Can you please share photos of the planting site as well as the symptoms they developed? The construction trash issue is probably not responsible, though if the soil was badly compacted by heavy foot traffic or machinery, that could be part of the problem. (It sounds like you may have amended the soil upon planting, but we aren't sure.)

Irrigation systems don't always provide water at the right time or in the right amount of they are run on a timer/programmed schedule. It's also possible that soil drainage varies enough between one end of the planting to the other such that some plants, even if watered the same, are getting too much or too little water compared to the rest. (Buried construction debris is a common culprit, but isn't necessarily the reason.)

Is the irrigation using a drip line, soaker hose, or sprinklers? When are they programmed to run, and for how long each session? As an example, in this heat, shrubs that are still establishing (during their first 2-3 years in the ground) may need several gallons of water per plant per watering, and might need a soaking once or twice a week depending on soil type and drainage. Ideally, the soil should be checked before watering is done, to make sure it's really needed. Feel the soil around six inches deep next to the roots. If it's somewhat dry to the touch at that depth, watering is probably needed, at least during warm weather. If damp when checked, watering is probably not needed. Both over- and under-watering can cause very similar symptoms, and plants that wilt or suffer leaf browning or shedding can do so under both circumstances, so the appearance of foliage and branch dieback alone can be hard to diagnose with regard to which situation contributed to the damage. Inspecting the roots directly can help separate the two, but that requires digging the plant up, so is a last resort if a plant is going to be replaced.

If any stem tips have turned brown (not just the leaves, but the stem itself), then they will not recover and need to be pruned off down to wherever they connect with still-living branches. Foliage that has partially browned (scorched) won't recover either, but the branch itself is probably alive and may leaf-out again either later this year or (more likely) next spring, and the damaged leaves might shed before winter or during new growth next spring. If too great a portion of any shrub has died and turned brown (or the paler, sickly gray-green or olive-green that is typical of dying cherrylaurel leaves, which precedes them dying fully), then the plant is best replaced as it will not recover well, if at all.

We suspect the issue centered around watering, though can't tell yet if it was too much or too little. Were the root balls loosened well before planting, if the laurel were container-grown? (If burlapped instead, was the burlap removed, along with any twine or wire ties around it, before planting?) If not, this can hamper establishment and make it harder to water the plants, since the soil type within the root ball is very different than the soil surrounding it, which impacts how each absorbs and retains water that the roots can access.

If you're able to share photos, we'll try to narrow-down the diagnosis. You can attach up to three photos per submission/response, but you can always reply again to this same question thread with other attachments if you want to submit more than three pictures. (Please have photo files be at least 1MB in size so we can see enough detail.)

Miri