and NACP,the limitation of DCPI reveals owing the minimum manually measured value of 1 mm/blow.Additionally,the evaluation of the deflection modulus(EFWD)using NACP is more efficient than that using DCPI.Based on the result of this study,we suggest that ACP can be used for strength and stiffness evaluation of railway subgrade rapidly and reliably.

A Proposed Framework for Optimised Utilisation of Materials for Low Volume Roads Using the Dynamic Cone Penetrometer

In order to minimise the cost of constructing low volume roads (LVRs), it is essential that optimum use is made of locally available, naturally occurring materials. However, conflicts often arise between material acceptability, as defined by conventional test methods and specifications, and material suitability in terms of actual engineering performance as a “fit-for-purpose” road construction material. To avoid such conflicts, it is crucial to adopt appropriate test methods and specifications for selecting construction materials. This paper presents a materials evaluation framework for optimizing the utilisation of materials in LVR pavements based on the use of the Dynamic Cone Penetrometer (DCP). This approach enables materials to be selected on the basis of their “fitness-for-purpose”. It avoids suitable materials from being rejected based on one or other of the traditionally specified parameters not being complied with, even though their strength, as measured by the DCP penetration rate (DN value in mm/blow) of the soil, may be adequate. Investigations of the properties of a wide range of locally available materials that have been used successfully in the construction of LVRs have confirmed the validity of the materials evaluation framework.

Performance evaluation of cone penetrometer device for measuring the subsoil compaction in mulched plots

Soil exhibits layers of extreme compaction from both natural causes and wheel traffic.These compaction layers impede root growth,thereby reducing the plants capacity to obtain water during drought.Subsoil tillage is a remedy for adverse soil compaction that results in improved conditions for crop growth.Mechanical disturbance of subsoil increases water holding capacity and reduces impedance to root penetration.Vertical mulching is a technique that can be used to partially alleviate soil compaction within the critical root zones of deep rooted crops.A study was conducted by placing raw and composted coir pith using a two row subsoil coir pith mulching machine in three different soil depths(250,350,and 450 mm)at the three application rates of 15 t/ha,20 t/ha,and 25 t/ha and the effect of soil strength was investigated.The experiment was conducted for a rainfed cotton crop.The soil strength profile was recorded in all the treatments.The cone penetrometer resistance was measured for each increment of 10 mm and recorded manually from a digital force indicator during maturity stages of crop in all the treatment plots.The cone penetrometer resistance was measured directly on the row and the cone index was computed.Deep placement of mulch reduced the soil strength as compared to shallow placement.The lower soil strength(0.5 kPa to 0.8 kPa)in the loosened and mulched zone provided an impedance free zone for the root to proliferate.The rapid increase in cone index values at depths immediately below the respective depth of placement(250,350 and 450 mm)of raw and composted coir pith mulch indicated that the existence of undisturbed soil profile below the mulched zone which could be potential limiting factor for root development.

Application of wave equation theory to improve dynamic cone penetration test for shallow soil characterisation

Among the geotechnical in situ tests,the dynamic penetration test(DPT)is commonly used around the world.However,DPT remains a rough technique and provides only one failure parameter:blow count or cone resistance.This paper presents an improvement of the dynamic cone penetration test(DCPT)for soil characterisation based on the wave equation theory.Implemented on an instrumented lightweight dynamic penetrometer driving with variable energy,the main process of the test involves the separation and reconstruction of the waves propagating in the rods after each blow and provides a dynamic cone load-penetration(DCLT)curve.An analytical methodology is used to analyse this curve and to estimate additional strength and deformation parameters of the soil:dynamic and pseudo-static cone resistances,deformation modulus and wave velocity.Tests carried out in the laboratory on different specimens(wood,concrete,sand and clay)in an experimental sand pit and in the field demonstrated that the resulting DCLT curve is reproducible,sensitive and reliable to the test conditions(rod length,driving energy,etc.)as well as to the soil properties(nature,density,etc.).Obtained results also showed that the method based on shock polar analysis makes it possible to evaluate mechanical impedance and wave velocity of soils,as demonstrated by the comparisons with cone penetration test(CPT)and shear wave velocity measurements made in the field.This technique improves the method and interpretation of DPT and provides reliable data for shallow foundation design.

Development of a New Dynamic Lightweight Penetrometer for the Determination of Mechanical Properties of Fine-Grained Soils

Dynamic cone penetrometer is mainly used as an in situ device and laboratory application, in a mould, has rarely been reported due to the confining effect. In this study, a dynamic lightweight cone penetrometer that can be used in a CBR （California bearing ratio） mould in the laboratory as well as in the field, with similar results, was developed. The results show that the influence of the mould confinement can be eliminated when the hammer mass is 2.25 kg. A strong correlation was found between CBR values and the new dynamic lightweight penetrometer index, for six fine-grained soil samples, with different moisture contents, used in this study.

Deterioration mechanism and rapid detection of performances of an existing subgrade in southern China

To relieve the increasing traffic load, many early built highways need to be widened or reconstructed. The rapid performance detection to existing subgrades is important to their reasonable evaluation and maximized utilization. Based on five kinds of soils taken from an existing highway in southern China, three commonly detecting methods were used to determine their moisture contents, compaction degrees and resilient moduli. The results showed that the measured moisture contents were greater than the design value, and the compaction degrees decreased sharply compared to the original ones. The moisture and heat exchange produced a decrease in the resilient modulus of plate loading test(PLT) from the standard 60 MPa down to 40 MPa. Afterwards, the portable falling weight deflectometer(PFWD) and dynamic cone penetrometer(DCP) were used to evaluate the subgrade performances. The measured PFWD moduli and the DCP penetration rates were correlated with the resilient moduli of PLT, deflections of the Beckman beam test, compaction degrees and moisture contents. The correlation analysis indicates that both of two methods are suitable in rapid detecting subgrade performances, but PFWD method is more recommended for it has higher accuracy and efficiency.